Re: [APML] 12' & 14' LX200

[Chris1011@aol.com]

In a message dated 12/17/2002 4:52:27 PM Central Standard Time, MalcS@Blinkinc.com writes:


> From what I've heard, the new carbon fiber tube C11 is more thermally
> stable than metal-walled tubes, and it has suberp optics


Sub erp? 

Personally, I think superb is being overused these days. I recently talked to a fellow ATM who knows his optics, has made quite a few high quality mirrors (one 12" I tested myself). He wanted a quick-look scope, and checked out at least four C9.25's. Yes, indeed the spherical aberration part of the overall figure was quite close enough, but when you are cutting out 38% of the mirror diameter, you really can't go wrong. Any errors in the central portion are masked off, and the remainder does not have much of a chance to deviate from theoretical. What he did find, and what turned him off was the extreme roughness of the wavefront. Microripple out the wazoo. He sent me some knife edge images, and they look like the surface of an orange. High speed polishing does this to the glass surfaces. To get a really smooth result takes hours at slow speed with pitch - translates into lotsa bucks. On top of that, none of them was without a certain amount of astigmatism (no, not on-axis com!
a because that can be collimated out). He finally decided to make a fast 8" Newt for his needs. 

For film imaging, this microripple probably does not affect the image a whole lot, perhaps you can see it with Tech Pan. With high resolution CCDs, the result is that bright stars grow in size quickly. The microripple throws a lot of light far out from the central Airy disc and causes a faint glow that reduces contrast a bit, but also raises the base noise level drastically outward from the central Airy disc of a bright star. 

Let's redefine optics as poor, adequate, good, very good and superb. I would say that any optic that has good spherical correction should be put in the good or very good category, but to be very good or superb, an optic also needs to be very smooth or super smooth, as well as having a reasonably flat field with no coma or astigmatism. SCTs by the very nature of the chosen design have significant field curvature, and off-axis coma. I would therefore not rate them at the very top, even if the optics were super smooth. 

Roland Christen