16.5" Cellular Mirror Project

It is funny how fate, karma or plain good luck comes into things I attempt. My wife says that this has to do with putting these good vibrations out into the universe, and it is repaid with good fortune. I’d rather be lucky than good.


I have in my covetous possession a large, cast cellular blank. The cells are truncated cones some 2″ in diameter tapering to 1 5/8″ at the bottom, arranged in a hexagonal array, there are six kidney-bean shaped holes at the edge which go into the glass which form part of the structure. All in all, there are no parts of the glass thicker than 3/4″ thick. The white marks on the back are the remains of the parting compound to keep the glass from sticking to the mold. The company that made it, EmbeddedRF, made 10 or less to test out their procedures before moving on to larger ones. They were sold some company in 2004. I found this one on Ebay for $350.

Rough Mix:

It has some small defects, 2 cracks in the rear next to some of the small pockets at the edge, and around a dozen hemispherical bubbles that didn’t quite level out on the surface. The bubbles are more serious, as the larger ones are around 3/16″ across, and may not fully grind out. There is around 3/4″ of glass at the surface, and this may be thick enough to get those bubbles out. Even if it isn’t, a few small defects like this would not affect the image in any serious way.

Additional, the insides of the various cells in the rear of the blank had a number glass “bubbles” adhered to them. I looked at them in dismay, knowing I couldn’t grind the insides smooth with them in there. I thought about it for a bit, and looking at a carbide-tipped scribe (Lee Valley Tools, thank you) I used this with a wood block to chip some 50 or 60 or these out. All were removed without serious incident. The bottoms were mostly flat or had a slight concave depression in them. I made a wood tool to match the conical holes and covered it with a mix of silicon carbide and epoxy. It cleaned the bottoms well enough but the sides were still somewhat rough. I may have to have a diamond tool made for this if I want to improve the situation.

I have also been lapping the back flat. I used a mix of 46 and 70 grit from a lapidary shop to start with. It left a pretty rough finish, so I changed to 80 grit. This worked better, but left most of the holes with small chips around their edges, somewhat spoiling the smooth look I had envisioned for it. I used a 220 grit water stone from my woodworking tools to level it further. I used so much of this stone, it is now less than 3/8″ thick, after starting at 1 “. I infilled the bottoms of the holes with sawdust and topped them with plaster of paris, giving some support to the edges to get a smooth finish. This is a technique I thought of some while ago, but wondered if it was smart. As it happens, the people who ground the 200” Hale telescope mirror did exactly the same thing when then prepared their blank for work, as detailed in the book, “The Perfect Machine”. I have built a tool to grind out the holes also, and you can see the results of that work. My guess is it will take about another 10 hours of work to finish this part.

The blank seems to grind slowly compared to other Pyrex I have worked, and its appearance is white without the usual yellow cast of Pyrex or faint green of plate glass. Internally, it lacks the characteristic ‘swirls’ that come from the molding process used on typical cast Pyrex blanks. There is a faint web-like pattern similar to what occurred when GE attempted to make fused silica blanks in the 1930’s. I did save some small pieces that clung to the inside of some of the rear depressions in case I want to test the glass.

I also want to edge the blank properly, and would like to borrow a potters wheel, which is normally used in wet conditions. I have a diamond lapping plate of 1000 grit, which I will use as a final step. I might need to buy a large 200 grit diamond stone, otherwise it will take a very long time.

My plans were to make either a 16.5″ f/4.5 Newtonian or a 16.5″  f/8.3 Ritchey/Crietien. The latter is a more complex instrument to be sure, and with the blank being cellular, perforating the blank may be impossible or next to it, so I have made tenitive plans for it to have a Coude focus. Since I have a 12 1/2″ full-thickness blank, I’ll use the smaller for a Dobsonian.

I have played around with the optics on a program called ATMOS. The conclusion is that to get an f/8.33 telescope, you need to make an f/2.75 or so primary (very fast, very deep) and a 5″ secondary, leaving you with a central obstruction of around 32%. My big Maksutov has a CO that big and I find its contrast pretty good. With the Coude focus I will need to find a way to

Keeping the weight of the scope down will be problematic, as I will have to work hard in all areas to do this:

The main tube will need to be a truss type, with aluminum front and rear plates connected with carbon fiber tubing. The rear cell will be an open frame CNC’ed out of a plate of aluminum with an attached plate at right angles to hold the focus as there will be a tertiary mirror to direct the light path from the secondary mirror outside the truss.

