How to apply flats in Photoshop ?

[skysurfer]

I always use DSS to stack frames for an astrophoto and post process the result (Autosave.tif) with Photoshop. But recently I discovered that Photoshop (CS6 or CC 2.017), called PSD hereafter, is also able to stack CR2 frames.
In some cases (e.g. when using a short 85mm tele) the stack from PSD is even better than from DSS.

See attached 100% crops of the Crux constellation taken from 26 frames of 30 sec each with the Canon 6d + 85mm f/1.8 squeezed to f/2.5.
The center of the full image is slightly right from the center right of this crop.

But it lacks the possibility to apply flat frames. I tried with it by making a layer mask created from the flat and applying with 'Overlay' or 'Screen' in PSD but it does not help very much. Even when playing with the 'Opacity' slider or playing with the 'Curves' in the flat mask itself.

The excellent site 'clarkvision.com' from the veteran Photoshop astrophoto processing guru does not say a word about flat frames as he only uses camera lenses with a PSD (or Canon/Nikon) supplied lens profile which does the work a flat normally does.
But using a telescope and photographing in prime focus does require flats (and I have them, which work excellently in DSS).

My question is : How do I apply flat frames to the PSD stacked image when using PSD ?

[Merlin66]

You need to use the divide function which was only introduced in CS5.

[rustigsmed]

Here's a vid I did on creating and applying artificial flats but it should work with normal ones as well

https://youtu.be/GW67PwUP8Ww

Cheers

[Merlin66]

Russell,
You appear to be subtracting the artificial flat rather than dividing?

[rustigsmed]

Hi Merlin, that's correct. I think you can use divide as well.

[Merlin66]

Subtraction is usually used for Dark corrections, and the flats are always normalised then divided into the Lights.......

[rustigsmed]

yes i think that is the case when doing full callibration (master bias, darks and flats or no flats) this question was simply for flats from a light stack in PS. happy to be corrected but as you could see it works fine - well in the case of an artificial flat anyway.

[Poita]

It's an old thread, but there is a good method here that works a treat https://pduncan.com/2015/11/20/vignetting-4/