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A self-made Novosibirsk TAL optics-based 4” achromat
A self-made Novosibirsk TAL optics-based 4” achromat
by Hans Radowan hans.radowan@hp.com
In the early fall last year (2002), I acquired a MEADE 102/ACHR optical tube assembly (OTA) with rings for €235. This f/9 achromatic commonly has a not-so-good reputation, and this unit was typical. I tested it and found what it was famous for – a lot of false colour, astigmatism, and a flimsy focuser. Although you probably can get good samples of this scope, and it looks nice at first glance, when you take a detailed look you detect all the problems a cheap Taiwanese refractor usually has. But overall it was a good deal, and it had a lot of potential to become a fine scope after some improvements.
First I tried to get rid of the astigmatism. In comparison with my TeleVue Ranger, the Ranger showed textbook patterns in a star test while the MEADE presented all its problems. When I watched the big planets (Jupiter and Saturn), in contrast the MEADE was the clear winner due to its larger aperture. But whatever I tried, I couldn’t adjust the astigmatism with the push/pull lens assembly adjustment screws. When I tightened them so strongly that the lens was more or less bent, there was some “improvement”: the in/out focus images weren’t looking like lemons, but like cherries!
So I tried to open the lens assembly and rotate the crown/flint lens against each other to improve or correct the astigmatism – an impossible mission. The assembly was so tight (maybe the reason for the astigmatism in the first place), that I was simply not able to unscrew it. I had read some reports about this scope, where some users heard something rattling when they shook the tube. They found that it was the lens, where the crown and the flint glass were loose! So much for quality control and final adjustment by MEADE.
It wasn’t really as bad as it sounds, but I wanted to have a perfect scope.
Before I bought the MEADE, I had a TeleVue Genesis (the older, non-sdf f/5 version) from a friend for testing purposes. I remember that its images were very similar, especially on Saturn. Just a tad less colour on Jupiter, and the Genesis was only a little crisper. My shortest f/l eyepiece, a 3mm Radian which delivered 163x, obviously didn't show all that the Genesis could deliver.
Nevertheless, my original ideas was to rebuild the MEADE and make it an ED semi-APO. My budgetary manager (wife) wouldn’t allow me to purchase the Genesis ;-)). Well, that’s easier said than done. When I tried to get ED lenses as “replacement” parts, the manufacturers – even MEADE, who also produces an ED sibling of that scope -- were more or less unwilling to help. MEADE Germany for instance (I live in Austria) told me that they could improve the optics, but they wouldn’t replace the lens. No way.
I decided to ask several other manufacturers: Optolyth (who makes an 4” ED achromatic for birding and hunting), the general importer of Vixen Germany, another semi-professional astronomer and dealer in Austria, TeleVue, Takahashi, and Pentax. While TeleVue and Pentax didn’t even respond, Vixen Germany told me that they could help. However, it would be cheaper to buy a new OTA, because the price for the replacement lens is equal to the price of a complete ED OTA. The Austrian dealer started helpfully, but sooner or later we ended in a price discussion over how much a new (or even used) Pentax would cost. Takahashi simply wrote back that they use their lenses solely for producing their own scopes. Mrs. Roth, the owner of Optolyth, personally responded several times, even with a very attractive offer telling me that they use a fluorite glass for their doublet. The only problem was that I needed a lens holder. Optolyth spotting scopes are waterproof, so the lenses are completely sealed and fitted into the tube. Having no optical workbench for alignment killed this project. But my experience with Mrs. Roth was the same I had with Al Nagler, when I called TeleVue once with a question about my Ranger. The owner of that company responded and was very friendly. So was Mrs. Roth. That’s customer care!
But back to my project. The MEADE 102 ACHR has a 1.25” focuser. When I started the redesign, I found out that with the 635mm focal length Optolyth lens this focuser would vignette the images. I began looking into a real APO lens, like the TMBs from Markus Ludes in Germany. Well, for these lenses, costing around US$2,000, the mechanics of the MEADE were too cheap. If I had to spend so much money, I wouldn’t spend it for an ATM project with cheap components. In that case, I would buy a brand new, perfect, high-quality scope with a warranty. I also found an American who promised me he had a producer at hand who could make an APO triplet at f/7.5 for an affordable price. But he kept putting me off, and now he offers short tube achromats, like the f/6 ones Ludes sells. Eventually I stumbled over some reviews of the Russian TAL 100R scope. The most attractive review to me was that at http://www.cloudynights.com.
