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I have a revelation (GSO) coma corrector that I use on my 130 pds with a Canon 1000d. I have noticed that I am still getting some coma. My question is, do I need a spacer between the corrector and the T ring. I can't find any definite information on this but I read somewhere you need 75mm between the rear element and the camera sensor

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I think I used a 15mm or 20mm spacer when I used mine during the last Mercury transit.  The results looked pretty good, but there were no stars in the corners to judge it critically.

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According to the specs you need 70 mm backfocus. The Canon has 44 mm and your T ring probably 11, so you will need 70 - (44+11) = 15 mm spacer to get it working correctly.

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