Saturday, March 12, 2005

Big Bertha Behaves

Okay, maybe the additional edge supports for the mirror solved (or mostly solved) the turned edge problem. I find that masking the mirror, even down to 13", did not do any noticeable good. Collimating the mirror on stars worked rather well, at least for low power observation. At higher powers, I still need to put an off axis mask on the front of the telescope--but when I do so, the performance becomes at least acceptable, although not spectacular.

Saturn now delivers a very crisp image at 285x, and a reasonably crisp image at 333x. Cassini's Division isn't inky black at 333x, but when the atmosphere goes into brief periods of calm (for a second or two), Cassini's Division is clearly visible all the way around the planet, and quite dark. I think I can see a bit more detail in the rings--perhaps something like additional divisions? I don't know.

Another measure of performance is to ask what the lowest power that you can see detail. At 57x, Cassini's Division was not visible. At 80x, it was clearly visible, and at 111x, it was obvious that it went around the whole planet.

Jupiter has been too low in the sky to see much, but by the time I finished last night, Jupiter was up about 30 degrees above the horizon, and starting to be pretty astonishing. At 333x, the image was not as crisp as I would have liked, although at 285x it was acceptable. At either magnification, there was more detail visible in the cloud bands than I could even begin to describe or sketch--lots and lots of whorls and dark dots against the pale background. Unlike Saturn, I believe that I am seeing a good bit more detail on Jupiter than I can see with my other two scopes--or, for that matter, with any other scope that I have used on Jupiter.

I am not quite sure why the off-axis aperture mask makes so much difference. Without it, by 160x, the image degrades substantially. A mask on the mirror itself made no substantial difference, but the off-axis aperture mask made an enormous difference. I am wondering how much of this might be caused by the diagonal mirror (which is pretty large) or the spider that holds the diagonal mirror. One way to test this is to build an aperture mask that can be centered in the tube, and operate off-axis. If the problem reappears when centered, then I need to figure out if the diagnoal is too large, or if the somewhat clumsy and homemade spider might be the source of the problem.

It does seem as though the focuser is a bit coarse. Perhaps going to a lower profile focuser and a smaller diagonal would help--but that would require changing the telescope tube. It might make more sense to buy one of the helical focusing adapters that go into the 2" tube, and let you make fine adjustments to 1.25" eyepieces.

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