Update: June 25, 2009

I am taking interest in this project again. I now have a mat on tiles cut for a 16″ tile tool, and more grit, polish and pitch for use. Astromart is great for finding these things.

I have now plastered over the holes but I partly filled them with sawdust! instead, seeing as how my woodworking area is filled with it. In the interests of not making extra work for myself, I plan on grinding the back of the mirror as I change grits.


I have been adding other pieces of pyrex to a growing mound of future work. On my honeymoon to Seattle in August 2009, I got 5 10″, 4 8″ and 2 6″ pyrex blanks as well as some grit. This brings my current tally to some 20+ blanks.

I have 2 4″ pyrex blanks plus correctors for Maksutovs, 4 6″ pyrex blanks and one polished but untested 6″ plate mirror;

This was taken some time ago, and you can see the back has been flattened most of the way across with some edge pockets left behind. I plan to continue grinding at an angle with a diamond stone to get a more uniform look.

It is strange looking, but lookslike a prototype of the ones made by Dream Cellular. This one is a more simple design, with a thicker front to allow deeper ratios. It has a cast f/5 curve already and that would save time for whoever starts grinding.


March 2012

All projects stalled over the winter due my being diagnosed with Type One Diabetes. Things are returning to normal, whatever that will be remains to be seen

Another ATM had a second blank like this for sale for $450 plus $40 shipping and ten a gouge by Canada Customs for $112 Ouch, bastards. Et Tu Brute?

He got as far as starting to polish it before putting it aside. he did jst as I had and ground the back flat first and did a nice job. It has its native curve of f/5 still so it will need a step to make into a Dobsonian. This is a dilemma. It has a few small marks made during polishing, which may not come out with more polishing. I could make a tile tool to regrind, but then I would want to go deeper to say a f/4.3 c urve to make this useable without a step or ladder.


The polish on the surface does look slightly off, like it still has an incomplete polish in the surface. My 12.5 inch mirror shows a much better and clearer look to it.

12 1/2" Dobsonian Project

October 15th 2008

This project has been ongoing for some time. I actually started on this one first but decided against grinding it before I had experience with a smaller size.

It started as a full thickness Pyrex blank from Vancouver Telescope, which I got for $100 because someone had started it, and made a mess of it. It had an uneven grind to it, resulting in an off-center depression in it that required some 5 hours to center up. Once that was done, it only took another 5 hours of grinding with 46/70 and 80 grit to get the f ratio to around f/5, giving it a 63 inch focal length, a good compromise in shortness of tube and magnification, a wider field but still doesn’t have large amounts of coma.

There were pits left from the 46/70, so I gave it 5 more “wets” of 80 grit to level it a bit more.

The tile tool was easy to make but I learned the hard way to take my time. I poured the base out of plaster in 4 separate pours to give me a 1 3/4 ” tool. I used epoxy to fix the titles but there was one problem with them as they tended to ‘float’ a bit and slide down the slope of the tool. I solved this by using a fingertip to bed them in when placed. None moved after that. I heard later on that some have problems with the pieces falling off, but those in ‘pools’ of epoxy did not tend to do this.  Given the size involved, I didn’t bother to go completely to the edge with tiles by cutting tiles, just used the broken ones. This way, the tool acts a bit like a sub-diameter tool. After 220 grit, I cleaned it up with care, using epoxy to fill in holes or fix suspicious areas.

I used a bit of extra weight on it in the form of one of my mounts counterweights, one that was around 15 pounds. Later on, I used just the weight of the tool and my hands on the back of it. I theorize that a turned edge is caused by excessive overhang on a tool that has too much weight for its thickness, which lets the tool flex slightly on the overhang, resulting in the start of a turned edge. I also took my time in doing the 1/3 strokes with the tool. I did make a mistake in this stage though. I was letting the blank rest on the bare formica of my plywood backed surface thinking there would be no trouble with astigmatism. The blank has a 1/4″ raised rim on its bottom, which supports ALL the weight of the glass blank while I work on it. I checked contact when I changed grits at 220 and found I had good contact, so I used some old rubber sheeting and cut three circles of it to rest the mirror on when working “tool on top” or TOT.

