31.8.16

2" shaft mouting pt.28: 7" PTFE clutch-stabilizer.

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As an aid to truing the run-out of my 11.5" wormwheel I added a low friction disk between the wheel and cylinder. PTFE [US Teflon] is a low friction material which is readily available on eBay[UK].

I order a piece 20cm x 30cm in 3mm PTFE [8" x 12" x 1/8"] on Friday afternoon and received it in the post on Monday. Even ignoring the weekend that is a delivery speed record!

Marking a thin circle on the the snow white sheet proved slightly difficult. I tried felt pens, pencils and Biros in the compasses without much success. In the end I used a large pair of screw adjusting dividers to scribe the circle. This was then filled with a magic marker pen and rubbed away to leave a thin, sharp line. [Engraving style.]

The image shows the disk with the cylinder lifted clear for the photo.

I then sawed out the circle using a hand fret saw. Cleaning up the sawn edge was going to be time consuming. So I hung the disk on the expanded jaws of the lathe. Being slippery and floppy the PTFE would not stay in place. PTFE is not remotely as rigid as materials like Perspex, for example. I added a 2" V-belt pulley with a sunken face held by a live center in the tailstock. A piece of sponge trapped between the pulley and the PTFE kept it safely in place without slippage.

Turning it required a very small, sharp pointed tool. Even this flexed the material and left a radius. So I backed up some coarse emery paper with a batten. Then used that to quickly obtain a neat edge using a  high lathe speed to keep the plastic straight by centrifugal force.

The brass sleeve I used to adapt the 60mm diameter bore to my 50mm shafts was originally [and deliberately] left too proud but was quickly turned down to be flush.

The large wormwheel is now beautifully stabilized and runs perfectly true thanks to being pressed against the 7" /180mm diameter cylinder. The PTFE adds very little friction and this will actually help to stabilize a long and heavy OTA. The brass sleeve rests on the top bearing to provide the upward thrust necessary to bring the wormwheel, PTFE disk and cylinder into intimate contact. In fact I had no need to use 3mm / 1/8" PTFE but decided I liked the stability it offered. The rest of the material will be handy for Dob bearings. My earlier 2mm thickness was too thin to allow CSK wood screws to hold it fast.

Click on any image for an enlargement.
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28.8.16

2" shaft mouting Pt.27: Bearing housing options.

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I thought I might seek out a square tubular metal option to compress between the flange bearings but have drawn a complete blank. Size would be critical but I can't find anything remotely suitable! My usual metal outlets had only flat plate or small profiles in aluminium. No combination of my larger and heavier alloy profiles suits the lateral spacing of the studs nor the size of the bearing flanges.

These would suit an [almost empty] 8" diameter round PVC pipe but a 180mmx189mm [7"x7"] square [structural] PA tube makes far more sense. A square, steel tube of that size, in any reasonable wall thickness, would probably need a crane to lift it.

Another alternative for the bearing housings is to use laminated kitchen worktop. I have a complete length of unused hardwood 60cm/24" wide x ~30mm thick which could be routed with longitudinal slots to house the threaded rods. [Studs.]

Applied compression forces by the studs should be much better resisted by the hardwood end grain than the weaker edges of the very thin laminations of the birch plywood which I had originally intended to use.

This very rough drawing shows how a 30mm thickness of board fits the flange bearings with a suitable overlap. The step between them will be disguised by the presence of alloy end plates sandwiched between the flange bearings and their [square] tubular wooden housings.

The inner edges of the 45° mitre joints need a pocket to clear the studs. This narrows the width of the mitred joints but probably leaves enough material for a strong joint. A second square tube of 30mm thickness could be fitted inside the outer tube to reinforce both and enlarge the compression area. Slight clearance issues exist with the axis shaft, inner bearing race and the rubber seals. Slight relief is easily managed with the router at both ends of the inner tube once glued together.  While slight shaft clearance should be routed out prior to assembly. Making both tubes larger [but still nested] solves clearance problems without any bearing protrusion, clearance routing being required.

