31.1.16

Smart & Brown Sabel belt replacement Pt.3

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To allow more room for more images without causing slow loading I have added a third part to this series on the 'Sabel' lathe.

I have taken more images with the lathe headstock partially dismantled. I also wanted to be sure I hadn't forgotten anything since I changed the belts back in January. Now some nine months ago.

The first image shows the compete headstock shaft withdrawn after removing the threaded locking collar at the far end. [see last post for details of removing the collar screw and unwinding the split locking collar]

Note the two shoulders which will make replacement seem far more difficult.

Force is NOT required to replace the shaft. It requires that each end of the shaft is lifted to allow the two shoulders to slide through the intermediate components and headstock bearings.


The next image shows the felt oiler for the front, plain headstock bearing. The lower end of the felt sits in a reservoir of oil filled when each of the knurled plugs is removed.

The ends of each of the bearings in the casting has an oil catcher rim and drain hole for oil which leaks out from the bearing. I cleaned these before replacing the shaft. Note the white metal bearing liner with lubrication slots.
The iron headstock pulleys and back gear removed from the headstock.

Note the click spring for the back gear engage/disengage button. The button is normally pressed home to the left. When the back gear is engaged the button must be pulled out to free this gear to rotate in mesh with the back gear layshaft gears. Otherwise the headstock will be locked up solid.

A close up of the key in the headstock shaft. This key engages with the large [back gear] gear shown above. Normally the large iron back gear rotates with the main shaft without touching the teeth of gear on the back gear lay shaft. Only when the button is pulled out and the back gear knob rotated, will the back gears be engaged in mesh with each other.

Oil the complete shaft well before reassembly in the headstock.
The lower end of back gear layshaft pin is exposed by rotating the back gear knob completely. [as shown] This tapered pin must be tapped out from the underside of its normal position to free the back gear shaft. I like to use a small riveting punch with a spherical hollow in the tip to avoid mushrooming the tapered pin. There is no need for a big hammer nor any force. If the pin won't come out easily then check you aren't trying to drive it further in from the wrong end.

Both back gear eccentric bushes must be aligned with each other to allow the back gear knob to operate the backwards and forwards movement to engage and disengage back gear.

The inside of the headstock casting showing the smaller, plain, headstock bearing on the left.

When reassembling the headstock shaft it will feel as if it will not go back in. The shaft must be raised at both ends for its steps to enter the main bearings.

Note that the 3 components of the linear thrust bearing must be slipped onto the tip of the shaft just before it can be passed through the left headstock bearing. The three parts want to face each other in the same way as original to avoid causing problems. Just look for the wear tracks and arrange the rings and central ball bearing race the same way. I lay the big iron gear, the iron pulleys and the linear thrust bearing in the headstock casting before re-inserting the shaft. If you had the patience they could be packed up to their correct height. Though you'd have to be able to remove the packing once the shaft is safely back in place.

Remember that the threaded locking collar must also be slipped over the tip of the shaft just before it passes the tumbler gears. The tumbler gears can be removed but it is much easier to leave them in place to avoid extra and unnecessary work. The white washer is PTFE low friction plastic. It goes behind the collar against the headstock casting.

It feels like a real fiddle to get the headstock shaft back in but it is only a matter of patience lifting each end in turn. It must obviously be  parallel with the lathe bed to go in smoothly. Try rotating the shaft back and forth as you push the shaft gently to the left to allow the key to find the slot in the large iron gear. I can assure you that absolutely NO force is necessary to get the headstock shaft back in. Just try lifting the iron pulleys, the large iron back gear and the linear thrust bearings in turn to allow the shaft steps to pass each hurdle. Three hands may seem useful but are strictly unnecessary. I removed the rear belt shield for the photographs. You may find it helps as long as you remember to thread the belt through it or you will have to start completely from scratch!

Disposable rubber gloves are handy when working on dismantling and reassembly of the lathe. The thin rubber aids sensitivity of touch while still protecting the hands from dirt and oil.


Click on any image for an enlargement.
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Smart & Brown Sabel belt replacement Pt.2

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Try refreshing the page to ensure you see the latest edition.

Click on any image for an enlargement. I have kept the images deliberately small to avoid slow loading.

I offer this advice for owners of 'Sabel' lathes only and using your own manual skills and intelligence to avoid injury and damage. Since I cannot supervise your work I accept no responsibility for your own actions. I am always happy to advise with this gentle warning clearly in mind. NO force is required to accomplish anything during dismantling and reassembly.

Part 2: A request for advice on fitting a new continuous belt to a S&B 'Sabel' lathe required rather more room than I could easily add to the already long, original post on linked belts. See next Older Post. [and next NEW Post for Pt.3]

http://fullerscopes.blogspot.dk/2016/01/my-smart-brown-sabel-lathe-images.html

NOTE: YOU CANNOT USE A NuTLink LINKED BELT ON THE S&B 'SABEL'. [As seen in these images] There is not remotely enough room in the headstock casting below the iron pulleys. It will fit on one pulley but is then impossible to change gear on the pulleys. The NuTLink is considerable oversized [and expensive] compared with a standard 13mm wide 'A' belt. I understand the NuTLink WILL fit on some Boxford lathes.

After struggling endlessly with the linked belt I bought a normal "A" size [13mm wide] replacement belt from a local car spares shop. These were available in a wide range of lengths in small increments. I chose one of 1325cm after measuring the circumference of the linked belt with a cloth tape measure. Remember to measure on the same pulley size if you want to double check your own lathe.

Another reason not to use linked belts: They rub away the pulley cover speed plate inside the hinged headstock cover. I was lucky to avoid this problem. Sorry about the quality of the image. Dark and flash caused flare and the overhead lights didn't help.

NOTE: that belt this size may not match your own layshaft:headstock spacing depending whether you have the S&B cabinet stand and/or a different layshaft placement.

Removal of the pulley layshaft is very easy.
The bearing are plain bronze bushes with nothing loose to fall out. Make sure there is no belt tension first. The large lever on the right pulls forward to remove belt tension for easy gear changing. There is a screwed belt tensioner just behind the lever. Pull the lever forwards before trying to adjust tension. Turn the adjuster and then push the lever back again to check the tension. I adjust the belt tension so that the lathe can run up to the highest speeds in a couple of seconds.

Then undo the grub screw [arrowed] on the small end of the alloy pulleys, just enough to withdraw the large pulleys to the left taking the shaft with them.

Once the multiple pulleys are free you can thread your new belt around them. Then replace the shaft by pushing it back in to the right. You may choose the dismantling moment to clean the pulleys and support casting. Re-tighten the grub screw and that part is finished.

The headstock is more complicated.

NOTE: I usually leave the small 3-jaw chuck in place to balance the headstock shaft and heavy iron pulleys. I just find this easier than struggling with the bare shaft. You may not agree but you can't easily change your mind half way through the job.

Undo the two screws arrowed and remove the small tumbler gear cover.

