CALIBRATING VLA DATA

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Calibration Source List (VLA). Last version 1615 sources

I. CALIBRATING VLA DATA

A. FLUX DENSITY CALIBRATION

1. Flux Density Scale used at the VLA

The flux density scale used at the VLA is based on the flux density and spectrum of 3C286= 1328+307 (B1950), or 1331+305 (J2000); 3C48= 0134+329 (B1950), or 0137+331 (J2000); and 3C147= 0538+498 (B1950) or 0542+498(J2000). The flux densities of these sources are based on the work by Baars et al., Astron. Astrophys., 61, 99, 1977. The flux density of these sources at a given frequency is given by the relationship
Log S = A + B * Log v + C * (Log v) ** 2
where S is the flux density in Jy and v is the frequency in MHz. The parameters for the sources are

          Source         A             B              C

          3C286        1.480         0.292         -0.124
          3C48         2.345         0.071         -0.138
          3C147        1.766         0.447         -0.184

NOTE:
The frequency range over which these expressions are valid is 408MHz to 15GHz. Extrapolation of these expressions to 22GHz will result in a small, but unknown error.

Note that the AIPS program SETJY with OPTYP = 'CALC' will calculate and insert the relevant flux densities into the database. Do not use this option if you are switching frequencies within the observing run. In this case you must calculate and insert the appropriate values for each frequency and IF with OPTYP = ' '. Note also that the 15OCT89 and preceding versions of AIPS do not correct the calculated flux densities for errors in the Baars scale (see next section), nor account for special variations based on UV restrictions.

2. Adjustment of the Baars Scale to 3C295 and NGC7027

Careful measurements made in the 'D' configuration in 1983, 1985 and 1987 have shown that the flux densities given by Baars et al. are slightly in error, and that this error is a function of both time and frequency. We have decided to fix our flux density scale to the Baars value for 3C295 for P, L, C and X bands, and on the Baars value for NGC7027 for U and K bands. Observations to determine the correction factors for 3C48, 3C147 and 3C286 will be made during furture VLA 'D' configurations.

Below are listed the RATIOS between the true and Baars et al. value for 3C48, 3C286 and 3C147 at the various frequencies for 1987 and 1989. Multiply the Baars et al. value by this ratio to obtain the correct flux density: Time variability is small compared to the offsets. See R. Perley if you need more information.

                    1987                                    1989

          P     L     C     X     U           P     L     C     X     U

3C48     0.95  1.02  1.04  1.06  1.10  

3C147    1.00  0.97  0.95  0.97  1.01         To be determined in Jan. 1990.

3C286    0.95  1.00  1.01  1.01  1.02

3. Accurate Flux Density Bootstrapping

Because of source variability, it is impossible to compile an accurate listing of flux densities for most calibrators. The values given in Section II of this manual are only approximate. We strongly recommend bootstrapping the flux density of a calibrator by comparing the calibrator observations with one or several observations of 3C286, 3C48 or 3C147.

Careful observations have allowed the following set of rules to be established for accurate bootstrapping of flux densities using 3C286 or 3C48. For 3C147, use the rules for 3C48.

3C286 is partially resolved to most combinations of configuration and band. Its resolution occurs on two different scales - there is a weak secondary located 2.5" from the core, and the core itself is partially resolved on longer baselines. Nevertheless, 3C286 can be used as a flux calibrator for all VLA observations providing the rules laid down below are followed.

3C48 and 3C147 are heavily resolved to some combinations of configuration and frequency, but nevertheless can be preferable as a flux calibrator over 3C286 since they contain no extended structure on scales greater than 1". Use of 3C48 or 3C147 for flux calibration is preferred over 3C286, providing the former can be used for the array configuration and band under consideration.

SITUATIONS WHERE 3C48, 3C147 AND 3C286 CAN BE USED DIRECTLY

The following combinations of array configuration and band have no restrictions in number of antennas or UV range:


	3C48/3C147     90cm     All configurations
		       20cm     C and D configurations
		       6cm      D configuration
		       3.6cm    D configuration


	3C286          90cm     B,C,D configurations
		       20cm     C and D configurations
		       6cm      D configuration
		       2cm      D configuration
		       1.3cm    D configuration

SITUATIONS WHERE 3C48 AND 3C147 CANNOT BE USED

The following combinations of configuration and band should not be calibrated with 3C48 or 3C147.
2cm A configuration
1.3cm A configuration

SITUATIONS WHERE SPECIAL RESTRICTIONS ARE NECESSARY TO ALLOW FLUX CALIBRATION

The following rules must be carefully followed to ensure proper flux bootstrapping in the combinations of array scale and band noted below.


