This set of posts is about the TV Lens we were generously given by CTV to make into scopes.
Myself excluded, most people with these will need to make a tripod, and some kind of mount, probably a fork mount for sake of simplicity.
I recently acquired a planer as well as a second table saw that I can keep in Vancouver, so I have enough tools to build parts for people without such tools for the cost of material.
The tripods can be made of 2 X 6 lumber cut at an angle for legs, two pieces of plywood glued together for the mating plate. Door hinges can be used for the tops of the legs and a chain connected to eye bolts to prevent the tripod from collapsing.
The fork is a special problem. Mine is made out of a piece of aluminum tubing rectangular in cross-section. To make the U shape, I cut 22.5 degree wedges of material out, bent it to shape and had it welded by Pro-Tec Marine Welding in North Vancouver for $140. I didn’t cut through completely, but left the cut so that one side of material was still there and with enough room between the cuts so that when they were brought together, a proper weld with filler rod could be done. Mine ended up being not quite parallel but close enough I can probably cold-set them.
One problem with this lens is where the focus comes to. I think it may be too close to the last lens element to make for a convenient position, so I think some kind of negative lens element is needed to add some back focus distance.
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.
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:
This is a photo of a frost pillar taken above Grouse Mt. It is caused by ice crystal of a flat hexagonal form acting like an array of tiny mirrors above the light. There was one shown on the Weather Channel, but their explanation left something to be desired.
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.
Paul Sykes was born in Hummelston, Pennsylvania USA in 1918. He acquired his interest in astronomy at an early age. During his teens he published his own monthly astronomical column and gave at least one lecture.
He was an officer in the United States Air Force, served in the Pacific during WWII attaining the rank of Captain. He was awarded a Presidential Unit Citation, the U.S. Air Medal, the Oak Leaf and Cluster and the Bronze Star. Following the war he attended UBC earning a degree in Physics in 1948. He rejoined the United States Air Force and attended the Oak Ridge School of Reactor Technology, studying nuclear physics. He worked on the NERVA Project, a nuclear rocket development effort and rose to the rank of Major.
Paul was appointed a lecturer and administrator in Physics at UBC and remained there until retirement in 1983.
Paul actively pursued his interest in astronomy, attending conferences and joining the R.A.S.C., where he became a Life Member.
Paul Sykes passed away in October 2005 at the age of 87 and left the Vancouver Centre a generous gift.
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.
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.
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.
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.