Once the tumbler gears are exposed you will see a split lock ring with a cross screw. Loosen and remove the locking screw with a good screwdriver to avoid damage to the slotted head.

The locking ring has a fine thread but this is cut deliberately tight on the headstock shaft.

You need to insert a wedge gently into the split to free the locking ring just enough to allow it to be undone. I use a slow tapered screwdriver as a lever in the split and turn the chuck by hand to unwind the locking ring.

Once the ring has been removed the headstock shaft can be withdrawn in the direction of the chuck.

Note that the iron pulleys are quite heavy and also keyed to the headstock shaft. Replacement can be fiddly.

NOTE: There are two SPRUNG felt oilers underneath the headstock shaft within the plain headstock bearings. DO NOT damage these when replacing the stepped headstock shaft.

DO NOT TOUCH the large bearing clamping screws. It is completely unnecessary and will undo any former careful adjustment for headstock play. YOU HAVE BEEN WARNED.

The back gear shaft is held with a cross pin. Use a punch to remove the pin then wiggle the back gear knob to withdraw the back gear shaft.

NOTE: The back gear shaft is held in eccentric bushes at both ends. These must be replaced in the correct rotation relative to each other to ensure the back gear will move correctly in its eccentric bushes when the knob is withdrawn and rotated. DO NOT be in a hurry to replace the tapered pin until the shaft moves back and forth correctly as the back gear knob is pulled out and turned.The pin on the inner face of the back gear knob is the location device for each position in its rotation.

Click on any image for an enlargement.

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My Smart & Brown 'Sabel' lathe belt problems.

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I have posted these rather poor flash images to show a belt clearance problem with the headstock casting since changing to Nu-T-link 'A' belts. It is completely impossible to change gear without parting the belt, re-threading it onto another pulley pair and then rejoining it.

The view from the front shows close but just acceptable clearance.
The rear view shows a thicker shelf in the bottom of the casting below the pulleys. [Arrowed]

I have repeatedly promised myself that I would repaint the lathe at some point. This "roundtoit" has lasted for nearly 30 years! The cleaning has gone downhill too over the past two decades. Normally nobody else would ever see it.


The motor's two speed pulleys x four pulley layshaft offers eight gears plus back gear. The large, cast handle on the right releases the belt tension. Changing the belt from one pulley pair to another is entirely manual but normally quick and easy. A turnbuckle sets belt tension.

The older "B" round riveted black linked belting is also unsuitable for the 'Sabel.' Being undersized, noisy and very prone to slippage despite nominal 'A' specification. I had quite forgotten that the higher speeds were possible so severe was the slippage as the rivets bottomed in the pulley grooves.

Overview of headstock. The Nu-T-Link belting is so bulky it will not move between the four, headstock pulleys. This is mostly due to the over-sized belt dimensions plus the projecting riveting striking the casting below the pulleys.

The Smart & Brown "Sabel" offers 8 speeds on the pulleys but has no gearbox. Plus back gear of course. The reversible lead screw provides reversible apron/carriage and cross slide feeds. A full set of change gears provides different feeds and thread pitches. The sable uses  split, plain bearings with a ball bearing linear thrust bearing.
Closer view of over-sized Nu-T-Link belting on the 'A' sized headstock pulleys.

The large gears are for the back gear drive to the headstock. A pull-out button disengages the belt drive to the pulleys to avoid clashing between two, very different driving speeds. A large indexed knob at the far left of the headstock engages back gear.


Expensive box of 5 meters of Nu-T-Link belting.

It clearly states 'A' size and 13mm but the Nu-T-Link is actually 14.5mm wide at the tops of the links. The depth over the rivets is 15.5mm! 'A' Spec is 1/2" [12.5mm] x only 8-9mm deep. A very expensive mistake!

I have now ordered a 13mm 'A' x 1325mm belt online from a local car spares outlet. The same outlet which claimed they couldn't help me because they don't know the dimensions of their V-belts! This, despite having dozens of sizes available and all sold by their precise dimensions. Downside with a simple belt is having to completely strip and rebuild the headstock including the back gear. Removing the layshaft is relatively easy. That said, a modern V-belt belt should last for years with reasonable care.

The 'toothed' V-belt arrived next day and it took me only half an hour to fit including some cleaning. The racket coming from the NuTLink belting on the motor pulleys had me quickly reverting to a simple V-belt. I now have no idea why I ever bothered with this expensive and noisy alternative belting. It might make some sense in a commercial environment, where costly dismantling or downtime must be avoided. Elsewhere the 'humble' V-belt is very obviously superior. It is difficult to photograph the lathe due to the narrow space between it and the opposite wall. The overhanging layshaft and steel cabinet add to the problem. Seeing these images jolted me out of my laziness and I gave the painted parts of the machine a spray with water based, engine cleaner and wiped it all down.

Click on any image for an enlargement.
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27.1.16

7" f/12 iStar folded refractor 10: Achieving real instrument mobility?

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There still remains the matter of optical layout. I had intended a 3-fold parallel layout with two mirrors.  A crossed or figure-of-4 layout has certain advantages for relaxed viewing of objects at modest altitudes. Over a certain object height it is arguably best to use a normal star diagonal for comfort. Below that point the star diagonal become a nuisance unless the observer sits to one side of the OTA. The more complex folding forms, mentioned above, require larger flats or a more bulky OTA. The simple 3-fold design is compact and maximizes the sizes of the folding mirrors by having smaller angles of reflection. The layout can also be neatly arranged in a simple, narrow, but taller, rectangular, plywood box. The dewshield can be added as a friction fit once the box is on its mounting.

I have to admit to not really wanting a "Newtonian" folded design. Simply because I hope to be able to sit behind the OTA while observing. This allows a quick squint up the tube possibly through simple ring sights. It also provides easy access to a straight through finder. The relatively short OTA has only a small arc of movement at the eyepiece. Rather like using quite a modest refractor but far more bulky in girth only. Provided the observer can reach the eyepiece, in the middle point of its travel, then any object above or below should be reachable by bending or stretching slightly.

This small movement can probably be accommodated from a comfortable chair. Or even an adjustable height, padded stool. I have an example of the latter but it needs attention to make height adjustment far more sensible. At the moment it requires a long bolt be withdrawn, the seat refitted as the bolt is pushed through the seat support and a finally, a knob tightened for security. Better, by far, to be able to simply lift the seat off and then hook it back on over any one of a series of rungs. This is easily achieved with a few lengths of studding and  a piece of channel section fitted to the back of the seat.

Such seats are available but cost silly money which I'd much rather spend elsewhere. These seats are really designed to cope with working people of different heights. Once set they tend to remain that way. They cannot be raised far without the user's feet losing contact with the ground. Sideways stability can then become an issue on soft ground. An adjustable or folding footrest would allow greater variability of seated height.

There is little point in allowing horizontal viewing angles except for bird watching. So that reduces the eyepiece arc quite usefully to [say] 30 to 90 degrees viewing altitude. Lower objects usually suffer from poor seeing anyway. Besides, viewing objects at lower altitudes can be achieved simply by raising the observing stool. Or removing the star diagonal and looking straight through the eyepiece.The short OTA removes the problem of tube balance with changing eyepiece weight. Balance was a serious issue with the long straight tube and required a hefty counterweight.

