Help please: Coma Corrector Owners

[rfdesigner]

Hi

I am trying to sort out my imaging train, and could do with a couple of dimentions that simply don't seem to be published.

I was about the purchase an SXOAG, with adaptors for use with a DSLR until some time next year when a 8300 based camera will appear (there are some advantages to getting older).

Now both my 30D and the KAF8300 sensors have more or less the same dimentions.. (yes different back focus distances.. but that's relatively easy to work around)

What I'm trying to plan in is zero.. or very nearly zero vignetting.

What I've realised is that a t-thread has about 35mm internal clear aperture, where as I believe I will need 40mm at the top of the 2" focusser in order to avoid vignetting at this point.

So... I'm concidering not going with T-thread at the scope side of the OAG, but with something a bit fatter.

Question is, as I'm going to require a coma-corrector, sunk into the focusser.. what is the internal diameter of these.

So far as I can tell there are three contenders:

RCCI (which I understand would require the scope to be strengthened to maintain collimation as it's very picky)

MPCC

SkyWatcherComaCorrector

So does anyone know what the internal clear aperture of any of these is, and what threads do they present to cameras/OAGs

Oh and the scope is the 12" F5 newt which has a 2" JMI Crayford focusser, and will get a new secondary to give zero (or no additional) vignetting when the time comes.

Thanks in advance.

Derek

[Bukko]

I have built my imaging train to ensure the OAG prism is on the long side of the CCD, so minimising its effect.

It is important that the OAG is after the corrector, as the guide camera also needs corrected stars.

I am not sure if having a bigger secondary and/or 3" focusser would help. with an f5 ratio, it will cast a longer light path than my 12" f4 does and my vignetting is tolerable...

If you really want me to measure the lens diameter of the MPCC, then please let me know.

Hope this helps.

Gordon.

[Bukko]

Sorry, Derek,

the message took so long to prepare, the system logged me out. I only had half saved, so the above makes no sense.

I have a similar setup to your proposed with the MPCC. It fits inside the T-thread system, so I think you would not benefit from having a bigger feed into the coma corrector. Unless you went to a 3" system, with an appropriate sized corrector/OAG, etc.

I see vignetting, but using flats, it is easily corrected in software.

Gordon.