Friday, October 27, 2006

Astrophotography Again

I believe that I mentioned the struggles that I have enjoyed in the cold, figuring out how to get the Losmandy mount aligned on the North Celestial Pole. I also discovered that the weight of the camera caused the diagonal on my reflector to unscrew--which had all sorts of shattering glass potential.

So, I pulled off the shelf the reflector that my father and I put together many years ago. I know it works for astrophotography; I've used it for that purpose before--and it is a pretty impressive optical system, especially for something that is only 8" in aperture.

I'm afraid that this telescope is at the upper end of what the Losmandy GM-8 can carry. Partly this is because of the weight; it weighs about 25 pounds. Partly this is because of its length, with the moment arm problem of something with a fair amount of mass at the ends of a fiberglass tube. (Fiberglass is stiff for its weight, but still not stiff in the sense of steel or carbon fiber composite.)

Still, for visual use, it works, and if I gave the timer on the camera a few seconds to fire the shutter, there was no visible motion, even with the wind blowing. (The camera is a Pentax ME Super, so I have to get the film developed at Wal-Mart. How quaint!)

Anyway, I shot some pictures of the Moon at prime focus, where you use the telescope by itself as the camera lens. Michael Covington's astrophotography exposure calculator says that I should use 1/250th of a second, so I did 1/500th, 1/250th, and 1/125th second exposures.

I dragged Big Bertha out as well, and since it can't track across the sky, this limits exposure time to 1/4 of a second or less. But hey, it's got plenty of light for the Moon. I used a 3x Barlow lens in between the camera and the telescope, not because I wanted the magnification, but because the focuser won't go far enough in to put the focal point into the camera body. A Barlow lens effectively pushes the focal point out a few inches. Covington's calculator suggested 1/30th of a second, so I did exposures at 1/60th, 1/30th, and 1/15th of a second. With the combination of Big Bertha's 2000mm focal length, and the 3x Barlow, I could not get all of the Moon on a single frame, so I may have a little less brightness than optimal.

The last set was to use eyepiece projection on the 8" reflector, where you attach the camera to an adapter that carries an eyepiece. In this case, a 25mm Plossl eyepiece made the Moon fit nicely within the film size. The calculator suggested 1/15th of a second, so I did 1/30th, 1/15th, and 1/8th of a second exposures.

I don't know if viewing conditions are better, or if my glasses are better than they used to be, but I had no problem getting what seemed like a very sharp focus with the Pentax. Focus is critical in astrophotography, and I have often found myself frustrated at this, but these seemed pretty decent.

Viewing conditions weren't great, primarily because the Moon was headed down, and there seemed to be quite a bit of turbulence above about 250x. At lower magnifications, however, such as 56x with the 8" reflector, and 80x with Big Bertha, the level of detail was breathtaking!

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