Astigmatism

Hi All,

Last night while attempting to view Mars I noticed the image flares out one way when short-focused and 90 deg rotated when long-focused. Ah-ha! I thought, astigmatism. Then I wanted to find out where it was coming from. So I shuffled my feet and turned my head until I was looking through the eyepiece 90 deg rotated and refocused. Same problem (did not rotate with my eye). Next I carefully rotated the eyepiece 90 deg and tried again. Same problem (did not rotate with my eyepiece). I have a 10” dob made from an old Mead Starfinder OTA. Magnification was 120X (10mm Plossl) Any suggestions? (I have tried very hard to collimate using a barlowed laser, and I think it’s pretty close). TIA

Darryl

Darryl Simmons said:

Hi All,

Last night while attempting to view Mars I noticed the image flares out one way when short-focused and 90 deg rotated when long-focused. Ah-ha! I thought, astigmatism. Then I wanted to find out where it was coming from. So I shuffled my feet and turned my head until I was looking through the eyepiece 90 deg rotated and refocused. Same problem (did not rotate with my eye). Next I carefully rotated the eyepiece 90 deg and tried again. Same problem (did not rotate with my eyepiece). I have a 10” dob made from an old Mead Starfinder OTA. Magnification was 120X (10mm Plossl) Any suggestions? (I have tried very hard to collimate using a barlowed laser, and I think it’s pretty close). TIA

Darryl

I suggest checking for astigmatism with a star. The behavior you describe sounds typical of a Newtonian that is still cooling down.

Check a star image just inside and outside of focus, with about three rings visible. Is the Fresnel pattern round on both sides of focus? The closer to focus you are, the more sensitive the test.

Clear skies, Alan

Well, I will add my $.02. If the scope is using the secondary from the old Meade as well as the primary, then the secondary might be the problem. If it is pinched in the holder for instance, then it can give this type aberration.
Also, if the problem is temperature related, much better at one temp than another, the secondary might be the problem.
I have seen this before and by remounting the secondary I did get resolution with one scope, but with another the secondary had to be replaced as it was effected by dropping of temperature due to internal stress I believe.