A lower eyepiece height usually requires a lower mounting. There is no point in stretching just to be able to look at the zenith. Pointing at the zenith always expects the use of a star diagonal. This sets the minimum comfortable height of the eyepiece in use. The builder must decide in advance if that height should be reachable by nearly sitting on the ground. Or more comfortably seated at a normal office chair height. The former reduces the overall size of the instrument when mounted but my age greatly reduces its desirability. A more normal seated height makes a lot more sense.

I plan to mount the folded refactor on a Dobsonian style of mounting. Though probably not of the normal reflector style. That would demand very high sideboards meaning considerable weight to be dragged around. The rocker box would also get in the way of my knees. I am thinking more along the lines of a Berry-style, counter-weighted, offset fork. How to lift the fork to the required height needs consideration.

A tripod is a serious encumbrance for the user's legs when carrying or using it for observing. It is also difficult to make light enough while retaining enough stiffness. Nor does it readily lend itself to having wheels fitted. A four legged pier is perhaps more desirable. Four feet is far better than the usual three even if it does add some weight. The problem is a getting a stiff joint between the horizontal legs and the upright pole without adding weight.

Or, I could have a few prepared holes lined with loosely capped PVC pipe. These would allow the fork, on a pole, to be carried and simply dropped into place. There would be no need to carry the OTA from place to place until the mounting is safely set up. Nor even having the fork itself attached, if it can be easily fitted or removed from its supporting pole/pipe. The sunken PVC pipes must be capped to avoid becoming wildlife traps, tripping pedestrians or filling with muddy water. 15cm, 6" drainage pipes and fittings are readily available and I already have some useful lengths. I can't imagine a mere four foot length of 6" PVC pipe flexing much if securely mounted in the ground. My recent back pain has taught me a valuable lesson. I had completely failed to consider my expectations of weight lifting capacity into increasing old age. How I expected to lift a 40lb, 7' long OTA above my head well into my 70s I have really no idea.

Click on any image for an enlargement.

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25.1.16

7" f/12 Istar folded refractor 9: Folding mirror shell backing.

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Progress has been non-existent due to a combination of an extremely painful back, constant sub-zero temperatures and now snow. The forecast is for rising temperatures so I may be able to play in the workshop quite soon.

A new week and a completely new temperature. 43F @ 11.30am! Last week it was -8C, 17F with wind and 3" of lying snow.

A smart, 6" compact, 2-mirror, folded refractor for sale but with a serious problem with the objective. I would have mounted the OTA with the objective at the low side of the main tube and the eyepiece much more readily accessible. In its present arrangement the observer's head will hit the OTA backplate except at very low observing angles.

I was going to rout out the plywood backing disks for the mirror retaining shells. This would leave only a 1/8" 3mm hole in the middle offering some flexibility in layout and mirror support. The problem then is working out of doors in drizzle. It involves carrying a folding workbench outside too which is never kind to my back. The old B&D benches weigh a ton and are an awkward carry. Using electric tools out of doors in the wet is neither safe nor sensible and will rust the cutting bits. Not to mention soggy sawdust collecting everywhere.
 
I'll just have to drill out the center hole and spin the disks on a threaded rod mandrel in the lathe. This also provides fine tuning of disk size and fit in the shells. The router is more hit and miss and can leave a ragged edge which when sanded can cause rounding. Which doesn't provide quite the same support as a nice true parallel/cylindrical edge. Choice of material thickness of the disks suggests extra weight should be avoided. Though hole saws can considerably reduce the volume of heavy Birch plywood. Or I could even use a hand or electric saw to remove material in larger segments.

I'm still undecided as to retaining the shell's rolled rims. The baking tins were 3" deep because I couldn't find any 2" deep in the sizes I wanted. The extra inch adds another inch to the OTA's length unless the collimating cells project. Not a serious problem except that it adds unwanted vulnerability. The OTA cannot be simply put down on one end without considerable care. Better, surely, to sink the collimation screws just beneath the OTA's end faces?  Simple hex socket head screws should suffice to allow occasional, mirror collimation. If it needs collimating every time it is moved I shan't have done a very good job of supporting the tertiary mirrors.

The image shows the shells with carefully turned disk of 12mm [1/2"] birch plywood. I overshot on the first trial despite taking only tiny cuts. So I approached the final sizing on the next two disks by sanding the last ten-thou, or two, with a flat backing plate. The disks were spun on a 10mm stud in the 3-jaw chuck with external jaws fitted. These helped to ensure the disks stayed flat by pressed the plywood hard against the projecting jaws as the nut was tightened. The large washers ensured the nut did not dig into the face of the plywood and made the nut easy to turn. The image is an illusion as the smaller shell is in the foreground.


Click on any image for an enlargement.

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22.1.16

Fullerscopes reflector on eBay

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Fullerscopes Newtonian Telescope | eBay

"Fullerscopes 8.5" f6 Newtonian Reflector Telescope. Mounted on Original refurbished Mk111 Equatorial mount. Slow motion controls, pedestal base, setting circles. All in good working order.
Excellent David Hinds optics. Which may well need a recitation after all these years! Collection only please."


Apart from the items listed there is a finder just out of sight. There are also reinforcing cast end rings and an extra ring to allow the tube to be rotated without it sliding down through the normal tube rings. Ideal for obtaining a comfortable eyepiece position. Roughly translating "recitation" suggests the mirrors need re-coating. The tube looks to be unpainted grey PVC. The De-luxe version would have been painted gloss white.

I have considerably brightened the single auction image to bring out the detail. I can still remember looking though one of these belonging to members of the Bath Astronomical Society. It seemed like a very exotic and expensive bit of kit to me at the time. 

The seller is only looking for £149 "Buy it now." Collection only. West Sussex.

The seller has accepted an offer. Congratulations to the new owner!  


Click on any image for an enlargement.
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14.1.16

7" f/12 iStar folded refractor 8: Thermal effects of the folding mirrors.

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I have been considering the thermal mass of the folding mirror blanks as a potential problem. Perhaps one which would require forced ventilation or skeletonizing of the mirror cells and retaining shells. Then it occurred to me that I usually keep the OTA in unheated accommodation. The temperature of which hardly varies by more than a degree or two from inside to outside during the cooler half of the year.


Slitting the shell went smoothly and rather quickly using a fine blade in the jeweller's saw. Marking the slightly tapered tin needed a postcard to draw a line from both sides and then midway between the two with a sharp pencil.  Having to drag the workbench outside into better light did my back no good at all! A folding chair allowed me to get closer to the work without too much pain.

In summer the very short nights of semi-darkness at 55N do not make observation very practical anyway. Moreover, the OTA will be constructed from birch plywood. Which provides easily enough  insulation, in a well closed box, to slow any heat loss or gain from or to the air or glass within the [sealed] OTA. Since the flat mirror blanks are already near ambient they have little need to radiate any stored heat away. Nor are they exposed or enclosed in a [super-cooled] metal OTA. So need not radiate to the cold night sky.

