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Hello Yan,It is not common to use both the prescan and overscan columns at the
same time. The standard IRAF tasks for handling bias (colbias and
ccdproc) do not allow use of more than one contiguous region for
bias. However, if you wish to do so you can use FIT1D to do what you
want. The parameters I recommend are:
I R A F
Image Reduction and Analysis Facility
PACKAGE = imfit
TASK = fit1dinput = Images to be fit
output = Output images
(axis = 1) Axis to be fit
type = difference Type of output (fit, difference, ratio)
(interac= no) Set fitting parameters interactively?
(sample = 1-48,2096-2158) Sample points to use in fit
(naverag= -100) Number of points in sample averaging
(functio= chebyshev) Fitting function
(order = 2) Order of fitting function
(low_rej= 0.) Low rejection in sigma of fit
(high_re= 0.) High rejection in sigma of fit
(niterat= 1) Number of rejection iterations
(grow = 0.) Rejection growing radius in pixels
(graphic= stdgraph) Graphics output device
(cursor = ) Graphics cursor input
(mode = ql)The type of "difference" subtracts the bias, the "sample" parameter
defines the data to use for the bias. The value of -100 says to take
the median of each bias region (a median is good to avoid bad pixels
and cosmic rays), and the function and order fit a line between the
two regions. The value of the fit is subtracted from each pixel in
the image. Read the help page for this task for the various options.To later trim away the bias regions you would use imcopy with image
sections.cl> imcopy <image>[49:2095,*] <newimage>Yours,
Frank ValdesOn Nov 9, 2005, at 6:37 AM, Yan Grange wrote:> Dear IRAF crew,
>
> I have a question concerning the colbias command in the noao.bias
> package of IRAF. I am working with a telescope which uses columns
> 1-48 and 2096-2158 as overscan regions. The bias is found by the
> colbias command. I was wondering if it is poissible to set up the
> parameters in such a way that both sections (so 1-48 and 2096-2158)
> are used for this purpose. Could you help me with this question?
>
> thank you in advance,
>
> Yan Grange