Later I read some literature about achromatic refractors and found a book, Astrooptik, by the German Uwe Laux. I also read a lot about high quality achromatic lenses and the materials used. In checking what glass the Russians use for their TAL 100R, I found that the TAL 100R might be similar to the ZEISS E-TYPE f/10 design. I asked several users. One of them has a TAL and works also with an ancient ZEISS refractor. He told me that the yellowish tint the TALs show when used during day is also visible in the old ZEISS, but this doesn’t affect night use. The colour correction was very good, he said. He has a web page with a lot of shots, especially of the moon, taken with the TAL. Most of the owners criticised the rude Russian mechanics, the finish of the paint, and the short travel of the focuser. Was this the ideal scope? How could I get the lens? What about the difference in the focal lengths of the lenses? (The TAL is f/10, the MEADE is f/9.)
I questioned Mr. Wolf at Urania Telescopes of Moscow. He told me that he could provide me with a lens, be he couldn't help resolve the different focal length problem. Mr. Wolf offered his lens for US$150.
I ordered the lens and started to investigate the possibilities of getting a “tube extension”. I received several offers when I asked at the German “astronomie.de” discussion board. Most people were willing to make the change for a certain price, but Thomas Kroh of Germany said “Just send me the material, I’ll make it for free !”
Meanwhile I received the lens, after more than 2.5 months. Take a look at the two lenses -- the Russian lens on the left, the MEADE on the right. Notice the purple coatings of the Russian lens, while the MEADE coatings look like the old MgF2 – some kind of bluish. (I used the underlying black fleece to line the tube extension to reduce stray light and reflections -- like TeleVue does, but with another sort of material)
The next job was to acquire the material for the extension (a 140mm-long cylinder of machine-quality aluminum) and to produce a technical drawing of the design. It’s not easy to find such material in amateur quantities. I contacted Aluminium Ranshofen, one of the Austrian producers, where a nice production master offered to produce the tube extension. I first thought to agree. But then I thought that Thomas Kroh, being an amateur astronomer, would best know my problem. So I finally decided to ask him to do the job. Because I live in one of the most famous wine regions in Austria, I promised to send him a few bottles of the finest wines as some compensation for the time he spent. He didn’t like that, he said – “I don’t want to be paid.” So I sent the wines as special greetings from Austria. Really a nice gentleman, this Thomas. By the way, shipment of the lens was around US$90 by DHL from Russia to Austria, not including customs and import tax. I could have bought a complete TAL tube assembly for the same overall cost ! But "Nothing ventured, nothing gained," as the old proverb says…
This was my drawing:
Meanwhile, I improved the mechanics of the focuser. The original had too much radial and axial play. I simply inserted two M5 screws to reduce the radial play (note the small screw right to the focus knob in the image). I replaced the focus knobs with brass models and replaced the finder with an 8x50 finder I had from my 8” Dobsonian. I also drilled new M5 holes and inserted brass set-screws to replace the single small original one.
I reduced the axial play to an amount I can live with by tightening the rack and pinion focuser after cleaning and relubricating it. Now the focuser works more smoothly and precisely.
I also bought a used Vixen Great Polaris mount and set it up on my old wooden tripod from when I had a Celestron C5 (which I sold and replaced with the Ranger). These days, the Ranger resides on a Manfrotto/Bogen tripod with a fluid head. Quick and easy to use. Just for comparison – take a look at both of my beauties now. I’d say both are small scopes, but while the Ranger is of exquisite build, the TAL/MEADE provides more aperture.
This is the business end of the new scope, the final lens cell painted matte black matte and blackened internally with fleece.
What about optical quality ?
I did a short optical shootout between the Ranger and the modified MEADE/TAL. After collimating the MEADE/TAL, which I now call the Russky, I compared them on astronomical objects. While the Ranger is a very well colour-corrected scope, I had the impression that the Russky shows less colour – frankly, when I did the tests, it was somewhat turbulent, so mother nature filtered out the blue part of the spectrum a little. On Jupiter and Saturn the Russky looked a tad sharper and crisper, with somewhat less colour than the Ranger. Bright stars like Rigel and Betelgeuse showed nearly the same image. Both performed wonderfully for not being APO´s! On Jupiter I pushed the magnification up to 333x with my 3mm Radian. I'd heard that the Radians introduce some lateral colour, but there was less visible than with the Ranger at 160x with the same eyepiece. The image in the Ranger was dimmer, although the exit pupil was a little larger. When I reduced magnification to 166x in the Russky (with my 6mm no-name Plössl), for the first time in my life I was able to see more than 2 bands in a refractor. I could see two small additional bands (very fine), and I even saw some brownish and grey shades. Again, seeing conditions were fuzzy, but I felt that this scope had a lot of potential. I pointed it back to the stars, looked at Alkor and Mizar, and easily resolved the small companion of the latter. All that with the wonderful black velvet background for which refractors are so beloved. (That's why I sold the C5 and kept the Ranger.) The stars just snap into focus.
