Focus with a mask was pin sharp but M42 looks like I'm viewing through fog-

[Swoop1]

I don't understand what has occurred here.

Celestron C6 SCT on HEQ5Pro with ASI290MC. Dew heater strap at corrector plate end.

30 X 30 sec lights, 10 X 30 sec Darks 10 X Bias. Stacked in DSS and levels adjusted via GIMP

[vlaiv]

What is troubling you?

Fact that stars look bloated? Let's see - you used ASI290 and 1500mm of focal length. That combination gives 0.4"/px.

In good to average seeing - you are probably going to have to sample at about 1.6-2.0"/px in long exposure, so you are over sampled by factor of x4 to x5.

Here is your image at 1.6"/px instead:

That looks much better, doesn't it?

at 2"/px it looks even better, detail is sharp and stars are point like.

[Swoop1]

Thanks @vlaiv

How did you do what you did?

[vlaiv]

1 hour ago, Swoop1 said:

Thanks @vlaiv

How did you do what you did?

I just took your image and software for Windows called IrfanView (very nice image viewer application that can also do basic manipulation like cropping / scaling and such) and down sampled it by x4 and x5 (to 25% and 20% of original size).