I used a straight-edge and some feeler gauges to measure the depression, but I now own a spherometer good to 5 decimal places. I found I overshot my f/5 to f/4.8, so I did 120, 220 and now 320 grit with the tool on top. This is actually easier but I am not learning what I wanted from this. By changing TOT and MOT, you get control of the depth of your curve, very important if you plan on making refractive optics.

After 220 grit, I poured plaster and dental stone for the backing of the pitch lap. The finer grits after this will not change the curve all that much, so the plaster cast will be close enough for the pitch to conform properly. I found the dental product, while being slightly more expensive, is much more fluid when mixed. I plan on using it for all my later projects.

I have also made plans for the construction of the mechanics  of the scope. I really like some of the light-weight upper cages connected to the mirror enclosure with trusses. I plan that route. I found some of the upper cage assemblies a bit flimsy though. Too much empty space with the eyepiece looking at open air behind the secondary. Lots of people put up thin plastic shields to combat this issue. My plan is to use my router and a 1/4″ straight bit on 5/8″ baltic plywood. I will make two rings with a shallow cut in the middle. I will then bridge the two parts with an epoxied piece of formica (cuts well with a good blade on a table saw), and use truss poles to link upper and lower sections. I will build a small version of this for my polished 6″ f4.7 to see how well it works before commiting to a big version.

For grinding with 320 grit, I started with Al2O3 instead. I found this was a bit slow, so I returned to silicon carbide after just 4 wets. I did 15 more this way, then finished up with 5 wets of aluminum oxide. I got this in a box of grits, from Aurora Astro who got it from Bill Cook, of the ATM book fame. He’d had enough and cleared out his stuff. I did this as an experiment mostly. The Al2O3 grinds more slowly in a more controlled fashion, important for getting to and keeping a curve at the correct amount. For a tough glass like Pyrex, it is rather slow at 320 grit. It also seems to break down a bit slower, confirming what more experienced workers say about it. It has grains which have a flat red blood cell shape, not a sharp, angular form.

I now have to clean up my work area to prepare for the fine grinding stages. I don’t want to return to 220 grit because of a mistake. I set the tool aside on a shelf where it can keep until I’m certain I won’t need it again.

The plaster for the polishing tool is dry but needs some clean-up to make it nice and smooth. I nearly used SANDPAPER on it but used files instead.

I used the 25 micron on the mirror and did some 25 wets. Some people like to use more but I plan to go through 15, 9 and 5 micron as well for a smooth surface. I had to take a break and repair old holes in the tool with epoxy resin to trap any old grit in them permanently.

January 11th 2010

I have gone through 15u, 9u and 5u; 30 wets each with one minor mishap. The tool stuck fast to the mirror at around the 4 1/2 minute mark of a 9u wet,  needing force to separate them. I had been playing with the ratio of grit to water and I believed it was due to the grit being worn too far, so I shortened the wets to 4 minutes from 5. No further problems arose. The mirror now has that buttery smooth surface to my fingers and the hint of mist it holds disappears when I breath on it, the signs of proper fine grinding. No pits seem to remain.

I built a 8″ and 12 1/2″ pitch laps. That went well, in spite of forgetting to use parting compound (an alcohol spray mix) on the mold. It came off but took a few tiles near the edge with it. There was one damaged in the interior area and I might have to replace it to avoid a zone being created.

I ran out of my old hard pitch, so will need to open a container to make other laps. I did add a few tablespoons of turpentine to each batch to soften it up a bit.

It did a variation of warm pressing by using a bowl of hot water that was steeply curved, allowing the lap to be suspended over the water, making it a bit more controlled with how heat is applied.

I did try to do some polishing and it seemed to work but with catching and uneven friction,  the classic problem with incomplete contact between mirror and polishing lap. It will need more pressing to even it out. The main problem now is while humidity is high enough to slow evaporation of the rough/water mix, the temperature is too low to get effective polishing action. I will have to wait for better weather to allow the shop to be warmer. It is also quite hard to move. With 4 times the contact area of a 6″, the amount of work is that much larger. I seriously doubt I could do a mirror much larger by myself, and I was optimistic enough to buy 2 more 16″ blanks. I took some photos that show the wide polished area on the outer portion, with a central blank area that doesn’t show a reflection at all.  This is the classic result of only polishing with the pitch lap in the upper position. Most books recommend you swap them to go evenly. I did it this way because as you polish pitch lap on top, the polish moves from the outside to the center of the mirror. If you check the center for remaining pits, if the center checks out fine, then you KNOW the rest has a complete polish.

