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Design and construction of a 20 inch ball scope

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#1 Pierre Lemay

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Posted 05 April 2015 - 11:13 PM

20 in looking high small.jpg

 

20 in looking S small.jpg

 

A very long time ago, like many other amateur astronomers, I developped the desire to own a large amateur telescope. The definition of "large" in the early '70's, when I started out in this hobby was not the same as today. In those days a 12.5 inch was considered large and a 16 inch, enormous.  It was natural to mount our instruments on equatorial mountings, with all their advantages and invonveniences.  Sometime around that period John Dobson and his San Fransisco Sidewalk Astronomer friends forever changed our perception of "large". Suddenly, we were thinking in terms of 20 and 24 inch F/6 behemoths. A decade or so after this, Al Nagler started introducing revolutionary eyepiece designs that would allow shorter and shorter focal ratios. It is during this exiting period that my dream telescope took shape in my mind.  The final result can be seen in the above pictures. Although I followed the Dobsonian revolution as it unraveled, I had gotten used to equatorially mounted telescopes and wanted my instrument to track. I had also enjoyed using the 16 inch Ottawa (Canada) center club telescope with it's rotating Upper Tube Assembly (UTA) with dual eyepiece turret and also longed for this feature.

 

The end result was a large, tracking ball scope. Little did I suspect I would spend more than 20 years building it. That to attain my goals I would have to litteraly invent a new tracking platform, as well as a few other innovations that had rarely, and in some cases never been seen before. Of course I did not actually work on the project for 20 years! I had a lovely young family to look after, a demanding carreer, and very little time. I would sometimes work for a few months on my project and then do nothing, sometimes for several years. But the dream lived on and I persevered.

 

In the coming weeks I will describe the design and fabrication techniques used to manufacture this unique telescope. I will demonstrate how I made the large 30 inch diameter hemisphere that constitutes the heart of the mount, the f/3.9 conical mirror and the fabrication techniques I used to grind, polish and parabolize it, the Optical Tube Assembly (OTA) including the unusual primary collimation system, the rotating turret eyepiece holder, and, of course, the large, dual-axis, multi-lattitude equatorial tracking platform specially designed for ball scopes.

 

20 inch project goals

My 20 inch telescope making project evolved from a desire to own a large instrument that would also be portable and easy to assemble, disassemble and use. I had built and owned telescopes from a 60mm toy store department refractor and small Newtonians from 6 inches to a 12.5 inch f/5.2 Newtonian on driven equatorial mounts.
The main design goals for the new telescope were:

  • Aperture: 16 to 24 inches
  • Equatorial tracking
  • Rotatable eyepiece orientation for viewing comfort
  • High optical quality maintained through all tube orientations
  • No ladder required for observing
  • Assembly/disassembly by one person in less than 5 minutes
  • Transportable in a small hatchback automobile
  • Weight: operational tube assembly 75 pounds; mounting: 25 pounds

After studying hundreds of telescope designs in the 16-24 inch sizes I came to the conclusion that the ball scope was the best candidate, with a horseshoe mounted telescope as a plan B and a Dobson on a Poncet as Plan C.

 

Short history of Ball Scopes

 

Although he is rarely mentioned, credit in popularizing the modern ball scope we use today goes to Thomas P. O’Brien who published an inspiring article describing his 14 inch in issue #29 of Telescope Making magazine. This is the telescope that inspired the late Peter Smitka who later launched Mag 1 Instruments, makers of the Portaball line of telescopes. Others had experimented with ball mounted telescopes in the past, starting with Sir Isaac Newton himself when he invented the optical system that bears his name. Other, early inspirations for me were: Berton Willard (S&T March 1967) with his bowling ball 4 inch, Normand James (April 1972 Riverside and Stellafane August 1972 TM conventions – see S&T Oct 1972) with his 12.5 inch driven hemisphere floating in water and Alphonse Pouplier (S&T August 1993) with his computer controlled Astroscan showed us ways of mounting mirrors in the sphere and, in some instances, making the scopes track. Edmund, with their Astroscan and Peter Smitka with the Portaball telescopes had demonstrated commercial products that were easy to use. Smitka’s telescopes, made to apertures of up to 18 inches, were convincing evidence that this underused minimalist design held promise as an optimal instrument for visual observing.

 

In the next installement, I will describe how I made the conical primary mirror.



#2 allardster

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Posted 06 April 2015 - 08:18 AM

Thank you Pierre for documenting this telescope. This is going to make for many nights of interesting reading.



#3 starman345

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Posted 06 April 2015 - 08:27 AM

Thanks for doing this Pierre, very interesting.



#4 woodscavenger

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Posted 06 April 2015 - 09:08 AM

Hah,  I am the first to follow this thread.   I am looking forward to your details.

 

I cant wait to see how you dealt with the balance of weight to see how you got enough in the rear to counterbalance not only one EP but 2 big EPs with your rotating assembly.

 

I am interested in your rotating EP configuration and how that affects viewing comfort.

