[bratislav (Bratislav)]

Boy oh boy.

It looks like we have had lots of "entertain" from Don so far, but not much of "inform" and "educate" despite vailant efforts of Janoskiss.
I guess I'm to blame as well as so far noone apart from Steve actually understood what I was talking about.
I can't really make it any simpler, I'm afraid if anyone wants to know something about optics I can only suggest a good book like Rutten and Van Venrooij or Conrady.

Let me but in one more time, just to try to clear the confusion.

BTW, I don't really need to talk to an "optical designer friend" - I have designed coma correctors (single element - subaperture menisk, two element - Ross family, and three element units - Wynne/Gascoigne type) for various systems (sphere, paraboloid, hyperboloid, Dall-Kirkhams and Schmidt Newtonians). I've been designing optics for decades, and when I say something, I usually know what I'm talking about.

So let me offer something even simpler to understand. I can show you spot diagarams of two optical systems using the same coma corrector.

Although I have in my posession reverse engineered data for Celestron/Baader and Lumicon units, I don't know the prescription for Paracorr (as noone volunteered a unit to be pulled apart, and I'm not going to shed $$$ just to try to convince someone who doesn't really want to learn anyway).

So, here I will use a well known coma corrector published several times - in May '85 Sky and Telescope (under a peer review of late Bob Cox) and Telescope Optics book (again peer reviewed by R & vV). It is designed for 1200mm focal length (author designed it for 12" f/4 scope).

Let's see what Zemax says about two systems. 100mm f/12 (reverse coma anyone?) and 100mm f/4 (which should be spot on if we are to believe Don).

Vertical bar on the left gives you a reference size (200 microns). Black circle is Airy disc size. I've used monochromatic light not to confuse people even more with polychromatic aberrations. Five spots are spread as to cover field from dead on axis to approx 14mm off axis (very edge of the 1-1/4" size eyepiece and approx. corner of a typical DSLR chip).

A 100mm f/12 scope (attachment 1)

{{{ I'm hopeless, can't get the attachment to be where I want them }}}

Reverse coma ? Hmmmm .... Sure. Want to see f/16 ? F/20 ? It only gets better, trust me.

100mm f/4 scope, focused for best images on a flat focal surface (attachment 2)


Hmmm. But it should be perfect, it is f/4 after all !!!! And those off axis images look like -er- coma ????
Don't let me show you what a 50mm f/4 looks like.