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"Copy of post I made on Cloudy Nights":
The Paracorr's superiority over other coma correctors is its ability to work with all eyepieces from all companies and still give excellent results. Only one distance from the focal plane of the eyepiece to the coma correcting lens results in the best coma correction, and since eyepieces vary in their placements of this focal plane, you need to be able to adjust that distance for best results. Only the Paracorr is adjustable in that parameter.
Photographically, you only set the distance once and leave it. Hence, a less-expensive coma corrector, like the Baader or the Lumicon, will work just fine.
Technically, a coma corrector is engineered to best correct a particular f/ratio, but there is some flexibilty, with longer f/ratios being better corrected than shorter ones.
Perfect coma correction is when coma is cancelled to the edge of your widest field eyepiece (i.e.the coma is smaller than the size of the Airy disc in that scope).
Which brings up the issue of field width. Coma increases linearly with distance away from the center point of the focal plane. If the eyepiece has a narrow field that does not extend very far from center, then much less coma will be visible than if the eyepiece has a wide field extending quite a ways out.
In particular, eyepieces with large field stops, like a 41 Panoptic, 55mm Plossl, 31 Nagler, 35 Panoptic, etc. will show more coma at the edge of the field than a 10mm eyepiece with a small field stop. True, the comatic image will be magnified in the higher power eyepiece, but ultimately it is the field stop of the eyepiece that counts.
Many people with f/5 scopes do not feel the need for a coma corrector, I believe, because they are using narrow field eyepieces with relatively small field stops. Because the comatic star image at the edge of the field of a 35 Panoptic is 81X as large as a star image in the center of the field. It may not appear that huge because some of it is invisible to a particular eye, but it is measurably there.
All people who put a Paracorr into their scopes see an improvement in star images. How much is that worth? I see people spending hundreds of dollars to get better eyepieces with less inherent astigmatism and better transmission. Why not spend the price of just one premium eyepiece to improve the image from EVERY eyepiece you put in your scope?
When I saw the improvement in my scope's images, I almost cried. True, it extended the focal length 15%, which ticked me off. And it meant my 35 Panoptic went from a 1.4 degree field of view to a 1.22 degree field, but that was a reasonable sacrifice to get star images tightly focused all the way to the edge instead of only the inner 10%.
Coma correction has a place all the way to f/8 if perfect star images are the goal. People spend a lot of money for a premium mirror from someone like Carl Zambuto or Steve Kennedy in order to get better, more tightly focused, star images. It seems a shame to sacrifice that by not using a coma corrector.
As for the visibility of coma, it gets more visible if:
--the sky is darker (dark skies allow the fainter outer edges of the comatic star image to become visible)
--the aperture is larger (same reason as sky darkness)
--the viewer has better dark adaptation or genetically better night vision (more sensitivity to low-light aberrations)
--the field of view is wider
--the edge of field star images are carefully compared to the center-of-field star images
--the eyepiece has inherent astigmatism or field curvature (these both bloat star images--add them to coma, and the edge-of-field images get ugly)
--you expect the edge of field star images to be perfect
--your other scope is a long focal length scope and you can regularly compare.
--the f/ratio gets shorter (shorter f/ratios have more coma and coma starts closer to the center of the field)
I observed all night last night at a site with 11 other telescope users. 7 of them were dobs; all were larger than my 12.5".
Coma corrector use was 6 out of 8 dobs. Of the remaining 2, one wanted a Paracorr but couldn't afford it, and the other one had an f/6 dob (long tube!). The point is, that being exposed to a Paracorr by comparing your scope with and without one, or seeing its effects in other scopes (by viewing regularly at a place where a lot of others view) tends to result in the observer seeing its benefit. I've watched the percentage of Paracorr use steadily climb over the last few years, and, except for the economy dobs where justifying a $300 purchase might be difficult, I think it will eventually become nearly universal for users of f/5 and shorter scopes.
Does it cause light loss? Yes, about 3% on axis, but the star images elsewhere in the field are so much better focused that, overall, fainter stars are visible with the Paracorr.
Does it distort a planet's image as it nears the edge of the field? Less than without one.
One tip to make a Paracorr easier to use: put a number label on each eyepiece that corresponds to the particular setting of the Paracorr for that eyepiece. Number the stops on the adjustable top of the Paracorr with the same labels. You'll never need to refer to a chart or memorize which setting applies to which eyepiece ever again.
Hope all that digression helps.
Don Pensack
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