Coma

[Andrew*]

There seem to have been some misconceptions arising about coma recently, and I would really like to put things straight before they get confused.

Coma is an aberration caused by, and inherent in, parabolic mirrors - most commonly found in newtonian telescopes. No matter how well-figured a newtonian mirror is, coma will always be present unless a coma corrector is used.

Coma is seen more at low powers and gets more severe off-axis, i.e. further from the centre of the FOV. Coma becomes more severe in faster scopes. An f/4 scope will show much more than an f/6 scope.

When coma is seen, it will almost ALWAYS result from the parabolic mirror of the telescope itself - nothing else. Eyepieces rarely introduce coma, and, even rarer, correct coma. There are one or two very rare eyepieces on the market designed to correct coma. The only common way to remove coma is using a coma corrector.

I repeat - eyepieces do NOT commonly have excessive coma. When a high-power eyepiece gives a bad view in a fast newtonian, it is because it is not designed to cope with such a steep light cone. Aberrations you see will include astigmatism, field curvature and other things, but rarely coma, and if so it will be masked by the other aberrations.

Conversely, an expensive very well corrected low power eyepiece designed to cope with a very fast scope will still show coma in a newtonian. This is not the eyepiece's fault, but it is inherent in the newtonian's parabolic mirror.

A final point: For visual use, coma correctors are not usually necessary unless you are using a fast newtonian with good, low power eyepieces. I say this because:

- in a poor eyepiece the coma will be mostly masked by the other aberrations present, so a corrector will help very little as it won't correct those.

- in a slow newtonian much less coma will be visible

- in medium to high power coma will not affect views very much

- in a fast non-newtonian there will likely not be much coma

I hope that helps to clear things up. I hope I have my facts spot on and would encourage people to challenge me on this.

Cheers

Andrew

[Andrew*]

I've made some minor edits.

I would like to stress that I don't mean to offend anyone in any way. I just don't like people trying to give advice, but having the wrong idea themselves, and then people going away with the wrong information and passing it onto others!

I only came to understand coma very recently and was under the impression that the eyepiece was responsible for coma. It's not. Usually an eyepiece's aberrations are to manifold to easily identify - I just call it "bad performance" rather than trying to pin down the aberrations responsible!

Andrew

[John]

This is useful stuff Andrew

It's easy to get confused between abberation which is due to the mirror and those caused by the eyepiece - like you, its taken a while for it to sink in with me

I found this useful web page giving some more information and some illustrations:

Got Aberrations?

I guess this means that investing in a coma corrector is only worthwhile if you are going to be using top quality eyepieces - the corrector won't help with eyepiece induced abberations. Thats the theory any way - I wonder what people's real life experiences are ?

[Steve]

Parabolic mirrors do reduce 'spherical' aberrations.

I think the confusion comes from people not understanding the difference between coma and spherical aberrations.

[zhgutas]

An interesting read, Andrew, and really helpful. I certainly do come across this or that eyepiece sometimes blamed for "coma" here and there. And when this is mentioned in an eyepiece review of some sort, it is almost automatically seen as a deficiency in an EP itself, rather than a telescope (usually when the EP's are only compared in one 'scope). Must go re-read some of the reviews with this new info in mind then! Thanks

Marius

[albedo0.39]

Interesting thread andrew - what sparked this line of thought off for you ?

I've only been seriously into astronomy for about a year and discovered pretty early on, that fast mirrors make coma not lenses, so sort of assumed it was common knowledge.

Fact is, when out observing recently with some red hot guru's, I found the value in my mirrrors coma - it can actually help with collimation !

Still, I'd be interested to know what's behind the post.

Cheers

S

[Shibby]

Don't forget about the 190MN, which is supposedly coma free. However, this is really down the to coma correction built in. I'd love one of these things

Maksutov Newtonian - Skywatcher Explorer 190MN DS-PRO Mak-Newt Astrograph

[Andrew*]

Interesting thread andrew - what sparked this line of thought off for you ?

Hi Steve, this post sums it up quite well:

An interesting read, Andrew, and really helpful. I certainly do come across this or that eyepiece sometimes blamed for "coma" here and there. And when this is mentioned in an eyepiece review of some sort, it is almost automatically seen as a deficiency in an EP itself, rather than a telescope (usually when the EP's are only compared in one 'scope).

Basically I don't want people to go off with the wrong ideas about coma and get it confused. People might start buying coma correctors thinking their Moonfish Ultrawides will turn into Naglers!

Andrew

[barkis]

Correcting the figure to perfection, particularly on a fast Newtonian, is very difficult and Time consuming. The corrected spherical surface has to reach the very edge of the mirror, and may well take several hours of localised hand polishing, with lots of testing between corrections, and done in temperature and humidity controlled conditions. Unless your mirror is made of Cervit, or other exotic glass

It's no wonder large top notch optics cost a fortune.

Unless your proposed purchase is a very long focus

Newtonian, then opt for a paraboloid every time. Of course some buyers are governed buy price, which I appreciate.

Ron.

[Macavity]

There are indeed folks who freely (and innocently) talk about "aberrations" but, if you pose the question (As I once did!), many of them cannot describe the VISUAL appearance of such things. Fortunately I found some(one) who could / would!

Being VERY familiar with "astigmatic" Corneas, I could never figure out WHY (how!) anyone would grind lenses with a cylindrical profile. Of course, it's "different for eyepieces" etc.

Personally, I think it no bad thing to study textbook examples of diffraction patterns -

Distance views of electricity pylon struts are of endless fascination here...

[Andrew*]

This is useful stuff Andrew

It's easy to get confused between abberation which is due to the mirror and those caused by the eyepiece - like you, its taken a while for it to sink in with me

I found this useful web page giving some more information and some illustrations:

Got Aberrations?

I guess this means that investing in a coma corrector is only worthwhile if you are going to be using top quality eyepieces - the corrector won't help with eyepiece induced abberations. Thats the theory any way - I wonder what people's real life experiences are ?

As do I, John. I toyed with getting one for my f/4.5 dob, but concluded that without really good eyepieces, there's probably not much point as I won't see much difference... Open to corrections of course

I've just checked out that website - excellent resource. I wish I'd found it a year or two ago!

Cheers

Andrew