Orion XT-12 and Coma

Hi,

I have been looking at various options for a large aperture, though portable scope and one option is an Orion XT-12. My question relates to coma. I have heard it said that large aperture scopes like the XT-12 suffer from coma, and thus an additional outlay is needed to correct this, normally in the form of a coma corrector such as the Televue Paracorr. Is this likely to be the case with the Orion XT-12, and are their more cost effective solutions to removing coma than an expense Televue option?

Many thanks,

Malcolm

You could make a blanket statement that any reflector under F/6 will likely have some coma. Whether it is enough to justify a coma corrector is a personal decision.
Floyd

"I have heard it said that large aperture scopes like the XT-12 suffer from coma,"

It's not the aperture, it's the f-ratio. Actually, ALL newtonian reflectors have some coma, at least in principle, but it only gets noticeable at short f-ratios.

At f4.9, some folks would want to correct the XT12, others wouldn't care.

People often conflate astigmatism due to the eyepiece/scope combination with coma inherent in the scope. In viewing through other people's scopes, my conclusion is that fast scopes require high-end eyepieces (simple designs suffer from way too much astigmatism for my tastes at about f5 and lower), but if you use good eyepieces I've never seen coma that was intolerable. Whether it's worth the extra cost is a personal thing.

I'll second what Jim said.

I have an f/4.9 telescope with a larger aperture than the Orion. To me the coma is noticable if you look for it at the edge of the field with low magnification. BUT when I'm using a max field (low magnificatin) EP, I'm either finding an object or looking at a large galaxy or star cluster, which are not critical uses. When you move to higher magnifications, you are looking at smaller FOV which are relatively coma free.

At f/4.5, I start to want a coma corrector sometimes.

HTH,
Kevin

,

I have been looking at various options for a large aperture, though portable scope and one option is an Orion XT-12. My question relates to coma. I have heard it said that large aperture scopes like the XT-12 suffer from coma, and thus an additional outlay is needed to correct this, normally in the form of a coma corrector such as the Televue Paracorr. Is this likely to be the case with the Orion XT-12, and are their more cost effective solutions to removing coma than an expense Televue option?

Many thanks,

Malcolm
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I use a Paracorr with any Newt faster than f/6! One thing i hate is coma.

Chas P.

hi,

i have a 12.5' f5 dob and the coma is not very noticable and i don't use a paracor. In my 17.5" f4.1 dob I do use one.

clear skies,

Larry Citro

Many thanks for this input. Its useful information, and probably I will hold off on a coma corrector until I really do need one. They cost around £316 over in the UK, so its not a trifling amount.

A lot of the discussion went over my head technically. Is there a good book I can get which gives me an introduction to concepts like Coma, F Ratios, focal lengths, fields of view, etc. I've been observing for some years, but am only now looking at spending more on gear, and thus want to be as fully informed as possible.

Malcolm