WHAT CAN ONE EXPECT FROM AN ISTAR 150 F/15 STANDARD ACHROMATIC DOUBLET?
ATM Build by Jack, September 2014
It’s been obvious for several years that my TV85 and cheap 6 inch Dob in our Florida home consistently outperform (on the usual lineup of urban targets) my TV102 and 10 inch Dob - Newt with a premium mirror in our upstate NY home. Atmospheric and local seeing is very good most winter nights on the east central coast of FL, and poor (on those few nights when stars are visible) in NY. So I transported the best scopes to FL last winter, brought the TV85 to NY, and decided to build a scope optimized for my local conditions.
With poor seeing conditions on a typical night in NY, the 4 inch refractor had consistently performed as well as the 10 inch Dob (no DSOs here). So I thought I would a build a large refractor, a 6 inch, which hopefully, could outperform the 4 and 10 inch scopes.
I'm not an experienced ATM, having built only a 10 inch Dob, and have a small workspace equipped with hand tools only. Storage for the scope is limited to a 24 x 36 inch floor space on one side of a small garage. My only option, if I wanted a large, easily moved refractor, was to fold it. And since the outcome of the project was uncertain, I wasn’t about to spend serious money for an APO objective. A long-focus achromat would be the glass of choice.
Pictures of folded refractors on CN and Google searches led me to a decision to build it in Dob style. I ordered a 6 inch F15 objective from iStar, a 3-1/2 inch folding mirror from Antares, and other components from vendors listed on the Stellafane links page. Wood came from Home Depot,and the dew shield vendor was Walmart (plastic paint bucket). The Dob base was salvaged from the 10 inch I moved to FL.
Total construction time was about six weekends. Had I known I would post this on CN, I would have taken pictures during construction. But it’s quite simple. A wooden box holding the two main optical components, each in their own independent wooden support, and a long narrow box on top holding the focuser. Three doors on the side and top rear of the OTA give easy access to optics and anything else that might have to be adjusted. An easily removable counterweight hangs on the top-rear of the OTA. Nothing complex here. Anyone with the most basic tool set can do this.
There have been four nights to star test the scope since its completion. And I’m amazed by the results. Star tests inside and outside focus look very good. No CA visible on mag-1 stars at 150x and 225x. Double star performance is excellent. The close component of i-cass (2.5”) is easily resolved at 150x (not bad for our poor seeing), and views of M13 and other clusters rival those seen in my 10 inch Newt. Views of the 10 day moon last night in clear skies were as good or better than anything seen in the 10 inch, and most surprising, no CA. Can’t say with authority there wasn’t any, it’s just that I couldn’t see it.
Overall, the results have far exceeded my expectations. I have the impressive resolution and contrast of a 6 inch refractor, no obvious CA in any of the objects I’ve observed, in an easily transportable package, at a very reasonable price. I fully understand the advantages of shorter tube APOs, and the reality that “aperture rules” in many situations. But for light polluted urban and suburban skies with less-than-ideal seeing conditions, a long focus achromat can be the ideal choice. It certainly was for me. And if a 90 inch OTA is too long and heavy - just fold it.