Stan,

I want to calibrate my data taken with an STL-11K camera. I shoot flat fields from a light box and have the aquisition software (Maxim DL) automatically subtract a dark frame from each flat field image. I do not use bias frames when calibrating because I do not scale my image dark frames and the flat field images have been dark subtracted.

I want to know if I need to subtract a pedestal value (which I assume would be 100) from the flat field images.

Richard

Hi Richard,

I presume that MaxIm still uses a 100 ADU pedestal, so enter that value in the Calibration form:

check "Subtract Bias/Pedestal from Flat";
check "use pedestal";
enter "100".

Stan

Stan,

Here is the FITS header from one of my flat field images. For pedestal, it says "0". If this is the case, then when calibrating I should not subtract a pedestal value from the flat in CCDStack, correct? Please confirm.

Thanks, Richard

SIMPLE = T
BITPIX = 16 /8 unsigned int, 16 & 32 int, -32 & -64 real
NAXIS = 2 /number of axes
NAXIS1 = 4008 /fastest changing axis
NAXIS2 = 2672 /next to fastest changing axis
BSCALE = 1.0000000000000000 /physical = BZERO + BSCALE*array_value
BZERO = 32768.000000000000 /physical = BZERO + BSCALE*array_value
INSTRUME = 'SBIG ST-L-11K 3 CCD Camera' /instrument or camera used
DATE-OBS = '2009-06-28T10:28:12' /YYYY-MM-DDThh:mm:ss observation start, UT
EXPTIME = 14.000000000000000 /Exposure time in seconds
EXPOSURE = 14.000000000000000 /Exposure time in seconds
SET-TEMP = -20.000000000000000 /CCD temperature setpoint in C
CCD-TEMP = -19.977100736655856 /CCD temperature at start of exposure in C
XPIXSZ = 9.0000000000000000 /Pixel Width in microns (after binning)
YPIXSZ = 9.0000000000000000 /Pixel Height in microns (after binning)
XBINNING = 1 /Binning factor in width
YBINNING = 1 /Binning factor in height
XORGSUBF = 0 /Subframe X position in binned pixels
YORGSUBF = 0 /Subframe Y position in binned pixels
FILTER = 'Lum ' / Filter used when taking image
IMAGETYP = 'Light Frame' / Type of image
OBJCTRA = '18 04 38' / Nominal Right Ascension of center of image
OBJCTDEC = '+43 02 46' / Nominal Declination of center of image
EGAIN = 0.80000001192092896 /Electronic gain in e-/ADU
FOCALLEN = 750.00000000000000 /Focal length of telescope in mm
APTDIA = 150.00000000000000 /Aperture diameter of telescope in mm
APTAREA = 17671.459168195724 /Aperture area of telescope in mm^2
SWCREATE = 'MaxIm DL Version 4.51' /Name of software that created the image
SBSTDVER = 'SBFITSEXT Version 1.0' /Version of SBFITSEXT standard in effect
HISTORY Dark subtraction (Simple Auto-dark)
CSTRETCH = 'Medium ' / Initial display stretch mode
CBLACK = 21192 /Initial display black level in ADUs
CWHITE = 22231 /Initial display white level in ADUs
PEDESTAL = 0 /Correction to add for zero-based ADU
SWOWNER = 'Richard J Fletcher' /Licensed owner of software
INPUTFMT = 'FITS ' / Format of file from which image was read

Unfortunately, a few parties use a weird and obsolete definition of "pedestal" and they always set it to zero, regardless of the actual pedestal. SBIG does (or did) this too.

I am not at all current with MaxIm, but the last time I used the program it was unable to handle negative values (e.g. inputting a floating point FITS with negative resulted in MaxIm inflating all ADU to lift the negative into positive territory).

So I suggest a simple test to see what MaxIm is really doing: Take any frame and use it to dark-subtract itself in MaxIm. Then read the resulting values, which will be the actual pedestal. I’ll bet that it is 100 ADU just because MaxIm has such a long history of doing that.

