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An easy to build 8 inch tracking ball scope

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#1 Pierre Lemay

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Posted 08 August 2016 - 11:28 AM

Back in the mid-90's I was looking for a simple, novel way of driving a ball scope. The purpose was to eventually incorporate this drive to a 20 inch ball scope I had started making, which I would eventually finish, twenty years later. You can see construction details of the 20 inch on my website (see my signature) and in this CN thread.

 

I presented the first world prototype ever made of a ball scope driven by this novel roller drive, a 6 inch f/3.7 Newtonian, at the 1995 Stellafane convention. Not only could it track the stars, just like a Poncet platform, but it could also have the potential of doing astrophotography since it would also incorporate a dual axis. Unlike the Poncet and other tracking platforms, it is ajustable for any latitude from about 10 to 60 deg (northern or southern hemisphere), and it doesn't need to be reset every hour or so, like all those other platforms have to. In addition to introducing it at the 1995 Stellafane meeting, the 6 inch was also briefly described in a January 1996 issue of Sky and Telescope magazine article entitled "The ten top telescope ideas of the year".

 

Here is what the fibreglass ball mounted 6 inch telescope looks like:

 

IMGP0267.jpg

 

The ball scope tracking mount didn't catch on and, as far as I know, no other ATMs built one after I described it. About 10 years later it was "rediscovered" by, now, S&T Telescope Making Telescope Workshop editor Jerry Oltion who described his ball scopes in a few articles between 2005 and today.

 

Many comments I got after describing my 20 inch ball scope, including after publication of the article in this year's March issue of S&T, told me that this is a very nice concept but so complex to build, particularly the hemisphere, that it scared people away.

 

So, in the fall of last year I decided to pool all the knowledge I had acquired over the past two decades making tracking ball scopes and design an instrument that would be so simple to make, practically anyone could do it with very simple hand tools, in only one or two weekends and for a cost of about $300, excluding optics.

 

The resulting telescope is an 8 inch f/5 newtonian mounted in a 12 inch diameter aluminium hemisphere.

 

8 inch ballscope.JPG

 

The telescope OTA weighs exactly 17 pounds (including eyepiece and finder). And, of course, being a ball scope, the OTA does not need anything else to operate. A shallow hole in the ground makes a great base! However, in my version, it rests upon a plywood base which holds the tracking platform, which weighs about 5 pounds. The OTA comes apart in less than a minute and will eventually store in a one cubic foot box that will cover the entire telescope for storage. The soon-to-be-built box will also double as an adjustable tripod base that levels the tracking platform and brings the eyepiece to a confortable height. More on this later as I finish making it.

 

The equatorial platform incorporates a single, 5v geared stepper motor which beautifully drives the scope in a very consistent way, half-stepping every 0.88 seconds (both the 6 and 20 inch use two synchronous motors). I have not included the dual axis function in this telescope because I doubt it would be used for astrophotography, but that feature can easily be added later if wanted.

 

In the coming weeks I will describe how you can make this telescope using commercial off-the-shelf (COTS) parts and where to get those parts. My version is entirely machined but all the machined parts can be made with alternate materials, including wood. The OTA can be made first and used on the ground, sitting the hemisphere on nothing more than a small plastic cover you can find in your kitchen shelves. However, I highly recommend you build the drive platform which can be made for about $30 and which will provide continuous tracking for your telescope, from almost anywhere you live or travel in the world. 

 

And, of course, this being a ball scope, the eyepiece position is always in the most confortable position and, unlike a dobsonian, there is no gimble lock ("Dobson's hole") when observing near the zenith. I'm looking forward to sharing with you some of the new innovations I had to come up with to make this scope work, especially the very clever, and simple, primary mirror collimation system which is at the heart of the success of this project.

 

So please stay tuned as I write up the details and provide photos and sketches. As I did with the 20 inch, I will then incorporate the material I write here to my website so that it can easily be found at a later date, once this thread as migrated deep down in the archives of CN and becomes harder to locate.


Edited by Pierre Lemay, 08 August 2016 - 12:29 PM.

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#2 tomykay12

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Posted 08 August 2016 - 03:43 PM

Thankyou...



