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Mak-Cass Image Circle

Started by ebuehrens, 12/31/2009 09:52AM
Posted 12/31/2009 09:52AM Opening Post
I have a 10" f/12.5 Mak-Cass with Intes Deluxe optics and Matthias Wirth tube & mechanicals. Central obstruction is 25%. Great visual instrument but now I would like to try imaging with it. Can anyone tell me - from calculation or experience with similar instruments - what the effective image circle is for this scope? I'm considering using it with the AP .67 reducer, trying to decide what CCD camera to acquire and would like as much useful field of view as possible.


Posted 12/31/2009 10:13AM #1
Quick rule of thumb - multiply the aperture of the telecompressor by the power of the telecompressor to get the illuminated image circle.

Posted 01/06/2010 08:46AM #2
Generally speaking all Mak Cassegrains that are optimized for best visual performance by implementing smallish co. and narrower center baffles have a very restricted fully illuminated fields, that is the main tradeoff with this design. Reducers can not perform miracles as the restrictions to the illuminated field occurs at the front end.

Take a high contrast image of cloudy sky with a digital camera, preferably with a full frame sensor, and see how much visible light fall off there is at corners of the field and decide what would be the optimum CCD chip size, or acceptable image circle, that clears most of the corner fall offs.

The fully illuminated image circle of my TEC 250/20 is roughly 1mm in diameter or about the size of Jupiter at the focal plane, visually the light fall off is not at all visible at moderate to high powers, the co is 22%, yours being 25% is fairly close to mine.