I was able to get a couple of photos of the moon through my Skywatcher 80mmED refractor a few nights ago. These are worlds better than the pics I took with my camera lenses, but they still lack a lot of the detail I was able to see visually. There were some rilles (long, winding canyons) in particular that were striking through the eyepiece, but which didn’t show up in the photos.
Feb. 2009 moon
And, for fun, here’s the same photo with the colour saturation artificially enhanced. I don’t know how representative of reality the hues are, but this is what came out of my camera:
Chris Graham is a long standing amateur astronomer with a keen interest in astrophotography. In 2005 he approached Council with an opportunity to collaborate in the use of a robotic telescope located in the United States using internet links. The U.S. location was subsequently replaced by one in Australia. In both instances Chris provided the telescope and imaging equipment and software while the RASCVC provided some setup expertise, operations labour and processing experience. It was a match that enabled both sides to learn a great deal about managing a remote telescope facility.
Remote access to dark skies via the Chris Graham Robotic Telescope (CGRT) has allowed the RASC to complement the “hands-on” local observing at the AOMO facility and interest a greater circle of members and public who are unable to travel to dark skies. Having had such a facility in the portfolio of RASC -VC resources has enhanced our services to local members and contribution to the community. The CGRT has served as an important training and development facility in collaboration with our partners, which include the H.R. Macmillan Space Centre, UBC, SFU and the Canadian Astronomical Data Centre (CADC) of the NRC. Lay persons and students have been invited to learn to use sophisticated telescope equipment and advanced robotic operations software. Captured data becomes available to all Canadians for study and processing via the CADC. Additionally, RASC Centres across Canada have also been invited to request project data capture from the CGRT.
Chris’ generous donations enabled Vancouver Centre to pay the site rental fees. Although our current arrangement will be suspended at the end of 2008, we are optimistic that there will be new opportunities in the future.
Simon Fraser University (SFU) will host astronomy workshops for grade-school students, to be held at SFU
and at schools throughout the BC Lower Mainland. Schools which attend these workshops will receive (at no-charge) a set of basic but high-quality refractor telescopes, and SFU staff will provide students and teachers with the training that is crucial to ensuring a positive first experience with a personal telescope. (First-come, first-served,
while supplies last! See below for details on the telescope to be donated.) At our daytime workshops, to be held during school hours, students will learn how to point and focus these telescopes using terrestrial objects, and will be engaged in an interactive multi-media presentation on the stars, planets, and other celestial objects currently in the
night sky. Students will receive and learn how to use a star wheel and “Sidewalk Astronomy” booklet provided by the Royal Astronomical Society of Canada (RASC). In addition, we will host evening “star parties” at SFU and at various community locations, in collaboration with volunteers from the Vancouver Centre of the RASC. Students
who attend our daytime workshops will be invited to these star parties with their teachers and families, where they can use their school’s new telescopes under the guidance of our experienced staff. Our star parties will also be open to the general public, and our guests will get to look through high-end amateur telescopes that will be operated by
staff from SFU and the Vancouver RASC. SFU and its funding partners will donate 10-15 telescopes to each school that attends a workshop (first-come, first-served, while supplies last!). These are basic but high-quality telescopes with a 50mm objective lens, and which come with a finderscope, tripod, and two eyepieces. Teachers will be asked
to be responsible for loaning the telescopes that are donated to their schools to their students, after they complete our workshop, for use either on their own, or at one of our evening star parties.
When the astronomy bug bit me again (or did I have a relapse?), I was pretty content with the two scopes that I bought, a 6″ Maksutov/Newtonian and an 8″ Maksutov/Cassegrain.
Both are great instruments, excellent visually but also capable of being used for astrophotography. The 6″ is great to just grab. All I need is the smaller mount, eyepieces and rings to start looking, with little cool down required. The dew heater is nice for those damp, cold nights here in Western Canada, but I find I can do without it for some hours.
The 8″ has a better optical reach but with its extra weight, 25 lbs plus the diagonal, the Naglers for wide field, I use it with my big German equatorial only. Since I want to get good value for the effort of packing up, I usually need the dew heater due to longer times outside.
The 6″ is great for most things. The 8″ needs extra care when focusing due to mirror shift a while the focuser on the 6″, a Crawford of dubious parentage, also shifts when heavily loaded with an eyepiece and a barlow.
I want a simpler instrument, with a bigger light grasp on some kind of Dobsonian mount. To that end I have been scouring EBay and other sources I won’t get into here for the parts and supplies needed to further my ambition.
