Sources of astigmatism

Hi all,

I'm finishing a 10" f/8 and before I send this mirror for recoating, I want to make sure it doesn't need any refiguring work.

Basically, I bought a "pig in a poke" off Ebay four years ago. It appears to be an old Cave 10", judging by the type of full-thickness molded pyrex and the mirror mount it came with. (Paid about $125 for both as I recall.) The coating was bad, of course.

Last night the images showed a very small amount of astigmatism on star images. The moons of Jupiter were tiny crosses. I'm using a 1.3"ma Protostar secondary (new)with their curved spider, in their "Blacklite" tubing (great stuff!).

Not having a laser collimator, I used a simple cheshire crosshair collimator, and everything lines up.

So, what are the possible sources of astigmatism in this system? I'm really hoping to prove the mirror good, but want the brutal truth.

Tonight I will try again, and rotate the mirror in its cell to see if the "crosses" rotate. (Not easy with this Univ. Optics mirror mount.)

Many thanks!
Steve House, St. Charles, MO

Hi Steve,
I think the first question must be, are there 4 clips on the primary cell. If there are, could they be pinching the optics? This is how a 4 clip cell would effect a mirror if the mirror was too tight in the cell or the clips too tight on the mirror.
Then, the more common source of astigmatism is the secondary mirror. But being a new one of reputable source, it should not be the problem unless it were pinched.

This is somewhat redundant with earlier comments, but anyway:

Most cases I've seen the astigmatism is from the diagonal
mirror. Even if you have a good one, improper mounting can
cause it to warp and show astigmatism. If mounting with
adhesive, you want something like silicon rubber that remains
flexible. If you use a shell mount with polyester
stuffing, make sure theres just enough stuffing to keep
the mirror in place and no more.

Rarely I've seen problems with primaries. Ideally the
primary would be supported at only 3 points towards the
outer edges of the mirror (three pads positioned at 70%
to 80% of the radius).

If the primary mirror glass was not properly annealed,
you can get into astigmatism problems, too. The figure
of the glass can change over time from heating / cooling,
etc. You might weigh this in your decision whether
to have it re-figured, vs. starting over with a new mirror.
I have seen one 6" mirror from an old Telescopics scope
that will not hold its figure during re-coating. It had
bad astigmatism, so I had it re-figured, only to have it
warp again during coating; expensive lesson.

Air currents in the tube can also make astigmatism.
If the scope was not acclimated for very long, or seeing
was really bad, this is a possibility.

Hello:
In my scopes, the primary source of astigmatism is me. The prescription for my glasses is old so there's quite a bit of residual astigmatism in the images. There are two easy ways to test. The first is to rotate your head, and see if the astigmatism rotates with you (best done with a star a little out of focus). Another way is to increase the magnification, and see if the astigmatism goes away. At higher powers, the exit pupil is smaller, so you're using less of your presumably astigmatic cornea+lens, so the image quality is better. If adjusting the mirror cell doesn't provide relief and rotating the mirror doesn't help, the problem may be you.

Cheers
Mike Connelley

Hi All,

Many thanks to you for your replies. I attempted a star test last night (July 4), and before the clouds set in I thought the image much improved. I will try another test tonight. If it looks good, I'll send the mirror off for recoating and finish the restoration job on this mount and tube. This mount was supposed to be made by Kenneth Novak's brother in Chicago. It is rusty and needs a new paint job. This should be fun.

Thanks again.

Steve House