Considering coma when buying a barlow

[smolloy]

Hi all,

I spent part of my observing time last night exploring the quality of the scope rather than actually observing, and I noticed that my scope (unsurprisingly) suffers from very noticeable come for the outer third of the view when using the 10 mm EP. My scope is a 250PX F/4.8 Dob, so this isn't all that surprising.

I am considering adding a 2X Barlow to my collection, and I was wondering what this would do to the coma. Since there is a region of the FOV where the diffraction limit is greater than the coma, I would imagine that halving the FOV with a Barlow would shrink my view to the region where coma is unnoticeable, but something feels very wrong with this line of reasoning.

Perhaps it isn't strictly related to the FOV, but is dependent on the magnification of the EP -- with larger magnification making it worse?

In other words, will the purchase of a 2X Barlow automatically require the purchase of a coma corrector?

Thanks,

Steve

[John]

Coma is produced by the scope. Eyepieces, especially low cost wide angle ones used in fast scopes, produce astigmatism. A coma corrector won't cure the eyepiece astigmatism.

You probably need to invest in better quality eyepieces that don't produce so much astigmatism rather than invest in a barlow. The BST Explorers are an improvement, Celestron X-Cel LX's better still.

[Trull]

Have you thought about a paracorr?

[smolloy]

Thanks guys.

I understand that coma is entirely due to the parabolic shape of the primary, but I think you're right to point out that there are other sources of error that could be more important. When viewing tonight, I paid attention to the shapes of stars as they drifted out towards the limits of the FOV, and I only really noticed the comet-like shape appearing very close to the edge. Before that point, false-colour artifacts were very noticeable especially in big, bright, Jupiter.

False red was visible on the edge closest to the centre, while blue was visible on the opposite edge, and the strength of this effect was proportional to the distance from the centre of the FOV.

This definitely doesn't sound like coma to me, and the fact that I am seeing false colours proves your point that it is the EP that is the problem.

I need to do more learning methinks

Thanks,

Steve

[Chris H]

How much chromatic aberration and in particular coma you are prepared to put up with is down to personal choice, the less you want to see the more it is going to cost you. If you don't mind the outer 10% of the field of view to exhibit coma then you will be happy with the performance of most ep's, if you want to see as little coma as possible/none then your looking at Televue ep's (all of which are tested down to F4) and if that is not enough then you'll need some sort of coma corrector for example the Parracorr.

[John]

The irony is (if you can call it that) that lower cost wide field eyepieces produce astigmatism which often masks the coma that the newtonian design produces.

So you invest in premium eyepieces which don't produce astigmatism in fast scopes, eg: Tele Vue Naglers or Pentax XW's or similar.

Then you can see the coma that the scope is producing !. So you need to invest more £'s in a coma corrector.

It's a bit of a "money pit" at times isn't it

[smolloy]

It really does seem like this hobby could cost me more than my house

Given that the aberration isn't visible at the centre of the FOV, then I don't think it is astigmatism, since that should be visible everywhere, right? Given that it is an off-axis phenomena, perhaps it is more likely to be related to excessive field curvature?