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#1
13-10-2012, 07:15 PM
 roughy (Mark) Registered User Join Date: May 2011 Location: Hobart Posts: 112
Flat Fields Integration Time

Is there a formula for calculating the integration time for a flat based on your chip and camera characteristics? I have been perusing the "Handbook of Astronomical Image Processing" but cannot seem to find a specific equation.

Cheers

Mark
#2
13-10-2012, 08:08 PM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 15,476
Quote:
 Originally Posted by roughy Is there a formula for calculating the integration time for a flat based on your chip and camera characteristics? I have been perusing the "Handbook of Astronomical Image Processing" but cannot seem to find a specific equation. Cheers Mark

I generally shoot until I get about 20,000 ADU. His paper suggests to do more as CCD responses are linear.

On the other hand I have had flats that are too bright cause unwanted effects in my images. Not 100% sure why. So I tend to stick to around 20,000 ADU plus or minus a bit. I also take a dark and subtract it when doing the master dark.

Also using good exposure darks helps with the flat fielding. A poor dark seems to throw off the flat fielding as well.

Greg.
#3
15-10-2012, 11:39 AM
 solissydney (Ken) Registered User Join Date: Mar 2006 Location: Sydney Posts: 660
Flats

What?
#4
15-10-2012, 12:00 PM
 [1ponders] (Paul) Retired, damn no pension Join Date: Nov 2004 Location: Obi Obi, Qld Posts: 18,778
I'm with Greg, though I go a bit higher with 25-27K ADU. I have my Flats settings in Maxim set to 0.3 sec and adjust the brightness of my flat box to achieve the 25-27K. Any longer than about 0.5 sec and I take flat darks. For me using the SX cameras 0.3sec seems to be fine without flat darks.

ADU = Analog to Digital Unit. A conversion factor that converts your analog camera signal to digital units. These days most seem to be in the range of around 0.3 - 0.9 conversion rate (= gain. thanks Rick ). Short and simple, if your camera has a full well depth of 20,000 electrons to convert that to 16 bit digital units you need to divide your 20,000 by approx 0.3 to get 65535 ADU (or 16 bit). The deeper the well depth the larger the ADU conversion (gain) can be.

Last edited by [1ponders]; 15-10-2012 at 12:24 PM.
#5
15-10-2012, 12:09 PM
 RickS (Rick) PI cult recruiter Join Date: Apr 2010 Location: Brisbane Posts: 10,584
The stuff by Richard Crisp is here: http://www.narrowbandimaging.com/flat_fielding_page.htm

It's not easy reading for the new or faint-hearted

Quote:
 Originally Posted by solissydney ADU's ???????? What?
ADU = analog-digital unit. When your image analysis program tells you that the value in a pixel is 10456 then that's measured in ADUs. The other measure that is used is the number of electrons captured. The number of electrons is scaled by a gain factor to give ADUs. The gain factor varies depending on the characteristics of the camera sensor.

Ideally you want your flats to have a high average ADU count without going so high that you stray into the non-linear area of your CCD's performance curve. There isn't a specific formula for this because it will depend on your sensor. The usual advice is to target around 20,000 to 30,000 ADU. Richard Crisp's stuff talks about how many flat frames are needed to reduce flat-fielding noise to an appropriate level.

Cheers,
Rick.

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