elongated stars and blue star halos

[alexd106]

Hi everyone

I am fairly new to astrophotography so hope members here can shed some light on a couple of issues I am having.

My imaging setup is as follows;

Skywatcher equinox 80ed -> OVL field flattener -> 16.5mm extender -> 21mm filter drawer (with Astronomik L2 filter) -> 11mm extender -> asi294mc camera.

The OVL FF recommends a 55mm back spacing between FF and camera which I think I have achieved. The Astronomik L2 filter is 1mm thick so google recommends adding 0.33mm to the backspacing distance which I currently haven't done.

I think I have 2 issues. The first issue is that I have elongated stars in 3 corners of my images (see images below) - bottom left, top left and top right (worst). The stars in the bottom right corner look ok to me. Might this be a backspacing issue and I need to move the camera back by say 0.5mm or is this an issue with tilt?

top left
[img][\img]

top right
[img][\img]

bottom left
[img][\img]

bottom right
[img][\img]

My other issue is the strongish blue halos around some of my stars seen in the image below.

[img][\img]

Again is this an issue with the back spacing or is it just a limitation of my fairly old refractor? Might an astronomik L3 filter help with this problem?

Many, many thanks for any insight you can provide.
Cheers
Alex

[UlteriorModem]

Need to double check the back focus spacing and spacing in general for the field flattener.

Also might be some minor tilt issues. Those are a little tougher to deal with.

ps; A filter wont do anything save change the focus train length.

[alexd106]

Need to double check the back focus spacing and spacing in general for the field flattener.

Also might be some minor tilt issues. Those are a little tougher to deal with.

ps; A filter wont do anything save change the focus train length.

Hi Tom, thanks for your help. I'll have a play with the back focus spacing and experiment by adding some of the thin zwo spacers that came with my camera between the FF and camera. I'll also have a trawl through google to figure out how to determine whether I have tilt and what to do about it. I suspect its probably due to the compression ring in the focuser drawtube holding the imaging train at a very slight angle.
Thanks again
Alex