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Ceph and Cass

Focal reducer/coma corrector

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Need some help understanding these, are they the same thing or produce different results? I would like to be able to take wider field images through my DSLR and also reduce the Coma. Would the skywatcher coma corrector do both?

Thanks in advance

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A focal reducer reduces the effective focal length of the telescope they are used with, hence increasing their focal ratio F# and speed but often at the expense of vignetting the field. To work well they have to be designed to work with a particular type of telescope (refractor, reflector, SCT, Mak) as they have different aberrations e.g. field curvature. A coma corrector corrects the aberration coma found in fast telescopes that use parabolic mirrors e.g. Newtonians. Some change the effective focal length a bit some do not.

Hope this helps. Regards Andrew

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Andrew is right, but if we look specifically at your scope in your signature, the SW 200PDS..

From FLO, this is a 200mm f5 Newt.

A straight coma corrector, like the Baader MPCC, will flatten the field for imaging, reducing the "bubbles" and "tadpoles" seen nearer the edges.

The SW CC will correct for coma, but it also reduces the focal length (and so the f-number). It states it's a 0.9 FR, so the 1000mm fl will become 900mm and the f-no will drop from f5 to f4.5

As Andrew says, you will see an increase in vignetting, so more correction will be required to flatten (different flatten to coma...) the image. You will also widen your FOV, of course.


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There is one nifty bit of kit that both corrects coma and acts as a focal reducer- the ASA Keller ASA-2Korrr.

Are you sitting down?


They are not cheap!

I use one and it cost more than any of my scopes. The plus side is I can get images in minutes that might otherwise take hours.

Sadr, Cygnus- single sub 300 seconds at ISO800 12" F2.9 scope


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