 

And where did you get or how did you make the massive partial sphere.  I am thinking big balloon and paper-mache.... :grin:



#5 peleuba

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Posted 06 April 2015 - 04:08 PM

 

Excellent - Dave Jukem who now owns Mag1 Instruments has just completed an 18" that looks very similar to the telescope in the picture with the unequal length truss/strut poles and off-center ball.  Dave was the machinist that Peter contracted with to make all of the parts for the early Portaballs, including the sphere, focuser, upper cage truss poles etc.  Dave bought the business from Peter back in 2007 - it was a natural fit - and still manufactures and sells the Portaball line.

 

Peter was a friend of mine and sadly passed away from Leukemia back in October of 2013.


Edited by peleuba, 06 April 2015 - 04:14 PM.


#6 Dick Jacobson

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Posted 06 April 2015 - 07:02 PM

Beautiful work, Pierre! I am curious to see how you built the hemisphere, what materials you used to achieve proper friction between the rollers and the ball, and how you dealt with balance issues.



#7 Pierre Lemay

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Posted 06 April 2015 - 08:22 PM

Making the 20 inch mirror

 

The project began after I located a suitable slab of glass. In 1990 I saw a classified ad in Sky and Telescope by Dick Nelson, owner of the Optical Crafstman telescope making company.  He was going out of business and getting rid of two 20 inch, conical pyrex blanks. I got in touch with him and obtained one of them for $500.  The blank was a rough, cast piece of glass that would require a lot of work even before grinding could begin.  The blank was almost 2½ inches thick over the central 7 inch diameter portion and then tapered out to about 1 inch at the edge.  It weighed approximately 50 pounds. The front surface was far from flat and had “ripples” of glass with variations in thickness of up to 0.1 inch. The diameter varied from 20¼ to 20¾ inches. This picture (with permission from the now grown up lady!) shows the eldest of my daughters sitting on the just received blank:

 

rough 20 inch blank.jpg

 

I had read with interest Donald Dilworth’s relay lens article in the Nov-Dec 1977 issues of Sky and Telescope in which Dilworth had made his 16 inch blank lighter by tapering the edge of a full thickness blank to create a conical blank and supporting it through a central perforation.  I thought this was a clever way of both reducing the weight of the glass and simplifying the mirror mounting.  In his two part article Dilworth explained how he had used grinding wheels to grind away the excess glass and produce his tapered mirror.  I would do the same by grinding away the back of my already tapered glass blank to further reduce the edge thickness from 1” to ½” and I would trepan a 3½” hole for central support. Reducing the weight and thinning the blank as much as possible was important for two reasons: weight reduction would prevent the hemisphere's skin, in which the mirror would be supported, from buckling or bending (creating vibration). The mirror had to be heavy enough to balance the UTA but light enough not to put higher than necessary stress on the hemisphere. The 30 inch diameter of the hemisphere had been calculated to balance everything out (more on this later). Remember: one of the design goals was to have a 75 pound OTA, a formidable objective. A lightweight mirror blank was central to attaining this goal. After all the work described below, the 20 inch conical mirror blank ended up weighing 32 pounds.

 

The other reason for thining the glass was to allow it to cool as quickly as possible without resorting to fans blowing from the back. Indeed, the hemisperical ball's skin could not be perforated to allow vent holes if it was to be used with the tracking platform I had just invented (I had long discussions with Peter Smitka on this at the time this decision was being contemplated). Almost 70 % of the light gathering surface of the pyrex mirror is less than an inch thick and, the glass is suspended, with no mirror cell touching it, favoring natural convection with the surrounding air.

 

Preparing the blank for rough grinding

To do these operations I built a grinding machine which would be used throughout the project.  My grinding machine, shown in these pictures, was based on Dilworth's grinding machine seen in the Nov-Dec 1977 issue of S&T:

 

Grinding machine 1 small.jpg

 

Grinding machine 2 small.jpg

 

 

The grinding machine has three spindles. The right spindle is the one on which the mirror is mounted. It spins at approximately 2 RPM. The central spindle controls the back and forth movement of the grinding/polishing tool. It's stroke can be adjusted by sliding the crankset pin radially. The left spindle can also be used to induce an offset movement but I rarely used it. The pin stayed centered most of the time. In addition, the long push arm holds the pin that pushes the tool or mirror blank and it can be adjusted lengthwise to adjust for tool offset. A 2 HP motor provides more than enough power.

 

Before grinding the back of the blank to lighten it up, I paid a tombstone maker a visit and asked his help to remove glass to roughly “work in” the F/3.8 curve I was going to need for the mirror (ended up at f/3.9).  Using sandblasting techniques and a 24 inch long template with the correct radius, he was able to “generate” the rough radius of curvature after only a few minutes of work.  Using a diamond grinding wheel I also cut away the excess glass on the edge to regulate the diameter to a circular 20¼ inch dimension. If I had to do this again, I would use the curve generation device recently described by Chriske on CN: see here.

 

Glass from the back end of the blank was then removed by mounting a motor with a spinning grinding wheel attached to the arbor, just like in the Dilworth article. The motor was mounted on an angled linear guide. The blank would slowly rotate (2 RPM) while I would move the motor from edge to center at an angle, increasing the taper and thinning the edge down to ½ inch. 