May 23,2010

The intervening weeks have been a mash-up of work, weekends away, the flu, cool conditions and now a shoulder/neck injury causing a pinched nerve. With the shoulder usable again, I restarted my efforts. I had wanted to finish in time for Table Mountain in August, but that seems unlikely now. I did the usual warming of mirror and lap, 30-45 minutes, followed by at least 30 minutes for warm pressing. I used a red plastic mesh bag for onions between them to add micro-facets to the lap. I had wanted to do this before but didn’t have a piece of mesh that could cover completely. This does two things: it forms the pitch to the mirror AND brings them both to similar thermal conditions. This will give the best polishing action and a better spherical surface. I am trying to maintain a better form for the polish stroke also: 2 inches or so back and forth, some variation in length and placement of tool, smaller number of strokes say 6 to 18 per position before moving and letting the rouge thin out and adding some water to lengthen each wet.

The lap is very hard to push around and changes to the rouge mixture does little to alter this. In most cases I only managed 1 1/2 hours of actual polishing time. It is true that the longer you go, the better the two pieces accommodate each and the better the figure you have. So far, there have been a total of 9 sessions with perhaps 12 hours of polishing.  I have changed my cleanliness as well. My shop has woodworking tools in it and bits of grit on the floor. I have avoided cleaning it out, as    this would raise dust contamination making the possibility of scratches while polishing. In between work sessions, I put each piece into a plastic bag folded over and weighed down to prevent breezes or drafts from getting dust inside. I am considering using mirror on top to finish but that means pushing around a 24 lb mirror, not the 12 lb tool.. Also rouge would not go as far, and since the mirror has to be lifted more  often, it increases the likelyhood of dropping it.

Some recommend polishing the mirror face up and then face down. The mirror will polish from the other edge to the inner edge when the mirror is on the bottom and this will be reversed when the mirror is on top. I did it exclusively with the mirror on the bottom for this project. My rational was this: many mirrors fail due to poor or incomplete polishing. It LOOKS good but you can only know for certain once the coating is on. A bad polish is now unmistakable. if you do all your polishing with the mirror on the bottom, once the center polish has a complete polish, you know the edge parts must also be good.

Since the mirror now holds some polish across the entire surface, I have started testing it. The Foucault tester from RASC still works, using an old 12V lamp for light. It is a virtual copy on the one in Texeraeu’s book, right down to the brass pieces used to make the slit light source. I removed that to give more light to help me align the apparatus. I puzzled over how to do this and realized that I could use a sheet of paper to find where the reflection was ending up. I had to use a mirror support meant for an 8 inch mirror. I realized I could just tip the 12 1/2″ forward, rather than raise the tester up some 2 inches. There are some tricks to know: the distance between source and return is the geometric center.  The image should be in focus when it gets back to the tester, and pass between the light source and 1/8″ from where your knife edge is. You also need to have the TRAVEL portion of the knife edge at 90 degrees to the returning beam or the shadows that you see will be meaningless. Once I did that, I replaced the slit mask. With a few adjustments, I could now see a squarish oval with a series of interference bands running vertically, more or less what should be visible. I need make a knife edge, as this is missing. I though I could use a piece of stainless steel for it and tired to cut it on my table saw. It went flying off like the piece of shrapnel it had become.

August 20th.

I am using  a 8″ pitch lap for parabolizing/smoothing this large mirror and have found it has poor contact.  I rebuilt it using softer (added 4 Tbs turpentine) pitch for better conformity. I have used this for 4 spells of polishing totaling some 5 hours. With my semi-working left shoulder 80 minutes is the maximum I can go without serious pain or compromised polishing. Once you get to large mirror, sub-diameter pitch laps become the norm: they weight less, and need less force to push and pull around.

Tests have looked better now, with a trace of roughness present and it seems to be of the shape called an “ellispoidal”, half way from a sphere to the classic paraboloid. I am still thinking about my support structure. A classic Dobsonian would be easy but also a Porter Equatorial is workable with not that much extra labor.

