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
8 Inch: 6 Pyrex, one plate glass
10 “: 7 Pyrex, 1 Suprax 33
12 1/2″ 7 molded Pyrex
16″ : 2 Molded Pyrex 3 1/4” thick, 2 Cellular Blanks
My plans are as follows:
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.