It was two years ago I had a discussion with a friend of mine, David. He was an accomplished film astrophotographer, I an astrophotographer wannabe. He was lamenting the pitfalls of his 10" f/5.6 newtonian. The problems were vignetting, coma, and a curved focal surface that exacerbated the coma. A faster focal ratio would be nice, but would have made everything worse.
At about that time I had been reading Rutten and van Venrooij's excellent "Telescope Optics", in which they extol the virtues of the Houghton design. It's all-spherical optics are very forgiving for a newbie to make, it has no coma and a nearly flat focal surface. I told David about it, and we set out on a mission with stars in our eyes - to build the Houghton.
The design chosen was the Lurie-Houghton. After reviewing his pictures, we decided the focal length of his 10" was about what was desired, so we set out to build a 12-13" f/4, which would have just a bit shorter focal length. I ran through the numbers in R&V, and plugged them into Oslo. The results were a little disappointing, despite R&V's claim that the design wouldn't need much if any tweaking from the formulae.
Off to the ATM list with some questions, and a number of people gave me some great advice and help. I learned of several people who had built or were buliding Houghtons, and a few folk even ran my design through Zemax or Oslo and sent me optimized versions. The one I chose was designed by Mike McJimsey. I continued to tweak it, of course, moving the corrector in and out and so forth. The work to be done consisted of
It just so happens that an old Starliner equatorial mount fell into my hands as I was planning this. There's the hardest item, and it's already done! Furthermore, the mount was a twin of David's (although he had extensively modified his) so we knew what it could do and what to expect. We planned out a budget of $1000 for glass, grit, and raw materials, and started collecting things. I bought a test indicator for the wedge tester, called Salem Distributing and got a bunch of grit and pitch, and found a dirt cheap plate glass blank for the primary.
I optimistically sent out for quotes for the corrector glass. A houghton doesn't care what kind of glass its correctors are made of, so I thought I could get away cheaply. The only place that would quote the glass in plate was Newport, and they didn't give me warm fuzzies about what I would be getting and when I would be getting it. I decided to cross them off the list for now. Everyone else would only do BK-7 or better, and the quotes were in the thousands. Undaunted, I began hogging the primary and planning the rest, figuring that cheap glass would show up someday on the list or elsewhere, and I'd just keep my eyes open.
We still had to decide where to place the correctors. I had two designs. One with the correctors at the 1 focal length, and one with them at about 0.9 focal lengths. The closer one had the advantage that the secondary mirror could be mounted on the corrector itself, so we'd get rid of the spider and its diffraction. On the other hand, it's a lot easier to find/make a spider, and there was the issue that if the correctors weren't very precisely made, they might have to be "tuned" by changing the distance form the primary. If that were the case, we might get into trouble if the secondary were mounted to the corrector, because if we moved it too far from the design, the focal plane would either be too close to the tube or there would be vignetting.
Time went by. I traded (OK, mostly gained) knowledge with the fine folks on the ATM list. I spoke with Bill Cook, who had not only started a houghton, but runs the ATM Journal, with articles on the houghton (I bought a complete set of back issues). I spoke with Rick Scott, who built a houghton and was kind enough to spend quite a bit of time telling me of his experiences. Anthony Stillman warned me of some of the difficulties of fabricating the correctors. Dave Rowe gave me pointers on his mount and setup. Bratislav Curcic assured me that it was doable ;^) and gave me information on making a composite tube.
Dale Keller gave me tips on his horseshoe mount (even though I had the Starliner, it might be useful to make). Greg Jones sent me an optimized version of my design and lots of good advice, as did Kevin McCarthy, Roger West, Kristopher Barrett, Peter Smith and Mike McJimsey. Peter Hirtle gave me advice on mounting the secondary. Peter Smith gave me TONS of good advice. Mel Bartels advised me as to grinding and grits. Richard Schwartz also had a lot to say about glass. Roger Ceragioli told me the pitfalls of secondary mounting and corrector cells. Gary Wolanski was nice enough to design a special secondary holder for us if we decided to mount it on the corrector, and quoted a very reasonable price for building it.
Lorenzi Marco, Greg Jones, Kristopher Barrett, Kevin McCarthy, Bill Cook, Clive Milne, Roger Ceragioli, Rick Scott all played with or built houghtons, and were free with sharing their knowledge. No doubt I am forgetting to include some people who helped me along the way. If you are in that category, my apologies. My intention was not to name names, but some people (like Mike McJimsey) deserved to be mentioned because I used his design, so I figured I should make a list to the best of my ability.
Anyway, there was a serious illness in my family, a new baby (a boy) born to me, and a death in David's family, and things got shelved for quite a while. Almost two years passed.
Recently, I saw Dan Cassaro selling inexpensive good quality plate and my eyes lit up. I got the houghton bug again. I did more and more design, and the thing looked more and more real. Then, two things happened that caused me to put the project to bed for good.
First, The reality that there isn't a commercial 35mm camera in the world that we could use without vignetting seriously. Since one of our goals was to shoot unvignetted frames, we were now talking about a custom focuser, a medium format camera back, and a separate guide scope (since we now had no place to put an off-axis guider). I don't know why I wasn't thinking of this earlier. I guess I got so involved in making the optics work, I forgot about the film at the end of the road.
These new requirements also meant scrapping any idea of using the Starliner. We were pushing its limits as it was. Adding another scope and a heavier larger photo setup would be too much. Now the hard part that I thought was done was not done, and became even harder (as the mount had to manage two scopes without flexing).
The second reason is that David became disinterested in astronomy. I was now doing the project alone, with the goal of producing an astrograph that I didn't know how to use. Better to start with a small newtonian and learn the ropes first before going off and building the dream scope.
So, I consider the project permanently scrapped, at least until I get big time into astrophotography and actually have a need for such a scope. I figure by that time I'll have had enough experience pushing glass to try my hand at a schmidt derivative ;^). In order that the efforts of myself and all of the fine people who've contributed to this not be for naught, I've decided to publish the design as-is. There's no guarantee that this design is final, but perhaps it'll save a future ATM some time and effort in figuring out his or her houghton.
The final version had the following prescription:
*LENS DATA 12.5" f/4 Lurie-Houghton SRF RADIUS THICKNESS APERTURE RADIUS GLASS SPE NOTE OBJ -- 2.4047e+20 4.4074e+18 AIR AST 1.9686e+03 V 22.000000 152.500000 A K3 C 2 -5.1952e+03 V 3.726876 S 152.500000 P AIR 3 -1.9686e+03 P 13.246169 S 152.500000 P K3 P 4 5.1952e+03 P -- 152.500000 P AIR 5 -- 1.0560e+03 50.800000 AIR 6 -2.4243e+03 S -976.000000 P 171.189595 SK REFL_HATCH 7 -- 233.792884 S 50.800000 KX REFL_HATCH * IMS -- -0.019251 22.347877 S
Those of you who can read Oslo files can download the Oslo lens file.
The spot diagram looks like this:
As you can see, it is ``diffraction limited'' in that the entire spot on axis lies inside the airy disc. This is true inside a circle of about diameter 0.8 inches! When defocused by 0.05mm, the spots are under 20 microns edge to edge. The field shown in the spot diagram corresponds to a circle of diameter 44mm, which is corner to corner on a 35mm film frame.