I have an f4.5 13.1 inch old coulter dob I recently purchased. The other night I used a 9mm ultrawide Pan-Optic ep for observing. I noticed that as I went beyond focused in one direction on a star, the star seemed to draw a white line sideways (horizontal). The more out of focus in one direction, the longer the line got. Then when I focused the other way (sorry I don't know the term for being out of focus on either side of focused), the star drew a line vertically. Also focusing in general with the 9mm seems to be a chore. I saw a diagram of this in a book, but at the time didn't know I have this problem. Now I don't remember where I saw the explanation. Is this just a simple collimation problem? If someone knows what causes this and how to correct it, I would appreciate some help. Thanks
Need help on a focus problem
Started by david101, 03/09/2003 03:24AM
Posted 03/09/2003 03:24AM
Opening Post
Posted 03/09/2003 03:37AM
#1
It sounds like your scope has astigmatism. One one side of focus a star image will be a line, on the other side of focus the line will be at 90 degrees to the first side of focus. Less severe cases will show slightly oblong out of focus images, again at 90 degrees to each other on opposite sides of focus.
Chances are that it is due to improper mounting of either the primary or secondary mirror. An 8" Coulter owned by a friend had the secondary held distorted by the adhesive holding it and his scope showed astigmatism. We had first remounted the primary in a conventional cell (no adhesive) and found the astigmatism was still there. We rotated the primary mirror 45 degrees and the astigmatism did not rotate with it. We remounted the secondary properly and the astigmatism vanished.
Chances are that it is due to improper mounting of either the primary or secondary mirror. An 8" Coulter owned by a friend had the secondary held distorted by the adhesive holding it and his scope showed astigmatism. We had first remounted the primary in a conventional cell (no adhesive) and found the astigmatism was still there. We rotated the primary mirror 45 degrees and the astigmatism did not rotate with it. We remounted the secondary properly and the astigmatism vanished.
Posted 03/09/2003 01:47PM
#2
Hi:
Alan's comments are right on, but there is a couple of more things you may want to check before you start taking mirrors out, etc. Try turning the eyepiece in the focusser to see if the astigmatism is in the lenses of the eyepiece. Also, try switching eyes! You may have astimatism "built in" that you are unaware of. I used to have a 13.1 myself, and found it to be VERY critical of mirror "tightnesses" as Alan mentioned...
Good Luck...
Pat
Alan's comments are right on, but there is a couple of more things you may want to check before you start taking mirrors out, etc. Try turning the eyepiece in the focusser to see if the astigmatism is in the lenses of the eyepiece. Also, try switching eyes! You may have astimatism "built in" that you are unaware of. I used to have a 13.1 myself, and found it to be VERY critical of mirror "tightnesses" as Alan mentioned...
Good Luck...
Pat
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