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Old 30-10-2013, 11:18 PM
bloodhound31
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Blowing out stars.

Can someone please put it in simple terms?

Example: Horsehead and Flame Nebulae - How do you capture 20 minutes of photons for the faint areas without burning out the big stars in the region like Alnitak and the other bright objects next to it.

How do you tackle it?

Baz.
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Old 31-10-2013, 05:58 AM
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Obviously well depth and if your shooting narrowband or broadband has a lot to do with this. But as far as capture goes there is nothing you can do about it in the case of Alnitak. Of course with narrow filters you have a better chance of not saturating it, and higher well depth cameras will do better too.
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Old 31-10-2013, 08:56 AM
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multiweb (Marc)
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Just do different exposure times Baz then combine is PS in the end.
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Old 31-10-2013, 09:46 AM
WingnutR32 (Sam)
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Marc, any chance of a quick step by step to layer the different exposures? Or a link to one that shows how?

I have been in bloodhounds position before, even with M42 and showing the core via different exposure times, but layering for some reason doesn't provide the expected results.. Particularly with halos from the longer exposure.
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Old 31-10-2013, 12:34 PM
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multiweb (Marc)
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Quote:
Originally Posted by WingnutR32 View Post
Marc, any chance of a quick step by step to layer the different exposures? Or a link to one that shows how?

I have been in bloodhounds position before, even with M42 and showing the core via different exposure times, but layering for some reason doesn't provide the expected results.. Particularly with halos from the longer exposure.
Hi Sam, there are many ways to blend things in. PM me if you need specifics and I'll put some files together for you. Here's a broad explanation.

For M42 the standard practice is to shoot some shorter subs to highlight the core (trap) features then layer them in PS and apply masks with a feathered edge to blend in. You can then edit the mask with the brush tool to hide or reveal features and/or get rid of halos.

Or you can also use the Merge to HDR feature in more recent versions of PS to do the blending for you.

There is another technique to get rid of halos in a single sub that involves picking the background color in an area close to the halo then pasting as color then punching the stars back through. It's quite hard to do though. I'll find a link for this. Jason Jennings [Jase] did a tut.

Quote:
Click on the lasso tool and set the feathering to approx 20 pixels
Using the lasso tool, select an area that is of same size and is relatively close to the colour flare area you wish to remove. The purpose of this exercise is to match the background hue of the surrounding area so the proximity to the area you're trying to fix is fairly important. At this point, don't worry if the selected area contains stars.
Copy and paste the lasso area so you now have a new layer that contains the small patch of the sky in which you've just copied.
Hold down the control key and move the mouse to shift the small patch over the colour flare area you wish to remove.
Now go to the Filters | Noise | Median
Increase the radius until you see no more stars in the small patch of sky that you've copied. If you don't like the median tool, dust and scratches, or the minimum filters can do a similar job. Median I've found works well in general.
With the patch layer selected, change its blend mode from normal to colour
You'll now notice that your colour flare has disappeared i.e. has been blending into the surrounding background hues. If it hasn't you may need to increase your selection size or reduce the feather.
...but don't stop there...look what you've done to the stars in the area that's been repaired...they've inherited the same background tone you copied. Easily fixed...
Select the original layer and use the colour range tool to highlight the stars. Alter the fuzziness slider until you get a good match.
Expand the selection by 2 or 3 pixels (or to your taste)
Feather by 2 pixels (or to your taste)
Then select the patch layer (important, make sure its selected!!)
Hit the delete key
POW! You've now brought the star colours back in the repaired area...as you've deleted the selections from the patch layer so the colours come back through from the original layer.
If you find that the colour range tool selection wasn't ideal, you can always use the eraser tool to do a similar task, but I would recommend going back a few steps until you get the right selection i.e. manipulate fuzziness.
An easier way is to offset your framing so you capture the halo in another spot then reject it when stacking. That's the preferred solution.

If your stars are burnt during a long exposure you can do a series of shorter subs to retains star colors, then apply that layer as color in PS to put some colors in your stars with a star mask. Those are all masking techniques well documented.

Last edited by multiweb; 31-10-2013 at 12:45 PM.
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Old 31-10-2013, 01:01 PM
WingnutR32 (Sam)
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Thanks Marc,

Hopefully this helps Bloodhound as well.

Cheers
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