[codemonkey (Lee)]

Amongst all the overcast weather we've had lately, I've had brief opportunity to try and solve a couple of issues I've been having. One was focus related and in doing this I've noticed some odd results.

First is what I believe to be astigmatism. The star shape is elongated one side of focus, and the elongation switches direction (90 degrees) on the other side of focus. I'd seen what appeared to be off-axis astigmatism on this in the past, and is one reason why I'm shooting with the QHY178M -- tiny sensor meant the problem was outside the FOV. This is different and covers the frame.

The second is some asymmetry in the size of the CO inside and outside of focus. After analysing the same star in an image either side of focus, I determined the % of the star shape (length, not area) that represented the CO:

Outside of focus:
X-axis: 45%
Y-axis: 39%

Inside of focus:
X-axis: 54%
Y-axis: 54%

I've attached a couple of gifs, one demonstrating what I believe to be astigmatism. The other I believe indicates under correction.

So I guess I'm after three main things in writing this post:

1. Confirmation of my interpretation -- am I seeing what I think I'm seeing?
2. Thoughts on the significance of these issues? No mirror is perfect and neither is the environment they're in. Are these tests sufficiently poor to cause concern?
3. Thoughts on causes / corrective measures

I've read that a mirror that is hot and cooling to ambient can cause over correction. I appear to have the opposite problem. My ROR isn't well insulated and I recently noticed that condensation was forming on the primary some mornings, so now I leave the primary mirror fan running 24/7 which has effectively resolved that issue. Maybe my mirror's actually colder than ambient due to this, thus giving me under correction rather than the typical over correction?

I've also recently noticed collimation shift over different altitudes. After acquiring a laser I see this stays perfectly in place as I move the scope (relative to the center spot on my primary), but viewing through a cheshire I can see it move by what appears to be a couple of mm between zenith and horizon. Seems to be due to primary moving around... can't win! If you restrict the movement, astigmatism, if you give it a tiny amount of space, collimation shift. Anyway, my reason for mentioning that is... is it possible the collimation shift interacts with the coma corrector (paracorr type 1) to do weird stuff, like the issues I've presented here? Or must this problem be in the mirror or its housing?