Calibration sources
1. Flux Density Scales
2. L-band Calibration Sources
3. K-band Calibration Sources
Flux Density Scales
The NRAO list of VLA
calibrators have been recently upgraded (Perley, R.A, Taylor, G.B.,
The VLA Calibrator Manual, Jan. 23, 1996 ). It includes "five Prime"
calibration sources whith spectrums between 327 MHz and 15 GHz which are
based on the flux density scale defined by Baars et al (1977, Astron.&Astrophys,
61, 99) relative to the absolute spectrum of the CasA. At frequencies above
15 GHz the flux density is based on observations and emission models of
planets. The best analytic expressions for spectrums of these "Prime"
calibration sources between 300 MHz and 50 GHz for the epoch 1995.2 are
given by the formula:
The estimated accuracy of the flux density scale determined by these
"Prime" calibration sources is about 1-2% at L-band and ~ 5% at
K-band.
The Baars et al, 1977 flux scale has been recently extended for the frequencies
higher than 15 GHz (Ott, et al., 1994, Astron Astrophys., 284,331-339).
The fluxes of the VLA Prime calibration sources 3C286, 3C295 for L- and
K-band are in reasonable agreement (~5%) with the fluxes determined in this
paper. Since the fluxes of other calibration sources 3C48, 3C138, 3C147
signficantly differ (up to 20%), it suggests that they are possibly variable.
There are also other reports on possible variability of these sources.
L-band Calibration Sources
The list of L-band (1668 MHz) calibration sources for the DSN 70m antennas
is based on the list of VLA calibration sources which are regularly monitored
by the VLA. It includes all five "Prime" VLA calibrators and a
set of "Secondary" calibration sources with fluxes at L-band of
more than
10 Jy. The fluxes of these "Secondary" calibrators were taken
from the list of EVN Calibration sources by Pauliny-Toth, I. (1993) and
a paper of Kuhr, H. (1981) which also used the Baars et all, 1977 flux scale.
The fluxes of the "Secondary" sources are defined not as well
as the "Primary" and some of them are variable. Additionally,
some of the "Secondary" calibrators have large angular sizes,
so that using them for 70m antenna gain calibration will require the knowledge
of the size correction factor. Such sources are recommended to use only
for an antenna configuration/pointing check. When the "Secondary"
calibrators are used for the gain calibration, the measurements have to
be alternated with measurements of the "Prime" calibrators.
K-band Calibration Sources
The list of K-band (22256 MHz) calibration sources for the 70m DSN antennas
is based on the list of the VLA calibration sources and sources from the
list of Ott et al, 1994. The sources 3C286 and 3C295 are the only VLA calibration
sources which have not been reported as variable at K-band. NGC7027 and
Venus also are included as the prime calibration sources since their fluxes
may be calcuated relatively accurately. While using NGC7027 as the calibration
source for the 70m DSN, the sources size correction factor must be applied
and the value of the flux must be adjusted to the epoch of measurements
with an annual rate of decrease of ~0.5%. The flux of the Venus (see Figure
below) vs. geocentric distance (in a.u.) was calculated with the assumption
of a disk-like model of the sources with the radius 6120 km [5]. The sources
size correction factor is included (70m antenna HPBW ~ 43 arcsec or 12 mdeg).
The fluxes of the "Secondary" sources are defined not as well
as "Primary" and most of them are variable or have too large angular
sizes for the 70m antenna. (For variable sources, the available data on
a range of the fluxes change are given in the Table.) Such sources are recommended
to use only for an antenna configuration/pointing check. When the "Secondary"
calibrators are used for the gain calibration, the measurements have to
be alternated with measurements of "Prime" calibrators
to provide relative gain/efficiency measurements.
References
1. Perley, R.A, Taylor, G.B., The VLA Calibrator Manual, Jan. 23, 1996
2. Kuhr, H., et al., 1981, Astron&Astrophys. Suppl. Ser., 45,
367
3. Pauliny-Toth, I., L-band EVN calibration sources, 11/30/93
4. Ott,M., et al., 1994, Astron&Astrophys.,284, 331-339
5. Steffes, P.G., Klein, M.J., Jenkins, J.M.,1990, Icarus, 84, 83-92
6. Baars, J.W.M., et al., 1977, Astron.&Astrophys, 61, 99
7. Valtoja, E., et al.,1992, Astron.&Astrophys. Suppl. Ser., 95,
73-85