I started using my Vixen R200ss telescope (760mm, F3.8) a few days ago and I am having trouble with seeing-related star blurring. Here's what I see using my camera's live view mode (20-50 FPS):
- In-focus star: besides the diffraction lines from spider, there are little radial spikes that dance around. The entire star moves around a few pixels worth with both a high-frequency and low frequency component.
- Bahtinov mask: I can see the center line move back and forth, which means the focus is changing quickly
- De-focused image with secondary vanes clearly visible: When I wave my hand in front, I can see the heat waves, so I sort of have an idea what they look like. The image has brightness fluctuations that quickly move around, a more overall shimmering distortion like it is under water, and finally there is an overall translation of the image.
I can't get near anything good enough to see the airy rings, but the spacing for those is not much more than my pixel size anyway, so...
I installed two fans on this telescope: One is about an inch above the primary mirror, and small enough to just hide inside the shadow of the secondary (I think...). So this is supposed to "scrub" the boundary layer off the primary, but all the above mentioned effects are still present after 2+ hours. Maybe a little better, but hard to tell.
There is another fan that is mounted to the back of the telescope, blowing air in. There is only a narrow annular gap between the frame where the alignment screws are, and a thin sheet metal plate. So in short, air can only go in with some resistance and does not hit the back side of the primary, but only creeps up the sides. The airflow is only enough to lift a sheet of newspaper 1/2" off the front of the scope. Should I drill some holes in this plate?
The fans didn't seem to cure the problem. The only obvious difference is that the defocused heat waves now move quickly instead of slowly.
How can I tell what is the cause of the star movement? Whether it is still due to primary mirror boundary layer, or tube currents, or heat wave directly in front of the telescope (I have a cooled CMOS camera there shooting out warm air, but I also temporarily put a dew shield / hood on the telescope with no difference), or maybe rising heat from the concrete the mount + tripod are on, or maybe just something high up in the air. I suppose I can also compare with my 600mm camera lens, but I don't wanna have to dissemble the imaging train to put it back on the lens. The images from the lens seemed to have smaller stars anyway.