The worktop laminations should make stable but unusual and attractive bearing housings. Probably with far better cosmetic damp resistance than the birch plywood. The worktop has been standing on end for a couple of years in the workshop so should be well seasoned by now. The only real problem I foresee is deciding on corner treatments where each of the four boards meets on edge. A 45 degree edge [miter] would work best but would be critical of exact fit and finish for neatness. The overall dimensions of the wooden strip tubes have yet to be decided but ~180mm looks about optimal. Ignore the outer dimensions on the drawing.

Provided I get the sizing and grooving exactly right the studs and axis shaft can be easily fed endways through the assembled hardwood strip [tubular] monolithic housing. Then the flange bearings can be added and the washers and nuts tightened down to build a really rigid unit. The design of the bearing housings has been on the back burner while I considered alternatives and waited two months for the wormwheels to arrive. The plan is to bolt the declination housing on top of the axis joint cylinder. Major sinking of the cylinder into the Dec housing [to reduce overhang] is NOT easily achieved due to the enclosed compression studs. Though a circle could be relieved in the outer tube material with the router to reduce the degree of cantilevering slightly and perhaps to improve the appearance. This might weaken the joint between the alloy cylinder and the tubular axis housing.

It may seem odd but I have absolutely no experience of using mitre joints between the edges of boards. Edge butt joints are my usual skill level. Only the odd picture frame has enjoyed the manual mitre saw and that can't handle longer lengths. So it will have to be the circular saw or the bandsaw. Perhaps I should have a practice on cheap materials before wasting the kitchen top! I don't much fancy my chances of planing a perfectly straight edge at 45° on hardwood.

In the end I decided to cut strips of worktop 36cm x 95mm wide long to just fit between the studs. Careful balancing of the strips on each other showed they should be mitred to allow them to sink nearer to the center of the housing. A 6mm, 1/4" flat brought the strips to flush with the outside of the bearing flanges. After cleaning up the strips with a wood plane I made the first bevel. Then it occurred to me that I had a 45° router cutter with an edge support bearing. After careful depthing of the cut I turned the router upside down on the workbench. This proved far easier to work with by feeding the strips by hand than trying to balance the heavy router on the the narrow edge of the strip.

The rather poor image shows the strips are now flush but could be mitered further to sink them below the edges of the flanges. This would allow room for a second layer of mitered wood strips to be glued on on top. I took a second cut with the router's depth turret rotated by one more stop. The image alongside shows the result. I can keep on taking shallow cuts with the 45° cutter until I reach a suitable depth. Balancing the assembly on its nose allowed fine adjustment of the depth of each strip. The miters are still too narrow to be self-seating against each other. A light tap with soft hammer on each wood strip allowed them to be joggled into place. Once the nuts were tightened the wood made a much more solid feeling unit compared with just the studs and well tightened nuts. I'm rather pleased with how well it went. It is so long since I used my cheapo circular saw that the blade was rusty! As was my technique.

My 3mm PTFE has arrived from http://www.ebay.co.uk/usr/gfgplasticfabricationsltd via eBay[UK] Ordered on Friday afternoon and arrived in the post today [Monday.] Absolutely amazing service!

Click on any image for an enlargement. 
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26.8.16

Ronchi star testing filter update.

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I have been notified by the parcel service that my Baader SC filter can be picked up. Do other countries have such a sophisticated parcels/package services? When you buy something online you give your email address and telephone number to the vendor. These are then used to automatically contact you by SMS or email when the items is waiting at a local package/parcel outlet or post office. Far more reliable and user friendly than waiting in all day for the post person and then not get the package. Or they didn't bother to ring the doorbell and took it back to the sorting office. Almost every village has a collection center so they are usually within easy reach and particular outlet options are suggested depending on one's home address and can be specified when ordering. A further option is to have the item delivered directly to one's place of work. 
 
The absence of a green filter made my results of Ronchi testing achromats somewhat difficult. The multi-coloured images showed how each colour is brought to a different focus. This tended to soften the Ronchi bands.

After much reading online I placed an order for a Baader Solar Continuum filter. This transmits only a narrow band in the green. Hopefully, and according to reviews, it aids the testing of achromats. It also helps to bring out the Solar surface detail provided an IR filter is also used in tandem to stop reported leakage in that area of the spectrum. 


