catalog catalogBy the time you reach this chapter, most of your data will probably already be loaded into your catalog
either by reading an external tape or disk or by being generated by some task. Visibility data which are not
presently on disk may be read by the tasks FILLM, UVLOD and FITLD; see §4.1 and §5.1 for
details. Images that are generated by other imaging systems, (e.g., images from non-NRAO radio
telescopes or non-radio images) can be transported to by writing them out of the other imaging
system on tape or disk in the standard FITS format. The tasks IMLOD and FITLD can then be used to read
them into . These tasks are also used to read images saved with FITTP and FITAB from previous
sessions.
FITLD and IMLOD can read FITS-format images from external disk files or magnetic tape into your
catalog. The disk-file option is indicated by setting the adverb DATAIN to a non-blank value. Disk image
files must be read in only one at a time per execution of IMLOD, but FITLD can read more than one
FITS-disk file if the file names are identical except for sequential post-pended numbers beginning with 1.
DATAIN is a string of up to 48 characters that must completely specify the disk, directory, and name of
the input disk file to your computer’s operating system. See §3.10.3 for a discussion of FITS-disk
files.
One “feature” of complicates this otherwise straightforward disk analog of FITS tape reading. AIPS translates all of
your alphabetic inputs to upper case (this was demanded by users who otherwise became confused between upper and
lower cases).1
So if your computer distinguishes upper and lower cases for disk, directory, or file names, you probably should do
two things to prepare for this before running . First, restrict your external disk file names to
upper-case characters and numbers. Second, set an upper-case “environment variable” or “logical” to
point to the disk and directory where your FITS-disk images are stored before you run AIPS. You
may need help from your System Manager when doing this for the first time. A common strategy
on UNIX machines is to create an upper-case logical name after logging in but before starting up
AIPS:
% setenv MYLOGICAL myarea C R | if using C-shell, or |
$ export MYLOGICAL=myarea C R | if using korn, bourne, or bash shells, |
where MYLOGICAL is an all-upper-case string of your choice and myarea is the full path name of the disk directory
that contains your FITS-disk data. usually provides a public disk area known as FITS which you may
use.
Then, once inside AIPS, tell FITLD or IMLOD:
> DATAIN ’MYLOGICAL:IMAGE.DAT’ C R | to read in the FITS-disk file myarea:IMAGE.DAT. |
To check that the file name is correct, type:
> TPHEAD C R |
|
Your terminal will then list information about the image header of the disk file.
> OUTDI n C R | to specify writing the image to your |
> OUTNAME ’your-chosen-name’ C R | to specify the output disk file name in |
The string your-chosen-name can be any (≤ 12-character) title that you want to use as the image name within
and should be specified for images from other image-processing software systems. FITLD also
allows you to specify the 6-character image “class” parameter. Use OUTCLASS ’abcdef ’ C R, if you wish to
change the class from that in your input file as the image is read or if the image comes from a “foreign”
system.
> OUTS -1 C R | to keep the sequence number the same as that in the file; the
default is the highest unique number for images with this
name and class in your current |
> NCOUNT m C R | in FITLD only, to load m images consecutively starting with the file specified by DATAIN with a 1 post pended. The other files must be the same name but with sequential numbers post pended. If you use this option, do not specify the OUTNAME unless you want the same name for all the new images in your catalog. |
If OUTNAME is left unspecified, it defaults to the “name” of the image read from the FITS header — either the name
previously used in earlier image processing or the source name. If OUTCLASS is unspecified, it defaults to the Class
previously used in earlier image processing or to a compound name (e.g., IMAP, IBEM, QMAP, ICLN) which attempts
to describe the image. These defaults are frequently good ones when you are loading multiple consecutive images
with NCOUNT > 0. You may of course change the image and class names later by using RENAME (see §3.3.3 of
this 
ook
ook).
IMLOD and FITLD can also read images from magnetic tape; see the instructions in §3.9. Use the verbs MOUNT to mount the tape, AVFILE to advance the tape to the desired file, and TPHEAD to check that the tape is correctly positioned. Set NFILES=0, NCOUNT to the number of consecutive tape files you wish to load, and the say GO to either FITLD or IMLOD.
Images of the celestial sphere must be rendered with some sort of coordinate “projection.”
pioneered the handling of such coordinates, supporting true projective coordinates called
SIN (orthographic), TAN (gnomic), ARC (zenithal equidistant), and STG (stereographic) as well as
a special version of orthographic suited for East-West interferometers (NCP). also supports
modern2
interpretations of GLS (SFL Sanson-Flamsteed), MER (Mercator), AIT (Hammer-Aitoff), CAR (Plate carrée). MOL
(Molweide), and PAR (parabolic) “projections.” These are normally used to represent large sections of the
celestial sphere. Users should be warned that reference pixel values other than zero lead to oblique
projections.
Fits to the coordinates on optical images often lead to a modest amount of skew in the image. This is represented in
the PCi_j or CDi_j FITS header cards. If these cards contain any significant skew, IMLOD and FITLD will tell
you about it. In such cases, run DSKEW on the images after they have been loaded into . This is
the only task that understands coordinate skew and it will re-grid the images for use by all other
tasks.