 

A word of caution: use proper eye, ear and respiratory protection if you ever attempt this! In normal grinding, a wet slurry of carborandum is slowly used to grind out the curve and there are no health risks. However, in a mechanized operation like this, carborandum dust and glass quickly become airborne and can get into our branchi and lungs. Exposure to these aerosols could cause emphysema and/or lung cancer.

 

Trepaning

The blank was then mounted on a threaded pipe and a 3.5 inch steel trepanning tool, attached to a rotating motor arbor (≈150 RPM), was then used to “drill” a hole through the center by feeding #60 carborandum and water.

 

20 in trepanning small.jpg

 

As the trepanning tool spun, the blank, mounted on a non rotating table attached to a long, threaded 2 inch pipe, was slowly moved upwards towards the tool. It took several hours to pierce through the center of the 2½ inch thick glass blank. This picture shows the blank when it still had a 1 inch thick edge.

 

Final Curve Generation before Rough Grinding

Rough grinding the f/3.9 curve was first done with a spinning a 4 inch, followed by an 8 inch cast iron tool (a cast iron skillet with the handle cut off and slots milled in the sides). The tool was set at an angle in relation to the slowly rotating mirror blank surface, below and offset. This picture shows the 4 inch tool working the glass:

 

Curve generation 2 small.jpg

 

This is a technique used by professional opticians, normally in conjunction with large diamond wheel tools, to quickly generate the desired curve (“Blanchard curve generation”). I did not have a diamond wheel but, nevertheless, with this method I was quickly able to grind away the rough curve that had been initiated by the tombstone maker.

 

Next subject: conventional rough and fine grinding and polishing.



#8 Don H

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Posted 07 April 2015 - 08:08 AM

What an incredible scope, Pierre! 20" of portable, feet on the ground scope, with a tracking mount included. Seeing this makes me want to make my 12.5" f/5.9 thin mirror into a ball scope even more. I know you replied it could be possible with a 22" hemisphere, but I have questions about material options and if I can avoid fiberglassing work by me. One site I found, is called something like eztops, and they offer ABS, polycarbonate and Acrylic, with 22" available in 1/4" thickness, but maybe not in ABS. Can you offer an opinion on which material is best, or maybe point to more affordable sources? Congratulations again on your scope and thank you for sharing it with us.

 

Regards,

Don



#9 Pierre Lemay

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Posted 07 April 2015 - 06:52 PM

I know you replied it could be possible with a 22" hemisphere, but I have questions about material options and if I can avoid fiberglassing work by me. One site I found, is called something like eztops, and they offer ABS, polycarbonate and Acrylic, with 22" available in 1/4" thickness, but maybe not in ABS. Can you offer an opinion on which material is best, or maybe point to more affordable sources?

 

Don,

I looked for years and years everywhere for an already made hemisphere. Light globes frequently returned as an option but they were ofter to thin or made of a material that would either become very brittle in winter or very soft in summer. I wanted something stable in terms of strength for the 20 inch and ended up making it (it was particularly difficult to find 30 inch diameter ones). I also found some metal spun spheres on the Chinese website Alibaba but received either no response when I contacted the companies, mentioning I only needed one, or received an absurd high price for the hemisphere + shipping.

 

As mentioned in this thread:

http://www.cloudynig...e-build-thread/,

 

the following IKEA lamp shade (Brasa): should work for a 12.5 inch mirror:

http://www.ikea.com/...ducts/50140828/  

 

I looked at it in the store a couple of years ago and if I had a 12.5 inch to mount in a ball scope, I would try this first. The lamp shade is made of steel, it's inexpensive and looks well made and robust. Worst case, I would end up with a nice lamp.

 

Many techniques have been used by ATMs to make these hemispheres. Some folks start from an acrylic light globe and reinforce the inside with fiberglass. Others use one of these large exercise balls used in gyms as a form and build around this. I've even used a 10 inch earth globe as a form which I covered with fiberglass for my 6 inch ball scope.

 

However, I've developed a more systematic method. If you bare with me for a few days, I will explain this technique when I get to it. I started this thread by explaining how the heart of the telescope, the primary mirror was made. I know, it's not very exiting to read about this part of the construction but a ball scope is a holistic instrument. Every part plays a role in weight balance, especially the primary. After I've finished with the mirror making part, I will explain the adjustable mirror cell and then I will go into how to integrate that mirror support in the hemisphere and describe how the hemisphere was made. As a teaser, I will just say this: you can make any large (20"+) sized, very accurate hemisphere with this technique in 2 or 3 weekends, with very simple hand tools.



#10 Oberon

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Posted 08 April 2015 - 12:52 AM

No no no, this is the interesting stuff. Explain it all. Very interested down here... :waytogo:



#11 Pierre Lemay

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Posted 08 April 2015 - 11:41 PM

Conventional Grinding

 

 

After generating the f/3.9 curve using the methods described above, and thinning the edge of the blank to ½ inch, the 32 pound, 20 inch pyrex mirror blank was ready for conventional rough grinding with a full sized tool. Here is what the glass looked like just before conventional grinding:

 

blank before grinding.jpg

 

Two, 2-inch thick and 20 inch diameter tools were then cast out of a Portland cement mix containing no stones. I used the rough f/3.9 curve of the blank surface as a mold. One of the tools would be used to support ceramic tiles for grinding; the other cast as the pitch lap.  During casting of the cement tools, a 3.5 inch diameter plastic drainage tube was inserted at the center of the tools to create a hole, corresponding to the hole in the center of the glass blank.  This hole would not be covered with any ceramic tiles or pitch, to match the corresponding absence of glass in the glass blank. The holes would also provide a location to fix the cement tools during grinding/polishing.

