[APML] Re: Newton-cassegrains: opinions sought

[Emmanuele Sordini <vega@bloomingstars.com>]

Rick Krejci Scrive: 

> I just acquired a CN212 and use it for digital photography.  Here is a Canon
> 10D image using newtonian mode 
> 
> http://www.ricksastro.com/Gallery/htm/10D_M42.htm 
> 
> It is slightly cropped and still shows some elongation in the Corners.  I'm
> using the standard (if you can call anything standard in Tak land) Wide T
> ring, so I'm assuming the spacing from the corrector should be OK.  I may
> need to experiment some. 
> 
> Here is a SXV-H9 CCD image in cassegrain focus with their f9.9 reducer:
> http://www.ricksastro.com/Gallery/htm/SXV_m1_cn212cat.htm

Cogratulations on your shots! All in all, it looks like a very promising 
instrument. 

> 
> Thus far, after about a month of ownership, I like it quite a bit, but I
> might be concerned if I were using it for a full 35mm frame in newt mode.

Maybe, could a coma corrector/field flattener solve the problem? 

> The optics are fantastic, and the focus shift is minimal.  Collimation is
> very solid when changing optics and it takes about a minute.  I've had an

Yes, but anyway you have to tinker with the adjustment screws in order to 
achieve perfect collimation, don't you? And that operation must be performed 
with a star (as I was told any collimator will only get you close, but not 
dead-on), and only under good seeing.
IMO, the process is too dependent on atmospheric condition, so that I fear I 
could end up never switching between the two optical configurations. 

> 8", 10" and 11" SCT, and I would choose the CN212 over any of them, mostly
> for it's flexibility and pinpoint optics.

What about a slow (say f/9-f/10) Ritchey-Chretien? Could it be a viable 
alternative? 

Thanks,
Emmanuele 
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