November 12th 2010

I have decided a Dobsonian is the best choice for this scope. The upper cage is completed but still needs stain or paint. I build it out of two 5/8″  baltic birch plywood rings, connected by four 1/2″ fiberglass tubes I bought for some other project. The spider is a Gary Wolansky, attached to four 1/16 thick brass supports. There is a 3.1″ secondary to go with it but I am considering using a 2.6″ instead. Total weight is around 3 pounds.

The cage will connect to the rocker box/platform using three 3/4″ aluminum tubes. I am making my own split connectors but will have to modify them for a better result.

February 13th 2011

I did another spell of parabolizing using the 8″ pitch tool. The mirror appears very smooth now with a pronounced depression in the center 60% indicating I am most of the way to having the correct shape. There is May 7th star party at Aldergrove Regional Park which I hope to have this scope in attendance.

April 19th 2011

I have worked on it some more now. Our house is being demolished, so we have to find a new place to live by June 30th. Luckily, we seem to have found a house we can afford, in a place we both like. My back acted up again three weeks ago, so heavy lifting of a delicate Pyrex mirror was out of the question until it was better. I did 3 more parabolizing sessions totaling some 2 hours. The figure observed in the Foucault test is getting close an ideal appearance. There is a hint of astigmatism, but this could be due to its lack of support during testing. Here is a photo of it:

12 1/2″ f/5 Pyrex Mirror

The use of the 8″ pitch lap worked well until I rebuilt it due to the pitch being too hard for proper use. The first attempt resulted in one that was too soft, so I rebuilt it after simmering the pitch for 30 minutes. I had to also fix the tester as a bulb had burned out in it. The shadows after just another 75 minutes seems rather muted, like it has been SHALLOWED out and not deepened. This may be from the soft lap or the tester being changed. I don’t know at this point. It looked very similar to a 12″ f/5 done by someone else before the lap rebuild. In this photo you can see the central depression. The stuff on the lower right and upper right are just a bit of polishing compound left behind.

A telescope of this size has an advantage over smaller ones: you CAN star test it WITHOUT putting a coating on it. Even BARE GLASS will reflect around 6%, making its light gathering power equal to a 3″ COATED mirror.

October 2012

I have had the mirror cell welded up out of 1″ square 1/8″ wall thickness tubes bookended by a 1 1/2″ by 1/4″ aluminum bar. This will allow me to have the mirror ride in a swing–down tailgate. This can allow the mirror to be cleaned, even washed without it being removed from the lower plywood mirror box. With the final outside width established, I can make the box to hold it.

Nov 2012

I have constructed the mirror box now. It has an interior dimension of 15 1/4″ just over the width of the mirror cell framework. The material was baltic birch plywood, very dense and capable of fine joinery, in this case 1/2″ wide finger joints. This joint is very strong when done tightly and only requires a table saw capable of using a dado blade. The jig is simple and can be made yourself. The only mistake making it was when I glued it up and failed to arrange it so that the tear out was on the outside of one joint. This mistake can be mitigated with the use of a wood veneer covering over the outside. Not a trivial solution, as you need to “seize” the veneer to make is pliable and flat by wettening it with a diluted glue/water mix and then pressing it to flatten it and equalize the water content. I can also add wood trim top and bottom. For a more durable surface, I could put on formica surface.

The weight is a bit higher than expected, so I may have to lighten it somewhat.

Not much remains to be done; connectors for the three tube members connecting upper ring to mirror cell, attitude bearings and a rotating platform. Maybe even this year it will be done. I do want to refine the look somewhat to make it more like a profession woodworking/mirror making project but that may need to wait.








ATM 6" Mirror/Binoscope

Currently, I have a polished but not parabolized 6″ f/5 plate mirror made. I may get to test it later this year but I need to work out the bugs of the Foucault tester.

Grinding it went well as can be expected without any major problems, with no pits showing up after polishing. I used a tile tool, rather than the more standard glass tool as I wanted to use this small piece of glass for practice.

In other news, I will go ahead with the roughing in of the two pieces for a binoscope. I plan on using two tools, working each one for a set amount of time, switching, then continuing on. This should give me a pair of blanks with the same ROC, provided I switch them often enough. The blanks are both pyrex this time from Willman Bell and being tougher than plate, the ROC will only move with a lot of effort.