The mirror blanks are of Zerodur so there should be almost zero thermal effect on the mirror's optical flatness under any normal circumstances. If the telescope were to be used for solar observation, without a full aperture objective filter, then the mirrors might slowly warm up. The likelihood of my using the telescope like this is very small. The cost of a few square inches of solar foil is as nothing compared with a Herschel wedge. Any likely improvements in clarity over Baader foil are very unlikely to sway me to invest in a costly Baader solar wedge. So the foil filter will keep all of the sun's heat out of the telescope when I do choose to look at the Sun.

Adding hefty lumps of metal to the OTA, other than the inevitable objective cell, makes little sense. It would add unwanted thermal mass. So the cells are best made of plywood too. Which has low thermal mass and low thermal capacity into the bargain. Common sense suggests that the supporting plywood disks for the cells should still be well ventilated. Which is easily achieved in plywood by means of wood drills and/or augers. Leaving behind the supporting ring and the mirror contact pads.

The 5" mirror propped up in its newly slit shell indoors. The gap (at 5 o'clock) is almost non-existent. I'm rather pleased how well it turned out as I was expecting a much larger gap. A better image taken outdoors will follow.

I have come to the conclusion there is really no need to retain the full 3" depth of the [baking tin] mirror shells. So I shall turn [or rout] closely fitting disks to go behind the mirror banks when they are up against their retaining lips. I shall use felt support pads behind my 10" mirror at the calculated points. These pads are meant to protect furniture from ornaments or plant pots. The pads should be ideal to support the mirror blanks without local pressure. Distortion of the blanks must still be avoided regardless of OTA orientation or temperature.

A hinge will be fixed between the cell backing disk and the supporting structure for collimation. It is fortunate that the mirror cells do not need to be minimized in size nor cleared of protrusions to reduce diffraction effects. The cells do not obstruct the light path so cannot cause such problems. The rims of the shells will reduce the clear aperture of the mirrors by only 2mm. Many argue that the extreme edge of flat mirrors should not be utilized to avoid potential turned down edge from polishing. Many mirrors would probably have a far better figure if the edge is ignored. The flat, optical folding mirrors can be moved away from the objective slightly to avoid any vignetting. Which might add a little to the overall length of the OTA but will not reduce the clear aperture of the objective nor the folding mirrors. 

Would you believe it!?! The moon is high overhead in a clear sky and I can hardly lift myself out of the chair, let lone lift a telescope. I was going to cut the disks for the cells with the router but couldn't manage its weight with my bad back. This is confirming my need for a compact telescope which can be rolled out, ready for use, preferably already on its mounting.

Progress has been non-existent due to a combination of a painful back, constant sub-zero temperatures and now snow. The forecast is for rising temperatures so I may be able to play in the workshop soon.

A new week and a completely new temperature. 43F @ 11.30am! Last week it was -8C, 17F.

I was going to rout the backing disk for the mirror retaining shells. This would leave only a 1/8" 3mm hole in the middle offering some flexibility in layout and mirror support. The problem then is working out of doors in drizzle. It involves carrying a folding workbench outside which is never kind to my back. The old B&D benches weigh a ton and are an awkward carry. Using electric tools out of doors in the wet is neither safe nor sensible.
 
I'll just have to drill out the center hole and spin the disks on a threaded rod mandrel in the lathe. This also provides fine tuning of disk size and fit in the shells. The router is more hit and miss and can leave a ragged edge which when sanded can cause rounding. Which doesn't provide quite the same support as a nice true parallel/cylindrical edge. Choice of material thickness of the disk suggests extra weight should be avoided. Though hole saws can considerably reduce the volume of heavy Birch plywood. Or even use a saw to remove material in larger segments.

I'm still undecided as to retaining the shell's rolled rims. The baking tins were 3" deep because I couldn't find any 2" deep in the sizes I wanted. The extra inch adds another inch to the OTA's length unless the collimating cells project. Not a serious problem except that it adds unwanted vulnerability. The OTA cannot be simply put down on one end without considerable care. Better, surely, to sink the collimation screws beneath the OTA's end faces?  Simple hex socket head screws should suffice to allow occasional, mirror collimation. If it needs collimating every time it is moved I shan't have done a very good job of supporting the tertiary mirrors.

There still remains the matter of optical layout. I had intended a 3-fold parallel layout with two mirrors.  A cross or figure-of-4 layout has certain advantages for objects at modest altitudes. Over a certain height it is arguably best to use a normal star diagonal for comfort. Below that point the star diagonal become a nuisance unless the observer sits to one side of the OTA. The more complex folding forms mentioned above require larger flats or a more bulky OTA. The simple 3-fold design is compact, maximizes the sizes of mirror by having smaller angles of reflection. It can also be neatly arranged in a simple, narrow, but tall, plywood box. The dewshield can be added once the box is mounted.

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11.1.16

7" f/12 iStar folded refractor 7: The flat mirror retaining shells:

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Despite it being 38F in my workshop and suffering from Sciatic pain I decided to press on with the flat mirror cells. Or rather the shells which will contain the optically flat folding mirrors. These 4" & 5" baking  tins had already pledged their donorship to my cause. First I used a pair of screw adjusting bow compasses to mark the center of each base by means of arcs. I then used these as a center to draw a circle around the edge of the base. Ensuring that I left a generous width of rim to safely start my chain drilling. Chain drilling by eye is an inexact science so it pays to leave plenty of meat behind.

I used three small drills in turn in my dirt cheap, Chinese, bench, drill press. It must be nearly 30 years old by now but still behaves at least as roughly as it did as new. By using a couple of large nuts bolted down as guides on the drill table I was able to ensure concentricity of my first circle of closely spaced holes. Stops makes it so much easier than relying on eye alone. The first and smallest drill is inevitably springy due to its very small size. So it pays to be patient and allow it to start drilling where it will. Even if that results in slightly uneven hole spacing it hardly matters.

Once the first ring had been drilled I changed to the next drill size up and went round again opening up all the previously drilled holes. It is important to let the drill find its own center. Trying to force it to go where you'd prefer will only bend the drill. The larger drills to follow will ensure the disk is safely cut out. Thicker or stronger materials should have very much more care put into hole spacing on the first ring. Double-pointed center punches are handy for this job in thick, tough material. By steadily increasing the drill size on each run the gap between the holes is reduced by the difference in its own radius. In my thin alloy a third drill managed to break through almost all the way around. So that a light tap with a hammer knocked the ragged edged disk right through. I then used a large, coarse, half round file to smooth the ragged hole in the base of each baking tin.

In thicker and/or tougher materials a hacksaw blade should always be run along the previously drilled holes. This will ensure the disk is cleanly separated and can fall out of its own accord. Forcing a partially separated disk from the parent material will usual be costly if separation is not carried out properly. Patience and care will always be rewarded in a minimum of waste/wasted materials.

