Big Bertha

Okay, I pulled Big Bertha out this evening. The Foucault testing has not gone well, so I decided to try some star testing.

It would appear that the problem is a turned edge--meaning that the outer edge of the mirror is a bit flatter than it should be. This would explain why the image is less sharp than it should be, and the problem becomes more severe as the magnification goes up. It would also explain why stopping the mirror down seems to improve the image quality.

Finally, an interesting point: if I remove the aperture mask, the focal point changes slightly. This is not surprising; the turned edge has a very slightly longer focal length than it should, and trying to get a crisp focus on the entire image is going to be difficult--and it will be a slightly longer focal length than the stopped down mirror.

I guess the next step is to put a mask on the mirror face itself, and see how much of the edge needs to be masked. If this seems to help substantially, it may be worthwhile spending some money getting the mirror tested and refigured.

The good news, however, is that at low magnification, it works well. One of the few planetary nebulae that I can consistently find, even under light polluted skies, is M57, the Ring Nebula. It sits between the two stars at the south end of the constellation Lyra.

With my 8" reflector, M57 is visible, but faint. At 57x, it is bright but small. At 114x, it is larger, but hard to see, because the faint image is now spread over four times the area.

With Big Bertha, I am gathering more than four times the light--and at 78x, it is bright and not particular small. At 111x, it is pretty big, and still quite bright. Very nice.