I will probably be fine with with a little coma on the edges with the 28mm UWA but if it gets bad I will consider the GSO coma corrector. I have heard that this coma corrector does not come ready to use and you have to space it correctly with what ever you have. Are there any coma correctors that are adjustable?
werttt,
Yes, the GSO coma corrector has no simple way to adjust it. I mean, it's very easy to slide any 2" eyepiece up or down in a focuser. There's the bottom of it, of course, and then you can't move it down any further, but if you've ever put in an eyepiece into your scope and it won't focus due to loss of back focus (and you don't bother with an extender), you can simply slide the eyepiece out of your focuser a smidge until it focuses (assuming it was almost focused when you ran out of back focuser space). The same principal works with the GSO coma corrector. This is, however, rather tedious. And it gets even more tedious and crazy when you realize that other eyepieces have different settings, so the AT 28 UWA may use a different point than your 18mm 1.25" eyepiece (for example), or 12mm, etc, etc. For this reason, people who are seriously committed to a flat, non comatic view, will usually end up with the Televue Paracorr, because it has an adjustable top that one can refer to (assuming you've kept up with the various values), and adjust it for each eyepiece, or each eyepiece line. I believe parfocal eyepieces will have the same setting.
This is one of the reasons I, like John Dobson, prefer F/6 newtonians, and I own two of them. I have one F/5, a 130mm, btw, but I don't generally bother with the GSO coma corrector, which I own, but haven't found myself that concerned with. But coma, to me, gets exponentially bad as one goes below F/5. Some can tolerate it at F/4.5, others can't stand it at F/6, so, like chromatic aberration, there's no telling how you'll feel about it until you've played with your scope and either come to terms with the coma it has or bought the TV Paracorr to eliminate it once and for all.
This comes down, in my estimation, to the type of astronomer you are, and how much you like to tinker. It's kind of like guys who work on their cars and folks who simply drive them. Most people, of course, fall into the latter category (much to joy of those who do auto mechanics), but if you really want to control things, well, there are coma correctors. But that's how they work. I find coma in F/4.7 scopes a bit unpleasant, more than what F/5 presents, which is also unpleasant, but more like the chromatic aberration I see in my FPL-51 refractor -- it's there, but I'm not gonna spend tons of money to get rid of it. F/5 coma is like that to me, and F/6 is, as John Dobson found, not bad enough to impinge on the enjoyment of the view. But ALL newtonians produce a comatic field, just like all doublet and triplet refractors produce a curved field. The human eye is pretty good about ignoring/improving the view in mind, but I assure you that every F/10 refractor, if used photographically, will clearly produce field curvature stars in its final photographic product (unless one massages them out with software). The same is true with coma. I've noticed coma in an F/8 newtonian -- it's there. But I also don't find it bothersome down through F/6. Personally, I can tolerate it to F/5. At F/5, yes, it's getting pretty noticeable, but below that, well, I wouldn't want to own a scope like that myself, even tho I know for a fact that many 10" F/4.7 scopes have very, very good mirrors and produce better views of many, many targets than my 8" F/6, because, aperture rules. But a person's got to determine all this for himself/herself. There is no such thing as what's acceptable to anyone but the standards that that person settles on -- no one size fits all.
So, that's what you got. And the AT28mm UWA (like the Nagler T5 31mm or most any other excellent wide field eyepiece) will produce a giant, well-corrected field, but cannot fix the coma in your scope, and, in fact, because of their huge field, will exacerbate the problem. But the problem originates with the nature of the parabolic mirror, not the eyepiece. The AT28UWA, like the Nagler T5 31mm, works great in anybody's doublet refractor, and refractors, although they have to deal with field curvature, have no coma. The AT28UWA is great at what it does, but it doesn't fix coma. And its price is quite attractive.
Edited by CollinofAlabama, 09 June 2025 - 06:58 PM.
