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Is this normal?

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#1 aroughroad

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Posted 02 December 2021 - 12:00 AM

I am building a dark library.  I began with 10-min exposures at -15C.  All seemed fine.  Now that I am doing 15-min exposures (also at -15C).  The 15-min exposures seem a little brighter.  All variables are the same except for the exposure lengths.

 

I use SGP and the mean ADU value for the 10-min frames is 491, whereas the mean ADU value for the 15-min frames is 502.  See below for two examples.  Is it normal that the 15-min exposures are a little brighter?

 

10-min.JPG 15-min.JPG   



#2 Tapio

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Posted 02 December 2021 - 01:17 AM

I think yes.

But have to ask - what camera are you using for needing 15 min lights ?

These newer CMOS camera certainly don't need such long exposures.



#3 aroughroad

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Posted 02 December 2021 - 01:22 AM

I think yes.

But have to ask - what camera are you using for needing 15 min lights ?

These newer CMOS camera certainly don't need such long exposures.

I'm using ZWO's ASI6200mm.  The file sizes are huge (120mb), and the detail I get in a 15-min sub looks awesome.  I have a mount with encoders so 15-mins is easily handled.



#4 F.Meiresonne

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Posted 02 December 2021 - 01:35 AM

I think 15 x 1m  should be probably as awesome,but  more computer time to stack and handle. Also i would think with 15 min subs you will get saturated stars in some cases or overblown cores of certain objects.


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#5 OldManSky

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Posted 02 December 2021 - 01:38 AM

Yes, it's normal.  Dark current is a function of time and temperature -- the more time, the more dark current.  The more dark current, the higher the mean value.

And I agree with Freddy -- 15 min. on that chip is likely to blow out an awfully large bunch of stars.  But hey, if it's working for you, run with it.



#6 dcm_guitar

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Posted 02 December 2021 - 01:39 AM

I'm using ZWO's ASI6200mm.  The file sizes are huge (120mb), and the detail I get in a 15-min sub looks awesome.  I have a mount with encoders so 15-mins is easily handled.

I have the same camera and it's very nice!!  Yes, the sub files are BIG, and sometimes processing projects can take a long time when you take relatively short exposures.

 

The 15-min sub isn't about what a mount with encoders can handle.  Depending on how much light pollution is in your skies, you may simply be collecting unwanted light with such long exposures.  UNless you are at a very dark site, a 15 minute exposure is not really going to help.



#7 aroughroad

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Posted 02 December 2021 - 02:00 AM

Yes, it's normal.  Dark current is a function of time and temperature -- the more time, the more dark current.  The more dark current, the higher the mean value.

And I agree with Freddy -- 15 min. on that chip is likely to blow out an awfully large bunch of stars.  But hey, if it's working for you, run with it.

 

I have the same camera and it's very nice!!  Yes, the sub files are BIG, and sometimes processing projects can take a long time when you take relatively short exposures.

 

The 15-min sub isn't about what a mount with encoders can handle.  Depending on how much light pollution is in your skies, you may simply be collecting unwanted light with such long exposures.  UNless you are at a very dark site, a 15 minute exposure is not really going to help.

Interesting.  I live in Bortle 7/8 skies and use Chroma 3nm NB filters (imaging exclusively in NB given light pollution).  Below is a 10-min sub and a 15-min sub of the same object, taken in successive evenings.  10-min sub is without a reducer and with a system at F/6.9.  15-min sub is with a reducer operating at F/5.5.

 

Do you think stars are blown out in the 15-min example?  I guess it depends on the target (15-min definitely overkill for the Orion Nebula, for instance, but Heart Nebula is pretty dim relatively speaking).  What do you all think?

 

10 Min Sub
15 Min Sub


#8 bobzeq25

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Posted 02 December 2021 - 02:10 AM

It's absolutely normal for the 15' exposure to be brighter.  Darks measure the thermal noise of the camera.  The longer the exposure, the more thermal noise.



#9 bobzeq25

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Posted 02 December 2021 - 02:12 AM

Interesting.  I live in Bortle 7/8 skies and use Chroma 3nm NB filters (imaging exclusively in NB given light pollution).  Below is a 10-min sub and a 15-min sub of the same object, taken in successive evenings.  10-min sub is without a reducer and with a system at F/6.9.  15-min sub is with a reducer operating at F/5.5.

 

Do you think stars are blown out in the 15-min example?  I guess it depends on the target (15-min definitely overkill for the Orion Nebula, for instance, but Heart Nebula is pretty dim relatively speaking).  What do you all think?

 

You can't tell from a stretched sub.  What you want to do is measure how many pixels are clipped high at the maximum for the camera.  You want a few to be clipped, but many hundreds are bad.  I use PixInsight to measure that, there are other ways.


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#10 aroughroad

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Posted 02 December 2021 - 02:34 AM

You can't tell from a stretched sub.  What you want to do is measure how many pixels are clipped high at the maximum for the camera.  You want a few to be clipped, but many hundreds are bad.  I use PixInsight to measure that, there are other ways.

Interesting, helpful.  I have not yet explored PI or any other processing software (just learning how to take subs for now).  Guess it’s time I start with PI and figure this all out.  Thanks for the help!



#11 bobzeq25

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Posted 02 December 2021 - 02:45 AM

Interesting, helpful.  I have not yet explored PI or any other processing software (just learning how to take subs for now).  Guess it’s time I start with PI and figure this all out.  Thanks for the help!

Consider Astro Pixel Processor.  Has a major advantage of PI, calibrates/stacks/processes in one program.  Has a good gradient reduction tool, useful for reducing the effects of light pollution.  Capable of excellent results.  The numbered workflow actually teaches you how to process.

 

The major advantage of APP over PI.  Much easier to learn and use.  PI takes up rather too much of a beginners time, so learning other things gets impacted.




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