aberration

Last night was the first decently clear night in a long time.  I set the 20" to cool at around 5pm and started to observe around 9:30pm.  I thought this would be long enough to take care of any thermal issues, but I was wrong...  I still had the strange bar-like aberration when looking at bright stars and brighter double stars.  Each star would be elongated into a bar with two bright points at the end.

I remembered some of the steps in this article. I rotated my head and the bar didn't change.  I tried inside and outside of focus and the bar stayed in the same direction.  I rotated the primary slightly (maybe only 30 degrees) and it appeared to stay the same.  I loosened the secondary adjustment screws and nudged the mirror itself to make sure it wasn't pinched in the holder, and recollimated, but still no change.  I tried the 8" off axis mask, and while the star images cleaned up and I split it nicely (I was viewing STF 1639 as my test object, 1.85" 6.74/7.83), the image was not crisp and soon the bars reappeared.  I thought perhaps the bar was seeing dependent.

I went to M3, which looked really great with the mask on, better than Big Blue.  I was thinking I would see the aberration with all those stars, but I did not -- it must only show on brighter stars.  With the mask on the sky background was darker.  I removed the mask and the background turned to a milky glow, but at the same time M3 was brighter and more stars resolved in the core and more stars glittered on its perimeter.

Next I tried STF 1785 (2.78" 7.36/8.15) at full aperture and 333x, and the stars showed the aberration: each star seemed to have its own pair with a bright line joining them.  With the mask the bright line was dimmed but it looked like a double-double.  Adding the barlow to this (for 667x) the same bar-bell aberration.

Finally I rotated the primary a full 90 degrees, and the orientation of the aberration changed -- this points to the primary as the source of the error.  I don't think the sling or the cell is causing it.  The aberration was still there but it was smaller.  It makes me think the mirror is unevenly cooled because of the two boundary layer fans blowing across the mirror from the back.  I copied this design from Teeter Telescopes but it seems this is not the right solution.  I will try suspending a fan above the center of the mirror to see if that helps -- I need a trip to the hardware store to find the correct wire.

It's disappointing to have this defect (and a surprise since I didn't see it in the previous two years).  But I'm looking forward to fixing it.  The scope tracked really well and it will be a really satisfying instrument to use.  I also plan to put the Big Blue's Feathertouch focuser on it since last night I felt like I needed it.