Once the edges of the large holes were filed reasonably smooth I moved on to the lathe. Had I left the edges ragged they might have caught in the lathe tool. Holding such flimsy items in the lathe chuck requires some care. Concentricity is essential or the resulting rim will be lop-sided. An eccentric hole means loss of folding mirror aperture. Which could be critical for mirror position in the folded OTA.

A more fastidious builder might have used wooden/plwood/MDF mandrels to hold the tins concentrically and more securely. I decided to bypass this step to avoid making wood dust in my cold and damp workshop. Fine wood dust sticks like glue to cold, bare metal and causes rapid surfaces rusting by retaining moisture. In warm weather this is not remotely such a problem and the wood dust can be simply brushed off.

I used the 4" 3-jaw chuck for holding the 4" tin and my larger 6" 4-jaw chuck for the 5" tin. Tightening the jaws carefully avoided denting and permanent marking of the thin alloy walls. I cut very slowly and carefully throughout to avoid the tins being snatched out of the chuck jaws. The soft aluminium wanted to build up on the tip of the ceramic tool. So I just scraped it off at intervals with my finger nail after withdrawing the tool. Progress was slow but eventually I had reached a 98mm diameter hole on the base of the 4" can, leaving a 1mm wide rim. I finished off with rough abrasive paper wrapped around a cylindrical object. This safely removed the burrs which had formed both inside and outside the rim.

Soon I had a clean but much narrower rim without any burrs. Leaving only the thickness of the base material.

I stopped my cutting on the 5" can when I had reached 123mm. A 1mm wide retaining rim is fine for a 125mm mirror blank. Again I used the abrasive paper with the cylindrical former to avoid cutting my fingers as the tin was rotated quickly in the lathe. Burrs are always razor sharp so safety must be seriously considered. Sharp metal can cut through abrasive paper without any effort at all. Better it does this to an inanimate scrap object rather than a 6 hour wait at the A&E in a distant city!

I was also careful to keep the abrasive paper square to the tin to avoid damaging the anodizing. Even if it is only cosmetic it helps to maintain a longer life. So many telescopes look awful after a few short years of constant exposure to the cold and damp.

Unlike the 4", the mirror blank is a tight fit in the 5" tin. So I will have to split the shell to make room and allow for shell expansion and contraction. The Zerodur blank will hardly expand or contract at all relative to the aluminium. So one doesn't want to sacrifice optical surface quality by squeezing the mirror blank unduly! Screws into the cell base through the split shell will ensure there is no loss of strength. An alternative would be to seek out a slightly larger 5" baking tin.

Now I have my mirror shells finished I can start seriously thinking about the cell bases. I have yet to decide if I am going to retain the rolled rims on both baking tins for added strength. Or cut the tins down to greatly reduce their depths. If I do keep their present depths then some packing will be required behind the mirrors. This packing might well inhibit air circulation around the thick glass. Which in turn might reduce the glass cooling in typically falling temperatures. Resulting in thermal convection currents forming from the faces of the folding mirrors and rising into the light path.

I could drill a series of holes around the shell to allow air to reach the mirror blanks. Only a high quality hole saw will make a decent job in such thin, soft material. Trying to use twist drills will only tear the alloy shells to shreds and leave ugly burrs! It might still require a wooden mandrel to allow the holes to be cut neatly. Better perhaps to allow air to circulate freely behind the mirrors by means of large ventilation holes in the collimation cell bases.The backs and fronts of the mirror blanks have a much larger surface area than the edges alone.


Click on any image for an enlargement.
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10.1.16

7" f/12 iStar folded refractor 6: Folded OTA design considerations:

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A Birch plywood OTA "box" with glued, light baffle boards makes a lot of sense for a compact folded refractor. The plywood is inherently stiff and the baffle boards will eliminate inward or outward bending of all the other boards. Plywood is very easily cut compared with aluminium or any other metal. There is a long history of plywood construction where great strength and stiffness are required. Aircraft, boats and cars have all used stressed skin plywood constructions for over a century. The dynamic loads on these lightweight constructions absolutely dwarf anything a telescope is ever likely to meet. Though that is not an excuse to make the structure flimsy.

It just requires that OUR telescope tube uses best construction practice. It must be furnished with a number of incompressible baffle boards to resist skin flexure. Since the telescope needs light baffles anyway, the dual purpose of light blocking and adding strength can be safely met and should be encouraged. The loads in the structure must be spread as evenly as possible throughout. Which is where the skin comes into play. This will strongly resist stretching or compression but it has much lower resistance to bending.

The usual ATM arrangement of using heavy "planks" or thick plywood, adds considerable weight. Relying only on its considerable bulk to stiffen the structure. Which inevitably means it has to be strong enough to support its own [added] weight. Not an efficient use of materials where lightness can reduce the OTA's moment arm. Allowing smaller and cheaper mountings to be used. Not to mention making the telescope much lighter and safer to carry between its storage and the mounting.

Anything which comes between the amateur astronomer and using his telescope is a mental hurdle. One to be overcome every time the telescope could be put into use. Weight adds mental inertia to resist going out to observe on a whim. The best telescope is not the biggest, nor the best, but the one which gets used most often. How do we allow a big telescope to be more user-friendly? By reducing its weight and making it much easier to handle.

Skinned, cellular, aerospace materials, cardboard packaging and even flush house doors all use the skin effect. Which spreads the loads over a large area by the use of a mass of incompressible cells. The materials used can be very thin and even quite weak in themselves. Flush doors often use a cardboard honeycomb of small cells. Gluing the plywood facings onto the cells provides the necessary resistance to bending and compression. Aerospace honeycomb structures are incredibly strong and light despite using paper thin [or thinner] materials in its entire structure. The absence of thick materials allows air to fill the now empty spaces and air has no discernible weight. Bubble wrap would have similar properties if it were glued between suitably stiff skins. We all know how cardboard packaging uses the skin and cell effect to carry the world's goods safely around the globe. Yet it is constructed of thin but relatively weak [paper] materials glued between similar skins. While the common cardboard carpet or concrete form tube are also made of paper but have to be made massively thick and heavy to resist applied loads.   
 
A very experienced ATM contact has pointed out we live in a wet and cold Northerly climate. So wood and plywood are far more sensible than metal OTA constructions. Metal is very prone to condensation, dewing and frost. With the attendant risk of water dripping onto internally mounted mirror surfaces. We are used to seeing metal OTAs and probably admiring their neat and tidy cosmetic appearance. However, anyone who  has used such an OTA will attest to the heavy moisture load they seem to attract like a magnet. My habit of parking my refractors on their dewshields for compact storage, even in unheated accommodation, has usually resulted in water marking of the back of the objective lens.