#3 Pierre Lemay

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Posted 08 August 2016 - 11:26 PM

The hemisphere

 

I knew from the start of this project that the choice of the hemisphere would play a determining role in how well the end product would turn out. When building the 20 inch I had looked at many sources for these hemispheres and, in the end, had to make my own 30 inch hemisphere, using the technique I developed and describe in my website. I didn't want to make a hemisphere for this project, knowing it would be much easier for others to reproduce the instrument if they could purchase it already made. This was a guiding principle during the project: even though I had a well equipped home machine shop, all the parts I designed had to be easily constructed by folks having access only to small electrical hand tools. Major components had to be available as commercial products or easily made.

 

Metal hemispheres are made using the spin casting technique. A flat metal sheet is squeezed against a slowly rotating hemispherical wooden form having the desired diameter. As pressure is applied, it progressively generates the hemisphere as a tool squeezes the metal sheet all around the hemispherical wooden mandrel.

 

Here is a photograph of the raw aluminium hemisphere on a scale:

 

Aluminium hemisphere on scale.jpg

 

The aluminium hemisphere has a 1/8 inch edge thickness and weighs 1.12 kg (2.5 pounds). There may be other sources where you can purchase these but I bought mine from a metal forming company called Sharpe Products (www.sharpeproducts.com) , located in New Berlin Wisconsin in the USA. The model I purchased is the H1200-AL which you can see here: http://www.sharpepro...productId=10926 . At 106$ each they are pretty pricey but they are the cheapeast I found. If someone knows of a less expensive source, please let us know.

 

When I was at Stellafane Saturday, a few attendees suggested that salad bowls from IKEA might work as well. I had seen these bowls in their catalog but had never manipulated one. Being the dedicated ATM that I am, I stopped by one of the local IKEA stores on my way home from Stellafane on Sunday morning and picked two of them up. They are called Blanda bowls and are made of stainless steel. There are several sizes but the ones I purchased have an 11 and 14 inch diameter and they sell for $8 and $17 each + tax (Canadian dollars), about 1/10th of the Sharpe product.

 

Here is a picture I took this morning with the two IKEA bowls and a Sharpe hemisphere, side by side:

 

Various hemispheres.jpg

 

My first reaction is that the wall thickness of these Blanda bowls is so thin that they would not work with the way I've attached the single strut that supports the diagonal mirror/focuser unit. Indeed, a lot of pressure is concentrated where the strut attaches to the hemisphere (more on this support in a later post). There has to be a minimum of material there, or a very strong material (or both), to resist bending of the skin. If not, the strut will move and vibrate.

 

That is not to say the Blanda bowls could not work. They might work if the UTA is supported by a six pole truss that attaches to the very important reinforcement ring, which itself attaches to the lip of the hemisphere. I will discuss this attachement ring in a later post. How well the Blanda bowls would work also depends on the weight of the OTA. Indeed the thin wall of the bowls might be quite springy when the hemisphere rests on the three contact points that constitute the drive roller equatorial platform. 

 

The 14 inch Blanda bowl could probably accommodate a 10 inch mirror, but I suspect the weight of the instrument would be too much for the skin thickness of the bowl. As for the 11 inch Blanda bowl, it could accommodate a 6 inch mirror only. Also, please note there is a small flat on all the Blanda bowls. This flat would either have to be further shaped into a hemisphere of the same radius as the rest of the bowl or, not used on a three point support. It could, however, be used successfully on a ring support. That ring could be connected to a Poncet platform for tracking if desired.

 

In the end I'm very happy with the choice of the more expensive Sharpe hemisphere for the 8 inch telescope. It perfectly meets my design goals and is very robust. But I would encourage others to try the IKEA bowls in a scope project.

 

In my next post I will describe the single strut attachement bracket and the plywood reinforcement ring.

 

 

 

 


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#4 Rusted

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Posted 10 August 2016 - 11:17 AM

I spent ages searching for spheres and hemispheres. [for DIY speakers]

I found professional, spherical, plastic fishing floats were available in quite a range of sizes at quite modest cost.

They have a [rope hole] tube right through the middle but can easily be cut in half with a hand saw.

They smell of styrene when cut and only seem to come in bright orange or perhaps white.

The wall thickness is fine for supporting any reasonable weight.

