Roy Troxel said:
I am trying to locate some good technical information (and users' opinions) of Paracorrs. All I know is that they are supposed to eliminate the "coma" effect in low f-ratio scopes. One observer told me they are "mandatory" for use with any mirror under f5.
I searched Astromart forums for "Paracorr", but didn't find anything comprehensive enough. If someone could direct me to a web site or article, I would be most grateful!
Many thanks in advance,
Roy
Roy:
I have several Newtonians ranging in size from 3 inches to 25 inches. Several of them are in the F/4-F/5 range and I "always" use a Paracorr in a scope F/5 or faster. It is worth noting that coma is inherent in a parabolic mirror, a better quality mirror does not have less coma... It depends on the cube of the focal ratio. To understand the value of a coma corrector, one must first understand coma. While many have touched on the advantages of correcting coma at low powers with widefield eyepieces, I find it at least as important at high magnifications.
The diffraction limited or "coma free" field of a Newtonian can be calculated using the following formula:
CFf= 0.022 x F^3 x mm
Some numbers:
F/3 = 0.60mm
F/4 = 1.40mm
F/5 = 2.75mm
F/6 = 4.75mm
F/8 = 11.26mm
These are linear fields of view. It is obvious that there is a much larger coma free field of view at F/8 than at F/4. One also has to be careful here, "diffraction limited" means at that field, the coma degrades a perfect mirror (Strehl = 1.00) to "diffraction limited" (Strehl = 0.80), coma is there...
To compute a linear field of view to the angular field of view we are most used to, you divide by the focal length of the telescope and convert result from radians to degrees. For comparison purposes, I have used a 16 inch Newtonian as an example and included the "coma free field of view at 200x magnification:
F/3 = 1.6 arcminutes, @200x = 5.6 degrees AFoV
F/4 = 3.0 arcminutes, @200x = 9.9 degrees AFoV
F/5 = 4.7 arcminutes, @200x =15.5 degrees AFoV
F/6 = 6.7 arcminutes, @200x =22.3 degrees AFoV
F/8 =11.9 arcminutes, @200x =39.7 degrees AFoV
In your 12.5 inch F/5, the numbers look like this:
Coma free region = 4.7 arcminutes, @200x = 19.8 degrees AFoV
The first thing that is apparent is that coma free region is very small in a larger fast scope, at F/4, it is only 4 times the diameter of Jupiter, if Jupiter is not within the central 10 degrees of the eyepiece, the view is no longer "diffraction limited." With a tracking scope this is less of a problem but with a Dob and widefield eyepieces, it is tiny, tiny.
So, what does the coma actually look like when viewing a planet or double star? What all this means is that the sweet spot is small even with the best eyepieces. Whereas Naglers offer a very well corrected 82 degree field of view, the sweet spot is still only at the very center because of coma.
In the past, coma was not so apparent because the eyepieces of the time, Erfles, Konigs and the like were not "well corrected" in a fast telescope and so the off-axis eyepiece aberrations, primarily astigmatism, overwhelemed the coma so correcting coma was not of important.
But when the Nagler eyepieces that were corrected down to F/4 hit the market, all this suddenly changed, not only were the fields much wider, but the stars were round and sharp so coma now was the major aberration and it could be easily seen. And so Al Nagler took on the task of designing the Paracorr and making it a total solution.
What the Paracorr does is increase the diameter of the coma free field of view by a factor of 6 while adding a Barlow effect of 1.15x, this is huge... In your 12.5 inch F/5, this is what it does:
It increases the coma free field of view from 4.7 arcminutes to 24.5 arcminutes, or about 0.41 degrees, not much smaller than the field of view. If you own a 9mm Nagler, it increases the magnification from 176x to 203x and the coma free field from the central 17.5 degrees of the eyepiece to 104 degrees AFoV, this is greater than the 82 degree AFoV of the eyepiece, the sweet spot is now the entire field of view of the eyepiece.
At the eyepiece, the transformation is obvious, once only clean and sharp in the center of the field of view, is now at it's best nearly right up to the edge. And of course at low magnifications, the coma, most apparent as the stars with fan tails as one moves away is decreased by that factor of 6, the big widefields possible with eyepieces like the 31mm Nagler are now "refractor-like", sharp right up to the edge.
So... I hope this helps explain the reasons why those of us with fast Newtonians invest in a Paracorr to go along with a collection of Naglers and Ethos's. The first step are the eyepieces that are corrected to F/4, the finishing touch is a Paracorr, these go hand in hand and make today's fast Newtonians amazing performers.
Jon Isaacs
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