I have ground a pair of mirrors before, once in elementary school, and later on after college. I got busy with a new career of owning a small business, and did some more bits of grinding for other people to finish. I never have done it from the ground up before of taking a piece of pyrex, plaster, tile, pitch and rouge to make a mirror of my very own.
I did find a partly ground 12 1/2″, full-thickness Pyrex blank in 2006 at Vancouver Telescope for $100. It had a troubled history, with someone starting the roughing out but leaving it incomplete and somewhat off center. A good size for someone once they have a bit of experience at fixing problem like this that crop up. I needed some projects to refresh and learn from first. So, I sidelined it. It kicked around in some cupboards (bottom shelf) for some time.
I got a few books, not realizing some were in the library of the RASC already. A few articles, one in a defunct magazine called “StarTrails”, on the virtues of plaster/tile tools rather than a more expensive full size glass tool got me pointed in the right direction. A mis-step of using old plaster was the first mishap.
I got an old barrel from work, filled it with water and made a plywood/formica work surface and screwed the two together. I took extra care, left the top octagonal to make it easier to keep track of my movement “circling the barrel”. I put on counter top laminate (Home Hardware, ask to see their remnant pieces) to make cleaning easy.
Another great place to look for information is the www.Stellafane.org website with its complete listing of grits sequences to use, what the glass looks like as each one establishs a new surface finish, and how to modify your work to fix problems before they get out of hand.
A list of my stash of parts reads like a person who is destined for Dante’s 4th Circle of Hell, where hoarders end up.
I have, in the order I got them, a considerable wealth of the bits needs for mirror grinding and polishing. As an experienced woodworker I have most of what I need to build functional Dobsonians for most of these.
A kit consisting of two 6″ pyrex blanks, a glass blank, grits and rouge
A polished but not parabolized 8″ mirror; an 8″ f/8 mirror that has a worn and dirty coating
2 8″ f/5 mirrors, one plate glass and the other Pyrex,with a tile tool and pitch lap that are fine ground to 5 microns an need of a polish
A 6″ and 10″ 0.8″ fused silica blanks for two high performance light weight scopes
A 12 1/2″ full thickness Pyrex blank, now roughed in to f/4.9
A massive but light-weight 16 1/2″ cellular Pyrex blank cast to f/5
An old 11 1/4″ Maksutov Corrector blank made of Grade A BSC-2. In a fit of massive and stupid optimism I got this, not fully understanding what this last one gets me in in for.
Addendum: Nov 2012, I moved to Squamish in July 2011 and astronomy feel aside as I battled moving my life and business to a very permanent location. Nice and dark,except one offending street light bathing my otherwise pristinely dark yard. I got working on the 12.5 inch again getting a much smoother surface with a cumulative polish time of some 8-10 hours., mostly with a 8 inch sub-diameter lap.
I also added some MORE blanks etc to my stash. There are now 4 blanks in the 16 inch size, two of them identical cellular blanks (one polished), and two `classical` 16 inch full thickness blanks some 3 .25 thick, 60 pounds each, a 2.1 inch thick 18.625 inch pyrex blank, roughed out by Rob at Gold Mountain Observatory in exchange for 2 8 inch old mirrors and some small pieces of BK-7 and F-2 blanks.. I also found a 10.75 inch Maksutov blank, an 8 inch one, and a finished but unmounted 4.5 inch. There is also a 10 inch kit for $100 from Van Tele which has 2 10 inch blanks and grit and the old receipt for $82.50 from somewhere back east.
The tally is now: 3 small (4`inch or so blanks), 6 6 inch; 7 8 inch; 7 10 inch; 7 12.5 inch, 4 16 inch, 1 18.625 icnh and a 24 inch Zerodur mirror from an old Dall-Kirkham setup
3: 2 4inch or pyrex, one 5 inch of pyrex
6 Inch : 4 Pyrex, one fused silica and one plate glass
1. Learn to use the Foulcalt tester I “borrowed” from the RASC here and use it on my 6″ plate glass mirror and see what it looks like and send it to someone else to see if I tested it correctly. It is around f/4.3, so parabolizing is needed to use it. I plan to use this as practice of my methods. If I end up with a useable mirror, great. I’ll give it to someone at the club who needs one for a bigger scope than they have. I will then corrct to probable defects this mirror has. I will then look and fix the old 8″ I made years ago, hoping it didn’t get scratched in the intervening years. (Golden Years, bop bop bop)
The two 6″ Pyrex blanks would make a good bino-scope. This needs more planning, as bringing two images together is not a trivial task, no matter which of the S & T magazine articles you read. It will need its own mount with some arrangement for focus and alignment.
