28.2.13

10" f/8 Next comes the primary mirror cell.

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[8]

Automated Mirror Cell Optimization or 'PLOP'

A 10" mirror of 40mm thickness does not need a complex, multi-point, support cell. Though it does need gentle support and restraint. Thinner mirrors may well need 9 or more balanced and carefully calculated support points to avoid flexure. 

The need for fan cooling and avoiding poor mirror cell practice does not require greater complexity. Two pieces of 3/4" plywood plywood will form the simple, primary mirror cell. I have found some suitable coil springs, coach bolts and matching wing nuts. The three domed, coach screw heads will have rubber glued to them where they protrude through the mirror support board. Then they will be covered in felt glued on top of the rubber. The screws will provide 3-point collimatable support at the required radius. Avoiding the necessity of adding separate support points.

It is important that mirrors can slide freely within the restraints of the side tabs. Even if the distance they move is almost theoretical. The felt will provide this freedom to move. I shall not be following the modern practice of gluing the mirror to the cell with silicone aquarium cement.  If the plywood bends or there is any differential expansion between the mirror and its backboard then the mirror may be temporarily strained or warped.

The difference between a fine figure and a poor optical figure is less than the change in figure due to warping. Astigmatism is likely too. ie. Bending of the mirror blank across a particular diameter. Causing the surface to no longer be a figure of revolution. The shape of the supporting mirror blank could become a pretzel or a potato crisp, optically speaking. Only millionths of an inch separate the finest parabolas from the run-of-the-mill optical surface.

It is important that the side restraining tabs do not project over the mirror surface. Which is an all too common practice. Light obstructing tabs will cause diffraction. Provided the main tube isn't pointed downwards then the mirror is quite safe from falling out of its cell. The side restraining tabs will be rubber covered and reach almost to the front surface of the blank. This will ensure the blank remains seated against its support pads. The centre of gravity of the blank will remain behind the support tabs even with the tube horizontal. Again, the rubber will be cut from an old, car inner tube. The side tabs themselves will be right angle brackets screwed to the mirror support board beneath the rear edge of the mirror.

The cell base board (or perhaps two laminated boards for greater thickness) will be round and fit the telescope tube nicely. The circle will have a central perforation for the mirror cooling fan. Since it is otherwise closed to the tube end, the backboard will direct the airflow from the fan efficiently. A bare fan hanging in fresh air has very poor efficiency. The air can easily curl around and around the fan edges (like an invisible doughnut) instead of being directed cleanly forwards and away from the fan to form a proper airflow.  

The mirror support board will also be perforated to allow the draught from the fan to reach the back of the mirror blank. The air will then curl naturally around the mirror sides and move on up the open tube. The draught will suck the boundary layer from the front of the mirror. (where it really matters) Once clear of the mirror the airflow will scour the inner walls of the tube before reaching the secondary mirror. Where it will scavenge any warmth from the elliptical secondary blank and its spider. Spider vanes which are warmer than the surrounding air will have their own boundary layers. Causing diffraction effects out of all proportion to their actual thickness.

The smallest air temperature differences, in the optical path, will have different refractive indices to the cooler air in the tube. As is easily visible if you look at the background beyond the top of a bonfire. The airflow from the fan is to prevent stagnant, warm air from clinging to the mirror surface, inner tube walls, the secondary mirror and its supporting spider. If air did not have a different refractive index when at slightly different temperatures then there would be no need for a fan. The Newtonian would then be an almost perfect telescope. Unfortunately life is not that simple and the warm air must be removed by using a degree of force. The trick is to know how much air movement is required under different temperature regimes. More is not necessarily better when it comes to fan speed and and quantity of the airflow required.  

Pictures will follow:

The flexible plywood sources, I hoped I had found, were dead ends. 8mm was the thinnest available from the builder's merchants. The other did not deal with the public. I may have to try and find full sheets of marine or aircraft ply and roll a tube in two parts. Or, rather, overlapping halves. I have never seen anything but 1.5m x 1.5m. Which is only about 5' square. The length joint could be hidden by a mounting ring. Ideally I would like to use epoxy to laminate two layers but it will be months before it is warm enough for normal use.

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27.2.13

10" f/8 Another Vixen focuser?

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[7]

Thanks to Dave and AstroBuysell(UK) I have a nice, sturdy, 2", rack and pinion focuser, for my long focus 10" f/8 Newtonian, without spending a fortune. The item arrived superbly packed in bubble cushions and bubble wrap in a strong box all well taped for security in the post. Some lazy optimists get slapdash with the packaging once they have their victim's money. Not so with Dave. He did a great job and the focuser arrived in perfect condition. Thanks for taking the time and care to do it properly, Dave.



Here are the three Vixen focusers. From L to R: The 2" eyepiece fitting reflector type, a 1.25" fitting refractor type and a 2" fitting refractor type. The reflector focuser includes a 1.5" (38mm) long extension, 2/1.25" eyepiece adaptor and dust cap. Which Dave kindly included in the sale. I have some long focus 2" "Plossls" which will sit in there nicely if I ever need very low powers and more drawtube length. It may also have uses for photography.


A view of the undersides of the focusers where they would normally fit on the instrument. They each use low friction slides to ensure stable movement of their drawtubes while being driven in and out by their rack and pinion.


The reflector focuser from the side showing the focussing and draw tube locking knob.
The drawtube has been retracted to its minimum height of 100mm or 4".

The adaptor has a locking ring allowing a small range of adjustment of minimum height by screwing the adaptor in or out.

It is important to ensure the engagement of sufficient threads to maintain enough strength to support a heavy camera or other equipment.



 The focuser showing the alternative eyepiece fittings.  The taller 2" fitting eyepiece adaptor came with the focuser.

The focuser is shown fitted with the 1/2" (12mm) lower, 2" fitting compression ring eyepiece adaptor, borrowed from the large refractor focuser.

This reduces the minimum focuser height to 3.125" or 80mm.

Drawtube movement is 26mm or just over 1". 


Here is the reflector focuser from the rack and pinion side with the compression ring fitting from the large refractor focuser.

The plate supporting the pinion which drives the rack is held firmly by the four screws.

Drawtube tension/friction is adjusted by the tiny Allen grub screws on the opposite side from the rack.

The large refractor focuser fitted with the eyepiece adaptor from the reflector focuser.

The image on the right shows the main tube adaptor for fitting the focuser onto the CR150HD Celestron 6" refractor.

Also on the right, the large drawtube has been raised to the maximum.

