HDR for just star cores?

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jewzaam avatar
I have been shooting short L, R, G, and B exposures at gain=0 to keep the star core saturation down but keeping exposure long enough to swamp noise.  It doesn't eliminate saturation.  This results in a lot of images!  I could try repairing in post processing but I wondered about acquisition changes.

So I started digging into HDR for this.  I've used it for bright nebula such as M42, but it would work for stars.  I found a recommendation to do 3 sets of subs, though this looks to be targeting narrowband.  For a 10 to 15 min sub take also one 3 to 5 min and one 10 to 30 second sub.  For broadband I'd probably just skip the long exposure, though I do shoot 10 min narrowband right now.

Is it worth it?  If I shot 3 to 5 min broadband and took some short 10 to 30 sec subs and combined later is that any better than doing something like Repaired HSV Separation script in PI?  I have also been playing with that, taking the result and using HDRC to tamp down the stars in the data I already have.  And it helps!  But before I spend a bunch of time changing up my data collection thought I'd ask what others think.
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Steven avatar
It's what I've ben doing recently, and it works quite well.
Mainly for images taking with my "Optolong L-Extreme", which has… meh.. star colours… 
So I use a broader filter (Optolong L-Pro) to get a little more colour. 

Now this is mainly a "filter issue" rather than an exposure/saturation issue.
no matter what exposure time I use for the L-Extreme, the star colours will always be a bit, meh..  because of the duo-band filter.
But in general, shorter exposures will work for stars, similar to the core of Orion like you mentioned.  


- I throw in my L-Pro in the filter drawer, take a few short exposures.
- Stack both versions.
- Make a starless / star mask version of the (L-Extreme) nebula, 
- And I plop the stars of the L-Pro into the final image.


I assume it would work the same for LRGB, but yes.. it might take some work, doing that for mono and all the layers individually.. but in the end, 10x 20 seconds per filter will probably do, so 10-15 minutes of imaging.. might be worth a try either way.
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jewzaam avatar
I did give it a go with M31 this week. Just on L to try it, I took 300s subs and paired with 15s subs, roughly 1:1 but did reject a few for clouds. I don't have a "normal" data set yet to compare with but the result was pleasing when the 15s stack was used in HDR Combination and the final image didn't have any saturated star cores after processing. I may get some 30s exposures (my normal for L) to compare since I only had a bit less than 2 hrs in this test.
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Tayson avatar
Try use in PS annies action reduce star bloating. I thing max 2times.
jewzaam avatar
Tayson:
Try use in PS annies action reduce star bloating. I thing max 2times.

Bloat isn't what I'm trying to control, star core saturation and overall color is the concern.  When I stacked the 300s L exposures and used in LRGBCombination I got more obviously saturated star cores and some weird artifacts in processing.  I didn't get either when using the HDR version with 300s and 15s stacks.