flatcombine: Combine and process flat field images

Package: ccdred

Usage

flatcombine input

Parameters

input

List of flat field images to combine. The ccdtype parameter may be used to select the flat field images from a list containing all types of data.

output = "Flat"

Output flat field root image name. The subset ID is appended.

combine = "average" (average|median)

Type of combining operation performed on the final set of pixels (after rejection). The choices are "average" or "median". The median uses the average of the two central values when the number of pixels is even.

reject = "avsigclip" (none|minmax|ccdclip|crreject|sigclip|avsigclip|pclip)

Type of rejection operation. See combine for details.

ccdtype = "flat"

CCD image type to combine. If no image type is given then all input images are combined.

process = yes

Process the input images before combining?

subsets = yes

Combine images by subset parameter? If yes then the input images are grouped by subset parameter and each group combined into a separate output image. The subset identifier is appended to the output and sigma image names. See subsets for more on the subset parameter. This is generally used with flat field images.

delete = no

Delete input images after combining? Only those images combined are deleted.

clobber = no

Clobber existing output images?

scale = "mode" (none|mode|median|mean|exposure)

Multiplicative image scaling to be applied. The choices are none, scale by the mode, median, or mean of the specified statistics section, or scale by the exposure time given in the image header.

statsec = ""

Section of images to use in computing image statistics for scaling. If no section is given then the entire region of the image is sampled (for efficiency the images are sampled if they are big enough).

Algorithm Parameters

nlow = 1, nhigh = 1 (minmax)

The number of low and high pixels to be rejected by the "minmax" algorithm.

nkeep = 1

The minimum number of pixels to retain or the maximum number to reject when using the clipping algorithms (ccdclip, crreject, sigclip, avsigclip, or pclip). When given as a positive value this is the minimum number to keep. When given as a negative value the absolute value is the maximum number to reject. This is actually converted to a number to keep by adding it to the number of images.

mclip = yes (ccdclip, crreject, sigclip, avsigcliip)

Use the median as the estimate for the true intensity rather than the average with high and low values excluded in the "ccdclip", "crreject", "sigclip", and "avsigclip" algorithms? The median is a better estimator in the presence of data which one wants to reject than the average. However, computing the median is slower than the average.

lsigma = 3., hsigma = 3. (ccdclip, crreject, sigclip, avsigclip, pclip)

Low and high sigma clipping factors for the "ccdclip", "crreject", "sigclip", "avsigclip", and "pclip" algorithms. They multiply a "sigma" factor produced by the algorithm to select a point below and above the average or median value for rejecting pixels. The lower sigma is ignored for the "crreject" algorithm.

rdnoise = "0.", gain = "1.", snoise = "0." (ccdclip, crreject)

CCD readout noise in electrons, gain in electrons/DN, and sensitivity noise as a fraction. These parameters are used with the "ccdclip" and "crreject" algorithms. The values may be either numeric or an image header keyword which contains the value.

pclip = -0.5 (pclip)

Percentile clipping algorithm parameter. If greater than one in absolute value then it specifies a number of pixels above or below the median to use for computing the clipping sigma. If less than one in absolute value then it specifies the fraction of the pixels above or below the median to use. A positive value selects a point above the median and a negative value selects a point below the median. The default of -0.5 selects approximately the quartile point. See combine for further details.

blank = 1.

Output value to be used when there are no pixels.

Description

The flat field images in the input image list are combined. If there is more than one subset (such as a filter or grating) then the input flat field images are grouped by subset and an combined separately. The input images may be processed first if desired. However if all zero level bias effects are linear then this is not necessary and some processing time may be saved. The original images may be deleted automatically if desired. The output pixel datatype will be real.

This task is a script which applies ccdproc and combine. The parameters and combining algorithms are described in detail in the help for combine. This script has default parameters specifically set for flat field images and simplifies the combining parameters. There are other combining options not included in this task. For these additional features, such as thresholding, offseting, masking, and projecting, use combine.

Examples

1. The image data contains four flat field images for three filters. To automatically select them and combine them as a background job using the default combining algorithm:

cl> flatcombine ccd*.imh&

The final images are "FlatV", "FlatB", and "FlatR".

See also

ccdproc, combine, subsets