Cardboard, plywood and battens do not promote condensation due to their low thermal capacity and insulating qualities. Such materials have a much lower tendency to radiate to the cold night sky. [Itself a black body absorber.] Exposed metal can actually drop below the local air temperature due to such radiation causing "super-cooling." As we all know, thermal differentials are what drive convection currents in the telescope's own light path. It might be instructional to mount two identical refractors side by side. With one objective mounted in a typical metal tube and the other in a plywood tube. Will they show equal thermal effects on the image?

The downside of Birch plywood is that it is rather dense. The finished OTA, using typical construction methods, is very likely to be heavier than an all-aluminium construction. However, the folded refractor is so compact, compared with a long, straight OTA, it should still be more easily managed via suitable "drawer" handles. The compact, stable, boxy shape also lends itself far better to being moved about on a suitable trolley than any 7-8' long, straight tubed OTA.

A compact, plywood, telescope tube could use 3 dimensional, cellular construction rather than simple baffles. It would  become inherently stiffer and all the materials could become considerably thinner. The design really needs to incorporate longitudinal internal structures to stiffen the "innards." The stressed skin fuselages of aircraft use stringers over plywood formers. [Or baffles in our case.]

A compact folded OTA could be made usefully lighter by using thinner skins supported by thinner cellular construction. Not only would there be thinner baffles and more of them, but their own resistance to flexure would be increased by adding longitudinal structure. These could also be thin, glued plywood braces between baffles or even glued dowels or battens. Though the latter tend to get quite heavy, rather quickly, due to their use of solid material. A sheet of anything reasonably rigid tends to be very strong in its own plane. You could say it enjoys a myriad of triangulation over its entire surface. The trick is to use its inherent 2D stiffness by not allowing it to bend in the third dimension.

One still has to ensure that the light path is kept completely clear and extra weight is not added unnecessarily. Support for the heavy objective in its cell requires local strength. Though there is really no need for a massive counter-cell, type of tube adapter for a compact box. The objective cell's tubular rear extension and collimation screws must still be adequately supported. An objective board, made from a thick, cellular plywood sandwich, is perfectly suited to this task.

The objective board must then have its loads efficiently carried into the OTA box structure without flexure. Applying the typical, extra layers of thick plywood, to spread the loads, just makes the OTA even more nose heavy. Without necessarily spreading the loads further than the limit of their own extent. So a lightweight, cellular structure is called for to support the sandwich objective board. The loads from a lightweight, plywood sandwich must be carried back into the entire box to be evenly distributed via fully 3-dimensional baffling, or cells. They must be arranged in the horizontal, vertical and longitudinal orientations. All skinned over with thin plywood for stiffness.

It must not be forgotten that two layers of ply weigh the same as a single layer of twice the thickness. To reduce weight they must be separated and fixed to an indeformable, lightweight matrix. Then the ply skins themselves can be thinned to match the required stiffness and local resistance to compression.[ie.Impact denting.] The stiffness rises as the square of a structure's depth, or thickness. Which is why joists, studs and rafters are always placed on edge to the applied loads. They are then tied together to resist twisting out of their strongest plane.


One could almost imagine sculpting a fully 3D cellular material by cutting away only the light path but leaving everything else behind. Imagine firing a huge laser pulse into the objective aperture to burn away only the cellular structure where the laser light hits. The objective lens would focus the heat into its natural light cone. The lightness, strength and stiffness of the remaining cellular structure would be phenomenal! It is this idealized structure one must try to emulate to add more air at the expense of unwanted solid material. Though grazing incidence of the light must be avoided. Baffles with any reasonable thickness will often add their own grazing incidence. So thin metal baffles should be applied where the highest contrast is desired.

An alternative to adding baffles is to use an inherently stiff but lightweight material like rigid, closed cell, extruded insulation foam. NOT the common, polystyrene, bead foam which has rather low strength and poor local compressibility. Lightweight, insulating materials could be skinned with very thin ply to achieve great stiffness without added much weight. Though the foam is never as light as "thin air" itself of course.

The only danger might be an internal fire if such a telescope is used for solar observation via a Herschel wedge. Such methods focus the entire sun's heat right through the telescope structure before reducing it to a safe level just before the eyepiece. Often with the addition of ND filtration to further reduce the brightness. The heat having already been reflected away by the prism in the solar wedge.

I am not talking here about so called "solar filters" supplied with small, cheap telescopes to fit over the eyepiece. These are exceedingly dangerous and should never, ever, be used. Better, by far, to obtain some commercial solar foil filter material from Baader. Which is extremely safe provided it is built onto a structure to place securely in front of the objective. Since the solar foil reflects 99.99% of the sun's heat and light NO heat enters the telescope objective to reach the vulnerable user's eye.

An objective support board could be built from dense foam and thin plywood. The risk of fire being almost nil at the "blunt" end of the light path. The weight of the objective could be easily carried into the objective board by its own cylindrical rear extension. The collimation screws would still need local load distribution to avoid flexure with changing telescope orientation. No point in building a very stiff structure if the heavy objective lens is then allowed to sag away from its supporting board via the collimation screws. Or cause local distortion which would alter collimation as the telescope was moved around on its mounting.


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8.1.16

7" f/12 iStar folded refractor 5: Buying optical flats from Nova in the US.

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"US Government Surplus" round, optical flats on Zerodur blanks to 1/20th wave accuracy are listed by Nova-Optical in the US. A direct link appears at the bottom of this page:

A range of surface options include plain glass, dielectric coated and aluminium reflective coating. I ordered the latter and it was later confirmed that it was 96% reflective "enhanced" aluminium. A useful advantage where two mirrors interrupt the light path. There will still be a loss of at least 8% but a lot less than "standard" aluminium coatings which are around 88% when fresh. Resulting in a light loss of 24% for both folding mirrors. This considerable light loss might be useful when viewing a bright lunar surface to save using ND filters. Most folded refractors are probably long focus and used primarily for planetary, solar and lunar observation. Whether the overall light loss is considered serious for these targets is a matter of debate and personal opinion.

Please note: All claims as to the quality and accuracy of these optical flats are entirely Nova Optical's responsibility. I am merely a paying customer and have no commercial interest in the supplier. I was personally assured that when a sample of ten flats were tested they showed between 1/20th and 1/26th accuracy and came originally from highly respected US sources. I do not have the necessarily expensive equipment to test the validity of the claimed precision of these polished optical surfaces. Few people do. You buy these optical flats entirely at your own risk.  Not mine. I mention this only in case somebody becomes unhappy with their purchase and decides that it must be my fault because they were disappointed.

I received two flat, round mirrors, within a few days of ordering, well wrapped and the coatings in pristine condition to my own, unaided eye. The orange colour of the blanks suggest they really are made from Zerodur. A stable, very low expansion, glass-ceramic material much used for astronomical mirrors and other optical research. An image search will show you the expected colour. I have borrowed these interesting images from Nova's own website since they are relevant to my own purchase for educational purposes.  

My enquiry emails to Nova were answered succinctly and in good time. The correct customs forms were attached to help the package through Danish customs. The parcel was sent Priority International Airmail @ $55US for a 6lb package. I have paid more postage for less weight in Europe and not enjoyed remotely the same, very quick delivery.