This focuser offers 78mm of drawtube movement and once contained a single baffle. This was removed by the previous owner for imaging.



The 1.25" focuser from the Vixen 90M refractor with the drawtube at full extension. And below, with the drawtube racked fully in. The focuser offers 135mm of drawtube movement.



Here is an indoor, flash shot of all three focusers together. The two refractor focusers are both by Vixen. A 2" on the right. Which arrived fitted to a secondhand, 6" Celestron CR150HD refractor obtained from Ian King. The 1.25" focuser on the left is from a Vixen 90M [90mm f/11] refractor also bought used from the UK.

I don't think there is much doubt that the reflector focuser, in the centre, is also a Vixen. The family resemblance, cosmetic finish, control knobs and adaptor threads all match perfectly. There is some slight variation in the pale green "hammered" finish paint from one focuser to the other. Interestingly (?) the flash produces a much greater variation in hue than daylight. All three focuser tubes were wound down to be flush with the supporting surface for this photograph. [This image has also been flipped horizontally]

Click on any image for an enlargement

10" f/8 Newt (for the) Web telescope design:

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[6]

While researching Newtonian design online I came across this very useful software:

Link: Newt for the Web

Here was my first attempt before I had bought a (used) reflector style focuser to actually measure:

Newtonian

Metric: 254mm F/8 47mm m.a. (minor axis of the elliptical secondary)
Imperial: 10" F/8 1.85"m.a. 

Primary Mirror Diameter...................254.000
Focal Length..................................2032.000
Focal Ratio..........................................8.000
Tube Inside Diameter.......................302.000
Tube Thickness...................................4.000
Focuser Minimum Height...................50.000 *
Focuser Inside Diameter....................50.000
Focuser Extra Travel.........................12.000
Diagonal Minor Axis..........................47.000
Diagonal Offset..................................1.469
100% Illumination Diameter..............22.251
75% Illumination Diameter................35.424
Front Aperture Diameter..................287.206
Mirror Face to Focuser Hole..........1815.000
Focuser to Front End of Tube..........100.000
Mirror Face to Back of Tube...........100.000
Tube Length................................2015.000

This would represent an ultra-low focuser height.

And again, but now with an actual measured focuser depth of 100mm. (4")

254mm f8 47mm(m.a.) 100mm focuser height

Primary Mirror Diameter................. 254.000
Focal Length.................................2032.000
Focal Ratio.........................................8.000
Tube Inside Diameter......................302.000
Tube Thickness..................................4.000
Focuser Minimum Height................100.000 *
Focuser Inside Diameter ..................50.000
Focuser Extra Travel........................12.000
Diagonal Minor Axis.........................47.000
Diagonal Offset.................................1.469
100% Illumination Diameter.............15.686
75% Illumination Diameter...............32.852
Front Aperture Diameter ...............283.985
Mirror Face to Focuser Hole........1765.000
Focuser to Front End of Tube .......100.000
Mirror Face to Back of Tube..........100.000
Tube Length...............................1965.000

Note how increasing the height of the focuser to 100mm has shortened the tube. The diameter of full illumination has also shrunk in comparison with the ultra-low focuser. Even so, no vignetting is reported by the Newt-Web software.

Careful choice of focuser height affects the necessary size of the flat, secondary, elliptical mirror. The higher the focuser the larger the required secondary mirror. The larger the mirror the greater the obstruction in the light path to the primary. Causing even greater diffraction effects from the secondary obstruction.

Those seeking the highest optical quality from their Newtonian telescope will choose a lower focuser. Which would  immediately allow a smaller diagonal mirror to be used. Which in turn reduces the obstruction and minimises diffraction effects. The spider vanes add their own diffraction effects but not nearly as much as the secondary mirror itself.

There is no free lunch though. The smaller secondary mirror may/will not illuminate the entire field. The light loss around the edges of the field may become noticeable. No great problem with small planetary objects centred in the field of view. Not so good for deep sky observation or photography.
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Now a range of eyepiece choices, when used with a telescope of 2032mm or 80" focal length. Magnification is easily found by dividing the focal length of the objective by the labelled focal length of the eyepiece.

A very simple example would be a 1" eyepiece used with an 80" focal length mirror giving 80x magnification. (or power)

80/1 = 80. Or, in metric terms: 2032mm/25.4mm = 80x  One inch is 25.4mm.

The higher the magnification the smaller the field of view. Useful powers on the planets run from about 120x upwards. This will vary with the object being observed and it's orbital position relative to the Earth. A smaller but sharper image is far more desirable than a large fuzzy one. Though higher powers can sometimes be used to examine a particular feature to confirm some detail. The seeing conditions usually place the limit on magnification. When the observed object is boiling from atmospheric disturbance it is a complete waste of time pushing powers too high.

The table below shows the eyepieces from my own inexpensive collection of secondhand Meade 4000 and no name Asian Plossls. Some of the higher powers employ a 2x Barlow lens. This is a negative lens in a housing which simply fits into the drawtube like an eyepiece. While simultaneously providing a matching socket for normal eyepieces to fit into. This doubles the effective magnification of any eyepiece fitted into the Barlow lens. There are higher power Barlow lenses available but I don't own any. Maximum power for a high quality mirror of 10" aperture is 500x and only possible in very good/perfect seeing conditions.  

Eyepieces:

Focal Length, Power, Exit Pupil,  True Field

32mm............63.5x.......4.00mm......0.551°
26mm............78.2x.......3.25mm......0.640°
20mm..........101.6x.......2.50mm......0.492°
15mm..........135.5x.......1.88mm......0.369°
10 mm.........203.2x.......1.25mm......0.246°
7.5mm.........270.9x.......0.94mm......0.185°
6.4mm.........317.5x.......0.80mm......0.157°
5mm............406.4x.......0.63mm......0.123°
3.2mm.........635.0x.......0.40mm......0.079°

With a focal length of 80" = 6' 8" (2032mm) the magnification soon rises. An 80" f/l is equivalent to a 6" f/13. Or 8" f/10. Even a 12" would have an f/ratio of 6.6 at this focal length.

My apologies for the awful formatting of the tables above. It is something to to do with the blog template software. The tables look far worse in the editing mode! I have spent half an hour fiddling with the number of full stops and still it looks completely askew. If I leave the full stops out everything shifts to the left. Completely Ignoring the spaces I carefully inserted between the columns. Another problem is that each digit and letter has its own allotted width. The software cannot automatically arrange numbers or letters in neat columns vertically above each other.

