IRAF help page for package images, program hselect

• images
• fields
• expr

hselect images fields expr

images

fields

expr

The function of hselect is to select a subset of images from a database according to a selection criteria supplied by the user, tabulating a subset of the fields of each selected image on the standard output. The resultant table is output in list form, suitable for further analysis or for use to generate a list of images to be processed by another task.

The form of the boolean expression expr is fully documented in the manual page for the hedit task. In the case of hselect task, however, the expression need not be parenthesized to be evaluated as an expression. The special operand "$I" denotes the name of the current image and is used in the fields list when the image name is desired as one of the columns of the output list.

1. Compute the mean exposure time for all the images in a database. Note that the argument "yes" is a trivial case of a general boolean expression and hence need not be quoted.

cl> hselect n1.* exp yes | average

2. Print the name, length of axes 1 and 2, and title of all two dimensional images in a database.

	cl> hselect n1.* $I,naxis[12],title 'naxis == 2'
	n1.0001	512	512	quartz
	n1.0002 512	512	"dome flat"
	n1.0005 384	800	"ngc 3127 at 45 degrees"
	cl>

3. Produce an image name list for use to drive another task. The selection criterium is all images for which the value of the parameter "q-flag" has the value 1. Note carefully the use of quotes. If the @ operator is unfamiliar read the manual page for hedit.

cl> hselect n1.* $I '@"q-flag" == 1' > imlist

If the parameter "q-flag" were instead named "qflag", the following simpler expression would suffice.

cl> hselect n1.* $I 'qflag == 1' > imlist

Since individual image headers are currently stored as separate files, selection from a large database is quite slow.

hedit, imgets, imheader,