When the package arrived i Denmark I was contacted by email and asked to pay the customs clearance charges, VAT and import taxes by the Post Office/Customs service. [Which I did very conveniently on their website from the supplied link.] The package was then released and delivered the very next day. Danish Post Office Parcel Tracking was provided within Denmark once payment of the charges had been made. I must say I was delighted with Nova's and the international postal/customs service I received. It was all completely painless even if it made a large, but expected dent, in my bank account. It was a case of do something, or waste the expense of the objective and time spent building the refractor. 

When the package arrived at my home, I saw the box had been opened for inspection and re-sealed with official, Red "Post Office-Customs" tape here in Denmark. Fortunately they had not fingered the Al coating and I am not sure they even opened the neatly taped bubble wrap and considerable layers of protective tissue paper. The customs form described the optical flats as 4" & 5" telescope mirrors.

Those importing such items from outside the US can probably find an online, import charges calculator provided by your own government. My own total [Danish] charges amounted to almost exactly 0.3 of the purchase price. VAT [Moms] is set at 25% in Denmark. There was a £16 [equivalent] charge by the Danish post office for customs clearance. Import duty was quoted as 2%. PayPal charged me about £20 to convert my currency and transfer US dollars to the supplier's email account. I have no idea if my bank would have charged that much but I imagined PayPal was offering me some protection if my funds had simply vanished. It was also extremely convenient to be able to do everything online.

Be aware that VAT is usually charged on the entire transaction. i.e. Purchase price + freight charges + all customs and delivery charges. This all adds to the purchase price but must be accepted as part of "the deal" in buying almost anything from the US. Don't be fooled into thinking you will somehow avoid these fixed charges by some lucky fluke. You cannot "get away" with the supplier under-pricing the value nor claiming the item is a gift. So don't even try. Gift allowance is far too low to affect any likely, optical purchase. Do you suppose that career customs officers are going to turn a blind eye in your special case?  The items are listed as merchandise on the customs form with the full purchase price and postage in US Dollars.

I hope all this information will help you to make a useful decision if you are in the market for such items. Even at the full price including all charges these [round] optical flats were considerably cheaper than any competitor's retail prices which I could find by searching online. I don't have easy access to the many small ads services in the USA where secondhand optical flats sometimes crop up. The Nova flats are unused and I just hope the remarkable surface accuracy is as claimed. Only after extensive testing, in the finest seeing conditions, at very high magnifications, will the quality of planetary or lunar images be likely to be discernible from using the objective directly in a straight tube. With reasonable luck and careful collimation, there will never be any visible difference at all when using 1/20th wave flats.

It is a risk I personally consider worth taking for the increased convenience of a far more compact OTA.The present long, straight tube is certainly impressive in size but is simply far too heavy and too unwieldy for me to handle safely. In icy conditions I seriously doubt I would be able to manage it at all. I have nearly dropped the OTA a couple of times in mid-carry out to the mounting. I cannot afford to replace a broken objective even if one was available.

It is highly recommended that, regardless of source, you always oversize optical folding mirrors to avoid using the outer edges or losing light due to the flats being too small. Tilting will always demand more mirror breadth than you might have hoped for. Draw yourself an accurate,  full sized light cone on a long roll of paper with a useful circle of full illumination. Then cut it out carefully to your perfectly straight lines. Now fold it where desired and measure the width of the fold directly from the drawing to find the minimum mirror size at that location. If you are restricted to a fixed OTA/tube size then be even more generous with your mirror size allowances. Measure twice. Order once.  
 
All enquiries to:

Optical Flats @ Nova Optical Coatings

PLEASE NOTE: A fellow ATM has contacted me to say that Nova is running out of optical flats. The useful 4" size is no longer available. Contacting Steve Dodds at Nova-Optical yourself will confirm which sizes he still has in stock.
Second update: 4" flats are still available but not coated. You would be responsible for getting your own flats coated.
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7.1.16

7" f/12 iStar folded refractor 4: Folding gets underway.

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The arrival of my 4" and 5" 1/20th wave flat mirrors from Nova Optical in the US prompts me to post yet another monologue. See the link in the next post for Nova Optical.

For those who have not read the entire saga I need to reduce the load on my aging bod. Carrying a long, straight tube out to the mounting involves several operations including dragging, lifting and handling considerable weight. Not least is having to lift the entire OTA above my head into the waiting tube rings. So I am going to fold the optical path into three shorter sections with two flat mirrors. The folded OTA will not need to be raised nearly so high so that I can "get under it" to look overhead. In fact I can probably sit comfortably on a chair when the folded telescope is mounted at a suitable height for observing with a star diagonal.

I need a means to support the folding, optical flats so that they do not sag or become strained from their containment or support. Mike Lockwood has much to say on the subject and his wise words have guided my own choices. Though he was talking about large Newtonian secondaries the same conditions apply. The 2nd folding mirror will be suspended face down just like a Newtonian secondary flat. Except that the 2nd folding flat will be round rather than an ellipse. Or any other similar "oblong" form typically used for large Newtonian secondaries. Grinding a traditional elliptical secondary out of a huge, round blank means lots of very careful work and expense. So some secondaries have only the corners relieved from a basic, rectangular blank. Strain may be built up or relieved accidentally in the mirror blank if any heat is caused by grinding.

Some might consider silicone adhesive to retain the flats but it has limitations and great care is required to avoid an unsuitable supporting material. The adhesive must be flexible enough to avoid causing optical strain in the mirror blank itself. This usually means applying thick blobs of adhesive with easily removable spacers to ensure the blobs are not flattened by the weight of the mirror blank. Thick blobs will act rather like coil springs once the adhesive has cured and the temporary spacers removed. Which will allow some lateral movement when the backing plate [or board] inevitably warps, contracts or expands.

The backing material must also be suitable for the silicone without loss of strength or adhesion. Which might mean solvent treatment or special priming to avoid the adhesive peeling off under load. A large secondary could easily ruin a very costly primary mirror [or objective] if it fell the entire length of the OTA when the telescope was pointing vertically! 

Mike Lockwood goes on to suggest a surrounding metal shell instead of using silicone adhesive. But one with a full surrounding rim which is carefully flattened and smoothed. The intention being to offer an evenly supporting rim around the entire edge of the secondary mirror. Sagging or warping of the suspended mirror blank must be minimized. Equal edge support all around the edge is probably the best and easiest way to avoid it.

To this end I have just ordered some small, anodized aluminium, baking tins in 4" and 5" diameter. I am hoping these will be the perfect size to match my optical flats. Though beggars can't be choosers. Particularly with online sales where one can't sneak a home made plug gauge into a shop to check actual sizing. Don't laugh! Been there. Done that. I have even taken a vernier caliper into charity shops when searching for ATM donor parts from the shelves.

If the baking tins prove to be undersized I can easily split the shells with a fine saw like most other commercial, elliptical secondary mirror shells. The shells can then be snugged up with screws to contain the mirror at the base of the anodized aluminium cans without distorting the glass. If the cans prove oversize then that is no problem at all and suitable packing can surround the glass blanks. I just hope they aren't too undersized!

