Handling Big Bloated Stars

[r3i]

Now that I have my observatory and with the longer nights returning I've been able to start getting much longer imaging sessions on a single target. 

Below is an example of the Cave Nebula (8"RC + CCDT67 + QHY8, 109*5mins).

There are no doubt many things wrong with it but it is much better than the results that I used to get before I had the observatory.  However, one aspect that I'd like to improve on is reducing the presence of bigger bloated stars, if this is possible.

I am aware of the technique of reducing stars using the PhotoShop Colour Range/Minimum Filter method and I can see how this works on the smaller stars, but I can't seem to get it to have any effect on the bigger stars.

Is there a particular technique for handling these stars somewhere during the processing workflow or is it due to problems further upstream during image capture or even just a consequence of the kit being used? Any suggestions are most appreciated.

[ollypenrice]

YOu copuld ma

[ollypenrice]

YOu copuld ma

 That was a useful post! Let's try again.

You can try to make a star mask for which there are various net tutorials. The idea is to protect the stars from stretching. They are semi effective but not a panacea. Watch out for artefacts.

Real blighters can be dealt with by making a special soft stretch which I call a core control stretch since it also works for galaxy cores. Here it's applied to the deadly Alnitak. Initially the curve rises as it did for the main image but it flattens early to hold down the star. Voila;

Once finished, the soft stretch has the main image pasted on top of it. You can then make a proper mask or, as I tend to do, just use a soft edged eraser of low opacity and iteratively diminishing size to let the  soft stretched Alnitak through from below in a series of operations. It gives something like this;

For a fix on an image you have, put a generoous, well feathered circular marquee around a your star and Ctrl H to hide the selection, as here. (Only the big star is selected.) In Curves, Alt Click on the outlying background around the star so as to pin that where it is and ease down the curve above that. A few iterations within increasingly small selections works well.

Olly

Edit; I forgot to say that in LRGB imaging the bad bloaters are best handled in RGB only if you intend to make a core control stretch to use as a layer.

[r3i]

Thanks Olly, very informative - you're a star (pun intended)

[Russe]

As the stars get overexposed in their center very early on in the game while stretching the histogram initially, what do you do to prevent this from happening? Or do you fix it later. I have the frustrating struggle of preserving star colour (DSLR). I seem to lose their colour from the initial stretches already. I've tried the whole night whilst on call and didn't manage it.

[ollypenrice]

As the stars get overexposed in their center very early on in the game while stretching the histogram initially, what do you do to prevent this from happening? Or do you fix it later. I have the frustrating struggle of preserving star colour (DSLR). I seem to lose their colour from the initial stretches already. I've tried the whole night whilst on call and didn't manage it.

Tricky. Try a special stars only stretch. Use a curve that rises steeply as usual but flattens early and just look at the stars. Stop once you have nice colourful stars. (You can lift the saturation big time because the stars themselves have loads of signal.) Paste the proper image on top and select the stars. (Noels Actions or MartinB's star layer tutorial on here.)

expand and feather this selection so that the stars inner cores and a bit beyond are chosen. Erase the top layer cores to let the coloured cores through from below. You can blur the bottom layer, vary the eraser opacity etc. Experiment!

Olly

[DMouse]

Hi, in pixinsight you can create a star mask. Then using this you can stretch the DSO and stars separately, to get control over the bright stars and help stop bloating.

Hope that helps

Clear Skies

[ollypenrice]

Yes, this is true. However, I find masks either in PI or Ps very hard to control. You easily end up with small stars but soft bloated haloes around them. Lack of skill on my part, no doubt.

Olly