                                            NUMBER OF
SOURCE      BAND    UVRANGE    CONFIG    INNER ANTENNAS       NOTES
	    (cm)      (K)                  (per arm)
--------------------------------------------------------------------------
3C48/3C147   90       0-40       All           All

3C48/3C147   20       0-40        A             7
		       "        B,C,D          All

3C48/3C147    6       0-40        A             3
		       "        B,C,D          All

3C48/3C147   3.6      0-40        A             2
		       "          B             6
		       "         C,D           All

3C48/3C147    2       0-40        A             1         Not recommended
		       "          B             4
		       "         C,D           All

3C48/3C147   1.3      0-40        A             1         Not recommended
		       "          B             3
		       "         C,D           All


                                          NUMBER OF
SOURCE    BAND    UVRANGE    CONFIG    INNER ANTENNAS          NOTES
	  (cm)     (K)                   (per arm)
-----------------------------------------------------------------------
3C286      90       0-18          A          7
		     "          B,C,D       All

3C286      20       0-18          A          4
		     "          B,C,D       All
		   90-180         A         All     Reduce flux by 6%

3C286       6       0-25          A          1      Not recommended
		     "            B          4
		     "           C,D        All
		  150-300         A         All     Reduce flux by 2%

3C286      3.6     50-300         A          3      Reduce flux by 1%
		   50-300         B          7      Reduce flux by 1%
		   50-300         C         All     Reduce flux by 1%
		    0-15          D         All

3C286       2       0-150         A          3
		     "          B,C,D       All

3C286      1.3      0-185         A          2
		     "            B          7
		     "           C,D        All

If these guidelines are followed, the bootstrap accuracy should be 1 or 2 percent at the two lower bands, and perhaps 3 to 5 percent at the upper bands. At 2cm and 1.3cm bands, other effects, such as dish efficiency, pointing and atmospheric absorption (1.3cm) are probably more important.

If one were to ignore the guidelines, and blindly calibrate the data on the basis of the available data, the flux error obtained would vary according roughly to how much resolution occurs but would not exceed 5% for 3C286. Bear in mind that there will occur a differential error as well, as the antennas at the ends of the array will be overcalibrated with respect to those at the center.

4. Flux Bootstrapping without 3C286, 3C48 or 3C147

If you have not observed 3C48, 3C147 or 3C286 within your run, you can still accurately bootstrap your fluxes, providing that you have not observed at K-band. To estimate the calibrator fluxes, use LISTR to print the calibrator raw data, averaged by scan, in matrix form on the line printer. There will usually be a discrepant antenna or two - discard these (temporarily), and average the rest. Multiply by ten to obtain the flux density, except at P-band, where you multiply by 100. If there are u-v limits for a calibrator, use them obtaining the listings. If you have observed at a non-standard frequency, a small adjustment is required to account for the fact the noise source temperature in each antenna varies with frequency. To correct for this, multiply you resultant flux by the following factors:

1)    L-band   1+(1.3x10-3)(freq-1465)  (1400-1670 MHz)

2)    C-band   1+(3.3x10-4)(freq-4885)  (4500-4950 MHz)

3)    U-band   No correction needed, 14650-15300 MHz

Due to uncertainties in system behavior, this scheme has not yet been implemented at P and K bands. The coefficients have not been determined at X-band.

B. PHASE CALIBRATION USING POSITION CALIBRATORS

The complete list of calibrator sources used at the VLA is given in Section II. In most programs, calibrator sources are observed at least once an hour and sometimes as frequently as every 10 minutes. Calibrator observations are not only important for decreasing instrumental phase and gain drifts, atmospheric and ionospheric gain and phase variations, but for monitoring the quality and sensitivity of the data and for spotting the occasional gain and phase jumps.