I must add the usual safety note here that the Solar Continuum filter is NOT a safe, solar heat rejection filter on its own. It still passes more than enough heat and light to instantly blind any user who has not fitted a full aperture safety solar filter as well. These solar safety filters are readily available in film or glass form. The very popular Baader AstroSolar Safety Film filter reflects away almost all of the heat and light before it enters the telescope. This is vitally important to avoid blindness or sharply focused heat which can rapidly cause a fire or damage the instrument's optics through rapid overheating. There is still enough light to safely see the Sun's image visually in white light and to safely take pictures. 


The Baader Solar Continuum filter appears mirror like from both sides. It comes, well packed, in a drawer type of case for protection.

Only when one looks directly through the filter can the strong green cast be seen. I held it up to my compact camera and took a snap of its case to show its effect. Note how the printed spectrum on the packaging is now all but invisible. 






I added an IR block filter to my order in case I wanted to do more solar imaging.

This will be dependent on having a driven mounting which can easily manage the 7" f/12 refractor. The Fullerscopes MkIV proved inadequate and the VFO drive box died.









Click on any image for an enlargement.
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2" shaft mounting Pt.26 Wormwheel options.

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Having the wormwheels finally available meant I have to seriously consider their placement. The large 11.5" RA wormwheel could be placed above or below the polar axis housing bearings.

Placing it at the top under the axis joint cylinder looks smart but increases the cantilever above the top [Northern] PA bearing by 45mm. To make it into a "boater" hat.

While placing it at the bottom shortens the distance between the bearings by an equal amount but limits overhang to a minimum. I don't believe that torsion loads on a 50mm [2"] shaft are worthy of serious consideration. So won't let that sway my decision about wormwheel placement.

By allowing the RA wormwheel to rest on the plastic cups on top of the studs any extra flexure for the Dec overhang will be reduced. With the top flange bearing reversed there is no bare shaft above the bearing. Though that makes a compressed bearing housing much more difficult. I shall have to reverse the top bearing back to 'normal.'

A Teflon/PTFE disk could be fitted between the wormwheel and cylinder to provide even less flexure. The wormwheel and cylinder effectively become one unit. Carrying the applied loads directly from the top [North] bearing into the declination bearing housing without reduction in cross section. Russel W Porter would approve.

The Tollok bush is a delight to work with and instantly releases its tenacious grip on the polar shaft with half a turn of a single extractor screw. That is, providing the tension screws are all slacked off first. Likewise, the large studs allow rapid adjustment of bearing spacing and shaft projection to try different arrangement in only a few seconds. The bearings are also captive on the shaft via their grub screws. Which gives considerable freedom to adjust friction.

It all makes for a simple and almost effortless assembly and break easily down into its separate parts without special tools. I say "almost" because the weight increases with each additional part. The Polar axis assembly, as seen here, is already reaching just beyond my comfort level [at 55lbs] to carry in and out of the workshop. Naturally this goes with the territory of building a heavy mounting to support larger and longer telescopes in particular. Otherwise, why bother?

The drive worms are easily attached via sturdy alloy plates threaded over the four studs and tightened with the existing nuts. The worm support plate could be used to further increase load spreading against the end of the [still potential] plywood housing.

After I had completed the brass packing sleeves the large wormwheel had some run-out. So I dismantled the arrangement, opened out the radial holes to 9mm. I tried a rotation of the sleeve by 120 degrees to achieve an instant improvement. I have ordered a sheet of 3mm PTFE/Teflon to at as a low friction filler in a sandwich between the bottom of the cylinder and the wormwheel. I am hoping this will act as a disk/plate bearing to further improve the truth of the wormwheel and stiffen the combined assembly. It will also act as a clutch to supplement the wormwheel's radial screws.  Since the cylinder runs perfectly true and it is rigidly attached to the Polar Axis shaft it should damp any possibility of the large wormwheel having any remaining play. It is currently of the order of under 2mm at the rim.

I thought I might seek out a square tubular metal option to compress between the flange bearings but have drawn a complete blank. My usually metal outlets had only plate or small profiles in aluminium.  No combination of my larger and heavier alloy profiles suits the spacing of the studs nor the size of the bearing flanges. These suit a 8" diameter round pipe but a 5"x5" square PA tube makes far more sense. A square, steel tube of that size, in any reasonable wall thickness, would probably need a crane to lift it.

