Star removal-replacement methods.

[ollypenrice]

In Photoshop I had more or less settled on the extraction of a stars-only layer for putting back onto the fully starless, highly stretched image. However, I had  an image which wasn't playing nicely this way so I tried the following.

Standard linear stack, partial stretch till stars are about where I'd want them at the end.

Copy Layer. Run StarXt on the top layer.

Further stretch and sharpen and generally process the starless till all the faint stuff is pulled out and details are sharpened, etc.

Change the blend mode to lighten. When you do this, only the stars are visible from the bottom layer and, using levels, curves, contrast, etc. on that bottom layer you can manage the stars as you wish.

I've now done this on my last three images and have found it to give a very clean, natural result. In a sense the stars in the final image have never been removed from it, they have always been present in the bottom layer.

Olly

[Elp]

This is useful, i've always put the star layer on top but will try the above.

[Adam J]

5 hours ago, ollypenrice said:

In Photoshop I had more or less settled on the extraction of a stars-only layer for putting back onto the fully starless, highly stretched image. However, I had  an image which wasn't playing nicely this way so I tried the following.

Standard linear stack, partial stretch till stars are about where I'd want them at the end.

Copy Layer. Run StarXt on the top layer.

Further stretch and sharpen and generally process the starless till all the faint stuff is pulled out and details are sharpened, etc.

Change the blend mode to lighten. When you do this, only the stars are visible from the bottom layer and, using levels, curves, contrast, etc. on that bottom layer you can manage the stars as you wish.

I've now done this on my last three images and have found it to give a very clean, natural result. In a sense the stars in the final image have never been removed from it, they have always been present in the bottom layer.

Olly

This is not how I do it. I tend to find that blend mode lighten is very often problematic in trying to recombine stars. Normally use a star only layer and combine using screen, seems to work every time. 

Adam 

 

[ollypenrice]

18 hours ago, Adam J said:

This is not how I do it. I tend to find that blend mode lighten is very often problematic in trying to recombine stars. Normally use a star only layer and combine using screen, seems to work every time. 

Adam 

 

The RASA does not naturally produce the best stars so this may be a factor.  A separate 'stars only' layer and blend mode Screen was how I was doing it as well. I agree that blend mode lighten is not good with a layer of extracted stars but, in the workflow I describe, the stars have never been extracted so they have never had their outer boundaries defined.

Olly

Edited December 17, 2023 by ollypenrice

[WolfieGlos]

I tried this method Olly on an image I processed yesterday for the IKO competition, and it worked really well. Thanks!

However, I did also try it on an image I have with a DSLR and it wasn’t very good. The stars have no noise but the background naturally did so it didn’t blend in very well. I suspect it works better on images with a high SNR? The IKO data was something like 100 hours. I’ll try it again, but like others, screen mode also works well but it’s harder (for me) to control star sizes compared to your new method. 

[Elp]

You can always denoise the layers prior to combining but care needs to be taken in not creating artifacts.

[ollypenrice]

17 minutes ago, WolfieGlos said:

I tried this method Olly on an image I processed yesterday for the IKO competition, and it worked really well. Thanks!

However, I did also try it on an image I have with a DSLR and it wasn’t very good. The stars have no noise but the background naturally did so it didn’t blend in very well. I suspect it works better on images with a high SNR? The IKO data was something like 100 hours. I’ll try it again, but like others, screen mode also works well but it’s harder (for me) to control star sizes compared to your new method. 

I denoise the starless layer whichever way I'm doing it. I do it as a top layer so I can erase the denoised regions of strong signal if necessary, though with NoiseXterminator this is rarely worth bothering with. What I do find is that a starless image which has what seems to be the right amount of noise reduction can often look over-smoothed when the stars are back in the image. I've been caught by this a few times and now leave a margin for a light final denoise at the end.

Olly