Stars like arrowheads

[Steve14]

Hi all
I have just got myself a new celestron c9.25 xlt which I'm primarily going to use for planetary imaging. I also have a asi1224mc. While it works ok- only shot the moon so far- i thought I'd try my full frame canon dslr and try some deep sky kust for fun. So i purchased a f6.3 focal reducer, got a 30mm spacer and gave it a go. YUK! A single shot of omega centauri revealed all the stars in the cluster were shaped like arrow heads. Not only that, the vignetting was terrible. Got the same result when i used a smaller spacer. Is this an issue with the optics of the scope? Poor polar alignment? Spacing? Collimation? Any advice would be appreciated. With thanks.

[JA]

Hi

With that description ("arrowhead" or seagull like stars) and especially if these defects are towards the edges/corners then what you probably have is a mixture of coma & astigmatism, which can manifest in what some call saggital coma flare, as shown in the diagram attached although in much greater detail than often seen with images of stars). As to the cause, you'd have to experiment a little; with/without reducer and also check that the collimation is as good as possible so as not to create a false impression re the amount of astigmatism, etc... Also Check if anything is loose or has play and whether that affects the outcome image.

https://www.handprint.com/ASTRO/IMG/comaflare.gif

Best
JA