Another alternative for the bearing housings is to use laminated kitchen worktop. I have a length of unused hardwood 60cm/24" wide which could be routed with longitudinal slots to house the threaded rods. [studs] Applied compression forces by the studs should be much better resisted by the hardwood than the weaker "end grain" laminations of the birch plywood I had intended to use. The lamination should make stable but unusual and attractive bearing housings. The worktop has been standing on end for a couple of years in the workshop so should be well seasoned by now. The only real problem I foresee is deciding on corner treatments where each of the four boards meets on edge. A 45 degree edge would work but would be critical of exact fit for neatness. Provided I get it right the studs and axis can be fed endways through the assembled hardwood strip [tubular] housing. Then the flange bearings added and the washers and nuts tightened.


Click on any image for an enlargement. 
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25.8.16

2" shaft mounting Pt.25: 60-50mnm wormwheel sleeve adapters.

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Since the wormwheels had been supplied with 60mm bores, instead of the 50mm I ordered, I had to make some brass adapter sleeves. This was very time consuming since so much material had to be removed from the only stump of brass bar I had left in a suitable size. This required a diameter and length which would provide two 45mm long adapters plus enough material for parting off and something to hold firmly in the 4-jaw chuck. Though I had a number of options to ensure concentricity I chose to bore and finish turn both sleeves without ever removing the brass bar from the chuck. 

I found a stump of yellow brass 65mm diameter and made a start on making the packing sleeves. First I center drilled the stump and took a cleaning cut. Then fitted a center and 'clocked' the piece in the 4 jaw chuck. The chuck jaws were tightened securely to avoid losing concentricity. Then I ran a 10mm drill in the tailstock chuck deep enough to give clearance for the nose of a boring bar.

In the picture above I have reached 45 mm bore and will continue deepening and widening the bore until I can part off two sleeves for the wormwheels. In fact I worked on the bore of only one adapter at a time due to the depth matching my medium length boring bar. That concludes tonight's effort.


Next day I finished boring and turning the first adapter sleeve. By sheer luck I was able to use the smaller wormwheel as a gauge to sneak up on the correct diameter. In between I had used a vernier caliper to reach the correct diameters. I also had my original, short stump of SS shaft which could be used to fine adjust the bore size.

Finally I parted off the first sleeve and started boring the second. This went much more rapidly because I could now trust my measuring tools to rough out to get close to finished size. Once that was finish turned to size I could part it off with the lathe in the slowest back gear. You would not believe how much swarf was left in the lathe's tray!

Now came the marking for the three radial holes in each sleeve for the adjustable clutch pads. These consisted of short lengths of ~8mm diameter nylon rod. Each of these is pressed against the axis shaft by a stainless steel grub screw to allow fine adjustment of friction/slippage so that the telescope can be pointed anywhere in the sky without having to retract the worms.

Instead of marking the radial holes with a marker pen I chose to tighten the grub screws straight onto the brass sleeves. This left clear imprints from the hollow nose of the screws which could then be carefully center punched. After using a small drill to ensure an accurate starting point I used a series of larger drills to open out each hole. Then all it remained to do was open out each hole with a tapered broach to aid entry for the nylon plugs. The alternative would have been to open out all the holes to 9mm which was slightly too large.

I have deliberately made the brass sleeves slightly too long so that there is a linear thrust bearing surface if needed. Should I later decide to make them flush with the wormwheels the sleeves are easily removable by backing out the grub screws. Meanwhile the nylon clutch plugs locate the sleeves securely in the wormwheels so that they do not rotate independently of the wormwheels. It is obviously desirable that the only bearing surfaces are formed by the close fitting brass sleeve and nylon plugs on the axis shaft. There is no slop either internally or externally on the brass sleeves to cause any eccentricity. Which would obviously badly affect the tracking.