 

 

Unglazed ceramic tiles, purchased from Willmann Bell, were glued to one of the convex cement tools with 5 minute epoxy. Although this worked to a certain extent, there must be better glues (I’ve since learned that adhesion is stronger when using slow hardening epoxy rather than 5 minute epoxy). Some of the tiles, especially near the edge, became undone during grinding.  I lost about 10 tiles (out of about 250). But I could sense several tiles were not well glued. I feel I was lucky to make it all the way to the end of fine grinding without losing more. Losing too many tiles towards the end of fine grinding would have forced me to completely renew the tiles, forcing me to go back to rough grinding.

 

Grinding and polishing were all done with mirror on top for two reasons:

  • I did not want the glass blank to be stressed under the weight of the heavy cement tools, possibly generating astigmatism during grinding and polishing operations.
  • I feared cement dust and chips falling from the Portland cement tool and scratching the glass.
     

Here is an overall view of the grinding operation at work:

 

grinding overall view small.jpg

 

 

And a close-up view of the blank being automatically worked by the grinding machine:

 

grinding close up small.jpg

 

 

For most of the rough grinding I placed a foam skirt around the periphery of the grinding tool to slow down water/grit runoff. I also covered the plywood turntable with plastic that I would change with each change in grit. It was great fun to see the machine working all by itself, having only to clean and renew the grit every once in a while. However, I found cleaning the tool a good weight lifting exercise. I did these operations during the summer months so I would take the tools out of the basement, go outside and water them down thouroughly with a hose. I can confirm that a 2 inch thick, 20 inch diameter cement tool is VERY heavy.

 

The overhead arm is made of two wooden beams separated by a slot in which I attach a bolt and fix a 3-inch diameter hockey puck (this is Canada, after all!).  The puck, made of hard rubber, was the ideal pushing rod during grinding and polishing. It fitted perfectly in the 3.5-inch orifice in the center of the blank and the rubber would not damage the inside of the cored glass blank.

Rough grinding with the full size tool went on for about 20 hours first with #80 carborandum, followed by #120 carborandum. A center over center stroke was used throughout. This was followed with fine grinding for about 30 hours, going through a number of abrasives from 180 carborandum to finer aluminum oxide abrasives. Nothing out of the ordinary here. In the end the glass was perfectly fine ground with no apparent scratches anywhere. The tool had survived the whole operation with only a few lost tiles.  It was time to polish.

 

Polishing

 

Polishing was also done with the mirror blank on top. A total of 30 hours were spent machine polishing with a center over center stroke using cerium oxide as the polishing abrasive. I used Gugolz 55 pitch, working in the air conditioned basement at about 20⁰C. The facets had to be re-opened several times using a soldering iron with a through attached. The facets became very thin towards the end of polishing.  Because of the central perforation in the mirror and the absence of pitch in the tool center, the area around the central perforation polished last. More than 10 hours of the 30 hours of polishing were spent getting this part of the glass polished. In retrospect I think using Gugolz 55 pitch was a mistake. It was too soft and may have been the cause of a severe, ¼ inch turned down edge which I was never able to get rid of. Have some of you expert glass pushers experienced this or something else caused this TDE?

 

At one point during polishing one of the bushings holding the gear reduction worm wheel of the turntable failed.  I had to take the gear reduction unit apart, machine new bushings and re-assemble. I think it is at this point that I made a stupid, beginners mistake: in my haste to resume polishing I did not properly wash my hands after re-assembly of the gear box and I suspect I had some small metal shavings on the skin of my fingers. I grabbed the almost-polished blank, set it up on the machine and resumed polishing.

 

After a few minutes I noticed the glass was scratched in small areas, scratches that were not there before. I had obviously contaminated the polishing lap. Scratches are not the defect with the most devastating effect on image quality and contrast but they look bad, especially after aluminizing. They show poor workmanship. I was angry at myself for my own carelessness. Removing the scratches would have meant going back to fine grinding but, on the other hand, polishing was almost finished. And besides I knew my grinding tool had to be redone entirely, which meant hours of grinding just to get the new tools to conform.  I decided to finish polishing and hope the instrument would still perform OK.

