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Old 30-05-2014, 11:30 PM
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There is a natural limitation of how many flats you can get if you take dusk/dawn flats. That is how long it takes to expose, download and save in rapidly changing light conditions.

This would also be an argument for using artificial light flats as there is no rush.

Greg.
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Old 31-05-2014, 12:25 PM
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Originally Posted by Shiraz View Post
.....
If this is correct though, the more flats the better and you will always gain something (even if small) by adding flat data. It also means that those who are forced to image under bright skies should stock up on lots of flats and that the "mega" brigade should also go mega on flats. And maybe I can get some better results out of the many sets of lights that I currently have by just adding more flats to the calibration pot.
Hmmm ok, will give that a go. I usually use around 7 or so flats on the RC12. This has been largely a guess. So I will get a bit more precise about this and work out the numbers required. On the TSA I have been using 16 flats on each filter.

These days I tend to do flats on the single colour I am imaging that night.
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  #23  
Old 31-05-2014, 02:16 PM
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That appears to reflect my recent experience, Ray. I'm away from my scope and data for a few weeks but will do some experiments when I get back. I look forward to your results...

Cheers,
Rick.
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Originally Posted by gregbradley View Post
There is a natural limitation of how many flats you can get if you take dusk/dawn flats. That is how long it takes to expose, download and save in rapidly changing light conditions.

This would also be an argument for using artificial light flats as there is no rush.

Greg.


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Originally Posted by Paul Haese View Post
Hmmm ok, will give that a go. I usually use around 7 or so flats on the RC12. This has been largely a guess. So I will get a bit more precise about this and work out the numbers required. On the TSA I have been using 16 flats on each filter.

These days I tend to do flats on the single colour I am imaging that night.
thanks for the feedback and info.

Reworked the theory and will attempt to validate it tonight if all goes well - will start a new thread, since it could possibly be useful to others who might not read this one.

Looks like you can control flat-induced noise simply by making sure that you have at least 10x as many total flat electrons as you have total sky electrons. eg, if you take 100 lights where the sky background in each is 3,000ADU above the bias, you need to take 100 flats if you use 30,000 ADU flats. It has nothing to do with how many subs - just how many electrons - so if you took flats with say 10,000 ADU in the above example, you would need to take 300 of them. If you image for a long time, take a lot of flats, if you have bright sky, take a lot of flats.

Not yet sure how much of an issue colour is for noise reduction - guess that dust on filters might make separate flats worthwhile for getting rid of bunnies if that is a problem. Quite a bit of other interesting stuff to look at as well, including the possibility that past images can be improved by using two different sets of flats - one to control noise and the other to get rid of vignetting etc .
Regards ray

Last edited by Shiraz; 31-05-2014 at 02:49 PM.
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Old 31-05-2014, 02:55 PM
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Originally Posted by Shiraz View Post
thanks for the feedback and info.

Reworked the theory and will attempt to validate it tonight if all goes well - will start a new thread, since it could possibly be useful to others who might not read this one.

Looks like you can control flat-induced noise simply by making sure that you have at least 10x as many total flat electrons as you have total sky electrons. eg, if you take 100 lights where the sky background in each is 3,000ADU above the bias, you need to take 100 flats if you use 30,000 ADU flats. It has nothing to do with how many subs - just how many electrons - so if you took flats with say 10,000 ADU in the above example, you would need to take 300 of them. If you image for a long time, take a lot of flats, if you have bright sky, take a lot of flats.

Not yet sure how much of an issue colour is for noise reduction - guess that dust on filters might make separate flats worthwhile for getting rid of bunnies if that is a problem. Quite a bit of other interesting stuff to look at as well, including the possibility that past images can be improved by using two different sets of flats - one to control noise and the other to get rid of vignetting etc .
Regards ray
Holy hell. My subs are around 3300ADU so I need 33000 ADU flats. So if I take flats at 30000 ADU I will need 10 flats; is that right? What about if I take the at 26000? Am I to assume it would 13? Confused now.
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Old 31-05-2014, 03:43 PM
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Holy hell. My subs are around 3300ADU so I need 33000 ADU flats. So if I take flats at 30000 ADU I will need 10 flats; is that right? What about if I take the at 26000? Am I to assume it would 13? Confused now.
yes, no and no you need only one 33,000ADU flat for each 3,300ADU light to get 10x. The only question is if you are including the bias in your 3300 for the lights - it will reduce the numbers a bit if you have not subtracted the bias. Regardless, a good enough rule of thumb for your system would be to take one 30,000 flat for every light - it is not critical that you get it exact but err on the "longer" side.

30,000ADU flats are good to get above read noise and below saturation - but you could use other flats if you compensate. For example if you wanted to use flats of 15,000 ADU, you would need twice as many of them as you would need of 30,000 ADU flats. the total ADU in all of your flats just has to add up to 10x the total sky ADU summed over your light frames.

making a few guesses on your imaging system characteristics, you would need maybe 30 flats of 30,000 ADU for each 10 hours of imaging.

Last edited by Shiraz; 31-05-2014 at 03:59 PM.
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  #26  
Old 31-05-2014, 03:56 PM
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Originally Posted by Shiraz View Post
yes, no and no you need only one 33,000ADU flat for each 3,300ADU light to get 10x . The only question is if you are including the bias in your 3300 for the lights - it will reduce the numbers a bit if you have not subtracted the bias. Regardless, a good rule of thumb for your system would be to take one 30,000 flat for every light - it is not critical that you get it exact but err on the "longer" side.

30,000ADU flats are good to get above read noise and below saturation - but you could use other flats if you compensate. For example if you wanted to use flats of 15,000 ADU, you would need twice as many of them as you would need of 30,000 ADU flats. the total ADU in all of your flats just has to add up to 10x the total sky ADU in your lights.

making a few guesses on your imaging system characteristics, you would need maybe 30 flats of 30,000 ADU for each 10 hours of imaging.
I will give it a go and get back to you with the results. It is easy to get all the flats I need for each project. Its worth a try to get a defined result.
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