The various 8″ blanks in varied states are a good step up from your garden-variety 6″ scopes. Capable of showing deep-sky objects yet small enough to be portable, you can build these in a number of ways: Long focal lengths, f/7 and up, will give long focal lengths needed for planets yet will still be useable for larger targets. The f/5 blanks will make great wide field mirrors. Here I can choose to make them solid wooden tubes or truss-types for lighter weight. I plan on building a nice wooden one on a Dob base for friend of mine next year, after winning for optcial and overall in the Table Mountain competition;).
The silica blanks are a great choice for a small, yet high performance and very portable telescope. Silica takes a finer polish than most other easily available materials, has a very low coefficient of expansion and is much thinner to boot. The material is much harder, making it slower to grind and polish but making it less likely that I will accidently go past my intended f ratio. Here I will spare little work and expense. These probably won’t be started before 2010.
The 12 1/2″ will be my primary OBSERVATION telescope. Large aperture on a Dobsonian mount will make most objects easy to find at a dark sky location. The figure on this mirror is very important, so I will leave polish and figuring to the end on 2009.
The 16 1/2″ cellular blank, made by www.embeddedrf.com . Huge yet light, only 20 lbs, lighter than my 12 1/2″ blank and will cool off quickly. It has some issues with the quality of the glass, namely some bubbles that didn’t quite make it out of the top surface of glass, leaving around a dozen hemispherical divots in the rough glass surface to be ground, the largest being around 3/32″ across. It has an f/6 surface cast into it, so some of these may remain after grinding. I don’t want to turn this into a second big Dobsonian, as a 16 1/2″ is “only” 74% more area, resulting in a 0.35 magnitude increase in light grasp. A better idea is to make a large, compound instrument like a 16 1/2″ f/8.33 R/C. I plan on buying a metal lathe sooner or later, so building a truss-type R/C would result in a light yet large aperture telescope. One problem with this is both mirrors need to be hyperbolic, which are hard to make and even harder to test easily. A second problem is this mirror blank may be impossible to treppan. This can be overcome by a third mirror resulting in a Nasmyth focus. I wouldn’t need to perforate the mirror and I could leave it open work mirror cell with cooling fans for all to see
The 11 1/4″ Maksutov blank is a special project in my mind, and beyond most what most ATM’s ever attempt. It is a compound optical system, with a corrector that requires special care to make properly. Starting in the late 1950’s with the so-called Maksutov Club and its “Circulars”, this fairly obscure design became widely known. Up until the 1970’s, when inexpensive and well-executed Schmidt-Cassegrains came into their own, a number of companies made Maksutov kits and/or blanks up to 11 1/4″ across. The early designs were slow f/12 or f/15 with easy to make spherical optical surfaces, pioneered by John Gregory. Later on, they made separate secondaries (not mirrored spots on the back surface of the corrector) to gain faster optics with better correction of images. Some, most notably Howard Louth, made ones with convertable Cassegrain/Newtonian focuses.
This type of instrument will test you. You need to master proper strokes in order to bring the corrector to bear, as it needs to stay very close to design. According to an number of experienced makers, you can “miss” the correctors R1 and R2 ROC by a fair amount and still end up in good position, as long as R1 and R2 are close to each other by some 0.5%. The only “easy” part is the spherical main mirror. One has to use care when selecting its size. Too small and the diverging beam the the telescope aperture will not be fully illuminated; too large and you make the instrument bigger and more expensive than need be. What gives me a headache, and probably others, is where you put the aperture stop, which limits the field of view. It is an attractive proposition to make. Its best function depends on getting a number of factors right. Lots of ATM people make Newtonian mirrors and get credit for making it right. I do take pleasure in learning something new but I usually lean towards more complex projects. I would guess a telescope like this one is probably less than one in a thousand. I do have plans to make a pair of smaller ones using a couple of old kits for a 4 1/2″ version. This way, I can make an f12 or f/15 all-spherical design, then take what I learn to do an improved version at f/10 or f/8, which require you to aspherize at least one surface, usually the secondary. That will be a challenge as these will have a 1″ and 1 1/2″ secondary respectively.
I fell in love with this type of telescope the first time I bought a commercial scope. Compact with lots of focal length to reach small objects. The corrector also makes for very low amounts of coma and other residual optical abberations. If you have the money, and want a telescope that is noticeable better inch for inch than your run of the mill Schmidt-Cassegrain telescopes made in the thousands by Meade, Celestron and others, you would be hard pressed to find a better telescope than a Maksutov by Intes or any good company. They leave little to be desired. They are heavier and therefore need a better mount and take longer to cool but the gains are worth it.
