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Old 24-05-2013, 06:38 PM
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Using HDR to avoid saturated stars

I often take a bunch of short subs along with my "normal" ones and then use HDRComposition in PixInsight to combine the integrated "short" and "long" images. This is an attempt to avoid those stars we've all seen which have a thin ring of colour surrounding a bleached white core where the star has saturated the CCD sensor.

When processing my recent image of Omega Centauri (http://www.iceinspace.com.au/forum/s...d.php?t=107621) I used this HDR technique and also did some 3D plots showing the star profiles with and without HDR. On the off chance that anybody is interested, here they are...

The first and third plots are two sample areas from an integration of 5 minute luminance exposures. The second and fourth plots are the same areas from a HDR combination of two integrations, the first of 0.5 second subs and the second of 5 minute subs. It's pretty easy to tell that the star profiles in the HDR version look much better!

Cheers,
Rick.
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Old 24-05-2013, 07:15 PM
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Fascinating Rick. It looks like the process overcomes star "fattening" due to increasing CCD non-linearity at high signal levels, as well as outright clipping when the ABG cuts in. Is this a reasonable observation? Regards ray
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Old 24-05-2013, 07:30 PM
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I am using this (or equivalent) technique all the time - when adjusting curves in DPP.
They are always steep at low intensities, while at the top they are more flattened but never completely flat (so, they are logarithmic-like).
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Old 24-05-2013, 08:03 PM
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Good post Rick. I've never gotten around to seriously trying this except ad hoc for objects with really bright stars in the field (e.g. M42). Make a lot of sense and good of you to show some detail in the process.
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Old 24-05-2013, 08:30 PM
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Quote:
Originally Posted by Shiraz View Post
Fascinating Rick. It looks like the process overcomes star "fattening" due to increasing CCD non-linearity at high signal levels, as well as outright clipping when the ABG cuts in. Is this a reasonable observation? Regards ray
Ray, I don't think it helps a lot with the "fattening" but it does replace the truncated top of the star profiles. Probably a second best strategy compared to your use of short subs with a very low noise camera.

Quote:
Originally Posted by bojan View Post
I am using this (or equivalent) technique all the time - when adjusting curves in DPP.
They are always steep at low intensities, while at the top they are more flattened but never completely flat (so, they are logarithmic-like).
Interesting, Bojan. I've never played with DPP. So you're doing something similar with a non-linear stretch?

Quote:
Originally Posted by RobF View Post
Good post Rick. I've never gotten around to seriously trying this except ad hoc for objects with really bright stars in the field (e.g. M42). Make a lot of sense and good of you to show some detail in the process.
Thanks, Rob. 3dplot makes it easy to experiment with these things and visualize the results. It's so hard to come by data when you don't have a permanent obs and live in the city so I feel obliged to try to make the best of what I can get!

Cheers,
Rick.
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Old 24-05-2013, 08:40 PM
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Quote:
Originally Posted by RickS View Post
Interesting, Bojan. I've never played with DPP. So you're doing something similar with a non-linear stretch?
Yes.. I developed this method to deal with LP, but it takes care of saturation as well see here:
http://www.iceinspace.com.au/forum/s...4&postcount=37


The 3D diagram is a nice way to visualise the result, though..
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Old 24-05-2013, 08:51 PM
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Thanks, Bojan.
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Old 24-05-2013, 09:19 PM
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Looks convincing.

Do you believe you need the ultra short subs? Why not 5 or 10 sec? Very short subs can cause uneven illumination from shutter errors?

DT
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Old 24-05-2013, 10:05 PM
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Quote:
Originally Posted by DavidTrap View Post
Looks convincing.

Do you believe you need the ultra short subs? Why not 5 or 10 sec? Very short subs can cause uneven illumination from shutter errors?

DT
David,

It doesn't take very long to push a bright star to saturation. Often a few seconds is more than enough. I usually watch the star profiles on Maxim to pick a good exposure time.

Uneven illumination hasn't been a problem for me. The HDR process only takes the very brightest bits from the short exposure stack - just the star cores typically. The HDRComposition process has an option (on by default) that shows you the mask that was used. The short stack often looks pretty noisy and horrible but as long as the star cores are OK it doesn't matter.

Cheers,
Rick.
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Old 24-05-2013, 10:35 PM
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Ta for that!
DT
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Old 25-05-2013, 08:02 AM
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Thanks Rick! I find my STF-8300 saturates stars very, very quickly so this will be a very useful trick to learn. I've been meaning to get started with PI so this is a great reason to do so
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Old 25-05-2013, 08:42 AM
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I tend to have the 3D plot of the brightest star coming up as info in the Maxim camera window, and as Dave says, doesn't take long for an 8300 chip to start turn stars into "mesas"!

Guess I'm usually more focused (no pun intended) on getting round stars than perfect profiles, but it shouldn't actually take that much more work as you say, particularly if you're running an automated profile for data collection. Was reminded last new moon how important it is to the run set to go to maximise every minute of dark sky when you're a city astronomer.

Another PI option is masked stretch. Can be very gentle on data, but prone to a little ringing around very bright stars on nebs sometimes.
Of course you can go crazy with starmasks and morphological transforms to "smooth" the sharp edges on your star profiles too if you're going to be artificial about it.
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Old 25-05-2013, 12:42 PM
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Definitely worth a try, chaps. Unless you have a camera with really slow downloads (I'm looking at you, STL11K!) it doesn't take very long to collect the extra data.
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Old 25-05-2013, 06:18 PM
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As a matter of interest (sorry for threadjacking) has anyone used the built in algorithms in Deep Sky Stacker to do this? DSS gives the option of using Entropy Weighted Average for specifically stacking images with different exposures to avoid burning out bright centres.

Any comparisons to this method here?
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Old 25-05-2013, 07:03 PM
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Quote:
Originally Posted by Garbz View Post
As a matter of interest (sorry for threadjacking) has anyone used the built in algorithms in Deep Sky Stacker to do this? DSS gives the option of using Entropy Weighted Average for specifically stacking images with different exposures to avoid burning out bright centres.

Any comparisons to this method here?
Sounds similar. Hopefully someone who knows more about DSS than I do can comment.
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