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My First Elephant's Trunk in Ha


Astrosurf

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There was a very bright moon and I only captured 10 subs, 8 at 8 mins and 2 at 5 mins, so not much data. Lots of processing in PS as StarTools didn't like it. I tried, and failed, to stop the stars bloating. Will do better next time. Criticisms please!

Alexxx

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  • 1 month later...

It's good. Simple as that.

There are a few suggestions I could make. (The white point slider - right hand - should only be moved if the data line in levels doesn't reach it. Sometimes this is the case and, when it is, move the slider left to meet it and then don't touch it again.)

There are mild dark haloes around the stars. This might just be a forum aretefact. If not, how did they get there? Did you use a star mask? That can cause them. Another possibility is deconvolution. That often creates them.

Personally I try to exclude stars from a large range of processing activities and, in particular, from any kind of sharpening. If you don't have Noels' Actions, MartinB has a tutoroal sticky on here on how to make a star layer selection. Once you have selected the stars you can expand and feather the selection to create a safe zone around the stars and then Select Inverse to operate on the rest of the image.

Olly

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Thanks Olly! (And I do know how to spell 'true'! :grin: )

I don't quite understand ' . . . if the data line in levels doesn't reach it.'

I don't understand deconvolution!

I'm not sure what caused the haloes. I'll have to look at my processing notes. I don't use star masks as they always mess things up for me. I've used the selection/expand/feather technique before, but I get haloes with that too. I keep changing the expansion to blur the area, but it's still not great, although I still use it from time to time. I'll check out Martin's tutorial as I'm probably doing it wrong.

What Noels' Actions action do you use?

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Thanks Olly! (And I do know how to spell 'true'! :grin: )

I don't quite understand ' . . . if the data line in levels doesn't reach it.'

I don't understand deconvolution!

I'm not sure what caused the haloes. I'll have to look at my processing notes. I don't use star masks as they always mess things up for me. I've used the selection/expand/feather technique before, but I get haloes with that too. I keep changing the expansion to blur the area, but it's still not great, although I still use it from time to time. I'll check out Martin's tutorial as I'm probably doing it wrong.

What Noels' Actions action do you use?

In this example the data line (the histogram, if you like) doesn't reach the right hand edge. That means you are not using the full range of brightnesses available to you. The same applies if you have an 'empty' gap at the left hand edge as well. In this case you'd just drag the right hand slider to the left (red arrow) until it met the end of the data. Click OK, open levels again, and now you'll see the data going right across, meaning you're using the full range of brightnesses available.

The difference between the flat lines to the left of the peak and to the right is than on the left you are looking at the dark stuff and very faint data. Much but not all of that flat line will be noise and can usefully be clipped out by moving the left hand slider to the right. But don't over do it! This is black clipping and throws away what may be precious faint signal.

On the other hand the flat line to the right will be mostly stellar cores. If you clip that you make the over exposure worse, effectively turning nearly saturated data into fully saturated. Throw nothing at all away on the right.

post-2393-0-50936500-1442507630.jpg

Noel has a 'Select Brighter Stars' action which you can run, expand and feather and then set to Inverse to exclude stars from processing.

Olly

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The only thing I can add to what Olly has said is that if you hold down the "Alt" key while moving either of the sliders in Photoshop, you will get feedback when you do clip the signal.  When you adjust the white point, the image will go entirely black - then as you slide the slider leftwards you will see pixels appearing on screen when you clip - these might be coloured pixels because you might clip the red channel (for example) first.  As you adjust the black point holding "Alt" the screen turns white and then pixels will appear as you move the slider rightwards, clipping the black point.  You can find the white and black clipping points and then just move the slider back a bit in the opposite direction until you see no clipping. 

Great image of the elephant by the way.  I may try that tonight if the weather holds.

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In this example the data line (the histogram, if you like) doesn't reach the right hand edge. That means you are not using the full range of brightnesses available to you. The same applies if you have an 'empty' gap at the left hand edge as well. In this case you'd just drag the right hand slider to the left (red arrow) until it met the end of the data. Click OK, open levels again, and now you'll see the data going right across, meaning you're using the full range of brightnesses available.

The difference between the flat lines to the left of the peak and to the right is than on the left you are looking at the dark stuff and very faint data. Much but not all of that flat line will be noise and can usefully be clipped out by moving the left hand slider to the right. But don't over do it! This is black clipping and throws away what may be precious faint signal.

On the other hand the flat line to the right will be mostly stellar cores. If you clip that you make the over exposure worse, effectively turning nearly saturated data into fully saturated. Throw nothing at all away on the right.

attachicon.gifWHITE POINT.JPG

Noel has a 'Select Brighter Stars' action which you can run, expand and feather and then set to Inverse to exclude stars from processing.

Olly

Marvellous! Thanks Olly!

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