1. Choosing a Calibrator

There are several criteria for choosing and using a calibrator. A list of guidelines, in decreasing order of importance, follows:

A) Choose the calibrator closest to your source. If it is within 10 deg., atmospheric phase fluctuations will be somewhat better calibrated. It is better to have one calibrator per source over the entire run. If several are needed, try to bootstrap their positions together. However, in the smaller configurations and at longer wavelengths, these criteria can be considerably relaxed, so a single calibration for a group of sources is often preferable. Furthermore, if your target sources can be self-calibrated, the need for rapid switching between source and calibrator is entirely removed. Hourly observations of the calibrator are more than sufficient for this case.
B) Choose a calibrator which has a P or S quality status for the desired configuration and frequency. The difference between P and S is minimal but P is preferred since fewer gain errors will result.
C) Different calibrator codes are used only to distinguish the accuracy of the calibrator position. If absolute positional accuracy <0.1 arcsec is desired, the position code should be an important consideration - use 'A' or 'B' calibrators. Note that all positions for sources with 'A' or 'B' PC codes are taken from the JPL astrometric list.
D) The intensity of the calibrator is of secondary importance. The only exceptions are when the calibrator will be used as a band-pass calibrator for spectral line observing, and for high dynamic range observations where closure errors must be measured.
E) For polarization calibration, it is important to use a strong calibrator (more than 1 Jy) and to observe it at least 5 times with a total range in parallactic angle exceeding 90o. Include at least one observation of 3C138 or 3C286.

2. The J2000 Coordinate System at the VLA

The VLA now supports the J2000 coordinate system. Details on this implementation are given by Barry Clark in VLA Computer Memorandum 167 of May 11, 1983. The complete description of the J2000 coordinate system can be found in the USNO circular 163 edited by G. H. Kaplan of the Naval Research Laboratory. In summary, positions given in epoch 2000 will be precessed in accordance with the recommendations of USNO 163. Positions given in epoch 1950 will employ adjustments, so that they are effectively processed by the recommendations of the Explanatory Supplement of 1960. Positions of any other epoch will, currently, be precessed by the recommendations of the Explanatory Supplement of 1960. The most serious consequence of this is that planetary coordinates given in apparent coordinates of observing date are assumed to be in the system of the FK4. It seems likely that as soon as the system of FK5 comes into greater use for the production of planetary ephemerides, we shall reverse this decision, and use the FK5 and the formulae of USNO 163 for epochs other than 1950.

It now seems profitable for anyone interested in the highest positional accuracy to use J2000 coordinates for all future observations, unless compatibility with previous observations is critical and the whole series does not span enough time to be unduly confused by the known error in Newcomb's precession constant. (It is probably less work to put previous observations in J2000 coordinates that to calculate the corrections to the 1950 coordinates of various dates, if the observations span more than a couple of years). For the general VLA user, the pressure to change systems is not so strong--he must consider whether it is more important that his observation remain compatible with previous observations of the object, or whether it should be compatible with future accurate astrometry, either radio or optical. We do not recommend changing to J2000 coordinates for an object which you have observed here before, and might conceivably wish to combine the old (u,v) data--there is no point in just asking for trouble. However, we encourage the use of J2000 coordinates for new observations unless there is a firm argument against doing so. It seems inevitable that J2000 coordinates are going to come into general use, and the sooner we can get through the painful transition period, the better off we all shall be.

III. VLA CALIBRATORS

Description of the Table

The following table lists all of the sources which have been found to be suitable calibrators for the VLA. The quality of some calibrators varies with frequency and configuration and the table includes comments pertaining to this. Several examples from the table are given below:

   NAME          EPOCH     PC        RA            DEC
   0038-213      2000.0    C   00 38 29.9524  -21 20 04.027
   0036-216      1950.0    C   00 36 00.4390  -21 36 33.100
   
   BAND      FLUX     A    B    C    D    UVmin    UVmax
   -----------------------------------------------------
   20cm  L   0.78     ?    ?    X    X       10
   6cm   C   0.34     S    S    S    S               200
   3.6cm X   0.22     X    S    S    S               200

   NAME          EPOCH     PC        RA            DEC
   0714+146      2000.0    T   07 14 04.6352  +14 36 20.629
   0711+146      1950.0    T   07 11 14.3000  +14 41 33.000
                      Also known as 3C175.1

   BAND      FLUX     A    B    C    D    UVmin    UVmax
   -----------------------------------------------------
   90cm  P   6        X    S    S    X        1        4
   20cm  L   1.90     X    X    X    S                 4