Click on any image for an enlargement.
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26.2.13

10" f/8 Tubing roundness continued:


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[5]

Despite a temperature of only 33F, +1C, in the shed I cut out six 12" circles from the waste 15" plywood cut-outs. I then ran the circles against the router with a central pin just to tidy up the jigsaw cuts. The saw cuts weren't bad but had a few lumps and bumps to tidy up. The blade also tends to lean over with added pressure. This is largely unavoidable. Since the 3/4", 18mm  plywood produces different levels of blade resistance at different angles to the veneer grain. Even taking it very easy with the forward pressure on the jigsaw the resistance changes constantly.

These circles are only to round out the very oval Biltema cardboard tubing. I pushed one circle to the centre of each tube and then pressed one each at about 4" from the ends. They were easy to fit by sliding them in on edge. More difficult turning them square to the tube to fill the inside tightly. I'll leave them in place for a few days to see if they improve the roundness of the tubes. The circles offer more far resistance to returning to oval than much thinner outer rings are likely to offer. The alternative tubing suppliers website insists that the tubing be stacked vertically before use.

If I was doing it all over again I would cut further outside the beam compass line. Then use the router to produce much more accurate circles. Not that it is at all critical in this context. It would just be nice to have accurate circles should they ever be needed for another project. I sanded and chamfered the sawn edges slightly to avoid damaging the inside of the tube as the circles were forced square to the tube.

The cost of the cardboard tubing, from the other dealer, is prohibitive despite the much more robust 9mm wall thickness. Though it does include "free" delivery. If I can arrange personal collection, at lower cost, it would be quite tempting. I could then ensure the tubing was round and straight before purchase. They also offer a 350mm /13.75") size. Which would ensure improved clearance between the light path and inner tubing wall. The weight increase is not dramatic.

If I stay with the existing Biltema tubing I shall have to use an exo-skeleton of ribs and formers to join the two halves. The external framework will help to stiffen the 4.5mm tubing into the bargain. As well as holding it round over time.

It has been suggested by a very experienced contact that I paint epoxy resin onto the tubes. It wouldn't be a bad idea to coat the exo-skeleton at the same time. To help to bond everything into one solid unit.

Royce recommends painting the entire tube with polyester resin but this isn't so tough as epoxy. Neither is likely to harden properly without a reasonably warm sunny day. Which is probably several months away in the present Danish climate!

The option still exists to wrap and glue another pair of tubes over the ones I have now. They would need to be neatly slit lengthways first. The round formers would maintain roundness. The joints between the tubes could be staggered to achieve high strength and stiffness. The long edges of the second layer would not meet but could be filled with strips cut from the waste lengths of tube. Epoxy might be a good choice of adhesive to ensure a solid tube.
_________________________________________________

Next day I removed one end circle and remeasured the tube. With two circles still in place the open end measured perfectly round across several diameters. Without removing all three circles it is difficult to say how soon the tube will assume permanent roundness. If ever. 

I have found another cardboard tube stockist but it isn't remotely local and only offers 1.5 metre lengths. Which is only about 5' long. Still well short of the 6'6" I need in one length.  

There is also a stockist offering 1.5m x 1.5m of thin aircraft plywood. I have previous experience of rolling a 6" tube out of 2mm birch aircraft ply. Though the meeting edges were very difficult to curve to match the remainder of the tube. I placed a batten along the join as I added dozens of car inner tube, cut-off rubber bands. While the glue dried between two sheets of the ply bent round circular formers. The tube still remained rather egg-shaped in cross section despite my best efforts.

Cutting the long meeting edges of the plywood tube after gluing is fraught with difficulty. Almost as difficult as cutting the width of the sheet accurately prior to wrapping and gluing. One simply cannot judge reliably where the edges will meet when the pressure is applied to the glued layers by tightening bands or straps. Too short and there will be a gap between the edges for the entire length of the tube. Too long and the joint will rise against the butting pressure of the edges. With two plywood layers each sheet needs to be quite a different width.

I have seen some suggestions, online, to soak or even to steam the plywood prior to bending. Not an easy task to steam a 5' square sheet. Though it would be easy enough to soak it. Even pour boiling water from a kettle over the sheet if it helped. It would be a hell of a waste if this damaged a very expensive sheet of plywood! I am loathe to get involved unless I can find a sheet of ply large enough and thin enough to form a full length tube around the cardboard core.

Probably using epoxy to bond it directly to the bare cardboard while lined with several circle formers to resist the crushing loads of the tightening straps. Where several laminations are desired thin aircraft quality plywood is so expensive that it would be cheaper to make a fibreglass tube. Or even a carbon fibre laminated tube. Though neither is as respectable for thermal neutrality as cardboard or plywood laminations.  

It might be possible to make a rotating head to allow the eyepiece to be always brought to a comfortable position. Rotating the entire tube in its mounting rings is never easy in larger diameters due to weight and friction.
____________________________________________

I have now removed the stiffening circles from one tube and left it standing upright to check for any subsequent change in form. It now seems perfectly straight and round. I wonder whether it will remain that way?  l'll leave the other tube with the plywood circles in place for a little longer to see if that helps.

It would be interesting to understand why these tubes do become so oval when laid flat. I think one can assume that the atmosphere in Biltema's warehouse storage is warm and dry. So moisture is not to blame. Yet the stock tubes were very oval indeed. While at the same time they resist hand pressure when trying to force them to become round.

Only the considerable pressure exerted by the plywood circles was able to force them back into roundness. I had to give the circles in the very oval tube a few good wallops with a hammer, via a roofing  batten, to get them to turn squarely across the tube.  

The rapid improvement in roundness suggests that the tubes will respond just as well to external plywood rings. Even more so with dowel stringers to help  to keep everything in shape and straight. A seal with the recommended epoxy should do wonders in providing a relatively cheap, lightweight thermally neutral and stable telescope tube.

Plastic tubes do not change shape noticeably despite lying for years out of doors. I had unused lengths of 14" and 18" PVC ventilation tube from unfinished Dobsonian projects. Which became rather brittle and bleached, after ten or more years in sunshine and weather, but never obviously changed shape.

________________________________________________

Another tubing update: The tube, from which I removed the plywood circles, has retained perfect roundness. The trick seems to be to leave it standing vertically. Had I laid it down flat then I am fairly sure it would soon become oval again. Which suggests that, for a telescope tube to stay perfectly round, it will require mounting rings. Or external plywood stiffening rings.

And another update: The tubes now seem stable provided they are standing up. I have failed to obtain flexible plywood to laminate over the cardboard tube. It is only available in 9mm which is far too heavy. It could be months in arriving if I special order 3mm.