When they arrived I found the 4" can provided a perfect fit with "shake" clearance. I had placed the mirror face up on a suitable plastic object to lift it safely off the table. The baking tin was then lowered very slowly, gently and squarely over the raised mirror. [Just as is done with object glasses to lift them out of their cells or to replace the glass elements afterwards.]

The 5" tin is slightly oval and refused to slide onto the mirror blank. I measured the tin and confirmed at least a 1mm of ovality. So there is still hope of it fitting if I can just make the tin perfectly round. If not I shall just have to slit the baking tin for clearance. Both mirror blanks are of exactly the correct diameter.

I shall have to make up some rough plywood mandrels for the lathe to hold the tins. Then I can cut an opening in the base of each can to leave a neat rim to contain its respective mirror. Both mirrors will be similarly supported so that the folded refractor OTA can be placed down in any orientation. Without any risk of a folding mirror taking a "nose dive" out of its collimation cell. I feel the encircling rim is kinder, more theoretically sound and more secure than traditional edge straps. Which usually have bent over tabs to stop the mirror blanks escaping. It is inevitable that such tabs will damage the mirror coating over time and may even add unwanted diffraction effects if they fall withing the light path.

The flats are shown resting on top of their respective baking tins. Which will hopefully be turned into retaining shells for their collimation cells. The remarkably strong, rolled rims will provide a perfect stop for a recessed [plywood?] ring to prevent the tin flopping about as the telescope is moved around. With a flat base behind the ring the mirrors will be held securely but gently. All it needs now is to turn the tin's bases into open rims to safely retain the mirrors. Which will of course be snuggled inside the tins with their faces almost flush with the open bases. The quality of these baking tins is truly remarkable. I am completely unable to make any impression on the ovality by pressing with my hands. It seems  almost a shame to have to spray the anodized aluminium tins matt black before they are used in anger. I am a great believer in using found objects instead of starting from scratch with cruder raw materials. Or even machining a component from the solid. If I were to remove the rolled edge, the cans would still have more than enough strength to support the substantial mirror blanks. Note the almost complete lack of taper in these 3" deep drawn objects.

The cell bases are likely to be Birch plywood turned to fit the can at the normally open end. The cell base board may be supported on traditional springs much like a reflector's main mirror cell. Or held by a hinge on one edge of the base and tilted for collimation via a sprung screw or push/pull screws. Rotation of the tilted cell will allow for lateral tilt during collimation.

Mirror cell hinging [hinge-ing] has several potential advantages: It is likely to need less depth than a conventional three spring cell. It avoids having to put a preset tilt on the mirror cell to match the required angle of reflection. It may also provide more secure collimation for an OTA. Particularly one which needs to be brought out from storage and returned there after the telescope has been used. It should safely avoid sagging of the folding mirrors as the springs settle and adjust to the suspended mass of the mirror blank and its cell. All pulling and pushing at different orientations of the OTA. Including being placed "nose down." [Objective facing downwards.]


Click on any image for an enlargement.
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1.1.16

7" f/12 iStar refractor 33: New year and pining for a sharp moon.

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I awoke at 6.15am on New Year's Day and saw the sky was quite clear. With the Moon and Jupiter just to the west of south it was probably worth going out. Clear skies being in short supply I went without breakfast and just took a cup of coffee outside.

Having dragged the pier over to my usual observing position I was set up by 6.30. The Moon's surface proved to be covered in rapid, though not particularly violent, thermal agitation [again.] Jupiter was at about 36 degrees high with the Moon slightly east and below at only about 33 degrees altitude.

After the usual hassle of dragging the massive pier and struggling to get the heavy OTA into the rings I was actually too warm in my big duvet jacket. So I had to remove my hat and jacket for a while to to cool off. Though it was far from warm with my hands getting cold whenever I touched bare metal. The car roof was still white with overnight frost at first. My inverted, plastic water butt mounting protector proved to be quite a handy table for eyepieces and accessories.

I homed in on Plato, as I usually do, in the hope of seeing some minor craters. It was not to be though as the Lunar features were slightly soft. Eventually the Moon became dimmer and I discovered the objective had misted over when I removed the eyepiece and looked though the open focuser. The OTA had come from unheated accommodation with temperatures inside and out reading 37F, 3C on my digital thermometer.

I had not fitted the full dewshield since dewing over had not occurred before today with only the stumpy permanent dewshield fitted. After fitting the longer dewshield I wiped the lens gently with a microfiber lens cloth and thankfully it remained clear after that. It would be handy to be able to keep the objective indoors and fit it to the OTA only for observation. Though this would require a complete redesign of the counter-cell and might well introduce collimation problems.

With the slightly soft images I was getting again I decided to use a thin feeler gauge blade to knife-edge the objective on a bright star. Examining the star first, with an eyepiece, had showed soft edges both inside and outside of focus. With any rings rather lost in the colourful mush.

The knife-edge required the 5" of straight extensions to be able to get the focus at the surface of the 2-1.25" adapter. The nearer I  came to focus the more convex became the shadow! Ideally one wants a straight, advancing edge. Then even darkening at focus. With the direction of the shadow movement reversing outside focus.

This is not quite so straightforward with an achromat because the different wavelengths [colours] each fall at different focal planes. This softens the shadow compared with a concave mirror and causes different colours to appear. Filters can help but I had no desire to get so involved with no bright star being particularly well placed for testing. I was half crouched, my back was aching and I was struggling to keep the star centered. So I didn't persevere with my testing for very long.

Perhaps I should set up an artificial test star at the far end of the drive to test the objective in comfort. A ball bearing or small Christmas ornament at 100 yards in bright sunshine should do. With the OTA horizontal and solidly supported on a workbench the "star" can be centered and kept there. There would be no thermal effects from the ground in weak sunshine with the drive itself being shaded by a tall hedge.

Back to the Moon and Jupiter and both remained rather soft. I couldn't tell whether the roof was responsible for the continuing thermal effects because it was now just below the objects I was trying to observe. Could the steel telescope tube be having some thermal effects on the image? It seemed unlikely unless I had brought the OTA out from the warmer indoors.

With slight fringing still visible at best focus I decided to try a 6" stop in front of the lens. It took only a few moments to cut out a cardboard packaging circle and then make a 150mm aperture in the middle with a craft knife. The Moon certainly became more monochromatic as a 6" f/19 [R35 equivalent] after the stop was fitted into the dewshield. Though it did not make much difference to the sharpness of the view. As usual, I rotated back and forth through my selection of secondhand, Meade 4000 Plossls. Neither Jupiter nor the Moon would take high powers and I topped out at about 175x without any increase in visible detail. The Fringe Killer filter just yellowed and dimmed the image without any increase in detail.

It quickly clouded over at 7.45 so it was time to pack up and go in for breakfast. The Moon made only one brief appearance after that but the sky was already brightening rapidly.


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