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To Paracorr or not to Paracorr?

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On 06/02/2020 at 22:39, Don Pensack said:

The Paracorr offers a top with a bit less wobble and slightly better correction of eyepiece field stops as large as 46mm and better correction at f/3-f/3.5.  Other than that, the HRCC would be fine IF you have sufficient in-travel of the focuser to accommodate it.

You should note that the visibility and appearance of coma is related to apparent field.  Use 50° eyepieces in your scope, and you likely won't notice coma.  Use 100° eyepieces in your scope and you will.  Though the linear size of coma changes at the edge of the field with true field (it gets smaller as the focal length of the eyepiece shrinks), the apparent size (what you see) of coma only changes with apparent field. 

Also, note that a short focal length scope will have inherent curvature in the focal plane that will mildly defocus eyepieces that have large field stop diameters.  Another way of looking at it is that coma is made worse in appearance by field curvature, so keeping apparent fields narrow will reduce the ability to see field curvature.

So, suggesting an apparent field wide enough to yield fairly large true fields, as well as avoiding defocus from field curvature, and having just about zero astigmatism, I think I would recommend TeleVue Delites.  You might find you don't need a coma corrector with them, at 62°, but a coma corrector would yield perfect stars from center to edge in them.  Also, they're all parfocal, so would use the exact same setting of the coma corrector for each +/- 0.5mm or so.

Thanks Don, too much to learn and digest from your posts. I just bought myself the HRCC and am waiting for a clear night to test It.


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