I weighed the cardboard tubes this morning. 120cm/47" lengths of 4.5mm thickness =  6.7lbs each. So a full length 2m tube of single thickness will turn out at about 12 lbs. If I laminate one tube over the other it looks more like 24lbs! Plus mirror, plywood rings, dowels, spider and secondary. Lumpen!

I have finally found a stockist of thin, flexible plywood online . (DK= Bøjelig krydsfiner)  Supposedly available in oversized sheets with a choice of wide or tall flexibility. With considerable effort I finally managed to get the local builder's merchants to make enquiries about pricing. I thought we were in the middle of a recession with a complete stop on house sales. Since the majority of building work follows a house sale builders are going bankrupt left right and centre.

You'd think the builder's merchant staff would be falling over themselves to supply a willing potential customer. But, no, I had to hand feed him the information and insist he look them up online so that he could make enquiries on my behalf!  The stockists have the builder's merchant's own company logo right at the top of their website!!! Why else would I bother to go to him?  Because the suppliers are wholesalers and won't deal with the public. So I am forced to use the builder's merchant. Arrgghh!

Now the price will probably be set unrealistically high! If it is cheap enough I can buy two sheets and laminate them together to make a really strong, but rather heavy tube 8mm thick. If it is just affordable I might laminate one sheet over the cardboard tubes to save weight. I tried lifting a sheet of 4mm ply while I was there and they are a bit heavy. Far heavier than my cardboard tubes! A standard metric sheet of plywood would be 2440mm x 1220mm.  A full telescope tube would only need 1981mm x 965mm (or ~78" x 38") Roughly 64% of a full sheet. Or 128% of one sheet with two layers. From searching online, an 8' x 4' sheet of 4mm (about 3/16") weighs from 20 lbs upwards depending on the timbers used.  

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23.2.13

10" f/8 Planetary Newt: The tube build:

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[4]

The 10" F/8 Telescope Tube design:

In my last 'essay' I tried to establish umpteen commandments for maximising the "humble" Newtonian for optimised planetary observation. So I had better follow the rules. No short cuts and no excuses. No sinking into bling for its own sake. If it does not add to the performance then it deserves no place on my telescope.

By chance I had found a (used) long focus mirror and matching flat online. They duly arrived in perfect condition and very well packed. The physical and optical dimensions of my planetary reflector were now set in stone. (Well glass, actually)

With the mirror diameter and focal length now decided I searched online for the concrete pile casting tubes. More commonly known as Sonotube in America. It turned out that there were two stockists of these spiral wound paper tubes. Both within half an hour's drive but in opposite directions. By far the cheapest option was Biltema. A discount DIY chain with a huge range of (almost) affordable products. They call them støberør in Denmark. (casting tubes or pipes)

They listed 12", 302mm internal diameter, spiral wound paper tube of 4.5mm wall thickness for about £12 or 120DKK. (in early 2013) The problem was the length. They only sell the stuff in 1.2 metre/47" lengths. My mirror required at least a two metre, 6'6"  length. I decided I could cope with this if I joined two lengths. I could choose where to place the joint later. After the entire OTA (optical tube assembly's) balance point had been established. At the time I thought that Biltema tubing had no plastic liner or wax coating, inside or out. So I could even glue the spare length of tubing over the joint. Or make a tube twice as thick from several tubes cut to size.

Why not just buy a suitable length of the 300 mm x 8mm thickness from the other nearby, but far more expensive, stockist? I like a challenge and prefer the thinner, untreated tubing for my initial tube design. If it fails I can always order the more expensive tubing. Possibly even going well oversize to 14". 350mm diameter. This would offer a 2", 50mm clearance between the full aperture and the inside wall. Not that vignetting is a problem even now.

Once in the vast Biltema shop I discovered they had absolutely no respect for the roundness of their stock! The tubes were stored on the very top shelf of a towering 20' high rack. The tubes were housed horizontally in a very tall and very rudimentary wooden frame on a pallet. This meant that all of the tubes were horribly oval from constantly pressing down on each other over time! Even the top tubes were oval so they may warp out of round naturally over time.

Why bother to stock something if your means of storage was so obviously ruining the product for its intended use? Oval concrete piles or pillars, anyone? Are oval pillars even legal under Building Regs? Why not stack the tubes vertically in a suitable wooden or metal cage? It would take up exactly the same amount of storage space. All safety factors taken into account, of course. You don't want these tubes dropping from a great height onto unsuspecting customers! How difficult would it be to run some strapping around the cage to allow vertical storage?

I decided the tubes were cheap enough and of suitable material quality to experiment with further. The spiral wraps were very neat and without any overlap. Making for a much prettier tube once filled and painted to keep it protected from inevitable moisture. They also had no (obvious) waxy or polythene lining. So would be much easier to work with and produce a half decent finish. (more on this later)

I'd probably start off by making some internal disks of plywood to force the tubes back into acceptable roundness again. So I bought the best two lengths in stock. (Believe it or not!) The rest were very much worse! In case you were wondering these tubes are far too stiff to make much impression by hand pressure alone. They hardly change shape at all no matter how hard I try to press them round. This is a good thing when the tubes are round. Far more difficult to deal with when they are this oval!

Given the very poor roundness of the tubing and the serious lack of length I decided to house the tubes in a series of plywood rings. These rings would be joined by several wooden dowels. You can call them stringers or longerons, if you like. Model plane, fuselage style.

A telescope tube must be very stiff overall. Even a large, thin-wall tube can become oval and bend regardless of its material. I have seen very long lengths of thick wall, steel tubing, up to 24" diameter, hanging from a crane strop, bend dramatically! Trust nothing to chance. A flexible telescope tube will place the mirror's optical axis outside the field of view. The telescope will constantly change its collimation depending on its angle of inclination. Any attempt at accurate collimation will be an exercise in frustration!

Perhaps the rings and dowels idea seem like overkill but I was determined to avoid an open tube. Nor introduce any unnecessary metal into the tube structure. I was holding a keen eye on the tube's neutral thermal properties. The intended plywood mirror cell was going to keep the bottom of the tube nicely round.

Now I needed the external rings to avoid introducing inner rings or baffles. I still wanted maximum clearance between the light path and the inside walls of the tube. Baffles would only push thermal currents out into the light path under the influence of the cooling fan. Turbulence would make this almost inevitable. A smooth bore tube would allow any warmer air to exit smoothly.