The material will also take a thread from a hand tap.

 

Spherical polycarbonate light globes are also available in different sizes.

These might be too thin-wall for a telescope and wouldn't be a patch on your superb hemisphere.

 

Could the Ikea SS bowls be nested in pairs for extra strength?

Perhaps with a thin interstitial material or adhesive to bond them?


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#5 Pierre Lemay

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Posted 10 August 2016 - 10:44 PM

Could the Ikea SS bowls be nested in pairs for extra strength?

Perhaps with a thin interstitial material or adhesive to bond them?

I hadn't thought of that. Thank you for the suggestion. A great idea, well worth a try. Next time I'm at IKEA I will try nesting one 14" bowl in another to see if that might work. If it has a chance of working, I will purchase a second bowl, find a proper glue and do a test.



#6 Mad Matt

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Posted 11 August 2016 - 12:18 AM

 

Could the Ikea SS bowls be nested in pairs for extra strength?

Perhaps with a thin interstitial material or adhesive to bond them?

I hadn't thought of that. Thank you for the suggestion. A great idea, well worth a try. Next time I'm at IKEA I will try nesting one 14" bowl in another to see if that might work. If it has a chance of working, I will purchase a second bowl, find a proper glue and do a test.

 

 

You might want to sandwich a fiberglas or Kevlar matt between the bowls with resin. That should make it substantially stiffer than the bowls alone.



#7 stormbird

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Posted 11 August 2016 - 05:01 AM

I have a number of these stainless steel bowls I have purchased in local thrift shops for a dollar or two each. I believe you could add, via bolts through the flat bottom of a bowl, your own material to complete the hemisphere. For example, add a dome of "purple heart" or some other beautiful hardwood to the bottom of a polished stainless steel bowl. The polished wood and metal would provide a striking two tone effect while allowing some adjustment in the friction coefficient based upon material choice. Also, no reason why a internal dome of wood could not serve as a adjustable mounting point for the mirror in smaller telescopes. My 2¢., great topic you have here.

Edited by stormbird, 11 August 2016 - 05:13 AM.


#8 Oberon

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Posted 11 August 2016 - 05:18 AM

 

Could the Ikea SS bowls be nested in pairs for extra strength?

Perhaps with a thin interstitial material or adhesive to bond them?

I hadn't thought of that. Thank you for the suggestion. A great idea, well worth a try. Next time I'm at IKEA I will try nesting one 14" bowl in another to see if that might work. If it has a chance of working, I will purchase a second bowl, find a proper glue and do a test.

 

 

Expanding polyurethane foam.


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#9 ckh

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Posted 11 August 2016 - 09:19 AM

Two thin hemispheres the same size will give you a crescent-shaped cross section when spaced apart. Stiffness will diminish near the edge, unfortunately.

 

gallery_240847_5047_12937.png



#10 Geo31

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Posted 11 August 2016 - 10:27 AM

Two thin hemispheres the same size will give you a crescent-shaped cross section when spaced apart. Stiffness will diminish near the edge, unfortunately.

 

gallery_240847_5047_12937.png

 

Indeed.  But the real question is:  Will it diminish enough to matter?

 

If a seatbelt degrades 50% over 5 years (due to UV and other environmental factors), is it no good after 5 years?  Yes, the strength is greatly reduced, but the real question is:  Does it still have enough strength to do the required job?

 

I use the point to illustrate that reduction can be noted, but never lose sight of the real question (and I admittedly don't know the answer here).

 

[edit] Also, don't forget that there was discussion of creating a sandwich composite here.  It may not matter that there is a different inside radius.  Once could create that composite with say 3-4mm of sandwich material in between (at the thinnest point) and simply cut off the protruding top of the inner bowl.  I would think the hard part is finding a material to adhere to the stainless steel well enough (but hardly an impossible task).


Edited by Geo31, 11 August 2016 - 10:32 AM.


#11 jdownie

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Posted 11 August 2016 - 10:44 AM

If 11" is enough internal diameter, why not mount the 11" inside the 14", using a foam core?  This would be extremely rigid.

 

Expanding foam would probably expand too much.  A regular or low expansion product would be better.



