First attempt at star mask...

[Atacamallama]

I have just managed to do my first star mask. Definite improvement, but I have some star haloes.

FWIW I was using the method described here:

What I wasn't able to grasp was how I was to apply the guassian blur to the mask, and am wondering if this is my problem.

any input greatly appreciated.

TIA

[ollypenrice]

With the mask active (click on the white icon in the Layers palette) just choose filter, blur, Gaussian blur.

I suspect that making a mask from a slightly stretched version of the image may be better than making the mask from the linear one. It makes for a bigger mask around the stars and is less likely to create hard 'inner cores' in the larger stars. So make and save a mask from a slight stretch and then go back to the linear image and apply the mask to subsequent stretches. Updating the mask at each stretch may also help.

When I use a star mask I also do an unmasked full stretch to be sure that the mask has not blocked any faint signal.

Olly

PS, In your image above I'd just try clipping back the green channel very slightly.

[Atacamallama]

With the mask active (click on the white icon in the Layers palette) just choose filter, blur, Gaussian blur.

I

Thanks for your reply Olly. I understand how to perform this step of the Gaussian blur, but what I am not understanding is what I am trying to achieve by performing it and what I should be looking for when setting the parameters. Do i blur more to make the stars cover a larger area and avoid the halos, or what?

[ollypenrice]

Thanks for your reply Olly. I understand how to perform this step of the Gaussian blur, but what I am not understanding is what I am trying to achieve by performing it and what I should be looking for when setting the parameters. Do i blur more to make the stars cover a larger area and avoid the halos, or what?

Ah, right, I follow your question. If you don't blur the mask it will, in effect, have hard edged 'holes' and when it lets light through between layers this hard edge will be visible in the resulting image. All selection tools should be feathered to avoid boundaries and a mask is, in effect, a selection tool in that it selects what will be allowed to pass from one layer to the next. You just have to try different blurs. There is no hard and fast rule because pixel sizes vary, the task in hand varies, etc etc.

I'm a recent convert to star masking but it can transform images in which stars are embedded in nebulosity. My previous technique was better for the background sky in galaxy images but masking works against nebulosity. Wonderful!

Olly