sticky exposure length or sticky gain/offset for sky-limited exposures?

Discussion of using SharpCap for Deep Sky Imaging
mAnKiNd
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sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:08 pm

Some advice would be greatly appreciated.

I'm about to start a new project and I'm contemplating which would be the best approach. I'll be shooting the Heart Nebula and wanted to see how long I could collect data for it. Each night, I'll go out and capture my respective mono channels (as 60 minute auto-saved livestacks) i.e LRGB, come back and process a first edition. Then, on subsequent nights, shoot respective filter live stacks again and then come back and doublestack them with the others in DSS, reprocess and release a new edition. When I 'feel' that the latest edition is no better in SNR, looks, detail etc.. than the previous iteration, I'll call it ends and move onto another target.

Now my conundrum is actually how to go about getting my sky limited exposures (as guided by the smart brain analysis).

One way, is to keep the same time in the exposures for each filter and use gain/offset to shape the histogram correctly. This means that I would end up with four exposures, each of them say 30 seconds and each of them having a different gain/offset. This would nescessitate different darks for each filter setting, but all of them 30 seconds. Probabity of matching guiding/tracking behaviour during the same exposure lengths also improves the chance of stars remaining sharp to each other between filters.

The other way, is to keep gain/offset the same, but this time vary exposure length to achieve that good sky limited histogram. However, this means that longer subs might have an increased probability of guiding/tracking issues. There's also a minor consideration of increasing dark noise/amp glow, which proper darks should take care of that. In this scenario, master darks for each filter will share the same gain/offset but have varying exposures. This approach is akin to how many shoot with their cameras set at unity gain and then vary the exposure length accordingly to compensate for the different filter bandwidths.

Considering I'll be doublestacking the live stacks, it means that if I choose to keep exposure times the same and vary gain/offset, then the 60m livestacks for each channel will all contain the same number of frame (120 if we use the 30s example above). If I go the other way, with maintaining gain/offset and varying the exposures (like the unity gain example), my 60 minute livestacks will all have varying number of frames for each filter. The number of frames contributing to the 60 minute livestack are obvious considerations for the sigma clipping and SNR aspects. This then begs more questions, such as how many frames to aim for to maximise my 60 minutes livestack? How many initial frames should you count from the livestacking tab, 10, 15, 20, or the max 25? Then the clipping sliders below come into question, how much pixel percentage should I reject? The goal here with the stack is to produce the best possible 60 minute live stack, for later doublestacking in DSS (or SC virtual monitor).

Also, I should say I'm willing to sacrifice maximum dynamic range, in order to keep my exposures relatively short (for better star shapes), up until say 3 minutes max, but ideally shorter. The goal for each individual exposure is to get a nice fat histogram off the left wall that occupies the good first half of the chart. I'll have to see how this works with sky limited exposures..

I know there are many questions in my text above, but it utlimate boils down to this:
For sky-limited exposures - sticky exposure length or sticky gain/offset?

..and how many frames for the best 60 minute livestack?

Hope this is clear and looking forward to any advice.

Cheers
Minos
Last edited by mAnKiNd on Wed Sep 12, 2018 9:33 pm, edited 5 times in total.
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

mAnKiNd
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:24 pm

Here's a working example of what I'm talking about but with imaginary figures as I've yet to determine actual values:

1hr Livestack comprised of sticky exposure lenghts:
L: 120x30s G50 O10
R: 120x30s G75 O30
G: 120x30s G70 O30
B: 120x30s G75 O30

1hr Livestack comprised of sticky (i.e. unity) gain/offset:
L: 360x10s G130 O30
R: 180x20s G130 O30
G: 240x15s G130 O30
B: 180x20s G130 O30

All of the above individual exposure settings would theoretically represent a sky limited exposure.
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

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oopfan
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by oopfan » Wed Sep 12, 2018 9:33 pm

There is a third option:

3. For all channels keep the same gain, offset, and exposure. Simply alter the stack size for each channel. For example:

L: 120x30s G50 O10
R: 180x30s G50 O10
G: 170x30s G50 O10
B: 180x30s G50 O10

Brian

mAnKiNd
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:39 pm

Hi Brian, thanks, that is correct indeed! I shortly considered that option, but since I'm going to rely on the auto-save feature (for ensuring consistency between stacks), it can only do x minutes so far, not auto-save every x amount of frames. Perhaps Robin can squeeze this one in when he gets a chance.
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

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oopfan
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by oopfan » Wed Sep 12, 2018 9:41 pm

Yes, I wouldn't mess with changing gain since it affects the full well depth.

mAnKiNd
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:44 pm

oopfan wrote:
Wed Sep 12, 2018 9:41 pm
Yes, I wouldn't mess with changing gain since it affects the full well depth.
Thanks, that is a very good point and could cause inconsistencies within the different filter datasets/livestacks.
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

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oopfan
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by oopfan » Wed Sep 12, 2018 9:47 pm

I've read about people keeping gain and offset the same. Some people opt for keeping the stack size the same and adjusting exposure, and others who keep the exposure the same and adjust the stack size. They are equivalent but keeping the exposure the same allows you to use the same darks.

Brian

mAnKiNd
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:48 pm

Having said, it is often recommended to vary gain/offset and is often done in imaging techniques, for example, Mr. Rista and others wrote many times on the CN forums about using the following gains for the ASI1600:

L: G50 O50
RGB: G75 O50
NARROWBAND: G200 O50

...all of them sharing the same offset set by the highest gain, 200, in the case of the ASI1600, gain 200 with an offset of 50.

I've done this myself a few times and it can work well to vary the gains accordingly with the filter bandwidth.

Cheers
Minos
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

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oopfan
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by oopfan » Wed Sep 12, 2018 9:52 pm

There you go! Everyone has an opinion with what works best. I found someone on the internet who had great photos and spoke in an intelligent manner. I am certainly open to new techniques.

Brian

mAnKiNd
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Re: sticky exposure length or sticky gain/offset for sky-limited exposures?

Post by mAnKiNd » Wed Sep 12, 2018 9:53 pm

oopfan wrote:
Wed Sep 12, 2018 9:47 pm
I've read about people keeping gain and offset the same. Some people opt for keeping the stack size the same and adjusting exposure, and others who keep the exposure the same and adjust the stack size. They are equivalent but keeping the exposure the same allows you to use the same darks.

Brian
Indeed, it seems there are three (more?) possibly equivalent avenues to achieving a livestack target.

Just to add keeping the exposure time AND gain/offset the same allows you to the the same darks.

Cheers
Minos
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
ASI1600MM PRO
EFWmini - HOLRGB
ASI120MM mini for guiding
SC CDC PHD2 DSS PI PS

https://www.astrobin.com/users/minoSpace/

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