im - image manipulate MWP & RWW
im mv: copy image 1 to image 2
im,3,5 copy image 3 to image 5
im add:1,3,4 mina:-1.5 minb:0
add images 1 & 3, put result in image 4, suppressing
data with pixel values < -1.5 in image 1, and data
with pixel values < 0.0 in image 3.
im mul: hl:"Go forth and multiply"
multiply images 1 & 2, put result in image 3, and
set the "im" HISTORY label of image 3 to the string.
String can be up to 50 chars.
im rs:4.2,-1 rescale (multiply) the pixels in image 1 by 4.2,
then subtract 1 from them.
im ph:2 print the header of image 2 (FITS format)
im ah:1,"Hello world" add a "user" HISTORY label to the image header of
image 1 comprised of the string "Hello world".
String can be up to 50 chars.
im si:1,2 an:1 Write a subimage of image 1, as defined by area 1,
into image 2.
im si:1,3 po:[-5,5,-4,7] Write a subimage of image 1, as defined by the given
x limits (-5,5) and y limits (-4,7), into image 3.
im si:1,2 po:[900*5:32, 900*5:30, 60*2:30, 60*3:00]
A subimage example for images in Total RA-DEC
coordinates (see below).
im temp:1,2,3 pow:-1.0 Compute dust temperature map from two IRAS flux maps
in adjacent bands (e.g. 60 and 100 micron). Shorter
wavelength is in image 1. Temperature map is put in
im cc:2,3,4 Perform color correction on IRAS flux map in image 2,
using temperature map in image 3.
im tau:4,3,5 Compute opacity map at wavelength corresponding to
(color-corrected) flux map in image 4 and temperature
map in image 3. Assume opacity varies inversely with
wavelength. Result is put in image 5.
im performs image arithmetic, makes a subimage of a current image,
and has functions for analysis of IRAS flux maps (which is just another
kind of arithmetic).
When pixels are changed by im, a HISTORY record reflecting the change is
automatically added to the header. The user may change this default
HISTORY record, with the h: flag. With im ah:, the user may add one
additional HISTORY record.
im reports the number of output pixels which have the value BLANK, even if
they were blank in the input image(s).
mina: and minb: have no effect on im mv:, im rs:, im ah:, and im ph:.
minb: has no effect on im si:.
im can add, subtract, multiply and divide two images, putting the result
in the specified image plane. The default images for arithmetic operations
are 1, 2, 3; that is image 3 = image1 (OP) image2. im rs: will multiply image
pixels by a constant and add a constant to the specified image (like 'rs' does
to stacks). The default image for im rs: is image 1.
im mv: copies an image from one image plane to another by linking the
image to the new pointer. It is particularly useful when making
overlay contour plots. The default for im mv: is image 1 to image 2
Note, however, that since this is a "symbolic" link and not a true copy,
any arithmetic operation on the image "copy" will also affect
the original (and vice versa). Recomputing a new image in the "original"
image plane will not affect the copy however.
Using the si: option, you can copy a portion of a given image into
another image plane. This is done either by specifying a defined area which
delimits the desired region, or by specifying the corners of the subimage
(left and right x and y limits) in the image units. The defined area does
not need to be rectangular, but the resultant subimage always will be
a rectangular region which encompasses the area given. With either method
of specifying the region, the subimage limits are rounded to the nearest
pixel. If an x or y boundary requested for the subimage exceeds that of
the source image, the request is reset to the source image boundary.
The default is to copy the region defined by area 1 from image 1 to
Note for images in "Total RA-DEC" coordinates (i.e., 'cp' shows HH:MM:SS on
RA axis), you must specify the subimage in arcminute offsets from
(0 hr, 0 deg). The resultant image will also be in "Total RA-DEC"
coordinates. See the example above for a simple way of doing this, using
the axes numerals that cp displays. For images in (l,b) coordinates, specify
the offsets in degrees from CRPIX1 and CRPIX2, even though 'cp' shows total
ANALYSIS OF IRAS IMAGES:
im has 3 special options for analysis of IRAS flux maps. im will
compute dust temperature and opacity maps from IRAS flux maps, and
perform the "color correction" to IRAS flux maps.
The method for computing dust temperature and opacity maps is described
in Langer et. al. (1989, ApJ, 337, 355 [Section V and Appendix A]).
It is assumed that the dust temperature along the line of sight can be
characterized by a single temperature and that the emission is optically thin.
Consult Appendix A of Langer et.al. for more information on the method and
Briefly, the method is as follows (using 60 and 100 micron as examples):
I(60)/I(100) = [B(T,60)/B(T,100)]*[tau(60)/tau(100)]
where I = intensity, B = Planck function, T = dust temp., and tau = opacity.
If it is assumed that the optical depth is a power-law with wavelength, then
the above equation can be solved for T at each point in the IRAS maps.
Temperatures are clipped at about 20 and 10,000 K, the exact clipping value
depends on the wavelengths of the input images.
Before calculating the opacity map at any wavelength, a color-correction must
be applied to that flux map to account for the effects of the finite IRAS
passbands. The color correction is performed as described on page VI-27 of the
IRAS Explanatory Supplement (1987).
Dividing each pixel of a color-corrected flux map by the Planck function at the
derived temperature of that pixel, converts the flux map into an opacity
map at that wavelength.
This document was last updated on
Tue Oct 17 16:24:05 EDT 2000
by Marc W. Pound.