I had several 15" waste circles already cut out in 3/4" 18mm, high quality plywood from another project. The problem was fitting enough dowels, of suitable diameter, into the annular space available. There was very little clearance between the inner cardboard tube diameter and the outside of the circles. So I chose to make new rings in 1/2", 12 mm plywood. Which I already had in the shed. This would  allow an increase of the dowels to 16 mm in diameter from my original plan of 1/2", 12mm.

I would have eight 16mm dowels running as stringers from end to end of the very tall tube. Using fresh plywood rings would allow dowels and rings strong enough to lift the OTA at any point which fell comfortably to hand. All telescope tubes without handles are a pain! Large tubes without handles are completely impossible to manage! The outer rings would help to force the tube round again. The inner roundness "taming" circles could then be safely removed. Or so I hoped.

Besides, the dowels would make useful handles when I wanted to move the OTA manually on its mounting to point to another position. None of this would add enormously to the overall weight of the cardboard tubes on their own. Nor add any metallic content. The cardboard tubes would become a light excluding core with OTA stiffness provided by the stringers and close fitting rings. (Provided, of course, that the cardboard can be made properly round in the first place)

So far I only have the tubes sanded smooth at the roughly cut-off ends. I also have the dowels. All beautifully straight and standing on end in the shed to stay that way. The problem is that the shed is still well below freezing. Working in two duvet jackets is still no fun if one's nose drips constantly onto the floor! I need to be able to rout new rings out-of-doors to avoid the usual "dust everywhere" problem. Routers are awful things for producing lots of dust. They are best used outdoors in the absence of a proper (industrial quality) extraction system. Dust on tools and metal surfaces quickly accelerates their tendency to rust. Since the dust absorbs and holds moisture. It isn't healthy to breathe the dust in either!


20.2.13

10" f/8 Planetary Newtonian Summary


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[3]

Summary of suggestions for an improved planetary Newtonian:

Gleaned from the CN thread and elsewhere:

High quality, preferably longer focus primary mirrors and preferably in the 8-10" diameter range are vital to image quality.  
Use a carefully sized elliptical flat of known high quality to match a low profile focuser.
Use a reasonably low profile focuser. No need for micro focusing in a slower telescope. Spend the bling funds on fast imaging systems.
A well over-dimensioned cardboard tube will help to maximise the potential of any Newtonian optics. External reinforcement may be necessary with thinner cardboard tubes. All tubes will sag to some degree. Particularly thin metal ones! Tube flex will place the on-axis area of best image quality outside the field of view.   
Avoid baffles in specialist planetary instruments. They hinder cooling airflow and push warm air currents into the light path due to turbulence.
Use full, well spaced mounting rings rather than small dovetails for maximum tube stiffness and stability.
Avoid large masses of metal in the area of the light path if possible. Heat will rise from it for hours.
A curved spider removes the visible diffraction common to typical straight armed spiders.  Particular curved spider designs are favoured for their specific geometry. Do some homework. 
Thin spider vanes are essential but must be stiff enough to avoid flexure. Steel vanes can be made thinner than aluminium if lightness is not considered vital. Hardened brass strip is another possibility.
Primary mirror clips/side restraints must not protrude onto the mirror surface because this causes diffraction too. 
A small cooling fan (or fans) will help  to remove heat from the mirror blank and sweep unwanted warmth out of the top of the oversized tube.
Large airflows are not necessary. The original instruments in the CN thread used thermostats to slow the fans. Computer fans are readily available. As are electronic speed controls for making silent computers. 
Blow air onto the back of the mirror and allow it to flow around the mirror blank and on up the tube.
Side fans will help to speed up cooling of the mirror blank but do not have much effect on improving detail and contrast.
Side fans did NOT replace a bottom fan in repeated tests and may be switched off during observation once the mirror is cool. Otherwise they will only stir the warm air currents in the tube and hinder airflow.
Avoid any vibration from the fans. Rubber band mounting can help. Pony tail hair bands have proved longer lived . Rubber O-rings can last for years.
Avoid open/truss tubes which allow body heat to pass through the optical path.
A rotating head or tube will allow you to place yourself on the opposite side of the tube. (Where your body heat is being carried over the top of the tube by a breeze) 
It will also improve comfort. A comfortable observer can see far more detail.
Concentrate on blackening the tube opposite the focuser to improve contrast.
Make the main tube longer beyond the spider/focuser to avoid stray light entering the eyepiece.
Avoid stray light entering the tube around the mirror cell.
A steady, driven equatorial mounting allows the observer to study an object continuously. 
An object moving across the field of view will not share its fine detail. Nor will a wobbling one! 
Avoid placing your telescope on sun-warmed concrete or tarmac.  
Store the OTA in an unheated room, garage or shed if it must be taken indoors between observing sessions.
Observing from a (safe) raised platform will put you above rising heat currents from the ground. It will simulate the benefits of a long refractor with its nose high in the air. 
A balcony is not the same thing at all. The wall below most south facing balconies will leak stored heat into your view. As will observing from flat roofs or roof valleys. It would be difficult to insulate any roof over a heated building well enough to allow a dome to be placed up there in winter. During warmer weather the roof will absorb vast amounts of heat. Then release it all day and night. 

Link to valuable Newtonian design software. Newt for the Web

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18.2.13

10" f/8 A planetary Newtonian?

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[2]
The need for such a tall pier, to raise a refractor mounting high above my intended platform, raised real doubts as to its practicality. The cost of a large achromat is closely matched by that of the high quality optical flats needed to fold the instrument. Folding makes real sense when dealing with a large refractor. How else is one to shelter the beast? Let alone provide any shelter from the wind and cold for the observer.

The giraffe may by tamed by folding but gains considerably more weight and girth. Folded refractors can be bulky and awkward in both use and appearance. The folded refractor also falls foul of thermal effects unknown to the pure, long tube refractor. Even baffling it against stray light can be a nightmare with some optical layouts. The difficulties involved suggested an alternative instrument be sought. One which would still offer high quality images of the planets and the moon.

Commerce and amateur astronomy folklore would have you believe that refractors leave all other telescope alternatives for dead on planetary detail. A Newtonian only provides crude and cheap aperture to gather the light for deep sky viewing.  Doesn't it? While the much more refined refractor is vitally necessary for the planets and Moon. Even better (allegedly) was an Apo (at even greater cost for even smaller apertures) for the study of exquisitely fine detail.

