Help a Refractorian reflecting... :)

[LewisM]

Had the Vixen R200SS out the other night - I knew my time to expose would be limited, so f/4 was the answer. Otherwise, I would have used my f/9.2 Fl102S

Anyway, during the focusing routine, I noticed that my airey discs were showing the spider and secondary perfectly - but to one side. Now, I thought collimation on this one was near perfect, and I started to be concerned. I mean SIGNIFICANTLY to one side.

After I finished my focusing routine, I set about my alignment routine. After alignment, I rechecked my focus again by using Markab (not too bight, not too dull), and now the airey disc was perfectly centred.

I don't know enough about the specifics of mirror cool down being predominately a refractor user, but would it most likely simply be differential mirror cooling that caused the initial apparent collimation issue, that settled down once the mirror reached ambient? Seems excessive to me, but like I said, I RARELY use reflectors, only when required.

Thoughts appreciated.

[LewisM]

I know the photo is horrible (I ended up with all of 20 minutes data before the rain came, and most subs were 60 or 80 seconds in suckerholes!), but it at least shows that, I THINK, my collimation is ALMOST spot on (maybe a LITTLE tilt still, judging by the slight asymmetric flare).

Yeah, stretched the living daylights out of the data I had in an attempt to get SOMETHNING - my first photo session in a month thanks to QLD's less than glorious night weather I do NOT like the photo, so no accolades on the photo will be believed 2013 off to a cruddy start

[Peter.M]

May or may not be a collimation issue, the secondary shadow may not be concentric if the star is not at the center of the camera. Then after the alignment it is more likely that the star will be in the center of the frame.

[LewisM]

Good point Peter - had not considered that.

I use APT to focus (and capture control) with the Canon 5D MkII (yeah, I'll go CCD one day!), and use the crosshair overlay as my alignment centre (just like a crosshair reticle) AND my focus tool (I find it MORE accurate than any Bahtinov). I think perhaps the star had drifted slightly that I was using to focus, and would no longer be centred in the field or on the chip, so would illuminate and shadow the secondary to one side.

Thanks for that - might have talked my way through it