SCHED uses catalogs to get source information such as names, positions, and velocities. These catalogs are in keyin format. Entries are terminated with a ``/''. Some or all of the catalog may be included in a section in the SCHED input file. SCHED will look there first for sources, and then go to the two possible external catalogs to find the rest (or all). Please note that the source catalog, along with the locations catalog of station positions, is updated approximately annually to new solutions. All source positions will change, although likely by small amounts. But relative astrometry projects should either use their own source positions to keep them constant over long times, or include processing steps that account for any changes in the catalog positions (the preferred solution).
As of Nov. 2011, there are three catalogs included with the SCHED distribution. One contains only a recent astrometry solution from the Goddard Space Flight Center geodetic group. Another contains only a recent solution provided by Leonid Petrov. This one typically has very nearly all of the sources in the Goddard solution (they are working from the same data sets) plus a significant number more. The third file, sources.vlba.all, includes all sources in the other two plus around 500 other sources. These are mostly VLA calibrator list or JVAS sources that, for whatever reason, don't have a good VLBI astrometric position. They should be considered suspect for VLBI and may not even be detectable with VLBI. There are over 7000 sources in the latter catalog, so there is a very good chance that a user's target sources will be in it and that there will be fairly nearby calibrators.
The sources in the catalogs from the geodetic solutions are based on the International Celestial Reference Frame version 2 or ICRF2 (these have CALCODE=V). These have positional errors of a fraction of a milliarcsecond to a few milliarcseconds. They include sources from the standard geodetic observing program, sources from the VLBA calibrator survey, the Australian LBA survey, and various sources specially observed in the RDV project on the VLBA. Most of the additional sources in the third file above, as noted, are sources that have not yet been measured with VLBI, or might be inappropriate for VLBI (too resolved), that are from the VLA calibrator list (CALCODE=Y) and from the JVAS survey (CALCODE=M; Patnaik et al. 1992, mnras, 254, 655; Browne et al. 1998, mnras, 293, 257; Wilkinson et al. 1998, mnras, 300, 790). The JVAS source positions are good to mas. The VLA calibrator positions are somewhat worse than the JVAS positions.
Users of accurate positions from the geodetic catalog are encouraged to reference the original papers, some of which are listed in the catalog headers. Information about, and data from, the GSFC solutions can be found at http://vlbi.gsfc.nasa.gov/dataresults_main.htm. Catalogs and much other useful information provided by Leonid Petrov can be found at http://astrogeo.org/. Comments in sources.vlba will let you know which solution the source positions are based on.
SCHED input parameters SRCFILE and
SRCFILE2 are used to point to any desired
external catalogs. A file name of up to 80 characters can be
specified. The default source file is:
On unix systems with the environment variable SCHED properly defined, this is the standard catalog distributed with SCHED. Users of the html version of the manual can read the catalog by clicking here..
Source catalog information can be given in the main SCHED keyin file. If the keyword SRCCAT appears, all input after the next ``/'' is assumed to consist of source catalog entries until a line containing the keyword ENDCAT and a ``/'' is encountered (don't combine this keyword with a catalog entry). Such ``in-line'' catalogs may appear anywhere in a SCHED keyin file, although it is probably best to put them near the beginning. SCHED will look in the external catalog for any source not found in an in-line catalog. If you wish to prevent SCHED from looking in external catalogs, specify SRCFILE=NONE (SRCFILE2 is not used at all unless specified.). It is common to provide in-line source catalog entries for unique program sources, but let SCHED find the fringe finders and other calibrators in the standard catalog. SCHED will look in the in-line catalog first, then in SRCFILE, then in SRCFILE2. It will use data from the first place a source is found.
The parameters for each entry in the source catalog are below. Lower case letters in this list are optional. All catalog parameters except EQUINOX and EPOCH revert to a default unless specified for a source. EQUINOX and EPOCH default to the last specified.
Note that the plotting capabilities of SCHED, specifically the RD plots, can be used to examine the distribution of sources in the catalog and to find catalog near any desired location. This is useful, for example, for finding phase calibrators near target sources.
SCHED will use whichever equinox coordinates are provided in the catalog and determine J2000, B1950, and DATE coordinates. All of these coordinates are listed in the .sum file. Various stations require different equinox coordinates. For example, the VLBA only understands J2000 coordinates while some other sites only understand B1950, although those may all be gone by now. Sched uses the SLALIB routines to make the coordinate conversions.
The J2000 and B1950 coordinates systems move relative to each other so an ``observe'' date is needed to make an accurate conversion. This is the date at which the coordinates of the reference sources were determined. For example, the VLA calibrator list coordinates were determined in 1979.9 originally, and all subsequent updates, based mainly on J2000 positions, were precessed to B1950 using an assumed ``observe'' date of 1979.9. A similar situation exists at MERLIN, where observing is done in B1950 coordinates. There, the calibrator positions (based again on positions provided originally in J2000) are precessed to B1950 using an ``observe'' date of 1950.0. Any B1950 source position determined on MERLIN should be precessed to other systems using that date. The user can specify the ``observe'' date using the main schedule parameter PRECDATE which defaults to 1979.9, the correct value for use with B1950 source positions determined on the VLA. For purposes of pointing antennas, the value of this date that is used is not very important. But if you are trying to phase reference, phase the VLA at a high frequency, or derive positions for correlation, this is an issue that should be dealt with properly.
The parameters of the source catalog are:
Be a bit careful about special characters included in source names. Some are benign. But, for example, the * and ? characters are used as wild cards in many programs so, if either ends up in a file name (as it will after SPLIT in AIPS), it could cause problems. The temptation to use * is high because of sources such as SgrA*. But the conservative approach is to avoid it. Also, do not include imbedded blanks in source names, as they cause problems at a variety of places in processing. Minus signs (dashes), plus signs, underscores, and parentheses should not cause problems and are the commonly desired characters.
Note that the number of characters you can have may be limited by downstream software. Snap files require source names of 10 characters or less, but are not normally written by SCHED any more. VLA observe files have a total of 13 characters for the source name plus qualifier. If the qualifier is 0 (default), no characters are needed for it. Otherwise, the qualifier needs however many digits it uses (e.g., 2 digits for qualifier ``33'') plus one for a blank to separate it from the source name. If either the VLA or a snap file needs shorter name, SCHED will use the first alias of 8 characters or less, so be sure to include such an alias if it will be needed. Finally, beware that the Mark III correlators and the geodetic software have 8 character source name limits.
Below is a sample of a few sources from the March 2005 source catalog.
SOURCE='J1629-2026' RA=16:29:03.0298580 DEC= -20:26:55.100570 RAERR= 29.510 DECERR= 13.450 CALCODE='V' REMARKS='VLBA Calib Survey - GSFC sols. - VCS2 - created 2004' / SOURCE='J1630+2131','1628+216' RA=16:30:11.2308370 DEC= 21:31:34.310260 RAERR= 4.370 DECERR= 5.380 CALCODE='V' REMARKS='VLBA Calib Survey - GSFC sols. - VCS2 - created 2004' / SOURCE='J1631+1052','1628+109' RA=16:31:18.7777000 DEC= 10:52:02.460000 RAERR= 4.370 DECERR= 5.380 CALCODE='M' REMARKS='JVAS - Browne et al. 1998, mnras, 293, 257; S8.4GHz= 72 mJy' / SOURCE='J1631+4927','1629+495' RA=16:31:16.5398860 DEC= 49:27:39.515680 RAERR= 0.520 DECERR= 0.630 CALCODE='V' REMARKS='VLBA Calib Survey - GSFC sols. - VCS2 - created 2004' /