Having had no chance to continue my shootout due to bad
weather, I packed the scopes and went to sleep. The next morning, I took
both again and pointed them at a hemlock tree in our neighbour's backyard.
Magnification used was 25x /24x with TeleVue Plössl´s in both
scopes (the 40mm in the Russky and the 20mm in the Ranger). I used a
TeleVue Amici Prism diagonal with both scopes. The Ranger showed some false
colour; when I looked at the needles against the blue sky the Russky
was crisper. Not much, but noticeable. And I know that my Ranger is no lemon.
Before I bought it I had a Vixen Geoma Spotting Scope, and I still have
an ancient German Nickel brand Supra 20-60x60 Spotting Scope. It's better
than the Vixen, but both fell behind when I compared them to the Ranger.
Now the Russky beat the Ranger! I believe it’s the good correction
of the lens and the larger aperture. Also, f/10 allows for good colour
correction. How the hell might a real APO perform, I thought by myself.
(My dream is either a TV127 or a 5” Takahashi!)
In the afternoon, when I was home from work, I performed the same shootout again with the same set up. The hemlock tree is about 30-40m (90-120 ft) away. I focused on a songbird that was sitting like me, enjoying the nice afternoon and singing its song. Even with the Amici Prism the view in the Russky was somewhat crisper and nicer. I could see feather details the Ranger didn’t show and the colours were brighter and more beautiful. I couldn’t find any trace of yellowish tint. I took my DOCTER 10x50 binocular, a reincarnation of the old ZEISS 10x50 DEKAREM and known for their yellowish tint. The yellow was visible there, but not in the Russky. The scope is a real keeper! I can’t wait until the Mars opposition this year.
31st March
I had the TAL out last night. After a forced cool down of 1.5 hours (my wife’s PC had to be repaired) seeing was about 8 of 10. At 91x ( TeleVue 11mm Plössl) Jupiter was beautiful and sharp. Added 2x Barlow, and now the image became blurred with a tad of yellow. The blurriness I assumed was a problem of the seeing. It also occurred when I used the 5mm Vixen Ortho and the 3x TeleVue Barlow with the 20mm TeleVue Plössl. I then tried several eyepiece and Barlow combinations. The more I increased magnification above 150x, the more yellow the image of Jupiter became. I didn’t see any bluish fringe until I inserted the 5mm Ortho, when the image of Jupiter got a barely visible blue fringe. Saturn was too low to see details, so I pointed the scope to Iota Leonis. I could clearly split the 1.7” separated stars with the 6mm Plössl (showing some blue halo around them, but clearly two stars). I’m deeply impressed, although this should be an easy task for a 4”.
Would I do this ATM adventure again ?
Honestly, no. Nowadays English shops advertise the TAL 100-R for sale at GB£199. This is a real bargain! I was lucky in finding several people who helped with my adventure. There was Mr. Wolf, who provided the lens, and Thomas Kroh, who made the tube extension/lens mount. But it was a lot of time and it really was a project. When you are ready to build your own scope, I’d suggest that you join an ATM group, or at least an astronomy group which has an ATM section. Otherwise you’ll have the same troubles I had. You need to look around a lot, and sometimes you feel the project will never get finished. Sometimes the problem is simple, e.g. where to find the raw material for producing the lens holder, and sometimes it's complex, like how to arrange pickup and shipping from Moscow to Austria. I needed a lot of talking and time to find somebody who could help with that. At the end of the day I didn’t save any money, but on the other hand this scope is wonderful. And it’s unique! No mass production, no Chinese plastic, no coma and other problems. Perhaps I need to rethink my "no" again. All in all, I believe it was a fun adventure, and it ended up fine.
But the next thing I need, maybe for my 50th birthday, will be a 5” Takahashi with Go To capabilities. Tell that to my wife, when you meet her!
Thanks for reading.
Edited by Jeff Verona