#12 ckh

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Posted 11 August 2016 - 11:13 AM

Yes that would be much better. It depends partly on how rigid and crush resistant the foam is.  The ball-drive supports put a lot of pressure on the outer sphere.

 

Another potential filler is lots of equal length slices of thin aluminum or hard plastic tubing.  Say 1" diameter x 1.5" long for this case.  Pack the slices like beehive cells and epoxy them to both spheres. Super stiff.

 

Some sort of packing like this one with 75 tubes in a hemisphere.  I think it's good enough just to let them slide together as you add them.

 

gallery_240847_5047_1609.png

 

I wonder if anyone makes thin steel or aluminum hemispheres that are draw-formed. Could be cheaper and stronger in this configuration than thicker spun hemispheres.

 

Carl


Edited by ckh, 11 August 2016 - 11:26 AM.


#13 ckh

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Posted 11 August 2016 - 11:36 AM

Here you can get a 9" and 10" aluminum hemisphere for $28 total.  Need bigger ones however for an 8" mirror.



#14 jtsenghas

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Posted 11 August 2016 - 12:29 PM

Regarding the use of doubled up bowls and adhesives between them; it appears to me that the stresses will be high only at the attach area of the post. I imagine a polyurethane foam for most of that variable gap as Jonathan suggested would do fine, but a low expansion version must be used. That's a very large surface area and the foam expansion could make it hard to keep the bowls nested as desired. I'm thinking that if the area that is bolted is filled with something that will cure more solidly- like an epoxy putty- issues of the walls compressing would be a non- issue. 

 

Do continue, please Pierre!

 

As for an approximately cubic box to house everything, it looks to me as if the square base of the platform could form the bottom of the case in the stored configuration,  and it could be placed atop that "cover" for observing. 


Edited by jtsenghas, 11 August 2016 - 12:34 PM.


#15 Geo31

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Posted 11 August 2016 - 09:25 PM

Regarding the expanding foam, it can create an incredibly rigid structure in a sandwich.  When building race cars, if the rules allow, builders will fill the voids in the unibody with structural foam and it creates a significantly stiffer chassis.  Choice of foam is certainly all-important.  About 18 years ago the electrical pole just the other side of my back fence spontaneously broke at the base.  The electrical company secured it back in place using ONLY foam.  No lie.  Stayed that way for about 6 months until they got around to replacing it.  Modern material science is astounding.

 

As for keeping the edges of the bowl in registry with one another, I'm sure a jig make of a wood frame could easily suffice.  Remember, as the foam expands it's going to take the path of least resistance - the open edge between the inner and outer shells.  Once hardened it could be cut and sanded and the air pockets even filled.  Should work like a charm.

 

I think the bigger issue (again) is using something that will adhere to the stainless steel sufficiently.  I'm sure there is a way to prepare both surfaces, but I could only speculate at this stage.



#16 Pierre Lemay

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Posted 12 August 2016 - 07:26 AM

Folks,

great information being exchanged here and lots of room for future experimentation, which I intend to undertake in a few weeks. In my next post, however, I will put the choice of the hemisphere aside and move on to a description of how the primary mirror is attached to the hemisphere and collimated. Unfortunately this will have to wait until next week as I am off hiking for an extended weekendin the next few days. 


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#17 ckh

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Posted 12 August 2016 - 02:24 PM

Regarding the expanding foam, it can create an incredibly rigid structure in a sandwich.  When building race cars, if the rules allow, builders will fill the voids in the unibody with structural foam and it creates a significantly stiffer chassis.  Choice of foam is certainly all-important.  About 18 years ago the electrical pole just the other side of my back fence spontaneously broke at the base.  The electrical company secured it back in place using ONLY foam.  No lie.  Stayed that way for about 6 months until they got around to replacing it.  Modern material science is astounding.

 

As for keeping the edges of the bowl in registry with one another, I'm sure a jig make of a wood frame could easily suffice.  Remember, as the foam expands it's going to take the path of least resistance - the open edge between the inner and outer shells.  Once hardened it could be cut and sanded and the air pockets even filled.  Should work like a charm.

 

I think the bigger issue (again) is using something that will adhere to the stainless steel sufficiently.  I'm sure there is a way to prepare both surfaces, but I could only speculate at this stage.

