Question regarding Skywatcher f/5 refractor

[ajay-mandke]

Hi,

I have a Skywatcher 150mm f/5 refractor scope that I used visually few times. I recently tried astrophotography using DSLR with that scope and I found out that I cannot focus to pinpoint stars. The stars show elongated shapes/tails no matter how much I try. I read more about the scope and found that this particular series had issues with comatic abberration etc.

Attached picture shows 10 sec. exposure when telescope was pointing north. The mount was roughly polar aligned using polar scope and I was trying to measure the polar alignment error using PhotoPolarAlign tool.

Is there a way to fix this issue with refractor?

Ajay

[edward-fraini]

I definitely am not sure of my facts here but it looks to me like you have CA, looking at the purple around the bright star. But it seems to me that the collimation is off as well. Note how there is flare off to the upper right quadrant. If this is really right I would attack that first then see how bad the CA is.

I will be watching to see if someone with more experience gives a response as well. Maybe both learn something.

Ed