Flat Field Study Page One
5,000 ADU
10,000 ADU
15,000 ADU
20,000 ADU
Lightbox
25,000 ADU
30,000 ADU
35,000 ADU
40,000 ADU
45,000 ADU
50,000 ADU
Sky Flat
25,000 ADU
55,000 ADU
60,000 ADU
The first variable that I investigated what the effect of varying intensity in the flat field. Each of the imaged below is composed of 12 luminance frames mean combined without noise reduction or deblooming after being calibrated with the same Master Dark Frame, but a Master Flat Frame created from a mean combine of 40 luminance Flat Fields aquired at the ADU level indicated using the Adirondack Video Digital Flat Fielder Light Panel. There is a single 40 frame flat field created from sky flats at 25,000 ADU for a comparison of Light box vs. Sky Flats.All flat fields were aquire using John Winfields Sky Flat Assistant plug-in with MaximDL. The OTA was a Takahashi BRC-250 with a ST-10XME CCD. All images were processed exactly the same in CCDStack and PhotoShop CS2. Clicking on any image will enlarge it for closer inspection. Clicking on the tab below will open the master Flat Field that was used for that image. As you will see, I really needed to clean the optical path so this is a hard test for a flat field.
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 blooming 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. More striking was that the Sky Flat did a better job of correcting the artifacts, especially the vignetting in my system. The histogram also has a sharper rise for the Sky Flat than the lightbox flat. Click of Page 2 to see the effect of number of flat fields.
Go to Second Page of Study
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. The estimates for ADU and for sufficent number vary from source to source. This
was an effort to answer the question for my CCD, the SBIG ST-10XME and other equipment.