 

 

Parabolizing

 

Following advice from Ottawa Optician Peter Caravolo who had done a few conic glass blanks, I glued the back of the mirror blank with very soft pitch to an 8 inch plate welded to a 2 inch diameter pipe flange. The flat on the back of the mirror blank being 7 inch diameter, this was a good fit to glue the blank to the flange. Before casting the pitch on the flange, I attached three small bolts at 120⁰ to the flange so that the small flat back end of the blank, where it is thickest, would rest. Without this, the glass would have gradually sunk in the pitch at an ever changing angle. I wanted the front surface to stay perpendicular to the axis of rotation of the turntables shaft, and concentric. The pipe flange would screw onto the spindle of the grinding machine (the spindle being a threaded steel pipe). A plastic tube was first fitted in the center of the flange to allow proper centering of the mirror and to provide latteral support in case the pitch flowed. I unfortunately don't have a picture of this setup but here is a sketch showing the principle:

 

mirror support parabolizing.jpg

 

The glass blank stayed attached to the pipe flange for almost 18 months while I parabolized the mirror (yeah, I know, I'm slow). I was a little fearfull of what would happen when I would separte them. After all this time, the pitch had hardened and who knows how much stress might get relieved when I broke the bond. But Peter had provided good advice. There was no sign of stress being relieved after separation. Pitch is very forgiving.

 

Whenever I needed to test, I would grab the blank by it's edge and simply unscrew it from the spindle. I would lift the 40 pound blank and attached flange and vertically attach it to the tester stand where a bolt would attach the flange to the test stand. When the mirror was in the test position, only the pitch was holding it thru that small bond in the back. I was always worried the pitch would break and the mirror come crashing onto the floor. I never left the mirror for more than one hour on the stand, knowing the pitch would eventually flow. The plastic tube in the center was a small security against a shearing break in the bond but nevertheless I remained worried. In the end the mirror always stayed perfectly glued and I never noticed any problems. This is how you hold a perforated conical mirror during parabolization.

 

Parabolizing was done with the mirror blank on the bottom, face up, and using sub-diameter tools. I had not made a mirror in close to 30 years (the last one being a 12.5 inch F/5.2) and had never worked with sub-diameter tools.  I worked the rough parabola in with a 8 inch glass tool, Gugolz 55 pitch and cerium oxide. Here is a picture of the 8 inch tool at work:

 

parabolizing with 8 inch tool small.jpg

 

The machine would push the tool from side to side but it would also create a kind of "W" through the center thanks to the adjustable crankshaft on the left spindle which is just out of this picture. Although I roughed the parabola in this way, I quickly found that my machine was very limited in terms of controlling parabolizing strokes. So off came the overarm and I did most of the parabolizing strokes by hand, with four or five different diameter tools, from 2 to 8 inches in diameter. Parabolizing around the central perforation was always tricky and I ended up with a turned down edge around the hole, as can be seen in this birds eye view of an early version of the surface with a ronchi screen, as seen through a Ross null compensation lens (the outer edge TDE is also visible):

 

roos ronchi showing TDEs.jpg

 

I wish to take a moment to thank Gordon Waite who came to my rescue at one point when I was stuck, unable to smooth out some zones without creating others. He provided some very helpfull advice. Thank you Gordon!

 

One lesson learned about sub-diameter tools: I should of made a 12 inch tool for smooting out the localized strokes with sub-diameter tools, which would tend to generate roughness (dog biscuit) on the surface. I liked the 8 inch tool because I could cold press it between the edge and the perforation. A larger tool would have required removing pitch from the center but I was lazy. I'm generally unhappy with the surface I created. It is rough at places (primary ripple), it has a TDE on the outer edge which I've had to mask (so far with ¼ inch mask but I will experiment this summer with larger masking) and it has some scratches. It performs OK, but it's not great. I know I can do better and it will have to go back on the bench sometime in the next few years. I have a few mirrors to make in the mean time (including a 12.5 inch f/3) so I will hone my skills before starting over on the 20 inch.

 

In my youth I had always used a Foucault test but remembered from the 12.5 inch how difficult reading the shadows of a fast mirror was like. The 20 inch was even faster. So I machined my tester so it could perform the Caustic test, which I would use as a backup. Other tests that were tried were the Ronchi and the Ross null test. The Ross lens compensates longitudinal aberrations seen through the Foucault knife edge and Ronchi screens. As discussed previously in another thread here on CN, the caustic made me loose a couple of months because there was something wrong with my tester (see this thread for a discussion on those difficulties). I will get back to discussing my caustic tester in a few months but for those who remember the discussion, I had concluded that my 3 inch diameter starret micrometer was no longer accurate. Turns out it's fine. I traced the problem back to the hard rubber feet on which the tester rests. They are too soft! Working to 0.0001" requires a lot of accuracy! But more on this some other time.

 

Here is a picture of me testing with a Ronchi grating. The large 6 inch Ross lens is set aside for now (I had not yet used it at this point, as you can see by the markings left by Peter Ceravolo when he had measured it) but it would be used later for testing.

 

Testing with ronchi.jpg

 

After I decided I had enough working the mirror, I drove over to Normand Fullum's house and left it with him to have it aluminized.

 

In the next subject, the mirror support, I will describe how I approached the problem of collimation without having access to the back of the primary mirror.

 



#12 jtsenghas

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Posted 09 April 2015 - 05:16 AM

Thank you, Pierre. I appreciate your thorough detailed account. Several months ago when you gave us a glimpse of this masterpiece in another thread I mentioned that this scope is "begging for a thread of its own". Thank you for your generous use of your time going to this level of detail.


Edited by jtsenghas, 09 April 2015 - 05:17 AM.


