star mask for "biggest stars" and mask for nebulosity

[lucchett]

hi,
I wonder if someone can share the best way to build a mask for:

-protecting the brightest stars (that apparently I can't get with the star mask tool)

-protecting/isolating a nebulosity from star field and back ground: here I've tried also wavelets, but I can't exclude the biggest stars.

I've also tried to import a mask from Photoshop, but when I invert it I get an error (memory overflow).

Thanks in advance, Andrea

[Nocturnal]

Hi Andrea,

I'm not sure I understand. Do you want a single mask that is only white for the background and black for stars and nebulae? If that's the case then I'd simply extract the Luminance and tweak it with the histogram tool to get the desired black/white levels.

[lucchett]

Hi Sander,
this is not the case.
I am talking about two different masks.

the one for biggest stars is to selectively apply an erosion filter: it's funny, Ive always had troubles in creating a good mask to protect small stars and today I have the opposite.

The one for nebulosity must act only on the neb.


Many thanks for your interest,
Andrea

[Nocturnal]

Hi Andrea,

those are indeed tricky masks. I hope someone else can help you with it better than I can. The only thing I can suggest for the nebulosity mask is to start with a Lum mask and then a generated star mask that you subtract from the Lum mask. It'll take some experimenting.

[Simon Hicks]

One suggestion for selecting large stars is to create a star mask (using StarMask or ExtractLuminance or whatever), then apply a bit of Histogram clipping to get rid of the fainter smaller stars, then apply some Erosion using the Morphological Transform to get rid of the smaller stars. You may want to apply some AtrousWavelets or Dilation to increase the large stars back up in size.

Alternatively create the a starmask and then use AtrousWavelets to kill the first 2, 3, 4 or however many layers to get rid of the smallest stars. Again, subsequent use of Erosion etc may help.

I would love a tool that allows me to say 'I'll mask that one, that one and that one please'. Maybe this is some sort of extension of the Paint tool with another layer enabled?

Cheers
         Simon

[lucchett]

hi, I've tried Simon's way and it works very well in isolating big stars!
Many thanks to both of you!
Andrea