I have been notified by the parcel service that my Baader SC filter can be picked up. Do other countries have such a sophisticated parcels/package services? When you buy something online you give your email address and telephone number to the vendor. These are then used to automatically contact you by SMS or email when the items is waiting at a local package/parcel outlet or post office. Far more reliable and user friendly than waiting in all day for the post person and then not get the package. Or they didn't bother to ring the doorbell and took it back to the sorting office. Almost every village has a collection center so they are usually within easy reach and particular outlet options are suggested depending on one's home address and can be specified when ordering. A further option is to have the item delivered directly to one's place of work.


Click on any image for an enlargement.

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24.8.16

2" shaft mounting Pt.24: Wormwheels arrive.

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Two months after ordering the wormwheels they have arrived. The first set were made with a 1.25" bore so I had to wait another month while another set was made. I ordered nominal 11" and 8" diameter wormwheels with a 50mm bore and matching worms and received 11.5" and 8.75" wormwheels [293mm & 222mm] with 60mm bores. Tooth count is 287 in both cases. This matches a 5rpm synchronous motor drive. Though stepper motors are possible and actually more popular these days.

I was told that the machinest is a gentleman in his mid 80s and now reaching the end of his desire to continue after two decades of producing these wormwheel sets in up to 14" diameter. The 14" are now discontinued. My own wormwheels are numbered 567 and 568 which is rather a lot of anything made to this level of precision.

The worms used to be made in stainless steel but these are now of brass. If I outlive them [as I approach 70 years of age] that will be a remarkable bonus!

They arrived in a very large box with masses of protection even including pipe insulation wrapping the circumference of the wheels.

I had no real idea of the true dimensions until they arrived so have postponed any serious progress on the mounting. I shall now have to make brass sleeves to match the 60mm bores to my 50mm stainless steel shafts. Making smaller holes larger is impossible if accuracy is to be maintained. Sleeving to make the central holes smaller is possible with care.

Note how the "threads" on the worms are coarser for the larger wheel. This makes obvious sense because the number of teeth on the wheel does not change while the circumference grows considerably.

The worm housings are simple U-channel profiles in sturdy cross sections to avoid flexure. The worms are held in journal bearings pressed into the webs of the channel section and fixed securely with tiny, SS grub screws.

 A view of the 'teeth' on the larger wormwheel. The thread on the worm engages over much its flanks to avoid play and ensures low wear in the wormwheel teeth.

Each wheel bush has three radial holes to take stainless steel grub screws with nylon pressure plugs. The latter press against the mounting axis shafts to provide a very simple clutch to allow the telescope to be moved easily, independent of, and without disengaging the worms.

The overall finish is very acceptable but the teeth could do with a scrub with a fine wire brush to remove some slight debris from the machining process. The vendor recommended lapping with plain oil rather than using any abrasive or even metal polish. His argument was that polishing particles would tend to bed into the materials and continue to cause wear over time.








Click on any image for an enlargement.

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18.8.16

Another [10"] OTA lash-up.

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It has long occurred to me that I could use four of the builders straight edge profiles to build a very stiff and light, four sided, altazimuth OTA. 

The images show a full sized mock-up using recycled rings largely held together with cord.  I have a sheet of 12mm birch plywood in stock to make smarter new rings.

The main problem with my earlier spar-type OTA was the torque forces around the doubled beam from the heavy 10" mirror's considerable offset. The beam did however suit a German equatorial mounting at the expense of some potentially odd eyepiece angles. 

The 2 meter, 6'6" long cardboard tube was simply far too heavy and awkward for comfortable handling. This four spar and rings OTA assembly is much lighter and readily provided spaces to grip it securely for transport to the observing site.

Having spars on all four sides immediately increases the stiffness of the 'tube' assembly. The spars have their own considerable depth to resist bending loads. 

The spars are also widely separated. Which naturally increases their 'moment.' So that bending of the OTA is greatly resisted in all planes.

My plan is to throw together a Dobsonian to get the 10" mirror back in action after far too long a hiatus building the 7" refractor and building mountings.  I don't think I ever once had the beam OTA correctly collimated due to flexure. The Dobsonian design provides the minimum possible ground clearance. Which obviously suits the very tall 10" f/8 OTA. The lower the eyepiece the less I need stepladders at modest object altitudes. A Dobsonian has no eyepiece access problems since it avoids odd tube rotation angles.