#13 Ravenous

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Posted 09 April 2015 - 07:35 AM

Fascinating thread, but the grinding/polishing post could be a thread of its own, full of the real-life hassles and experiences.



#14 Pierre Lemay

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Posted 09 April 2015 - 07:40 PM

Fascinating thread, but the grinding/polishing post could be a thread of its own, full of the real-life hassles and experiences.

You're right. I've only scratched the surface .  :lol:


Edited by Pierre Lemay, 09 April 2015 - 07:41 PM.


#15 Oberon

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Posted 09 April 2015 - 07:50 PM

Oh dear...



#16 jtsenghas

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Posted 09 April 2015 - 09:57 PM

 

Fascinating thread, but the grinding/polishing post could be a thread of its own, full of the real-life hassles and experiences.

You're right. I've only scratched the surface .  :lol:

 

Ouch....

After a groaner like that may I humbly ask you to leave such wisecracks to others?

Please continue, and please do so without glossing over any important steps.


Edited by jtsenghas, 09 April 2015 - 10:00 PM.


#17 Ravenous

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Posted 10 April 2015 - 03:57 AM

Yes these terrible puns reflect badly on you...



#18 Pierre Lemay

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Posted 11 April 2015 - 01:05 AM

Primary mirror cell

 

The primary mirror cell is a mix of simple and clever.  Due to the tracking equatorial platform design that the telescope is mounted on, the hemisphere surface acts not only as the tube bottom but is also an integral part of the pointing and tracking surface. It is preferable that the mounting surface not be compromised with large orifices that would both weaken the skin of the hemisphere and momentarily throw tracking off when a hole would be going over one of the rollers on the platform, below.  Mounting the mirror, including provision for collimation, had to take this restriction into account.

 

In a dobsonian, the mirror cell must prevent sag of the mirror surface when the instrument is looking near the zenith (worst case) and accomplishes this by distributing the weight of the glass on floatation pads in such a way that the image is not seriously degraded. This subject is frequently discussed on this forum and the mirror cell parameters can be calculated using a very practical Finite Element Analysis (FEA) shortcut developped by Canadian ATM David Lewis, called PLOP. The other mirror support necessary in a Dobsonian is the edge support which prevents the glass from sliding off the floatation pads located under the mirror. Depending on the glass thickness, diameter and focal ratio, the edge support of the glass can be anywhere from insensitive to extremely sensitive to image deterioration when the tube leaves the confortable position of verticality and adventures itself to a more inclined position. The sensitivity of the edge support can also be estimated using another FEA shortcut developped by Belgian ATM Robert Houdart and available on his Cruxis web site.

 

I'm telling you all this to emphasize the fact that a ball scope has many more degrees of freedom of movement than a Dobsoninan. A mirror that works in a PLOP and Cruxis developped Dobsonian will not necessarily work in a ball scope, or even an equatorial mounting for that matter. As a matter of fact, if the mirror is thin/large/fast enough, it may even have difficulty handling a Poncet type equatorial platform without introducing strong astimatism.  So when designing for a ball scope one needs to take the limits of the glass into account and use a sufficiently thick piece of glass whose entire weight can be supported on a single edge support without seriously deforming.  It's a paradigm shift for ATMs used to dobsonians.

 

To go bigger in diameter without a serious increase in weight or cool down time, one must use alternate glass shapes. Some cellular engineered blanks will be able to handle larger diameters in equatorial mounts or ball scopes (like Dream Cellular or Hercules) but they are very expensive.  The solution I chose (actually I think it chose me !) was the conical mirror blank described above. The mirror cell for this type of glass shape is extremely simple. There is no need for floatation cells and edge supports. Held only by the thick central part of the glass, the mirror deforms in an acceptable maner (more on this later). For those interested, conical blanks are available from Newport glass in California and conical mirrors from R.F Royce in Conneticut. Some professional mirror makers like Mike Lockwood also have experience working this type of glass.

 

Here is a sketch showing how the 20 inch diamtere conical mirror fits inside the 30 inch diameter hemisphere:

 

Mirror cell small.jpg

 

EXPLANATION OF THE MECHANISM: The 7-inch diameter flat central portion on the back of the mirror rests on a ½ inch thick, 8-inch diameter aluminum plate, attached with four hex bolts to a 3.5 inch diameter solid aluminum core.  In turn, the 8-inch plate rests on three, ½ inch bolt heads that thread into a ¾ inch thick and 10-inch diameter aluminum back plate, embedded in the epoxy that makes up the plastic hemisphere.  The ½ inch bolts mentioned above constitute the hub of 3 inch diameter spur gears. These three gears mesh with three brass splines, which are connected to long, thin shafts that stick out above the solid aluminum core, through three long holes.  The bottom of the brass splines were machined round and run in bronze bushings fixed in the thick aluminum plate. The top end of the shafts are knurled to provide a good grip while collimating and have central slots (slots had not been cut when these pictures were taken).  These slots serve to attach 3 foot long extension rods that allow me to collimate through a high power eyepiece when the telescope is pointing straight up. The shafts run through holes in the 3.5 inch central core much larger than the shaft diameters (to allow for inclination of the mirror support, without interference from the straight shafts). The shafts are loose but the bottom of the hole in the core is a little smaller than the spline diameter so the assembly remains captive. Each of the three spline/spur gear arrangements is installed at 120 degree angles from each other. A 5 inch long ¾ inch diameter stainless steel hex cap bolt screws into the center of the ¾ inch thick aluminum plate. When collimation is completed, this bolt tightens the whole mirror support assembly together, preventing any movement. Of course, when this pressure is applied the mirror remains unstressed, resting loosely on this support.

