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coma in fast newtorian

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The scope is designed to operate for objects on the axis, therefore anything off the axis is striking the mirror at the wrong angle to come to the same point as light striking at another position on the mirror. The design criteria for the mirror is only for on axis objects.

The mirror is the wrong shape for light to come to one focal point anyway. A parabolic profile is good, a hell of a lot better then a spherical surface, but not the ideal. So even on axis the image formed by the mirror is not a point. The faster the mirror then the greater this error is.

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If you look at this uncropped image at full size you can see that the stars in the corners of the image are a wierd, blobby shape but the stars in the middle 50% of the image are nicer. That's coma.


Careful framing and cropping of the image can significantly reduce the problem (see the final image here).


M3 globular cluster by porkyb, on Flickr

A coma corrector for around £150 should sort it out. I think that will be my next astro purchase.

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All Newtonians produce coma off-axis. Fast ones produce more coma. As I recall, coma is inversely proportional to the square of the focal ratio. For this reason it gets bad fast. Let's step through this stuff one point at a time.

First of all, what does it look like? Coma get progressively worse the further from the optical axis (field centre) you go. Coma causes stars to look like little comets whose tails streak away from the field centre. Here's an example: http://starizona.com...or-P3230C0.aspx Note that coma is sometimes confused with eyepiece astigmatism (which is also worse at the field edges). Usually, however, astigmatism causes stars to look like ovals or sea-gulls (scroll down here: http://stargazerslou...stron-x-cel-lx/). Typically, unless you have really good eyepieces, astigmatism will be worse than coma.

So much for what coma looks like. What causes it? To answer this you really need to look at the ray diagrams. First, let's consider a spherical mirror and why it's no good. Take a look at this link: http://farside.ph.ut...es/node136.html The image at the bottom of the page shows that even on-axis, rays from different parts of the mirror don't come into focus at the same location. This is called spherical aberration and is fixed by using a mirror with a parabolic, not spherical, profile. On this page is the ray diagram for a parabolic mirror, note that rays from all locations on the mirror come focus at one location: http://www.a-levelph...oncav-mirrs.php So that's why we want a parabolic mirror.

Both of the above ray diagrams show what happens to rays that come from directly in front: those rays are focused to a near-point on-axis. Note, though, that rays coming in a different angles are focused off-axis. So the image is formed on what is known as the focal plane. I could only find a diagram for a refractor that shows this, but it works the same with a reflector. see here: http://www.telescope...t/eyepiece1.htm See the dotted line before the eyepiece that says "object image" That's the focal plane (image plane, more properly). See how the two sets of rays entering the objective at different angles come to focus in different locations? That's how an image is formed on a CCD (which would go in the image plane).

So I've told you that coma occurs off-axis and I've shown you what the off-axis light is: it's light from different angles that corresponds to different locations in the sky. But what's coma and what's it doing off axis? Now we're ready to see what the off-axis ray diagram looks like. Here it is: http://www.astro.vir...coma-in-out.gif See the problem? Rays from different parts of the mirror come to focus in different locations. The consequence is that a star, say, won't look like a point but a comet-shaped blur. If you make enough of these ray diagrams for different angles you will see the effect is worse the further from the axis you are.

What to do about coma? You can't change the shape of the mirror: a parabola is the only optical shape that will bring in all on-axis rays to focus at the same point. So two options present themselves. Firstly, choose circumstances where coma is negligible. You can choose a focal ratio of f/6 or use higher powers at lower focal ratio. If you use lower powers at focal ratio of f/5 and below, you will just have to ignore the coma (or, more likely, the eyepiece astigmatism that likely swamps it). The second thing to do is add inverse coma to cancel it out. This is what a coma-corrector, such as the Paracorr, does.

I hope that's useful.

Edited by umadog
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