After all, "everybody" knows that Newtonians are crippled by the large secondary mirror slap bang in the middle of the light path to the primary mirror. The spider introduced diffraction effects which must surely destroy all chance of a sharp image of the planets. Newts can't even hold collimation for more than a few minutes at a time. All reflectors are total martyrs to thermal currents and cooling down issues. All reflectors suffer from the ill effects of needing those dreadful reflective coatings. With all the "well known" deleterious "glare" and "scattering" this must have a deadly effect on contrast and resolution. The Newtonian is a complete dinosaur. An instrumental throwback. Incapable of evolution or even subtle improvement. Or so "they" say.

I began searching online for possible alternatives to a large refractor. Hardly expecting anything fruitful to pop up. However, before very long I was deeply absorbed in an archived thread on Cloudy Nights forums: Cloudy Nights Shoot-out.. The thread discussed a comparison between some apparently humble Newtonians and the completely unaffordable toys of the comfortably rich, American amateur astronomer. These 'toys' were incredibly costly, large Apos (a 10"!) and very large aperture Dobsonians approaching 30"! All with premium optics by legendary opticians. Yet none of these mouthwatering instruments were able to match the remarkable planetary images provided by these carefully prepared, fully optimised, but far cheaper Newtonians.

These rather special Newtonians didn't look very special. They were of quite modest aperture but enjoyed very high quality, longer focal length mirrors. They used curved secondary spider supports, small secondary mirrors and forced ventilation from behind their plywood mirror cells.

Their tubes were made from lengths of that rather down market material called Sonotube. A disposable, cardboard former used for casting round concrete pillars and piles on building sites. The name Sonotube has become popularised amongst optical and audio hobbyists. Though are there are many other makers of these relatively cheap, spiral wound, paper tubes. Quality, thickness and even the different waxy/plastic coatings (or lack of) vary between manufacturers. None of them shout "quality" in the context of multi-thousand dollar telescopes!

The vital point about cardboard tubes is the lack of thermal capacity and quite reasonable insulation. In fact the cardboard is rather thermally (and acoustically) dead as far as telescope use goes. The draught from small fans, placed down at the bottom, pushes heat out of the mirror blank and on up the chimney formed by the cardboard tube. Clearing away all the usual heat waves clinging to both mirrors, the walls of the tube and the spider.

Moreover, the tubes of these very special Newtonians were deliberately oversized to place the telescope tube walls well outside the light path. The tubes were also made over-long to ensure a lack of stray light entering the field of view. Yet natural sky light, bright moonlight and street lights were cheerfully tolerated to ensure the observers' eyes did not dark adapt. The eye being much more sensitive to fine detail, colour nuances and subtle contrast when behaving normally. As they can only do in reasonable light. Rather than the coarse, grey, monochromatic view seen by the fully dark adapted eye.

The combination of all these simple "tricks" applied to these Newtonians produced planetary images which competed well against the finest instruments available to amateurs, at (almost literally) any price. Often beating those of twice the aperture.

Telescope mirrors are notoriously difficult to match thermally to the air in the tube with falling ambient temperatures. The thick mirror blank has considerable heat capacity and lags behind in heat loss to the air as temperatures fall. Most observing is done in the evening when temperatures are falling most quickly.

Serious amateur astronomers often put their instruments outside to cool down before they start observing. Despite this preparation, many mirrors never achieve thermal equilibrium in active use. Particularly those with full thickness mirror blanks. Not to mention closed off mirror cells when the telescope is brought out from a warm house, or vehicle, to a cool and cooling outdoor environment.

Boundary layers and rising heat currents exist with very low temperature differentials between the mirror and the surrounding air. These often rob a telescope of resolution, detail and contrast. Cooling mirrors often lose the near perfect figure placed there by highly skilled opticians. The telescope owner never enjoys the views which their large financial investment promised.

Could any amateur enjoy similar "giant killer" views with their own "humble" reflector? Well, the basic commercial Newtonian often comes in a thin, rather flexible and highly resonant metal tube. A tube which is often made rather small in diameter to avoid having a very large instrument to house indoors. So the standard tube handicaps the commercial telescope before it has even left the factory!

One can slide a flocked and insulated sleeve inside the original metal tube but it is unlikely to help much. Insulation has little effect where the tube is radiating to the night sky. The metal becomes supercooled relative to the air. Insulation further reduces the clearance between the light path and the cold metal. It is the differential temperature between air, metal and glass which wrecks the view. The metal tube amplifies these differences.

Ideally, you want an inch or more clearance between mirror diameter and the inside wall of the tube. Even more clearance with mirrors over 10" in diameter.

The major problem here is that few amateurs would really want a larger, spiral wound paper tube for cosmetic reasons. It doesn't look remotely posh or "techy" enough to show the large expenditure invested in their instrument. It simply does not impress (anybody) like a high gloss metal tube. Not unless a great deal of time is wasted on making the cheap, cardboard tube look prettier. Usually involving endless filling, laminating with glass fibre mat or cloth, more filling and spray painting.

All completely unnecessary tasks in improving telescope performance. Not unless the changes add extra stiffness to the finished tube. Wrapping the cardboard tube in a door skin veneer will be far more beneficial and will greatly improve the appearance. Thin, birch, aircraft ply would be an alternative but require great skill to hide the joints between the usually small sheets. Though some plywood stockists can offer much larger sheets, far more cheaply, than those usually found (at a very high price) in most model shops.

A cooling fan fixed directly to the mirror cell might well introduce vibration and this is usually exacerbated by the thin metal tube. Many thin metal tubes flex where they are fixed to the mounting. Large mounting rings will undo a lot of flexure associated with mounting small dovetails.

Those all-conquering Newtonians of the CN thread sometimes used a heavy plywood cell in the cardboard tube. Which would be naturally dead to such vibrations despite having holes to allow the fan to push air at the back of the mirror. Sometimes they used a very exposed mirror, in its skeleton cell, cooled by a large fan just sitting on the ground underneath the telescope.

Which raises another important point. The surface on which the telescope rests in normal use. Concrete and tarmac absorb the sun's heat and stores it in vast quantities. Releasing it later, so that it produces heat currents for hours after dusk. The same is true of roofs of course. Many surfaces and roofs are far too hot to touch during the day. They are usually quite massive, with a huge capacity for absorbing and releasing heat. Observing from such pre-warmed sites can be like looking over a garden bonfire. The built up heat rises, causing tremors and waves easily visible in the telescope eyepiece.