The spars suit clamp-on altitude bearings to achieve balance without permanent cosmetic damage to the spars. This is useful if I should ever rethink the design in the light of experience.

There are no torque loads applied to the new OTA. The mirror mass and secondary are both at the tube's center of gravity axis. I have simultaneously got rid of the thermal mass of the large and heavy aluminium saucepans and the aluminium plate of the primary mirror cell. I shall use a simple birch plywood cell for primary collimation.

The focuser will be fixed to the cardboard tube between two spars in the secondary cell to minimize the distance from the optical axis. This location will automatically apply a 45° angle to the focuser base for comfortable viewing at any tube angle.

The tube rings have no need to be made of heavy plywood if the short cardboard tubes are glued to become solid units with their pairs of rings. The cardboard tubes will provide some protection against dew and stray light. The plywood rings will each have four rectangles cut out to closely match the spar's outer dimensions. With the rings separated by the length of the cardboard tubes there is considerable natural resistance to any leverage applied by the spars to the ring assemblies. The reverse is also true.

No serious spar clamping is necessary due to friction in the rectangular cut-outs of the rings. In fact it is very difficult to move the cells along the spars with a quite normal tolerance sliding fit. Even with only two spars in rectangular cutouts I needed a rubber hammer to move the rings over and along the spars. Simple screws driven through the edges of the rings will easily suffice to fix the cells and allow easy, later dismantling if required.

I am trying to think how I could use a router to mass produce the 16 rectangular holes without wielding a hand saw. I have several bearing-guided, router bits which might work with a suitable template. Oddly enough I have never made or used a router template until now. Previous attempts to cut the holes with a coping saw were rather 'clumsy.' Requiring considerable coarse filing to smooth things out but left ragged edges from the birch plywood splintering.

I ended up using heavy brass plates with added barbel weights and G- cramps [C-clamps] to "box in" the router base plate as I cut each rectangle. An initial small drill helped to 'sharpen' the rounded corners left by the 1/4" 6mm router bit. Next time I shall make a square base plate out of Tufnol or plastic to better confine the cutter. The rounded base of the Bosch router does not slide perfectly straight against an edge. Nor does it work well inside a template. 

Click on any image for an enlargement.
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17.8.16

Ronchi [star] testing iStar 180mm [7"] f/12 R35.

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Using the same technique as with the Vixen last night I captured some Ronchi test images. All images were taken inside of focus with a Canon compact camera at full zoom through the Neumann 10 lines per mm Ronchi test eyepiece. 10 lpm =~250 lines per inch.

Using the 7" on Arcturus [low in the west] provided a much  brighter image than the Vixen had. The images were larger and still visible on the camera's LCD viewing screen after I had taken each 'snap.'

I was delighted to see the bands were straight by any reasonable standards when well within focus. Exactly how accurate these images are in giving an insight into the spherical correction of the iStar lens I have no real idea.

There was much more movement in the atmosphere tonight after a long sunny day when it reached 70F. Perhaps it was just the larger aperture exposing the thermal seeing?

The first two images have been converted to B&W and maximized for the colour which provided the greatest contrast. Usually green seems best.

All the other images are unfiltered and were brightened considerably in Photofiltre and then cropped for far greater image size on the blog. In each case I joggled between Gamma and Contrast to make the most of the visible bands.  

These coloured [white light] images exaggerate the colour well beyond anything I was seeing visually. The camera cannot ignore colour as can the elderly human eye.

The bands can be seen to spread apart and grow broader as one moves towards focus from well inside.

Note how the bands change from one snap to the next. I would probably be foolish to read too much into any particular test image.

I am looking forward to using the Solar Continuum green filter to reduce the multitude of rainbow colours and hopefully darken and sharpen the bands.

The bands are growing wider with each new image which should be a better indication of the actual figure on the complete lens.

In some of the images the edge looks rather soft while in others it looks slightly better.

There does seem to be a definite tendency for the bands to bend outwards towards the edge.

This area can be removed by stopping the lens down slightly. This also improves the colour correction since the lens becomes 'slower.'

Its aperture becomes smaller for the same focal length. Changing the focal ratio in this way reduces chromatic aberration and makes the lens less sensitive to spherical error.

