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Flats - Exposure Time


BlueAstra
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I have a query about Flats. I recently noticed that even though I had applied flats and bias to my images in DSS, I could still see dust doughnuts in the final PS processed image. I have processed images previously with DSS with no dust apparent on the final image (somehow!). In Maxim DL the image ADU range was 2500-13300, and the master flat was 37000-39000. Out of interest I took the Master Flat and changed the levels in PS so that the low and high cursors just touched the histogram, apparently giving an ADU range of 11000-30000, but when combined with the image this just made things worse. So does it matter what exposure level is used when taking flats? Should it be at a similar level to the image, or should it all 'come out in the wash' when submitted to DSS?

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It depends on the strength of the illumination of the flat field been used to illuminate the flat and what works best for your chip, its a bit of trial and error. it will also vary per filter.

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Probably best to google this using the specifics of your camera, when I used an SX-9 I got good results at 35000 ADU, but with my ATIK 338l+ the results of the Flats at 35000 were useless.  After searching on SGL and the web I found that 22000 worked best.

 

Using sequence generator pro (Flats wizard tool), I am able to get the correct exposure for each filter, at the desired ADU

 

Mark

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I agree with Mark - I found around 23000 was best for my 383l+ but I guess will vary with the chip.

APT also has a flat generation wizard and is free although I have found that the SGP one is more intuitive and can find the correct exposure more quickly.

Both control the filter wheel and can cycle through a complete set of LRGB flat generation.  You set the required ADU and simply run the routine. 

I use a LED white screen which I place over the top of the telescope with adjustable brightness to help the flat generation program keep the exposures in a practical range to achieve the set ADU value.

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Am I right in thinking that ATP and SGP will determine the correct flat exposure time IF you enter the desired average ADU? If you do not know the 'best' ADU for your camera it won't work it out for you? It sounds from the comments that an optimum 'flats' ADU for a particular camera type exists, and it is valid for any light exposure time. Is this correct?

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I, too, go for about 23000 ADU. Are you subtracting a master bias from your flats? (The textbook says it should be a dark at the same settings as your flats but this isn't necessary. A master bias at short exposures will be the same as a dark.)

Olly

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13 minutes ago, ollypenrice said:

I, too, go for about 23000 ADU. Are you subtracting a master bias from your flats? (The textbook says it should be a dark at the same settings as your flats but this isn't necessary. A master bias at short exposures will be the same as a dark.)

Olly

I also use 23000 as a target for my flats......probably because when I asked, Olly told me thats what he uses :) . 

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2 hours ago, BlueAstra said:

Am I right in thinking that ATP and SGP will determine the correct flat exposure time IF you enter the desired average ADU? If you do not know the 'best' ADU for your camera it won't work it out for you? It sounds from the comments that an optimum 'flats' ADU for a particular camera type exists, and it is valid for any light exposure time. Is this correct?

The Flats Calibration Wizard in SGPro will work out how much exposure you need per filter for a given ADU - but it won't tell you what ADU to aim for.

Peter

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51 minutes ago, Petergoodhew said:

The Flats Calibration Wizard in SGPro will work out how much exposure you need per filter for a given ADU - but it won't tell you what ADU to aim for.

Peter

Same goes for APT

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