advice needed...

[paul mc c]

Hello all,I am having some issues with my images lately probably to do with my calibration frames.I have recently changed scopes and didn't seem to have the issue before.The last scope had a 3" focuser,I now have a 2"focuser, don't know if that matters.Here is an quick image run through APP, just basic processing but still has gradient and the large halo...any ideas would be greatly appreciated.

[Budgie1]

Which scope and which type of calibration frames did you use? 

I assume the camera was the ASI533MC Pro you have listed but did you use any filters with it and when & how close to the Moon was the image taken?

[paul mc c]

13 minutes ago, Budgie1 said:

Which scope and which type of calibration frames did you use? 

I assume the camera was the ASI533MC Pro you have listed but did you use any filters with it and when & how close to the Moon was the image taken?

Equinox 120ed,l-pro filter,flat,dark flat and bias no moon near.

[Elp]

What does an extremely stretched version of the flat look like?

[paul mc c]

31 minutes ago, Elp said:

What does an extremely stretched version of the flat look like?

This what you mean..

Edited June 18 by paul mc c

[Elp]

The flat could be overcorrecting, the pattern on the outside of the flat looks similar to the stacked image residual pattern.

I take it you subtract the master dark flat from each flat prior to creating a master flat?

Though I wouldn't know why it's worked for you before and not now...

There's not a light leak anywhere, and also are you sure there was no stray light going into the objective, I know from mounting my optolongs that they don't like stray light, the moon also can have a similar effect but you've stated no moon near.

[ollypenrice]

I think you'll find that the problem lies in your blue channel. Certainly on the screen grab, looking at each channel separately shows that red and green are admirably flat but that blue is much darker (and less regular) in the region of the globular.

Before thinking about flats, can we clarify whether or not this has had any kind of gradient removal? Automatic gradient tools are prone to sampling the background sky too close to the bright target object and, in so doing, creating an over-darkened region round it. Since a globular is made of stars, and telescopes struggle to keep blue light in tight focus, a gradient removal tool might pick up too much background blue and erroneously pull it down.

If this hasn't had any gradient work done to it then your flats could be over-correcting, especially in blue. Finding out why can be a nightmare. One thing to try would be take your master flat, split its channels, and replace the blue with a second iteration of the green.

Olly