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Different sub lengths?


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In normal circumstances, for a specific Gain setting...

Darks are exposure dependent - so yes, 60 and 120s flats in your example

Flats are specific to camera / scope set up on the night (maybe a few nights if you have changed nothing in the optical train). But they are not different for any particular exposure length.

 

 

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Back in the day with CCD camera - you did not have to do it.

Set of darks would easily scale over range of exposures.

With modern CMOS sensors - it's not as easy as that. In principle, if you make sure your sensor is "well behaved" - like no amp glow that depends on readout - good bias signal and time dependent dark current, then yes, you could do the same and reuse (with care and special handling / math involved) - same calibration files.

However, due to short exposures and fact that most people don't bother with bias and just shoot darks (which is perfectly fine and proper calibration) - you can't perform dark scaling, so - redo darks for different exposure length.

Flats are fine as long as you don't mess with camera orientation / spacing / focus.

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Quick question: why would you take different sub lengths? It is very rare indeed that an object's dynamic range requires it. M42 is the obvious example of one which does, but the difference in exposure time needs to be extreme in order to solve the problem.

99% of the time it makes more sense to use a single exposure time.

Olly

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6 hours ago, ollypenrice said:

Quick question: why would you take different sub lengths? It is very rare indeed that an object's dynamic range requires it. M42 is the obvious example of one which does, but the difference in exposure time needs to be extreme in order to solve the problem.

99% of the time it makes more sense to use a single exposure time.

Olly

Hi Olly, it's was m42....just experimenting really, I would always take the same exposure times on any other object, but thought I would try and not blow the core, but failed again lol.

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Different sub lengths can be useful or necessary outside of high dynamic range targets. Lets say its windy and we are imaging under a Moon or light pollution so short subs of 60s are preferable as they swamp read noise effectively due to the sky being bright but have smaller chance of wind caused loss of exposure. Then lets travel to dark skies under no Moon and this time no wind. Now i might take 240s subs to cover read noise and without wind to ruin the subs there are no downsides.

Sticking to a single sub length is doomed to be a compromise in some cases, its better to adapt the exposure to the conditions in my opinion. But different sub lengths in a single night? Not sure i see the point, even for M42.

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2 hours ago, ONIKKINEN said:

Different sub lengths can be useful or necessary outside of high dynamic range targets. Lets say its windy and we are imaging under a Moon or light pollution so short subs of 60s are preferable as they swamp read noise effectively due to the sky being bright but have smaller chance of wind caused loss of exposure. Then lets travel to dark skies under no Moon and this time no wind. Now i might take 240s subs to cover read noise and without wind to ruin the subs there are no downsides.

Sticking to a single sub length is doomed to be a compromise in some cases, its better to adapt the exposure to the conditions in my opinion. But different sub lengths in a single night? Not sure i see the point, even for M42.

For M42 it's essential, I think. With a CCD I used 10 secs, 50 secs and 15 minutes. To be useful, though, there needs to be a vast difference in sub length. The difference between 2 mins and 3 mins won't fix dynamic range issues.

M42%20TEC%20140-600x396.jpg

 

M42%20WIDE%20COMB%20best-600x407.jpg

Olly

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3 minutes ago, ollypenrice said:

For M42 it's essential, I think. With a CCD I used 10 secs, 50 secs and 15 minutes. To be useful, though, there needs to be a vast difference in sub length. The difference between 2 mins and 3 mins won't fix dynamic range issues.

M42%20TEC%20140-600x396.jpg

 

M42%20WIDE%20COMB%20best-600x407.jpg

Olly

Oh yeah, with older cameras for sure. Newer CMOS with close to 1e- read noise and decent fullwell will swamp read noise by at least 3x while still not saturating the core with a carefully chosen exposure. At least with my IMX571 its the case and i dont have to think about it too much.

For very dark skies such as yours and a very narrow Ha filter might still be an issue in the fullwell department with the core though.

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1 hour ago, ONIKKINEN said:

Oh yeah, with older cameras for sure. Newer CMOS with close to 1e- read noise and decent fullwell will swamp read noise by at least 3x while still not saturating the core with a carefully chosen exposure. At least with my IMX571 its the case and i dont have to think about it too much.

For very dark skies such as yours and a very narrow Ha filter might still be an issue in the fullwell department with the core though.

I haven't tried M42 with a CMOS. I'm sure you're right about the longer exposures: no need to go beyond 3 mins, but how about the Trapezium and its colour?

Olly

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1 hour ago, ollypenrice said:

I haven't tried M42 with a CMOS. I'm sure you're right about the longer exposures: no need to go beyond 3 mins, but how about the Trapezium and its colour?

Olly

I will admit i have not had the chance to image M42 from dark skies, and i suppose i will not get the chance since it is around 20 degrees from the horizon here so that might play into the idea that i think its easy to expose for.

But my most recent M42 image with 60s subs and an f/5 scope appears to have around 10k median ADU on a small selection box around the trapezium, and of this value somewhere around 1000-1500 is light pollution. So plenty of room to take longer subs, but not impossibly long. Was not a particularly narrow filter either, with around 25-30nm fwhm passes, so your mileage will most likely vary but shouldn't be too difficult if some thought goes into the exposure length. Actually the problem with f/2 scopes and the like is that you dont want to swamp by only x3, since the subs might be so short its a pain to deal with in the stacking phase. Then i would probably bother taking the second set of short subs.

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