Tuesday, January 23, 2007

Big Bertha's Optical Problems

I mentioned a day or two ago that part of the problem that Big Bertha has for astrophotography is also a problem visually: the focuser that came on it is coarse. In addition, I can't get the focuser to rack in quite far enough for prime focus photography, so I am thinking of buying Moonlite Telescope's CR2 dual rate Crayford focuser. The racked in height of my current focuser is 4.5"; for the CR2, it is 1.45". The extra 3.05" will let me get the focal point well inside the camera body. The downside is that I will need the longest drawtube (the 2.75" one) to get back to where I am now with eyepieces--which means that I will probably need to use an extension tube in the focuser to get most eyepieces to focus visually. I have one, but it is a little clumsy.

However: what effect does the shorter focuser have on the rest of the optical path? I use Dale Keller's newtwin program (a Newtonian optical layout program for Windows) for this sort of thing--and I notice that it tells me that the current diagonal mirror is far larger than it needs to be for the current antique focuser. The diagonal mirror's minor axis is 4.25"; 3.5" is sufficient. The extra aperture both impairs resolution, and light gathering capacity, because it is blocking incoming light from hitting the primary.

With the lower profile CR2 focuser, I could reduce the diagonal to 2.8" minor axis, probably improving both the brightness of images a bit, and improving resolution. I suspect also that the diagonal contained in Big Bertha was whatever the people who made it found available for the lowest price, and a well-made diagonal might improve image quality slightly as well.

One thing at a time: first the focuser (which is the expensive part); then the diagonal (and sell the old one to someone who is building a 20" or 25" reflector). If there are continuing improvements in performance, it might justify replacing Big Bertha's overweight and overly large tube with something a bit more reasonably sized.

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