 

Here are a few self-explanatory pictures showing the mirror cell before it was imbeded in the hemisphere. Note that the central ¾ inch bolt that tightens everything down was a long threaded rod attached to a large knob when these pictures were taken. They were since replaced with a shorter stainless steel machine bolt.

 

Plate with gears small 2.jpg

Mirror on support small.jpg

Mirror on support close up small.jpg

Gears seen from top small.jpg

Mirror support small.jpg

 

 

One inconvenience with a perforated mirror is that collimation cannot be done with the usual Cheshire or Laser collimation tools since the center of the mirror is non-existent.  Instead, a simple peep hole is centered in the eyepiece holder and the adjustment screws turned until the 4 inch diagonal mirror image seen in the mirror is centered with the 4 inch diameter mirror retaining ring. This alone will get collimation in the ball park but, collimation must be followed up with examination of a bright star near the zenith for further fine tuning of the primary mirror alignment. I made three, 36 inch long extension rods that attach to the small, protruding collimation knobs. This makes accurate collimation using a star much easier and more accurate, because I can collimate the primary while simultaneously examining the out of focus star image. This collimation is made all the much easier because the telescope is driven.

 

Before I get into the technique I used to make the 30 inch hemisphere, I hope during the coming weekend to find some time to write about conical mirror bending on this type of mirror cell and attempt to calculate the impact on the optical quality of the mirror. I'm on shaky grounds here but I will give it a try.



#19 Pierre Lemay

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Posted 11 April 2015 - 10:03 AM

Additional discussion on mirror support

 

Here are a few thoughts that I forgot to mention on the mirror support. First a few pictures showing the collimation knobs today, with aluminized mirror in place.

 

This first picture shows the three knurled and slotted adjustment knobs sticking out over the central post, surrounding the ¾ inch diameter machine bolt that thightens everything together. You will notice there is a lock washer between the ¾ inch bolt and the 3½ inch diameter solid post so as to prevent mirror support from unscrewing under vibration. And yes, I know !, my mirror needs a good cleaning. I haven't cleaned it since it was first aluminized almost two years ago. One last detail: that yellow Canada sticker with a serial number, visible on top, was placed there by Canadian customs officers to register the telescope and certify it originaly came from within Canada when I first brought the 20 inch to the United States. 

 

Collimation knobs today small.jpg

 

In the second picture, two of the three extension rods are in place for remote collimation. The extension rods are simple, ¼ inch diameter threaded rods. On the mirror end I've slipped a teflon tube over the threaded rod, accross which a spring pin was pushed in. This pin inserts into the slot of the short adjustment knobs sticking out over the center support. I used teflon instead of aluminum as a safety measure: Should I unknowingly pull one of the rods out and it fell on the mirror, there would be less dammage and scratches with teflon (could have been nylon or some other plastic but I happened to have teflon tubes in my scrap box). On the upper end I currently have three large plastic knobs but they are unnecessarily big. I will soon replace them with ½ inch diameter knurled aluminum knobs, making storage easier and keeping the extension rods more within the shadow of the diagonal mirror during collimation:

 

collimation rods close up small.jpg

 

The last picture shows an overall view of the 3 foot long extension rods extending about halfway up the tube. I can collimate flat footed on the ground when the telescope is 10 degrees away from zenith. However I prefer to stand on a small stool and look for a star closer to zenith so that there is minimum disturbance of the collimated setting when I thigten down the central bolt. By the way the eyepiece height above ground is a little under 72 inches and my eye is at about 67 inches.

 

Collimation extension rods small.jpg

 

If I were re-designing this telescope today, I would probably use the Stewart Platform truss assembly, wonderfully described in the last few months by Oberon, to do all of the collimation. In fact my truss tube is already incorporating half a Stewart Platform, something I will describe in more detail later, and could be easily converted. However my adjutable mirror cell works really well and I don't see any reason to bypass it.

 

For now I would recommend to someone, wishing to make a ball scope with a conventional or a conical shaped mirror, to simply attach the mirror to a floatation cell (standard mirror) or a static central post (conical mirror) at the bottom of the hemisphere and mount the trusses the way Jonathan described for his Merope scope.



#20 ckh

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Posted 11 April 2015 - 11:00 AM

This is my first post. I'm following your thread with interest in every detail. :waytogo: Your engineering has inspired me to complete my first telescope with a ball mount and drive. (I polished a 6" f8 mirror 45 years ago. Now retired, I'm planning to figure it.)

 

Is the aluminum cylinder (that provides horizontal support) fit to the central bore in the mirror with a very small gap? Did you consider using a thin O-ring to define an area of contract near the lateral center of gravity?

