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Re: Newtonian Astigmatism

Started by jonisaacs, 10/02/2003 09:37PM
Posted 10/02/2003 09:37PM Opening Post
Several months ago I purchased a Star Hunter 10 (OPT) via astromart that had rather severe astigmatism. This is essentially the same scope as the Hardin Deep Space Hunter.

The problem was that the secondary was "pinched" and when this was relieved the optics turned out to be very good. I have heard that this is not uncommon with these scopes.

The secondary mirror slips into the plastic holder and there is a cylinder of foam that provides a spring action to hold the mirror in place. What had apparently happened with my secondary was first assembled, some of the foam had gotten of the foam had gotten out of its hole and was trapped behind the mirror, distorting the secondary length wise and causing astigmatism.

All that was necessary was to remove the secondary from its holder and reinstall it correctly.

I would suggest this as a first try as it is quite easy to do and apparently is somewhat common.

Posted 10/03/2003 09:19AM #1
Before doing anything, be sure to rotate your eyepiece in the focuser--does the astigmatic pattern move? If so--the astigmatism is in the eyepiece.

I think this will be unlikely--but it is the easiest thing to check.

And--as others have said, a mirror can have astigmatism--what it does not have is chromatic abberation.

Posted 10/03/2003 11:30AM #2
These scopes, available under different brand names (DSH, StarHunter, original Orion XTs, etc.) are all made by Guan Sheng in Taiwan, and have been notorious for too tight secondary holders. The foam Jon mentions is one possible cause, but in my case (and many others) the plastic secondary holder is just too tight.

It's an easy fix. Wear latex gloves to avoid getting fingerprints on the mirror. Remove the mirror, by prying it out if you have to. Heating the holder with a blow dryer may help it loosen up. Then sand out the inside of the holder with some 200 grit sandpaper until the mirror slips in and out without much force. Examine the inside ring of the holder closely and you can see where the mirror is still scraping against it. Concentrate your sanding in that area.

Once it slips in and out easily, reattach the clip so that it just barely contacts the surface of the mirror. The mirror can't slip out of the holder past the clip, and the (slight) force of the foam on the back of the mirror will hold it steady.

This eliminated any astigmatism from my XT10. It's also advisable to loosen the clips retaining the primary mirror, as they are overtightened to withstand shipping. The rubber should just barely contact the mirror surface. If you can slip a piece of paper between them and the mirror, that's about right.

These adjustments and more are described in detail on the Skyquest-telescopes Yahoo group at:

Jeff Chappell