Flat Frames Capture - impact of Dark Frames

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Luling_Skies
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Flat Frames Capture - impact of Dark Frames

Post by Luling_Skies » Fri Jun 08, 2018 3:10 am

When capturing flat frames, does it matter if a file is populated in the "Subtract Dark" field?

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Tim

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Re: Flat Frames Capture - impact of Dark Frames

Post by admin » Fri Jun 08, 2018 5:59 am

Hi,

current versions of SharpCap tell you to turn off dark subtraction when capturing a flat which is actually wrong. I have just written code in the last week that automatically captures and subtracts bias frames when making a flat frame which basically has the same effect as using a dark as flats are taken with short enough exposures that the main effect is the constant offset.

cheers,

Robin

jeff2011
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Re: Flat Frames Capture - impact of Dark Frames

Post by jeff2011 » Sun Jun 10, 2018 4:28 pm

Was about to post a question about calibrating flats and found this thread. In addition to the bias option, but be nice to be able to apply dark subtraction to flats. That is you could change your validation to a warning and then apply the darks selected to the flats. Would have to remember to change the dark selected for the lights. Alternatively and perhaps better, provide a selection for flat darks (or is it dark flats).

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Re: Flat Frames Capture - impact of Dark Frames

Post by admin » Sun Jun 10, 2018 8:35 pm

Hi,

my view is that the difference between a true dark (ie a frame taken with no illumination at the same exposure settings as a light frame) and a bias frame ( a frame taken with minimum exposure but otherwise the same settings as a light frame) is negligble in the case of taking flats (ok, I'm assuming here that flats are taken with a decent illumination and hence an exposure of < 1s). The contribution of sensor thermal noise in a sub 1s exposure is basically zero when compared to a properly exposed flat - all that we need to worry about is subtracting any uniform offset to the frame added by the camera (due to offset/black level/brightness controls) and for that job a bias frame will work nicely.

You don't even need stop the flat frame illumination - if you take flats at 0.1s and bias frames at 0.001s then the result of (flat - bias) is going to be 0.99x true flat signal with offset removed, which is itself a perfectly usable flat frame.

cheers,

Robin

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Re: Flat Frames Capture - impact of Dark Frames

Post by jeff2011 » Mon Jun 11, 2018 12:41 am

Hello Robin,

Your view is correct that the offset is the most important thing to correct in the flat but there are a few things to consider.

When taking narrow band flats it is not uncommon for me to take flats that are 20 seconds. My first generation ASI1600 mono has issues with flats between 5 and 20 seconds so if I can't get it under 5 seconds, I need to dim down my flat panel to get 20 second flats. This probably won't be the case for most people so I can understand not putting dark flats into the app as there are other more important features that you are wanting to add.

Some cameras (there are some Cloudy Nights threads about this) do not react well to biases taken at certain very short exposures. When you add the bias feature for correcting flats, it would be good allow the user to specify the duration of the bias.

From my perspective, I am a DSO imager. I have always thought of your software being specific to planetary imaging but recently a friend of mine demonstrated SharpCap to me which he used to capture DSO images on a C8 with an ASI294 camera with impressive results using live stack and short integration times. You also have some really invovative features. I just bought a lifetime license for the Pro version and look forward to using it.

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Re: Flat Frames Capture - impact of Dark Frames

Post by admin » Mon Jun 11, 2018 5:53 am

Thanks for the feedback - definitely a good point about the minimum exposure issues - I have seen that issue myself but had forgotten about it in the code for subtracting the bias frames :( Maybe 2 or 5x minimum exposure would be a safer bet.

Hope that you continue to enjoy imaging with SharpCap.

cheers,

Robin

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