A lawn or a field is far more forgiving and can provide a  much more stable and suitable base for the instrument. The heating problem and temperature lag also exists with concrete bases and supporting walls to observatories. Not to mention the heat waves emanating from human beings standing near truss telescope tubes. Or the warmth of the observer being carried across the top of the tube by a breeze. Why handicap yourself and your instrument without a second thought? A timber building hardly absorbs any heat. While metal and roofing felt will soak up heat like a sponge! Foliage will help to shelter walls subject to direct solar heating. Though it can take a long time to establish a dense enough covering to completely exclude the burning heat of the sun.

Then there is the problem of the popular Dobsonian mounting commonly used for Newtonians today. The altazimuth Dobsonian mounting is either standing still. With the object sailing rapidly across the field of view. Or wobbling jerkily to follow an object. Neither allows the observer to relax and stare continuously at the object centred in the sweet spot of the eyepiece. Particularly at very high powers while waiting for those all too rare rare moments of exceptional seeing. So a really steady, driven mounting, or driven equatorial platform, is also essential to the careful study of fine planetary or lunar detail. Can you use powers of 400x-500x on Saturn, Mars or the Moon with your present set-up? It seems that the humble Newtonians in the linked CN thread could.

A longer focal length mirror is useful for reasons which may not even occur to those who choose faster optics. The slower optics have a far greater depth of field. Requiring a much more relaxed focus setting immune to change or endless fiddling with slow motion focusers. Atmospheric thermal effects, which defocus the image, are ignored by the longer focus telescope.

The longer instrument also suffers from very little visible coma and astigmatism compared with a faster instrument. Coma correctors introduce more glass reducing contrast and detail. Slower mirrors are far easier to figure to a very high level of accuracy compared with faster optics. Ask any optician. Slower mirrors favour a huge range of inexpensive eyepieces. Offering lots of eye clearance and high powers from simple, relatively low powered eyepieces. Less glass is more detail and contrast. On axis is where the detail lies and where complex eyepieces offer no advantage provided the telescope is driven to follow the stars.

What is more, a carefully optimised and thoughtfully situated reflector can often dwarf the aperture of most Apochromatic refracting telescopes. Or even many high quality refractors costing many times as much as a "humble" Newtonian. Aperture provides greater resolution and light gathering power. Smaller apertures soon run out of useful magnification.

An 8" or 10" long focus Newtonian, with careful attention to all of the above, can often maximise the seeing under poorer conditions. Atmospheric convection cells have specific average dimensions. Too large an aperture will be hampered in all but the best seeing conditions. So the 8-10" long focus instrument offers even more bangs for your bucks/dead squid. Provided, of course, that you carefully optimise your Newtonian for observing. Rather than lavishing your telescope funds only on its decorative appearance. Or on expensive fixes for the instrument's own, designed-in faults.


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13.2.13

A telescope on a raised platform?

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[1]
The first of a series of essays on telescope design. Not remotely meant as lectures. More of my own "thinking aloud" about a forthcoming project.  I am indebted to Clive for sharing his knowledge and patiently acting as a sounding board for my own ideas.


They say that desperation is the mother of invention. My neighbour's security lights are a constant pain in my usual observing position. The trees surrounding the garden and nearby are all bad news for a clear view to east, south or west. The house is on the southern border of my garden. Further blocking the most interesting part of the sky. And, the damned lights!

So I decided on a raised platform shielded from the lights by the house. Possibly with a simple DIY observatory to keep me more comfortable in the harsh and constantly windy Danish winters. Snow is often lying for months and it is not a pleasant surface for extended standing about.

So I propped the ladder against the shed and went up to admire the view. With my feet only eight feet off the ground and level with the shed eaves, I had a panoramic view! Impressively so. Now I had to make the idea work in practice. No expensive white elephants for me. The Head Gardener would not approve either.

My massive welded steel pier is as solid as a rock standing on the ground. 8-10' up in the air on a wooden platform? Not very likely at all.  So I needed a tall new pier rising from the ground and sticking up another 7' above the platform somehow. Seven feet plus eight feet to the platform floor is over fifteen feet tall! But how to do it? No cast concrete piers on cubic metres of foundation block for me. It would take enormous resources (like a JCB) and completely wreck the drive, parking area and garden! We live on soft clay, with a high water table and have annual permafrost for literally months on end. I'd need a pump to keep the water low enough in the newly dug, bijou swimming pool while I dug furiously for weeks on end!

I needed something on much more of my puny, human scale. Something I could erect working entirely alone. (as usual) Timber is good. Timber is very manageable in suitable units. But you can't just prop a 15'+ wooden  pier up against  a hole in a raised platform. Isolation between the two is absolutely essential. Otherwise every movement by the observer and the slightest  breeze will lead to vibration up through the pier, the mounting and on into the eyepiece.

Tapered concrete post anchors are readily available at a remarkably low price at Jem&Fix (a discount Danish builder's merchant and DIY store) These are roughly 2' high x a foot square at the base. See the image alongside of a stack of the things lying on their sides in their outdoor department.

Once buried, and the hole properly backfilled with lots of ramming, it would take an awful lot to lift these lumps of concrete out again. These things reminded me of tent pegs on elephant-sized doses of steroids. Even skinny tent pegs can take enormous (steady) loads in a decent lawn. Imagine what pulls (or pushes) one of these huge, concrete anchors can resist. Now I had a plan!

I'd use these adjustable concrete anchors to support my platform legs. Not only holding the legs perfectly still but avoiding rot from exposure to the soil. Then I'd make a wooden pier 15' high. This would be anchored to the ground. Then I'd "guy" the diagonal timber struts to other concrete anchors just below and independent of the platform. The holes for the concrete anchors would be easily manageable. I have a long, narrow, post hole spade! While the adjustable ironwork straps on the anchors would solve any inaccuracies in height and position. Crackers? Quite probably! :-)

The stay anchors could even be leaned inwards to face the load. The square base would resist the pressure loads and the adjustable screwed rods and brackets used to apply even more pressure.

The relatively small area required by these concrete anchor blocks is far more friendly to the garden than a single, vast, cast concrete slab and a towering concrete pier. The anchors are far more user-friendly. What about those who end up inheriting the huge slab and pier if we should ever move away? The anchors can quickly disappear if the platform is no longer desired as a picnic spot with a view.

First I thought about using an old telegraph pole for the pier. Or perhaps buy a disused lattice radio mast. Or even buy a massive steel tube in a scrap yard. However, all of these would be well beyond my powers to lift them into place without the help of  a crane. Though there are boat winches, pulleys and timber tripods... Nah! Building the tripod would be as much work as lifting the pier itself.

