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

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

Post by oopfan » Thu Sep 13, 2018 1:46 pm

Minos wrote:
I think this will work well, but I need to decide whether to set the smart brain to analyse for Unity gain or Max DR.
My gut says Unity Gain.

Brian

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

Post by mAnKiNd » Thu Sep 13, 2018 2:04 pm

Thanks Brian, I might just go for unity then! I'm trying to consider what's best based on the fact that I won't be settling for a single 60 minite live stack, but will rather be doublestacking them plenty later.

Question; After I've obtained my sky brightness value by pointing at a dark patch of the sky (no bright stars or nebulosity) next to my target, wouldn't it better serve the smart brain analysis to then execute it on the actual target?

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Minos

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

Post by oopfan » Thu Sep 13, 2018 3:21 pm

Minos,

I think that you are making a brilliant observation. I liken it to my solution for matching lights and darks by temperature with an uncooled camera.

In Photometry there is a quantity called Airmass which is a function of zenith distance. Airmass affects the calculation of photometric magnitude by noting that the atmosphere reddens the color of a star.

We can extend this to AP by frequently rerunning The Brain on the same rectangle of dark sky and then using that calculated exposure for the next period of time.

How often should you resample? I would say at least once per hour. I know that Chuck from Chuck's Astrophotography has his SGP set up to refocus every half hour.

I don't know to what degree The Brain can be automated.

Brian

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

Post by admin » Thu Sep 13, 2018 5:02 pm

I feel that you are over-complicating things here - the Brain measurement needs to be run on something at least as dark as the darkest area in the field of view you are imaging - that will ensure that you get a gain/exposure combination that is not being affected by camera read noise. H

The only reason you would need to re-do is if conditions improve markedly during the night or you move targets to start imaging in a darker area of sky (nearer the zenith, further from the moon, etc).

For instance a worked example - with a sky brightness of 1e/pixel/s, I get a recommendation of

gain=251, exposure = 21.2s, faintest visible object in a 1 hour stack = 0.0176 e/pixel/s

If the sky brightness drops to 0.5e/pixel/s the recommendation is

gain=251, exposure = 42.3s, faintest visible object in a 1 hour stack = 0.0125 e/pixel/s

but, if you stuck with the old recommendation with 0.5e/pixel/s

gain=251, exposure = 21.2s, faintest visible object in a 1 hour stack = 0.0131 e/pixel/s

So, if the sky background brightness halves, you get to see fainter things in a one hour stack, but most of the improvement comes just from the sky getting dimmer (about 90% of the total improvement). Only the last 10% or so comes from increasing the exposure to match the new conditions.

Hope this helps,

Robin

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

Post by mAnKiNd » Thu Sep 13, 2018 6:14 pm

admin wrote:
Thu Sep 13, 2018 5:02 pm
I feel that you are over-complicating things here - the Brain measurement needs to be run on something at least as dark as the darkest area in the field of view you are imaging - that will ensure that you get a gain/exposure combination that is not being affected by camera read noise. H
Hi Robin, thanks for chipping in. So you're saying yes, run the smart brain analysis in the fov you are imaging (your target), but perhaps use the selection box to select the darkest area in that fov?

What am actually asking is whether or not running the smart brain analysis tool again on your actual target is better once you know the value of your sky brightness, which you would have obtained by running it on a dark patch of the sky.

This opposed to running the smart brain analysis for determining sky brightness and exposure/gain values but in a part of the sky that is NOT your target, rather something darker without bright stars and nebulosity.

Hope this is clear
Cheers
Last edited by mAnKiNd on Thu Sep 13, 2018 6:37 pm, edited 3 times in total.

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

Post by mAnKiNd » Thu Sep 13, 2018 6:17 pm

oopfan wrote:
Thu Sep 13, 2018 3:21 pm
In Photometry there is a quantity called Airmass which is a function of zenith distance. Airmass affects the calculation of photometric magnitude by noting that the atmosphere reddens the color of a star.

We can extend this to AP by frequently rerunning The Brain on the same rectangle of dark sky and then using that calculated exposure for the next period of time.

How often should you resample? I would say at least once per hour. I know that Chuck from Chuck's Astrophotography has his SGP set up to refocus every half hour.

Brian
Brian thanks, I was thinking about the effects of atmosphere and running it again when your target has significantly changed altitude and position, but I think running it regularly might wreak havoc with what master darks you can use. Unless you're up for re obtaining them every time..

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

Post by mAnKiNd » Thu Sep 13, 2018 7:15 pm

I started a new topic for the smart histogram question.

viewtopic.php?f=19&t=1117

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

Post by oopfan » Fri Sep 14, 2018 11:37 pm

Minos,

According to the forecast software that I use you are correct to believe that Friday night will excel. Good luck with that and happy stargazing!

Brian

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

Post by mAnKiNd » Sat Sep 15, 2018 1:34 am

Thanks Brian, same to you!

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

Post by oopfan » Sat Sep 15, 2018 1:40 am

You are clear but I am in thin clouds.

Better for me to do Darks tonight.

Brian

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