Saturday, October 28, 2006


I rolled out the 8" reflector this evening to finish off the roll of film in the camera. This time, I was using eyepiece projection with a 12.5mm eyepiece--meaning that all I could see was a portion of the Moon along the terminator. These can be spectacular photographs, if everything is sharply focused, as it seemed to be. The exposures I tried were 1/8h second, 1/4 second, and 1/2 second. I'll have Wal-Mart develop this tomorrow.

Every time I use this reflector, I am reminded of what a fine piece of optics it is. Big Bertha, having more than twice the aperture, will usually show more detail at the same magnification (as you would expect), Big Bertha seldom tolerates more magnification, under the same conditions.

The image through the 8" at 56x is breathtaking. The whole Moon is visible, with a bit of black sky around it, and one of the brighter stars visible off to the side. The features are tack sharp, with high contrast on everything. I kept putting in more and more powerful eyepieces, trying to find where the image broke down.

Turbulence is a bit of a problem, but you can see the image waving because of it. For brief moments, the air stops moving completely, and the image is flawless. But other than the turbulence, wow! At 354x, the image was still tack sharp.

The next step up, using a 9mm with a 3x Barlow, gave 471x. At this level, in addition to turbulence, the image was starting to soften--and no amount of focusing would make it sharp. But there was still a bit more detail visible compared to 354x. In particular, I was looking at a rille near the terminator. It was just a wiggling line at 354x--but at 471x, while the rille was not quite as crisply defined, there was definitely more detail at the bottom and the sides of the rille. In some stretches, I saw (or at least think I saw) differences in width and the shape of the bottom of the rille.

The next step up, using a 7mm with the 3x Barlow, was 606x--and here the softening had gone too far. I was seeing less detail at the bottom of the rille than at 471x.

I have decided that the Losmandy GM-8 mount is just a little light for the 8" reflector. If I adjust the focus, it can take a couple of seconds at least for the mount to stop shaking. There might be a CI-700 or a G-11 mount in my future, especially if I can find one used and cheap.

No comments:

Post a Comment