 

This certainly is the easy way to fill. There is a structural foam that is used in BMWs to reinforce some frame parts that proved too weak. Apparently it's an epoxy foam that sets very hard, but has some flexibility. Sounds perfect, but a little pricey. Don't know what the specs are.

 

Sheet metal spinning can be done on a lathe. I suppose the hard part is making the form which is usual made of wood (hardwood?). But doing it yourself would allow you to make very thin, light hemispheres from sheet aluminum (e.g. 1/32" that weights 1/2 lb/sq-ft) to your size requirements, in this case say 12" and 13" for a 1/2" fill.  The fill space is about 0.07 cu-ft. Could make an awesome hemisphere.

 

Enjoy your hike Pierre.  Meanwhile I'm looking forward to the rest of the construction.

 

Carl



#18 Pierre Lemay

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Posted 16 August 2016 - 06:59 PM

Primary Mirror Collimation

The primary mirror collimation system that would be used for this ball scope had to take had to take several limitations into consideration, including the following two:

  • Due to the drive roller equatorial tracking platform, much more advantageous than the Poncet type platform, I could not perforate the back of the hemisphere to reach adjustment screws;
  • As explained previously, the size of the hemisphere was limited to 12 inches in diameter, the largest commercialy available hemisphere I could find with fairly thick walls. This did not leave much room between the mirror and the inside of the hemisphere to add a primary mirror collimation system. Also, because of the relatively small hemisphere diameter, I wanted the full thickness mirror lodged deep in the hemisphere to use the mirror’s weight as much as possible to contribute to the balance of the instrument.

I considered all kinds of collimation systems, including mounting the UTA on three pairs of adjustable length struts, like Jonathan Pogson (Oberon) has been promoting through his Merope telescope (Stewart Platform). But, for all kinds of reasons, compromises (mostly weight related) made these solutions impractical.

 

I will always remember the Eureka! moment in my garage, while milling ever more complex prototype components, trying to fit them between the hemisphere and the mirror blank, when it finally dawned on me that the ball scope hemisphere not only offers the ideal pointing and tracking surface for a Newtonian telescope, but also the ideal and most simple, natural mirror collimation option as well.

 

The following sketch shows the fundamental principle I ended up adopting to collimate the primary mirror for this telescope. It’s so simple I suspect I just re-discovered something that has already been done by others before me yet, as far as I can tell, it is the first time a primary mirror has been mounted and collimated this way:

 

8 inch primary mirror collimation.jpg

 

As you can see, I ended up using the smooth inside surface of the hemisphere to collimate the primary mirror. Sometimes the simplest solutions to a complex problem are the best solutions!

 

The mirror sits on a ¾ inch thick plywood disc which is about 9 inches in diameter on one side, about 1 inch larger in diameter than the mirror. The other side is tapered to an angle approximately corresponding to the hemisphere radius. I turned this on my metal lathe but it could be turned on a wooden lathe or it might also be done with a jigsaw and a lot of sanding.

 

This disc is the recovered central cutaway section from the 12 inch OD diameter hemisphere reinforcement ring (more on this component later). Because the mirror I use in this telescope is a full thickness (1:6) pyrex blank, how the mirror is supported on this plywood disc matters little. No PLOP calculations required here to optimize back mirror support! If the mirror blank were thinner, one would integrate a 6 or 9 point floatation back support, whose supports would be calculated using PLOP. However, a word of warning: the mirror must not be too thin because, unlike a dobsonian, the edge support cannot be a conventional sling, or even whiffle tree, since the OTA is continuously spinning around the optical axis.

 

So the result of this setup is that a ½ inch plywood ring protrudes around the periphery of the primary mirror. The purpose of this ring was to provide a flat surface on which to push this simple mirror “cell”, to produce collimation (i.e. slight changes in angle of the mirror surface). The system is a “push-push” type of collimation. The pushing pressure is applied through three threaded rods that thread into inserts, imbedded in the plywood ring that serves to reinforce the soft lip of the hemisphere.

 

Inside hemisphere.JPG

 

Two of the threaded rods are fairly short and could probably be made from long, threaded machine bolts. The third rod is fairly long since it must reach all the way from the top of the ring to the plywood disc below. Also, the third threaded insert must be drilled at an angle of about 35 degrees so that its tip falls near the edge of the wooden disc on which rests the mirror.