Then I thought of using four, 4" square, pressure treated, vertical  posts. Each length of timber would be relatively easy to handle by one person. The four posts could be separated with a very strong spacers just below floor level. Right where the diagonal bracing timbers are fixed for maximum resistance. Then the top and bottom of of the pier could be pulled in with a metal strap to achieve a fair degree of triangulation. Festival of Britain 1951 'Skylon' style. (see image to the right) That should take out any resonant modes. Or any pier flexibility above the platform floor level!

If the raw, pier timbers above the platform offend my eye they can be simply clad in smart plywood. For even greater stiffness in a stressed skin, tapered box section. Nothing wrong with a bit of bling. Provided it has a useful purpose. Supporting a reflector would be much easier, of course. Requiring only a stubby extension above the platform. Instead of a 7' tall one!

The platform could also support an observatory of some kind. This would be built in place rather than craned in as a unit. A crane would be just as damaging as an excavating machine. And probably twice as expensive! There is no gang of willing astronomical society helpers here. Nor even the funds to hire a JCB and driver.

A hemispherical dome is a difficult chore of many, differently shaped gores. With (usually) far too much weight concentrated in the reinforcing rib. All just to support the cheapest possible roofing materials. So what about a semi-cylindrical, rotating 'dome?' No need to try and force a 3D curve onto flat materials. No buckled and ugly aluminium sheeting or soggy, heavy and disintegrating hardboard. No need for fancy geodesics either.

Suitably thin plywood can be easily bent to follow the gentle curves of a self-supporting half cylinder. The slit reinforcing arcs could form the central supports. Similar, outboard arcs can support the outer edges before the semicircular sides are added first.

If 4 mm waterproof plywood was chosen for the first curved surface then a second layer could be glued on top of the first. To form an incredibly strong curved laminate of even more layers. Though gluing such a large area might be better achieved on the ground. Or thicker plywood used in the first place. The gentle radius of the curve is hardly a difficult proposition up to say 12mm (½"). Vertical ribs could be added to the half circle "sides"of the turret to help improve stiffness in strong winds. Side cooling vents, or even an access door are a possibility.

 An up and over slide (shutter) can cover the viewing slit. Just like a dome but much more easily arranged. Or a shutter could slide horizontally on stainless steel or aluminium bars. Even allowing some variation in opening width in windy weather. Or to block stray light. The shutter will follow the line of the cylinder's curve without effort or wind-catching projections.

The half cylinder still needs a support ring and rollers to rotate on. Much like a round dome needs them to rotate on itself. This can all be kept safely within the footprint of the semi-cylinder easily enough. The supporting walls can be cylindrical or any variation typical of DIY domes. The semi-cylinder "turret" need not catch the wind much more than a true, hemispherical dome. A coat of white paint and the usual thermal problems are avoided. As well as improving the longevity of the plywood. A semi-cylinder also has plenty of storage space and headroom compared to a hemispherical dome of the same nominal diameter.

Click on any image for an enlargement.
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12.2.13

Another dome.

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I noticed a dome disappear from its roadside garden some years ago. Then in conversation  my wife said she remembered the unusual surname of the owner.  A search online followed up by Google Earth and I was able to pinpoint their new location. A superb dark site on a very minor road going from almost nowhere to literally nowhere in particular. 


As it houses a refractor there is no need for a silly door in a low wall.

Commercial domes usually have to cater for all kinds of instruments including Dobsonians.  
This forces the manufacturers to compromise on wall height.

The door faces south so could offer a limited view for a reflector if needed.











Here are the rails at the back of the dome for the up-and-over shutter.








A closer look at the other side of the dome showing the shutter itself.

Some domes have a drop down panel  at the base of the slit supported on cords. This can catch the wind and always looks very untidy to my eye. Usually the problem is that the panel is hinged outside the shutter rails. If it was simply hinged inside the rails it could drop down to rest flat against the wall. 









Click on any image for an enlargement.
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2.2.13

A 4" F:10 refractor.


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A new contact has kindly sent me some images of his 4" refractor on a MkIII Fullerscopes mounting.

The instrument is not labelled so it is uncertain whether the telescope originated with Fullerscopes. Some of the components are reported to be very reminiscent of Irving's. Beacon Hill telescopes may also have been involved. It is known that Fullerscopes sourced components from other, established manufacturers. Particularly in the early days. They would then undercut the original suppliers leading to their commercial downfall. (according to some sources)


The MKIII has worm and wheel slow motions with extension stalks. Fullerscopes graduated plastic circles are also present. These had the advantage of being much easier to read compared with the typically fine graduations on brass or bronze. Though silvering the circle helps to improve legibility dramatically. Given enough light!

The MkIII pot base sits on a smart, stainless steel tripod with removable legs. Stainless steel is much heavier and probably far stiffer than aluminium of a similar section. So will provide a sturdier, shake-free support for the telescope.

A star diagonal is always useful with a refractor to avoid uncomfortable contortions when pointing to high altitudes.



The finder/photographic guide telescope is a 20x60.

Examination of the image suggests that it was constructed from the objective, cell and housing of a powerful pair of binoculars.

It is not known whether Fullerscopes actually made this. The mounting rings are sturdy and have a reasonable stand-off from the main tube. This would allow the observer to more easily reach the eyepiece.


These adaptors are an interesting insight into the problems of managing afocal film photography at the telescope. This method provides larger images than photography at the focal plane. Useful for photographing the planets, Moon or Sun.

A 3-part tubular brass (bronze?) photo adaptor.
The colour is more reminiscent of bronze than brass.




The threaded section on the right would screw into a Pentax camera lens fitting. 











The adaptor separated to show the central position of the eyepiece.






                                                                             
Here is a sturdy (Fullerscopes) camera bracket. Presumably the brass rod would slide into a drilled fitting on the telescope. The slotted, flat bar would support the camera body and be clamped by the tripod fixing screw. The disadvantage on a lighter instrument and mounting is the imbalance from adding a cantilevered camera and support bracket so far form the centre of gravity. It would need a considerable sliding weight, on the main tube, to balance these forces out.








Here is the afocal photographic adaptor shown in the Fullerscopes catalogue.
(At bottom right)
It mentions a Pentax thread rather than Zenith.











My thanks to John for sharing these interesting images. 
Note that I have adapted the images in PhotoFilter to suit the blog format.
If anyone can confirm the instrument is a Fullerscopes product (or clearly recognisable as by another maker) then please do get in touch.
It has been confirmed as probably made by Barrie Watts of Beacon Hill.



Click any image for an enlargement.



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