So, I finally figured out the most crazy problem I have been having. Let me set up the story.
The last month or two, I have been having horrible stars in my images. They would usually be elongated from the lower left corner to the upper right corner across the entire frame. Sometimes, they would be stretched the other way. Rarely, they would be perfectly round. I was racking my brain, constantly fiddling with collimation, trying to account for sag and tilt in the focuser, and messing with guiding settings and PEC. I just could not figure it out.
However, one night turned out to be cloudy while I was out, so I decided to play around a little bit. Still super frustrated with my stars, I decided to slew over to Vega and defocus to see how my collimation was. I still didn’t trust myself or my cheap Svbony laser, although I had not had problems like this just weeks ago. The collimation looked good to me: the secondary looked pretty concentric to me in the defocused star image.
But I was curious about the tube currents that were produced by the dew heater I have wrapped around the secondary holder. Check out just how bad it is:
I definitely figured this probably wasn’t a good thing, but I didn’t know any better. I kept blaming focuser sag and collimation.
After researching the problem with my stars, all sources pointed to astigmatism. This is what makes the stars elongated outside of focus. In particular, the stars are elongated one direction inside of focus and are elongated orthogonally outside of focus.
So what could cause this? My mirrors are only a few months old, and I haven’t done anything different to them. Nothing much has changed with the scope at all over the last couple of months.
Last night, it was cloudy again such that I couldn’t image, so I spent a good bit of time playing around again. I watched intently an autofocus operation.
Is it just me, or do those defocused stars look really uneven? All of the light seems concentrated on one side of the Airy disc. Well, while wrestling with my thoughts in bed, it clicked. It has to be the tube currents!
So, I just turned the dew heater off on the secondary. And what do you know, the defocused stars are evenly illuminated and my focused stars are perfectly round. Score one for me!
I hope the 120mm fan can warm both the primary and secondary enough. It moves a lot of air, I think it will be okay.