Stan

Hi Stan,

OK, I dark subtracted my example flat field image from itself and here is an updated FITS header. It now shows the pedestal to be -100. Not sure what this means but I have noted that whenever I manually dark subtract an image during Image Calibration in Maxim (as opposed to having the software do it automatically during acquisition), it shows the pedestal to be -100. Perhaps you can make a recommendation based on this information as I don't fully understand what is going on.

Thanks, Richard

SIMPLE = T
BITPIX = 16 /8 unsigned int, 16 & 32 int, -32 & -64 real
NAXIS = 2 /number of axes
NAXIS1 = 4008 /fastest changing axis
NAXIS2 = 2672 /next to fastest changing axis
BSCALE = 1.0000000000000000 /physical = BZERO + BSCALE*array_value
BZERO = 32768.000000000000 /physical = BZERO + BSCALE*array_value
INSTRUME = 'SBIG ST-L-11K 3 CCD Camera' /instrument or camera used
DATE-OBS = '2009-06-28T10:28:12' /YYYY-MM-DDThh:mm:ss observation start, UT
EXPTIME = 14.000000000000000 /Exposure time in seconds
EXPOSURE = 14.000000000000000 /Exposure time in seconds
SET-TEMP = -20.000000000000000 /CCD temperature setpoint in C
CCD-TEMP = -19.977100736655856 /CCD temperature at start of exposure in C
XPIXSZ = 9.0000000000000000 /Pixel Width in microns (after binning)
YPIXSZ = 9.0000000000000000 /Pixel Height in microns (after binning)
XBINNING = 1 /Binning factor in width
YBINNING = 1 /Binning factor in height
XORGSUBF = 0 /Subframe X position in binned pixels
YORGSUBF = 0 /Subframe Y position in binned pixels
FILTER = 'Lum ' / Filter used when taking image
IMAGETYP = 'Light Frame' / Type of image
OBJCTRA = '18 04 38' / Nominal Right Ascension of center of image
OBJCTDEC = '+43 02 46' / Nominal Declination of center of image
EGAIN = 0.80000001192092896 /Electronic gain in e-/ADU
FOCALLEN = 750.00000000000000 /Focal length of telescope in mm
APTDIA = 150.00000000000000 /Aperture diameter of telescope in mm
APTAREA = 17671.459168195724 /Aperture area of telescope in mm^2
SWCREATE = 'MaxIm DL Version 4.51' /Name of software that created the image
SBSTDVER = 'SBFITSEXT Version 1.0' /Version of SBFITSEXT standard in effect
HISTORY Dark subtraction (Simple Auto-dark)
CSTRETCH = 'Medium ' / Initial display stretch mode
CBLACK = 21192 /Initial display black level in ADUs
CWHITE = 22231 /Initial display white level in ADUs
PEDESTAL = -100 /Correction to add for zero-based ADU
SWOWNER = 'Richard J Fletcher' /Licensed owner of software
INPUTFMT = 'FITS ' / Format of file from which image was read
SWMODIFY = 'MaxIm DL Version 4.51' /Name of software that modified the image
HISTORY Dark Subtraction (Dark 1, 4008 x 2672, Bin1 x 1, Temp -20C,
HISTORY Exp Time 14s)
CALSTAT = 'D '

Stan,

I think I've got it now. I measured the image ADU after making the subtraction described in my last email and was left with a value of 100 throughout the image, just as you predicted.

So, I will invoke subtraction of a pedestal value of 100 from my flat frames as you suggested earlier.

Many Thanks, Richard

quote:

PEDESTAL = -100 /Correction to add for zero-based ADU

OK, now MaxIm is correctly filling in that value (at least for their version of it):

PEDESTAL = -100 /Correction to add for zero-based ADU

That means it is necessary to *add* -100 to the FITS pixels in order to know the "zero based ADU" (i.e. pedestal = 0).

Unfortunately the pedestal keyword is implemented in (at least) 2 ways: additive and subtractive (applied to orignal value or applied to file value). And CCDStack uses the oposite interpretation than MaxIm.

Thus MaxIm pedestal = -100 is the same thing as CCDStack 100.

enter "100" in the calibration form.

Stan