 

The hexapod-truss collimation method (that Oberon has described) could provide an alternative that would simplify the mirror support for a mini version of your ball scope. For an f8, balance will require substantial extra weight. Perhaps an oversized ball would help.

 

Edit: Oops now I see that in your latest post you have chosen a teflon ring. Great idea.


Edited by ckh, 11 April 2015 - 11:03 AM.


#21 Jim Chung

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Posted 11 April 2015 - 12:30 PM

Just found this thread today, what a fantastic build!!!!!  Looking forward to future details.  Go Habs!!!



#22 Pierre Lemay

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Posted 11 April 2015 - 02:28 PM

Conical Mirror Deformation

 

How much does a conical mirror deform under it's own weight when it's supported only at its center and what effect does it have on optical quality? This is not an easy question to answer. With standard thickness mirrors one can use PLOP and Cruxis mirror edge calculator because someone has already done the mathematical approximations and assumptions. Not quite the case for a non standard shape like this.

 

To get an idea of the microscopic shape changes one must use Finite Element Analysis (FEA) techniques, where the mechanical properties of a material and its shape are dissected into very small individual micro-components which are then analyzed by a computer while applying pressures to the component being studied. I haven't done FEA since engineering school so my skills in this were pretty much non existent. However I have a work colleague who does this on a regular basis with components much more complex than my simple piece of glass. I provided him with an AutoCAD drawing and the physical properties of Pyrex and he did a mesh reduction that looked like this:

 

20 in FEA mesh.jpg

 

He then submitted the conical blank to several positions (in a virtual fashion) and looked at how much the glass was bending under its own weight.  As expected, the worst case position was a horizontal blank (looking at zenith).  Here is what he found:

 

20 in FEA 001.jpg

 

The last column below the half-blank image shows the peak to valley deformations from center (zone 1) to edge (zone 9).  This indicates a maximum of 178 nanometers of deformation, or about 1/3 wavelength at a wavelength of 550 nm. This is a lot. However, that does not tell the whole story.  Back in 2007 the deformation of conical mirrors was discussed in this thread: http://www.cloudynig...nded/sb/7/o/all and this one as well:   http://www.cloudynig...ed/sb/7/o/all. 

 

In the first thread, Robert Houdart (Cruxis) did a calculation on Mike Jone's 24 inch conical blank and found:

 

Before refocus: surface error RMS 107 nm (1/5 wave), P-V 380 nm (2/3 wave)

With refocus: surface error RMS 10 nm (1/55 wave), P-V 40 nm (1/14 wave)

 

Mike's blank has a similar shape proportion to my 20 inch so I am confident my blank is not seriously affected by mechanical deformation of its surface when supported only by the center.  However I would really like to know if someone in CN knows a way of analyzing the numbers in this FEA study to evaluate the performance after refocus. The software MirrorMesh 3D is mentionned as a tool. I will try and reach Robert Houdart through his website. I haven't seen his presence on CN for some time.

 

Next in line: how to make a hemisphere with simple tools.



#23 ckh

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Posted 11 April 2015 - 08:44 PM

Pierre,

 

At what radius are the "deformation"s listed above assumed to exist? At the average radius of the zone? Inner edge? Outer edge? Maybe we can figure out the deviation from a "best focus" parabola. :question:



#24 Pierre Lemay

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Posted 11 April 2015 - 11:39 PM

Pierre,

 

At what radius are the "deformation"s listed above assumed to exist? At the average radius of the zone? Inner edge? Outer edge? Maybe we can figure out the deviation from a "best focus" parabola. :question:

The changes in shape happen gradually from center to edge but are "indicated" by the software at the outer edge of each colored zone.  We fixed the outer edge of the central hole and measured deformation relative to that point. The change in shape is considered the same across each coloured zone even though, in reality, it would change within a zone. A similar approximation is done when testing with a zonal mask.

 

Strangely the change in shape is very regular for each zone, about 20 nm per zone. The hard part is calculating the impact of each zone change on the overall image. Each 20 nm drop means the glass is bending outwards a tiny bit, pushing the focus for that zone furter out by a very small amount. The error is cumulative and grows as you approach the edge. The focusing errors should normally be squared, added and the square root of the sum of all zones extracted, the definition of RMS.  It should be less tha 1/50 wave RMS. 

 

The part I'm having trouble with is determining the slope change and it's impact on focus for each zone. I tried using Autocad but the change in slope, relative to a sphere, is just to small for the software.

 

Does that make any sense?



#25 Pierre Lemay

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Posted 11 April 2015 - 11:55 PM

This is my first post. I'm following your thread with interest in every detail. :waytogo: Your engineering has inspired me to complete my first telescope with a ball mount and drive. (I polished a 6" f8 mirror 45 years ago. Now retired, I'm planning to figure it.)

I don't want to sound negative but a ball mount, with primary mirror nested inside the hemisphere is not the best application for a long focus telescope like a 6 inch f/ 8. The hemisphere would either have to be very large in diameter or loaded with lots of bottom counterweights. It's kind of like choosing an equatorial fork mount for a long focus refractor, not the most efficient application for this type of instrument.  Ball scopes are best suited to f/5 or shorter focal ratios. 




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