NAME

asncombine -- Process and combine images in one or two NICMOS associations

USAGE

asncombine association

PARAMETERS

association

calnicb style association file with list of images to be processed and co-added.

mask

bad pixel mask to be applied to all the images (in addition to any masks in the NICMOS image.

doerrors

set to yes to include error planes in co-added images error.

background

name of background image to be subtract from each image before. The background image will be scaled (i.e. multiplied with a factor) before it is subtracted.

keyback

An alternative way to specify a background file is to put the filename into the image header and specify the header keyword. This is useful to use different background subtractions for different images.

fheader

set to yes in order to multiply the background by the value of keyword "factor" in the image header. If the keyword is not present, the factor will be determined according to the setting of the following two keywords.

fmin

If not scaling of the background from the header keyword is desired (fheader=no) or the header keyword "factor" is not found, a number of different scalings of the background as specified in the following parameters will be tried. fmin is the

 minimum factor to be used.

fmax

maximum factor to multiply background

nf

number of factors for background scaling to try

bckdisplay

set to yes to display images after the background with different scaling has been subtracted.

regtable

The best factor for the background scaling is determined by computing the rms in two regions of the image and selecting the factor with the lowest value. If regtable is set to yes, then the regions to perform the statistics on are taken from the assoication table header. The regions in the table header are relative to the shifted images.

region1

If regtable is set to no, then the region specified with this parameter is used as the first region to perform statistics on. This refers to the unshifted image, and the same region is used for all images.

region2

If regtable is set to no, then the region specified with this parameter is used as the second region to perform statistics on. This refers to the unshifted image, and the same region is used for all images.

submedian

Set to yes to subtract the mode from each column.

modbefore

set to yes to use previously determined modes if available saved in the "subdir" directory (see below) backregion

rows

to compute mode for each column.

modcol

The fit to the modes as a function of column which will be used for the subtraction. The options are imsurfit|linfits|subtracted|gauss|median.

moddisplay

Set to yes to display modes subtracted from the images?

subquad

Set to yes to adjust the median in a region of each quadrant in order to balance the quadrants?

qheader

Set to yes to use quadrants statistics save in the image header from previous runs of asncombine.

regquad1

upper left quadrant for balancing the median

regquad2

upper right quadrant for balancing for balancing the median

regquad3

lower left quadrant for balancing for balancing the median

regquad4

lower right quadrant for balancing for balancing the median

quaddisplay

Set to yes to display quadrants subtracted from the images?

resolution

The shifting, masking and co-adding of the images will be done on sub-sampled images. This parameters specifies the resolution in fraction of pixel to be used for the sub-sampling.

rebin

After the co-addition, the image can be block averaged again so that the final resolution is reduced from the fully sub-sampled images. If this parameter is set to 1, the full resolution will be maintained. If it is set to the same value as the parameter resolution, the input and the output images will have the same resolution.

logfile

Name f log file with shifts and factors

nsigma

All the statics are determined in 2 iterations with rejection of nsigma deviations after the 1st iteration. If this parameter is set to a value < 0, only one iteration will be used.

nleft

The order of the fit to the median in column for x < 129

nright

The order of the fit to the median in column for x > 128

smlength

smoothing length for medians

split

combine 2 subset of images?

hupdate

if set to yes, the results of the statistics are saved in the image header for subsequent use. The following keywords are added to the image header: factor, region1, region1, quad1, quad2, quad3, quad4, regquad1, regquad2, regquad3, regquad4.

weightregion

A statistical weight is computed and put in the image header in this region of each image. This weight is useful for a weighted co-addition.

subdir

optional directory to save subtracted images

overwrite

If this parameter is set to "no", asncombine will skip the creation of output images is they already exist. If it is set to "yes", it will delete the previous images.

DESCRIPTION

Combines the images listed in an NICMOS "association" (see makeasn). The individual images can be background subtracted, bias subtracted, sub-sampled, masked and shifted before being combined using the imcombine task. The error estimates in the images are propagated. The output is a combined image which sci and err array as well as plain fits files with the same information.

From each of the images, a scaled background can be subtracted. The scaling used can be either taken from the image header keyword fheader, or nf different scalings can be tried and the one which results in the smallest rms in the image regions specified with parameters region1 and region2 will be chosen.

Other pre-processing of individual images is to subtract a background determined from the median in each of the 4 quadrants and/or in each column of the image. The former can be accomplished by setting the parameter subquad to yes. The constant subtracted from each header is either taken from the image header keywords quad1, quad2, quad3 and quad4, or determined as the mode in the regions specified with parameters regquad1, regquad2, regquad3 and regquad4.

The median of each column of each image is determined if the submedian parameter is set to yes. Only the lines specified with bckregion are used for that purpose. The medians as a function of column are subsequently smoothed and subtracted from each line. The kind of smoothing used is selected with parameter modcol. If it is set to "imsurfit", the imsurfit task is used and and functional form can be chosen by setting its parameters. If modcol is set to "gauss", a Gaussian smoothing with a sigma of smlength pixels is used. If modcol is set to "median" , a median smoothing with a filter of smlength pixels is used.

The images are enlarged by block replication with a factor of resolution, shifted by integer pixel as listed in the input association, and then combined with the imcombine task. The final result is block averaged by a factor of rebin", i.e. the size of the final images is the size of the input images times resolution/rebin.

EXAMPLES

TIME REQUIREMENTS

BUGS

SEE ALSO