 

The tip of the threaded rods were turned down to a small nipple, about 1/8 inch in diameter and 3/16 inch long. These unthreaded tips are inserted in small holes drilled in the edge of a large diameter steel washer.

 

Adustement screw close up.JPG

 

The center of the washer is screwed into the plywood disc where the disc is a little thicker. The plywood surface would not be robust enough if the steel tips of the adjustment rods bit in directly into the thin wood, thus the use of the thin steel washers. The washers are also about the same thickness (actually a little thinner) than the three 2 inch long angle brackets on which the mirror sits. Each washer can be turned slightly, one way or another, to better center the tip of the three threaded rods onto the mirror cell.

 

The picture above also shows that the angle brackets each have two ¼ inch nylon bolts threaded in them. The lower nylon bolt allows the primary mirror to be centered in its support. The higher bolts simply act as mirror clips to prevent the mirror from falling out of the mirror cell. I will eventually make fancier mirror clips that block less light, but this was a pretty easy solution to start the project with, so I went with that.

 

The way collimation is done is by unscrewing the threaded rod a turn or two in the direction we wish to move the mirror and tightening one or the other (or both) of the two other rods, while looking in a Cheshire or using a barlowed laser collimator. When everything is well collimated, the three collimation screws are then progressively tightened down until the plywood disc no longer moves, keeping an eye in the Chesire (or barlow laser collimator) to make sure collimation is maintained. I haven't used this telescope enough so far but I suspect collimation of the primary mirror will not need to be done very frequently after the initial collimation. The mirror is basically attached to the hemisphere once those push-push rods are tightened down. During tube assembly it is a simple matter to align the optical axis of the focuser with the center of the primary mirror (this procedure will be explained later). So once a laser dot is centered inside the small ring in the center of the mirror, the primary mirror should not need additional collimation (famous last words!).

 

At first, when I tried this system, I had a lot of difficulty getting the mirror to move during collimation. The friction between the tapered wooden disc and the interior surface of the hemisphere was too great. I tried greasing the disc but it did not help. Finally, I added three thin strips of Teflon spaced about 120 deg apart, near the threaded knob adjustment points, and it worked much more smoothly. To make collimation movement even smoother, I may eventually reposition the two adjustment knobs on each side of the UTA strut so that they push more in the direction of the movement, instead of leaving them perpendicular to the reinforcement rings, as they are now. This would make assembly of the threaded insert that guides the threaded rod a little more complicated (must be drilled at a compound angle) but collimation would be smoother and more direct.

 

 


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#19 Pierre Lemay

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Posted 16 August 2016 - 07:04 PM

View of collimation knobs.JPG

 

The size and position of the collimation adjustment knobs has also given me some headaches. Particularly the knob at the end of the longer threaded rod. This knob sticks out beyond the reinforcement ring at the top of the hemisphere. When using the scope, the protruding knob tends to interfere with the equatorial platform during scope movement in some tube positions. Because it’s a ball scope the tube can always be rotated around its optical axis to counteract this interference but this does not always produce the most comfortable eyepiece position.

 

In my telescope making “religion” I try to follow a few “commandments” that make telescope manipulation and observing much more pleasant. Two of those are:

 

  • “Thou shall not use tools when assembling/collimating a telescope”
  • “Thou shall only use captive components to assemble a telescope”

At the moment, the mirror collimation setup on the 8 inch follows those two rules. However, to prevent the inteference with the platform the tube rests upon, I’m thinking of removing the large, knurled aluminium knobs and replacing them with a removable tool, leaving only a small threaded rod sticking out above the reinforcement ring, on which to attach the tool. I still have the option of replacing the knurled knob with a wing nut, which would swing out of the way during the final tightening down. But, if this does not work well, I may have to revert to a removable tool.

 

In the next post I will discuss the wooden hemisphere’s reinforcement ring and its multiple functions and particularities.


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#20 bartine

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Posted 16 August 2016 - 07:25 PM

Inspiring!  I have a great 8" mirror that is in a not so usable home made scope.  I think it would be a great candidate for your design Pierre.