It remains a mystery why I saw such curved lines on the first attempt to using the Neumann Ronchi test eyepiece on the iStar lens.

I have been worrying about this for some time despite the very reasonable images I get visually and when imaging. Though admittedly the iStar is not a patch on my 10" f/8 Newtonian reflector.

I will update this post after further overnight thought.

I am a little confused by the concentric ring inside the image.
It is almost as if it is a double exposure at two zoom settings. Further reading suggests the parallel sides of the transparent media on which the Ronchi grating is etched cause this.

The 45 degree Baader diagonal has been eliminated for these images. I used a star diagonal instead. 

I think it is fair to say that the bands are curved inwards towards the center. With a stronger curve nearer the edges. If this was a mirror it would be an oblate spheroid with a turned up edge.

It is decades since I did any serious optical testing and that was almost always on my own ATM mirrors.

I shall have to do some reading on how the bands appear on lenses compared with mirrors.

Arranging multiple images is a complete lottery on Blogspot. Nothing I do with the usual editing facilities will allow me to place the images correctly. Once published or updated they look completely different from the editing mode! I have deliberately kept these multiple images small to avoid problems with slow internet connections.They can all be enlarged by left clicking.


Click on any image for an enlargement.
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Ronchi [star] testing achromatic refractor objectives.

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Since purchasing my Ronchi test eyepiece I haven't used it much at all. Normally [?] I have always had a habit of knife-edge testing every telescope [at the focuser] to see the figure of the objective. The problem with refractor objectives [rather than reflectors] is chromatic aberration. This effectively blurs the Ronchi bands because the light focuses at different points along the axis depending on exact wavelength. This is why a green filter is often suggested for star testing achromats. 

The Ronchi test would similarly benefit from reducing the test light to a narrow band of wavelengths in the visible spectrum. With all the rest of the spectrum discarded, the achromat is then effectively tested [for spherical aberration] in green light. Green-yellow is the light to which the human eye is most sensitive. Which means that it is more easily able to reject [subjectively] dimmer colours. The colours still have much the same brightness but the human eye/brain system can ignore them relative to the preferentially brighter yellow-green. The human eye is least sensitive to violet so can usually tolerate a violet wash. The lenses of older observer's eyes naturally yellow with age with a further subjective reduction of the purple haze. Which is why older users often report a perfect view from a faster lens. Where younger users would complain bitterly about the violet. 

The real experts in lens testing, and presumably achromatic lens manufacturers, use a large, precision optical flat and a laser as a light source. The light passes from the laser through the lens and is then bounced back through the lens again via the optical flat beyond it. Thence back to the camera or tester's eye. This is known as double pass testing.

The major advantage of such testing is the increased visibility of lens errors since they are effectively doubled. AND, that the tester does not have to wait for a test star to show between thick clouds. NOR, do they have to worry about the "seeing conditions" on that rare, perfectly  clear night. 

Thermal agitation in the atmosphere between the light coming from the distant star is perturbed by the difference in refractive index with changing temperature. Twinkling of the stars is the usual sign of thermal agitation. Using a telescope when the stars are twinkling strongly can add a "boiling effect" superimposed on views of the Moon or planets. Naturally this usually spoils the view!

When I first tested my iStar 180mm f/12 R35 with my new Ronchi test eyepiece I saw strongly curved test bands near focus. At the time I falsely assumed that this was the result of using a white light source. [A bright star without filtration.] In other words, my blind loyalty to my purchase was making me completely blind to reality. Regardless of colour [frequency] the Ronchi bands should still remain perfectly straight in a spherically corrected lens. 

Each different colour [frequency] of light will have slightly different focal point. The closer to focus the broader the bands appear and the further apart. These straight Ronchi test lines will all overlap in a confusing picture. But, they should all remain straight if the lens is well corrected for spherical aberration. Which it should be of course if it is to lay claim to being of astronomical quality and "diffraction limited". Which is the minimum standard expected of any decent telescope objective.    

Months later, while reading a thread on Cloudy Nights, I noticed an image showing nice straight and dark Ronchi bands from an objective test. I then asked "the tester" which green filter he had used. Whereupon I discovered that he used a green laser, microscope objective and optical flat to achieve his remarkably sharp test images indoors.

