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The Effect of ADU Count on Combined Flat Field

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Background

 

The purpose of Flat Field data reduction is to correct for optical defects in the system that alter the apparent light delivered to the CCD and to correct for variations in CCD sensor's response to light. As with any processing there is the potential to introduce noise. The point of making a master flat frame is to improve the signal to noise (S/N) ratio. This requires that one delivers enough signal to the CCD to overcome the read noise and that a sufficient number are added to average out the read noise that is introduced.


When I was just starting out everything was a mystery especially the concept if image calibration or image reduction. The way and specifics of dark frames (same time and same temperature as your light frames) and bias frames was relatively easy to grasp. However the best settings for flat field images was more difficult to pin down. I heard different average ADU counts, and different number of flat field frames to combine. Some people insisted that sky flats were better and others that light box flats were better. I decided to try and find out for myself, what worked best for my equipment. The following were my attempts to address the 3 questions:

  1. What is the optimal ADU count for my flats?
  2. What is the optimal number of flat frames to combine into the master flat?
  3. Are light box or sky flats better?
 

Methods

 

The first variable that I investigated was what the effect of varying the intensity in the flat field.

 

  • Each of the images below is composed of the same 12 luminance frames that were mean combined without noise reduction or deblooming.
  • These 12 luminance frames were calibrated with a Master Flat Frame created from a mean combine of 40 luminance Flat Fields. This is displayed in the center column.
  • The 40 luminance flat field images were acquired at the ADU level indicated using the Adirondack Video Digital Flat Fielder Light Panel.
  • The OTA was a Takahashi BRC-250 with a ST-10XME CCD.
  • All images were processed exactly the same in CCDStack and PhotoShop CS2. These images resulting from calibration with the varying ADU flat master frame are displayed in the right column.

 

Clicking on any image will enlarge it for closer inspection.

 

As you can see, I really needed to clean the optical path so this is a real test for a flat field. The flat fields worked amazingly well and the vignetting and dust motes are not visible to me in the final resulting image

Results

.
ADU Count of Flat
Field Subframe
Resulting Master Flat
Click on image for large version
Resulting Image Calibration
Click on image for large version
5,000


10,000


15,000


20,000


25,000

30,000


35,000


40,000


45,000


50,000

55,000


60,000



Impressions

It was apparent to me that, except for the extremes, there was very little difference.

  • At 5,000 & 10,000 ADU there was an increase in noise in the final calibrated image.
  • At 55,000 and 60,000 ADU there were artifacts created in the calibrated image due to saturation that occurred in the Flat Field image.
  • However, from around 15,000 to 50,000 ADU there was very little difference created by the change in Flat Field ADU.
  • Currently I take my flats around 35,000 ADU as this seemed the best compromise for quality and time to complete my flats.

Chris Abissi

 


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