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short subs for HDR composition

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#1 bcstein12

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Posted 04 August 2020 - 02:30 PM

Could someone give me some advice on taking short subs for HDR composition.  I am using a SBIG 8300M camera.  I have noticed that when I take subs <60sec my background seems excessively noisy as compared to my longer ones.  I only take 40-60 subs for integration.  Am I not taking enough to counteract the background noise?  Should I dither every sub?

 

Thanks


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#2 elmiko

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Posted 04 August 2020 - 03:18 PM

You should dither at least every other sub. It will take care of the walking noise in the background. And the more subs the cleaner the Integrated stack is. And of course make sure you are using calibration frames. Ie. Darks flats bias.


Edited by elmiko, 04 August 2020 - 03:20 PM.

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#3 nimitz69

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Posted 05 August 2020 - 05:47 PM

Ive done subs as short as 15 secs for HDR without issue. Of course I dither every 3rd frame, use darks, flats & dark flats and have hundreds of subs. My current M13 project has over 1,100 subs and i’m Not done yet ...

Edited by nimitz69, 05 August 2020 - 05:48 PM.

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#4 StephenW

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Posted 05 August 2020 - 06:45 PM

>My current M13 project has over 1,100 subs and i’m Not done yet ...

 

! :)

 

When dealing wit a single filter, do you process your subs in separate batches or process them all together?

 

So far I've maxed out at about 100 subs for a single filter, but you must be up into the 300 sub range...?



#5 nimitz69

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Posted 05 August 2020 - 09:33 PM

I built a desktop just for post processing so its pretty powerful. I haven’t got very far with processing yet but i plan to process all the subs for each filter together. My image method is for each hr of LUM i capture 20 mins each of RGB. I have`15 sec, 30 sec. 1 min and 2 min subs. I’ve got about 12 hrs at the moment and i’d Like to get to 15 total. Unfortunately weather is a real problem here this time of year, next clear night is predicted for anther week ...
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#6 WebFoot

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Posted 05 August 2020 - 10:55 PM

I image with SBIG cameras.  What exposure length I choose to use for HDR purposes is a function of many variables, including (i) how long my other exposures are, and (ii) how much dynamic range I'm dealing with.
 

Examples:

1. When I shot the Orion Nebula with my FSQ106, I used exposure lengths of 15-seconds, 90-seconds, 300-seconds and 600-seconds, because of the extreme range of brightnesses in that object.

2. For my recent globular clusters, with my f/9 14.5" RC, I took images of 1-minute and 5-minute durations.

 

3. For my recent M81 with the RC, I took a series of 15-minute and 5-minute images.

 

If you're worried about not swamping either background noise or read noise, you're taking exposures that are too short for the brightness of your object, IMO.

I dither between every subexposure.

 

Mark




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