For 2009, I plan on doing at least some of the following:
1. Test and repolish the 8″ f/8 for a nice wooden Dobsonian for a friend. Possibly enter it into the contest at Table Mountain. While doing that, I can polish and parabolize the two 8″ f/5 mirrors and test them while working on the first.
2. Finish the 12 1/2″ mirror and see if I can get a workable 3 pole Dobsonian that is more portable. I will need some other parts for it like the rings for the atitude bearings. I want to be able to collapse this into a small package, excluding the truss poles of course.
3. I actually did think ahead and I bought a very nice home built spherometer accurate to around +/- 0.05%. While this makes it easy for mirrors, it is not strictly required for them as you don’t need to hit a particular radius of curvature.
For any kind of lens or corrector, you can’t really do without this piece of equipment. This one is plate of aluminum drilled with 3 holes at a well-defined radius with three ball-bearings in them for the feet, and an dial indicator at the center. To use is simplicity itself: you zero it out on a precision flat surface, place it gently on what you wish to measure, and out comes a number, positive or negative called the sagitta, named after Sagittarius the Archer, as the curve you are measuring looks like a curved bow. You put this number into one of two formulas, depending of if you are measuring a positive or negative curve, and you get a radius of curvature, ROC.
This type is good, but the bar type can be used as well. It is simply a ground bar with a hole near the center for the indicator and pairs of holes starting near the middle and going out. You put feet into matching holes on either side and then zero it out.
4. The TV Lens Project is currently very stalled. I have a fork mount and tripod to put it on but I want it all. I want electric zoom and focus controls and possibly a tracking system that works by setting it on a star in a database, hitting a key, letting the star drift a bit, and hitting the key again. Assuming the mount is level and the digital encoders on both the azimuth and altitude are accurate, the computer should have enough information to continue tracking the object, and even tell you where to point the scope to get other objects in the database. I see some computer programming in my future. Maybe I can talk Kris into it.
I have in the past looked at the big, and I really mean big, Dobsonian mounted scopes and lusted. However, there seems to be a common problem with the really large ones, especially once people get above 16″. They tend towards using the big, THIN mirror blanks, usually less than 1:10 ration. I had a chance to look through a number of these at star parties, and have found while the images are noticeably brighter, they usually lack clarity. One beautiful 26″ in particular had won an award, for design. After looking at several Messier, NGC and Caldwell objects, I came away disappointed. A talk with someone very respected in the ATM and astronomy community (who shall go nameless, to protect the guilty) summed it up. He said in trying to gather as much light as possible, people often forget that RESOLUTION counts as much once you get to around 12″ or so. He said astigmatism and poor parabolization is the rule rather than the exception for most thin mirrors 20″ or larger. Beyond that size, you gather more photons but seeing ultimately limits you. Most places people go long distances to, go there because of darker skies, not necessarily better seeing. You need to go higher and drier to really make a difference. My belief is that trading resolution for more photons is foolish. The poor, HUGE mirrors by Herschel, Lord Rosse and others made before the Foulcault test shows this clearly. Read their descriptions of objects listed in Burnhams Celestial guides. I find their text matches up with what I can see at 47 years old with a well made 6″ Mak Newt from a fairly dark area (or at least as dark as it gets) 20 miles from Vancouver.
The best scope I looked through at Table Mountain was an 18″ f/6 with a 3″ mirror, a standard 1:6 ratio. This does put limits on things. Some of the scopes I saw there needed their own trailer to move in, which sort of defeats the purpose of the Dobsonian, simplicity and portability. A scope like this should be usable on 15 minutes notice. I have no qualms with people “going big” but there is an inverse relation between scope weight and amount I use it. I have only used my big 8″ 5 or 6 times since I got my 6″ and I have few regrets about it other than the money I spent on it.
I thought with the acquisition of a number of Pyrex blanks, (2 X 6″, 4 X 8″ and 5 X 10″) I was finished buying these. Discovering a 16″ 3 1/4″ Pyrex blank changed my mind. It is heavy, about the limit of what I can do with aging muscles and back. I will take a short cut by getting the initial curve pre-generated with a diamond tool. If that goes well, I plan on getting some of the other blanks started this way.
Full thickness blanks are unusual now, as most are thin Pyrex or even mere BK-7 glass. I *COULD* get the two thick ones cut on a diamond saw into FOUR 16″ X 1 1/2″ thick blanks, quite useable by most mirror grinders.
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.
Preamble:
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.
Addendum:
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.
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.
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?
