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fextract stsdas.hst_calib.foc.foccs



fextract -- Extract stars to another image.


fextract input output refoc coords maximages


This task copies a region around each star in the coordinate list from the input image to the output image. If the output image does not exist it will be created; otherwise, the existing file is modified. The stars in the input image are copied into a horizontal strip in the output image; the strips for different input images are stacked vertically. Because the input images are at different focus positions, the focus will change vertically in the output image. A global background value will be subtracted from the input image (unless the b_option parameter is set to "none"), and the data values will be divided by the exposure time.


input = "" [string]
Name of the input image.
output = "" [string]
Name of the output image. If this already exists, it will be modified in-place. If it does not exist, it will be created with a size determined by the value of the maximanges parameter and the number of coordinate pairs in the coords parameter.
refoc = [integer]
Position of the refoc mirror.
coords = "" [string]
Name of the file containing X,Y coordinate pairs. This is the actual file name; it should NOT be preceded by an @ sign.
maximages = [integer]
Total number of focus images. If the output image does not exist it will be created large enough to contain this many rows, one for each focus image. If the output image already exists this parameter is ignored.
(rownum = -1) [integer]
Output row for current image. The default of -1 means that the current image will be placed in the next available row. You can specify a row that has already been filled in, in which case you will be prompted for confirmation. The task does this be getting the value of the newrow parameter.
(box= 29) [integer]
Width of box to extract.
(box= 29) [integer]
Height of box to extract.
(b_option = "sigclip") [string,
Allowed values: none | mean | sigclip]

This option specifies how background subtraction is to be done. Specifying "none" means no background should be subtracted. Specifying "mean" means the background is approximated by the mean of all values between lowcut and highcut inclusive. The "sigclip" method will determine the background in a two-step process. The mean of all pixel values is first found; call it M. For a Poisson distribution the mean is equal to the variance, so the standard deviation is approximated as the square root of the mean. Then the mean is gotten by including values within the range from M - NSIGMA * sqrt(M) to M + NSIGMA * sqrt(M). (As the pixel values are integers, the range limits are actually taken to be the nearest integers to the above limits.)

(lowcut = 0) [integer]
When computing the background with b_option=mean, values below this value are ignored. See also highcut.
(highcut = 5) [integer]
When computing the background with b_option=mean, values above 'highcut'1 are ignored.
(nsigma = 3.) [real]
When computing the background with b_option=sigclip, values more than this many standard deviations from the overall mean are ignored. See als b_option.
newrow = -[integer]
Output row for the current image.

If the row number specified by rownum has already been filled in, this parameter is used as a way of prompting for confirmation.

(exptime) [real, min=0.001, max=INDEF]
If the value of the exposure time keyword EXPTIME in the input header is less than or equal to zero, that value will be displayed, and the user will be prompted to give the correct exposure time. The data will be divided by this value.




This task was written by Phil Hodge.


Type "help foccs option=sys" for a higher-level description of the foccs package.

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