   NAME          EPOCH     PC        RA            DEC
   1733-130      2000.0    A   17 33 02.7058  -13 04 49.546
   1730-130      1950.0    A   17 30 13.5352  -13 02 45.837

   BAND      FLUX     A    B    C    D    UVmin    UVmax
   -----------------------------------------------------
   20cm  L   5.20     S    X    X    P       40        3
   6cm   C   5.00     P    P    P    P
   3.6cm X   5.80     P    P    P    P
   2cm   U   3.70     P    P    P    P

   NAME          EPOCH     PC        RA            DEC
   1759+237      2000.0    C   17 59 00.3527  +23 43 46.974
   1756+237      1950.0    C   17 56 55.9320  +23 43 55.800

   BAND      FLUX     A    B    C    D    UVmin    UVmax
   -----------------------------------------------------
   20cm  L   0.70     S    S    S    X        6       90
   6cm   C   1.00     X    S    S    S                90
   3.6cm X   0.55     S    S    S    S
   2cm   U   0.00     ?    ?    ?    ?

TABLE HEADER

Line 1:  Source IAU name at epoch (2000).  Use
	 of this name in OBSERVE fetches RA
	 and DEC at epoch 2000.
	 PC = Position Code for coordinate accuracy.

Line 2:  Source IAU name at epoch (1950).  Use
	 of this name in OBSERVE fetches RA
	 and DEC at epoch 1950.

Line 3:  (Optional)-Other common names for the
	 source.  Desired epoch must be
	 explicitly specified in OBSERVE.
	 (The Dec. 1989 release of OBSERVE
	 does not recognize this alias name).

Position codes are:
   A = positional accuracy <0.002  arcseconds
   B = positional accuracy  0.002 - 0.01  arcseconds
   C = positional accuracy  0.01 - 0.15  arcseconds
   T = positional accuracy >0.15  arcseconds

   Note:    Errors in declination increase in the south, except for A
            and B calibrators.

TABLE FORMAT

Col 1 & 2: Band and Band code.  For 1.3cm use 2cm entry.
Col 3:     Flux = The approximate flux density of the source.
	   Use only as an indicator of the source strength.
Col 4-7:   Calibrator quality in the A, B, C and D configuration

	   P = <3% amplitude closure errors expected
	   S = 3-10% closure errors expected
	   X = Do not use.  Too much resolution or too weak
	   ? = Not observed

Col 8 & 9: Antsol restrictions.  These are suggested UVLIMITS in
	   K to use in CALIB to avoid data which are
	   contaminated by structure.  A UVMIN (Col. 8) generally
	   means the source is confused at short spacings.  A
	   UVMAX (Col. 9) generally means the source is resolved
	   at long spacings.

Particular comments on the above examples are:

1. Although '?' appears in the table for 0038-213 it is likely that the source is a fine calibrator at 20cm in the A and B configurations since it is unresolved at 6 and 3.6cm at similar resolutions. It is listed with a '?' because we have not confirmed its suitability. Many '?' entries can be interpreted in this way.
2. The X at 20cm C and D configurations for 0038-213 and the UVMIN means that the source is confused at short spacings at 20cm. The source could be used, but gain quality would be poorer.
3. The source 0714+146 is only a calibrator at 20cm in the D configuration and in B and C configurations at 90cm. Many similarly extended sources are included in the listings. Most are fairly strong and can be used as bandpass calibrators at 20cm.
4. The inaccurate position (PC-T) of 0714+146 is not a restriction for 20cm D configuration observing.
5. Notice that 0714+146 can be called with the alias 3C175.1. Only the most famous sources have entries under pseudonyms which are restricted to 3C names. We are trying to stick with the IAU designation names as much as possible.
6. Note the apparent conflict in UVLIMITS for 1733-130 at 20cm. This conflict is resolved by noting that two different ranges will allow a valid CALIB solution; the first, valid for the D array, is 0 to 3 k wavelengths; the second, valid for the A array, is 40 k wavelengths to the longest baseline (approximately 180 k wavelengths).
7. The 2cm listing for 1759+237 has zero flux density and '?' for quality, indicating it has not been observed. Because the source shows a flat spectrum, it is likely to be a good calibrator at 2cm.