I was going through old S & T magazines (a great resource BTW, a much better magazine then than it is now) and found an article [Dec 1979] by Sinnott on a 6″ binoscope. I had seen it before and was going to file it but its unusual optics caught my eye.

It featured an unobstructed light-path that allows the pencil of light to exit the tube through the side into prisms that rotate and invert the image. With 3 additional reflections (all internal, less light loss) you end up with a normal view like a true binocular.

The only problem is that while you grind them like a 6″ f/6, you need to parabolize them like a 12″ f/3. This means each mirror has to be deepened in an offset manner. How far offset depends on how much you want the light pencil from center. This also means a very difficult Foucault Test.


I have an idea what to do with this mirror: Build a scope out of it in a Dob base, and leave it in a case at the Elvin Lakes shelter in Garabladi Park

Lots of people with kids go up there and what better place to enjoy the wonders of the night sky than a dark place like this?

NOVA Newsletter: Mount Kobau 2008 Star Party Report, etc.

NOVA is the RASC Vancouver Centre bimonthly newsletter.

From the  current edition:

Mt. Kobau Star Party Report 2008

“…. Tuesday night was a wonderful night with the Milky Way bright and visible from horizon to horizon. Pomponia’s 12-inch dob and Ron Jerome’s 16-inch were having a great time finding faint fuzzies with M82 being especially entertaining. ….”

Previous editions are available in the archives.

October 9th: Observations of a Telescope Designer – Building the Thirty meter telescope

This is the Royal Astronomical Society of Canada – Vancouver monthly meeting.

Craig Breckenridge and members of the world’s premier Observatory design team at Empire Dynamic Structures took a trip to see the facility currently hailed as the largest optical telescope in the world. This marvel of engineering is soon to be eclipsed by the construction of the Thirty Meter Telescope which is currently being designed by this local company located in Port Coquitlam. Facets of the LBT’s design seldom seen by outsiders will be presented in all their glory with commentary by our resident expert designer.

We meet at 7:30 p.m. on the second Thursday of each month, downstairs in the auditorium of the H.R. MacMillan Space Centre.

Free Admission, Open to the public

Astro-coffee and astro-cookies will be served at the Southam Observatory after the meeting.

Sept. 13: THE 2008 PAUL SYKES LECTURE ON ASTRONOMY – "Mapping the Lakes and Rivers on Saturn’s Moon Titan"

Location: Room 201 of the Hennings Physics Building at UBC (adjacent to the SUB)

This year’s speaker is Guinness Book of Record holder Dr. Rosaly M.C. Lopes from NASA’s Jet Propulsion Laboratory. Her talk: “Mapping the Lakes and Rivers on Saturn’s Moon Titan” will show us just how similar the Earth is to the largest moon in the solar system.
Start Time: 19:00
Date: 2008-09-13

This year’s speaker is Guinness Book of Record holder Dr. Rosaly M.C. Lopes from NASA’s Jet Propulsion Laboratory. Her talk: “Mapping the Lakes and Rivers on Saturn’s Moon Titan” will show us just how similar the Earth is to the largest moon in the solar system.

Dr. Lopes is a Principal Scientist at NASA’s Jet Propulsion Laboratory and Lead Scientist for Geophysics and Planetary Geosciences. Her major research interests are in planetary and terrestrial geology and volcanology. Continue reading “Sept. 13: THE 2008 PAUL SYKES LECTURE ON ASTRONOMY – "Mapping the Lakes and Rivers on Saturn’s Moon Titan"”

July 10, 2008 – Great Moments in Planetary Exploration

Time: 7:30 PM
Location: H.R. MacMillan Space Centre, downstairs auditorium
Topic: “Come along for a trip down memory lane as we review the Great Moments in Planetary Exploration.  Our path will go from Mercury to Pluto and beyond.”
Speaker: Aaron Clevenson

Aaron Clevenson is an Astronomy Professor at Lonestar College in Houston, Texas in the US, Observation Chairman, and past president of the North houston Astronomy Club.

Free Admission, Open to the public

Astro-coffee and astro-cookies will be served
at the Southam Observatory after the meeting.