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Old 03-06-2013, 03:46 PM
E_ri_k (Erik)
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Refractors, Coma, and Field Flattener

Hi guys. I have been doing some imaging with my Vixen ED80. The stars in the corners appear to be affected by coma. I've been doing some reading on the net, some people say refractors suffer from coma, and others don't. I didn't think they did either. Or is it field curvature? Will a flattener correct for it?

Erik
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Old 03-06-2013, 03:51 PM
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multiweb (Marc)
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Yes there are correctors for refractors. William optics has a range of them. You can google the best flattener for your type based on other people experience. For my Orion ED80 I use the WO P-Flat IV.

Last edited by multiweb; 03-06-2013 at 09:11 PM.
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Old 03-06-2013, 03:54 PM
E_ri_k (Erik)
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Cheers Marc, I'll check it out
Erik
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Old 03-06-2013, 08:24 PM
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troypiggo (Troy)
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I've tried quite a few flatteners on the SW ED80 I had. Best I found was the WO FF4 Marc mentions above, and the Orion Flattener (not the reducer). I still have the WO FF4 and not using it. You're welcome to borrow it, see if you like, and if you do we can talk turkey
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Old 03-06-2013, 08:45 PM
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When we talk about refractors and coma it brings up the concept of corrected image circle. That is the diameter in mm of the circle of the image from the refractor that is aberration free.

This varies with different models and is a stat that is worth knowing about your scope. It becomes a problem if you want to use a camera with a sensor with a diagonal size greater than that corrected image circle size.

As pointed out flatteners can correct this but they also have their corrected circle. Again if that is too small for a large sensor you will still get coma in the corners.

Greg.
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Old 04-06-2013, 10:14 AM
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alocky (Andrew lockwood)
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I'm sure someone will correct me if I'm wrong, but my understanding is that coma is unique to parabolic (or the various hyperbolic surfaces in the exotic RC type designs), whereas the distortion in a refractor is spherical aberration.
Either way - field flatteners for refractors and coma correctors for newts are very different designs.
Cheers,
Andrew.
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Old 04-06-2013, 01:25 PM
E_ri_k (Erik)
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Thanks for all the info guys. I understand what you mean about an abberation free zone, Greg. Would be fantastic if I could borrow yours Troy, and test it out.
Erik
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