imstat / ccdproc

cristian wrote on Aug 05, 2008

I have created an image: dark_v_median.fit using darkcombine.
The parameters for this are:

PACKAGE = ccdred
TASK = darkcombine

input = dark_v_0*.fit List of dark images to combine
(output = dark_v_median.fit) Output dark image root name
(combine= median) Type of combine operation
(reject = minmax) Type of rejection
(ccdtype= ) CCD image type to combine
(process= no) Process images before combining?
(delete = no) Delete input images after combining?
(clobber= no) Clobber existing output image?
(scale = exposure) Image scaling
(statsec= ) Image section for computing statistics
(nlow = 0) minmax: Number of low pixels to reject
(nhigh = 1) minmax: Number of high pixels to reject
(nkeep = 1) Minimum to keep (pos) or maximum to reject (neg)
(mclip = yes) Use median in sigma clipping algorithms?
(lsigma = 3.) Lower sigma clipping factor
(hsigma = 3.) Upper sigma clipping factor
(rdnoise= 0.) ccdclip: CCD readout noise (electrons)
(gain = 1.) ccdclip: CCD gain (electrons/DN)
(snoise = 0.) ccdclip: Sensitivity noise (fraction)
(pclip = -0.5) pclip: Percentile clipping parameter
(blank = 0.) Value if there are no pixels
(mode = ql)

I also used the zerocombine and flatcombine tasks to create: bias_v_median.fit and flat_v_average.fit

Before using ccdproc I get the following statistics for dark_v_median.fit:

ccdred> imstat dark_v_median.fit
# IMAGE NPIX MEAN STDDEV MIN MAX
dark_v_median.fit 390150 106.9 4.563 92. 189.

But the problem is that after using the ccdproc task (where I use this three new combined images) to get the processed images I get the following statistics for the same image dark_v_median.fit:

ccdred> imstat dark_v_median.fit
# IMAGE NPIX MEAN STDDEV MIN MAX
dark_v_median.fit 390150 0.2917 3.996 -20. 18.

I don't know why are the statistics changing after I use ccdproc.
Please ...any ideas?

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Francisco Valdes wrote on Aug 05, 2008

If I understand you, you use ccdproc to remove overscan, bias, etc. You did not include the ccdproc parameters. Ccdproc normally subtracts an overscan from all exposures. It also automatically processes calibration files even if they are not explicitly specified as input. So my guess is that ccdproc has subtracted the overscan from the dark. This is normal and I would expect the mean value of a dark to be near zero.

So my interpretation is that the first imstat shows you the pixel values with an electronic bias of roughly 106 ADU and after overscan subtraction to remove this bias the second imstat shows the actual dark counts with an average near zero.

Yours,
Frank Valdes

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Last post on Aug 05, 2008