 

I believe I know what I'm going to do with it now!



#21 jtsenghas

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Posted 16 August 2016 - 07:45 PM

Very nice! I admire your desire to keep your adjustments tool-less,and think I have an idea or two to offer to avoid knobs that may interfere with your scope motion. I'd like to check out some hardware before describing it in more detail.

I like your collimation method and have no doubt the compressive loads against the wooden ring also provide additional stiffness to the hemisphere via the ring.

Do continue, please!

#22 mark cowan

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Posted 16 August 2016 - 08:29 PM

"Most admirable the method of making the mirror pointing correct."  (unattributed but believed H. Poirot c. 1905)  



#23 Geo31

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Posted 16 August 2016 - 09:15 PM

VERY clever!

 

[edit]  To solve the "stiction" problem with the mirror cell, you could consider UHMW plastic.  A 10"x10" sheet, 1" thick is $36US.  Certainly not as cheap as wood, but it solves some issues at the same time it can be machined like wood.

 

http://www.usplastic...23911&catid=868

 

Also, for folks who don't have a lathe, but have a router table, I can conceive of a jig that could be used to cut the curve on a router table. 


Edited by Geo31, 16 August 2016 - 09:20 PM.


#24 Pierre Lemay

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Posted 16 August 2016 - 10:15 PM

VERY clever!

 

[edit]  To solve the "stiction" problem with the mirror cell, you could consider UHMW plastic.  A 10"x10" sheet, 1" thick is $36US.  Certainly not as cheap as wood, but it solves some issues at the same time it can be machined like wood.

 

http://www.usplastic...23911&catid=868

 

Also, for folks who don't have a lathe, but have a router table, I can conceive of a jig that could be used to cut the curve on a router table. 

Thanks for the suggestion Geo31. I didn't know about that plastic supplier and yes, your proposal would be a very elegant way of mounting the mirror.

 

I haven't discussed this yet in the project description but, one problem that still needs to be solved in this design is primary mirror cooling. What I would really like is to replace the plywood board on which the mirror sits, with perforated steel or copper. I will go over this subject in a few posts from now, when I discuss tube balance for this particular telescope, but to balance the scope I had to add about 3 pounds of counterweight below the mirror. The only place I had available was the small cavity below the plywood collimation sled. But, that was the location I had planned for a 4 inch fan to be used for cooling!

 

So instead of UHMW, I would really like something heavy, and perforated which would eliminate the counterweights (I like it when an accessory in a telescope has two functions) and recover that space for the fan. Whatever proposal I come up with must be simple and reproducible by an average skilled and tooled amateur, so that this design remains accessible to almost anyone who wants to make one. I'm still working on replacing the plywood board with something heavy that can allow mirror ventilation and tube balance. I am fully opened to any suggestions you brillant ATMs out there may have.



#25 jtsenghas

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Posted 16 August 2016 - 10:42 PM

Here's a thought about cooling that piggy- backs on previous discussions about doubled thinner gauge hemispheres assembled with a gap:

Why not have spacers between the two hemispheres that give it radial stiffness, but allow for air to pass within that cavity? The inner hemisphere could then have multiple holes beneath the mirror for allowing convection. The mirror "board" could be a corresponding heavy open structure (I'm thinking of some kind of dense grate of some sort) that air can pass through. Finally, instead of fitting a single larger fan under the mirror, multiple tiny fans (totaling a similar CFM rate) could be placed on convenient spots around the mirror to blow air through holes in the inner hemisphere into that cavity. Those smaller fans would also be more easily able to be damped for vibration with thick foam gaskets.

Yes, this is a bit more complicated, but it shouldn't need fancy expertise, close tolerances, or special tools. Done right, it could really cool those thick mirrors.

I realize that such an arrangement raises the mirror significantly, but the COG of this assembly could be lowered by using massive spacers between the bowls at very low locations, right?

EDIT- Although boundary layer scrubbing fans on scopes are generally more effective blowing rather than sucking, fans drawing air from the regions adjacent to the mirror would almost certainly also help to get the air just above the mirror to be of more uniform density. Hmmm...

Edited by jtsenghas, 16 August 2016 - 10:49 PM.



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