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Eyepiece astigmatism


Astronut

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I couldn't pass up the opportunity to get the dob outside tonight despite the high cloud sweeping in, the almost full moon and the local light pollution.

I didn't see much - a quick look at M13 and M57, etc. M13 was not as bright and spectacular as I remember it 20+ years ago in an 8".75 newt but maybe that's down to the bad conditions... or could it be the stock 25mm and 10mm eyepieces supplied?

I was looking at the double cluster with the 25mm which filled the FOV with bright stars and gave me a chance to check out edge of field aberrations. As expected the stars did not appear focussed at the edge of field and as I de-focussed in and out the stars showed a vertical (radial?) line on one side of focus and a horizontal line (tangential?) at the other side of focus. I estimate this effect occured at the outer 20-30% of the FOV and I could only get perfect focus in the central 40-45% (estimate) of the FOV.

Am I correct in thinking this is astigmatism? Are the stock eyepieces supplied really that poor?

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What you saw could be signs of astigmatism, but that condition is usually a localised one and not present all around the field.

Coma is a possibility, due to collimation in a reflector being not perfect, but I guess you will know the difference.

Spherical aberration will be present all around the edge of the FOV.

Steve is right, any good scope is deserving of the best eyepieces one can afford, otherwise it will never perform to it's full potential.

Ron. :D

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Ron's right, I must have been speed-reading and picked up on 'as I de-focussed in and out the stars showed a vertical (radial?) line on one side of focus and a horizontal line (tangential?) at the other side of focus'.

This video shows astigmatism (thru an 80mm refracter):

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Thanks, that video confirms it. That's exactly what I saw. I don't think it was coma - isn't that where the stars at the edge look like little comets with pear shaped tails pointing towards the edge? I couldn't detect any coma but I suspect that's because the astigmatism was hiding it from view.

I'll definately have to get some better eyepieces asap.

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Also illustrated (rather well, though not by me!) here:

http://stargazerslounge.com/index.php?topic=11154.0

For a long time, I laboured under the mistaken impression that this was the same as my KNOWN(!) eyeball astigmatism. Couldn't figure out how anyone would/could make "cylindrical" lenses for eyepieces/telescopes. :)

Seriously though: Pylons, roof tiles, brickwork make quite an instructive target! :D

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