He did not recommend Ronchi testing on stars because of the "seeing" problems and chromatic aberration problem. The downside of his efforts is having to afford or borrow a large optical flat for testing at full aperture. Not to mention the skill required in setting up the indoor test system and then capturing the results.

Very narrow passband, green optical filters are available. However, I discovered a Baader Solar Continuum filter had useful testing properties for my intended use. The SC filter is meant to clean up white light, solar surface images which can be improved further with an IR/UV cut filter. [According to a comprehensive test of various green filters for solar imaging.] Since I hoped to do some solar imaging myself I took a very deep breath and placed an order for both filters online in the 1.25" size.

 CCD moon and planets

Last night, I tested my little Vixen 90mm f/11 refactor on the Fullerscopes MkIII mounting using the Neumann 10 lpmm/ 250 lines per inch Ronchi test eyepiece. Arcturus was shining brightly, though rather low down to the West, just above the local trees. The low altitude should have saved me some neck strain. It didn't, so I added a Baader 45° terrestrial prism to bring the test view almost horizontal. Capturing what was clearly visible with my eye was quite another matter! Starlight, spread over a distant lens, is both dim and small. [No eyepiece magnification!] The compact digital camera needed full zoom to make the test image [of the bare objective] large enough.

The problem then was the dimness. Every time I pressed the shutter button the image simply vanished from the viewing screen. Fortunately I managed to capture several full aperture images more by luck than skill. When considerably brightened, and further enlarged by cropping, I had a fuzzy and colorful record of my Vixen lens quality. Seeing conditions and the prism diagonal seemed to have zero effect on the quality of the lens test. Though the erecting prism caused a wavy line of discontinuity through the middle of the last image, the quality of the bands looked no different from the direct view. You can judge the images here for yourself. It should be said that for optimum accuracy the lines should really be expanded to show only a few by drawing closer to the focus. Having more lines visible shows the general nature of the surface including any edge problems. 

It follows that testing the bigger iStar achromat should produce very similar results. The Ronchi test lines should be just as straight as the Vixen's. I will share the images here when I have them. It was too late last night after practicing with the Vixen. The sky was still not fully dark after 11pm.

I really need to make a simple camera alignment adapter to center the 'nose' of the camera lens over the Neumann Ronchi EP. I have made a similar [recycled] detergent bottle top adapter already for [normal] 1.25" Meade 4000 eyepieces, but it won't go over the Neumann Ronchi test EP.

One slight oddity is the lack of a filter thread on the Neumann Ronchi test EP. The 'body' is very short and thick walled. It seems an oversight not to have provided a standard thread for a 1.25" filter.

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9.8.16

2" shaft mounting Pt 23: Adding a disk/plate bearing.

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Here I have fitted plastic 'feet' as friction pads to the tops of the studs. There was too much friction until I tried some silicone grease intended to stop car door seals from freezing together in winter. The silicone produced a silky smooth turning action on the shaft. The slight extra degree of friction may be useful for avoid too much sensitivity in balancing the OTA. The large flange bearings and their seals have very little friction in themselves.

I should have reversed the flange bearings by now to avoid having any bare shaft above the bearing. The minimum amount of cantilevering is desirable to avoid any chance of shaft flexure.


This image shows the brass ring I made in the lathe to resist the outward pressure from the Tollok locking bush's opposed cones. Which should ensure the Tollok bush contracts strongly onto the Dec shaft to hold the saddle very firmly.  That is not a gap between the shaft and the Tollok bush. It is just that the inner cone has withdrawn slightly inside the bush.

A thin, steel, expansion ring is supplied with these bushes but it was slightly too large in diameter to fit between the flanges of the channel section saddle. I had already had to grind small flats on the Tollok bush to fit the largest flange into the channel. Cutting away the light alloy saddle, to allow the bush to fit inside, would probably have weakened it. The small flats I ground and filed smooth will not affect the Tollok bush's strength at all.

Having the Tollok bush available has allowed the saddle to be supported with virtually no overhang. Every bit of unnecessary overhang means extra counter-weighting for the OTA. One just has to ensure OTA clearance for the large RA wormwheel.

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