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Calibration Scans on Continuum Sources

As with any type of observing, it is always a good idea to run a few calibration scans at the beginning of, and during, your observing run to make sure that the telescope and receiver are behaving properly. At Arecibo this is done by performing a ``spider scan" (a pattern of four scans across an object, each separated in position angle by 45$^\circ$) on a standard calibration source. The output of this scan is automatically displayed on the screen, allowing the observer to asses the current state of the telescope and chosen receiver.

Continuum sources in the list used for flux density calibration were selected in an ad-hoc fashion from a variety of places, but primarily from Kuehr et al (A&AS, 45, 367, 1981), the compendium of linearly polarized sources by Tabara & Inoue (A&AS, 39, 379, 1980), and an unpublished list of unconfused calibrator sources prepared from the NVSS survey by Jim Condon and Qi Feng Yin. For each source, all reliable flux density measurements from both the literature and web-available catalogs have been fitted against frequency using the following spectral form:

$$\rm log_{10}(S(Jy))\:=\:a0\:+\:a1*\left[log_{10}(freq(MHz))\right]\:+\: a2*\left(exp(-\left[log_{10}(freq(MHz))\right]\right)$$

The values for the constants are given in the displayed source list after the word ``flux:". All sources in the catalog listed as 'good' sources are believed to be non-variable. Additionally, if the source size is not given, it is believed to be less than about 15 $^{\prime\prime}$. It should be noted, however, that: (a) some of the sources in this catalog are significantly linearly polarized. If you wish to calibrate on a polarized source, you should determine in advance which source(s) are appropriately polarized sources; (b) We cannot guarantee that all sources are excellent calibrators at all frequencies. For example, you should bear confusion and source-size considerations in mind when selecting a calibrator source at any frequency. It is therefore highly recommended that you observe a few different calibrators. (And if you do find a source to be a poor calibrator at a given frequency, please tell us about it, via the ``gui comment" button.)

The frequencies at which the standard spider-scan calibration is done is set by the receiver chosen. Note that when running the calibration button you cannot change the board center frequencies, bandwidths, or scan length. The scan length is set to be six times the half-power beam width, and the center frequencies and bandwidth are listed as Table 2.

Table 2: Calibration Frequencies for Standard Spider Scans

		Center Frequencies/Bandwidth [MHz]
Receiver	HPBW	Board 0	Board 1	Board 2	Board 4
327	15$^\prime$	326/25	326/1.563	326/0.781
430	12$^\prime$	430/25	430/12.5	429/6.25	431/6.25
430CH	10.$^\prime$	430.0/12.5	430.0/1.563	430.0/3.125
610	8.0$^\prime$	612.0/6.25	612.0/1.563	609.75/0.781	613.25/0.781
lbn	3.4$^\prime$	1300/25	1375/25	1415/25	1500/25
lbw	3.4$^\prime$	1175/25	1300/25	1375/25	1415/25
lbw	3.4$^\prime$	1415/25	1550/25	1666/25	1610/25
sbn	2.2$^\prime$	2380/25
sbw	2.2$^\prime$	2212/25	2380/25	2690/25	2850/25
cb	1.0$^\prime$	4500/25	4860/25	5000/25	5400/25
xb	0.58$^\prime$	8500/25	8800/25	9000/25	9200/25

A typical output from a standard spider-scan calibration is shown in Figure 5. This information is obtained by running Carl Heiles' Mueller Matrix routines on the data to correct for instrumental polarization (described fully in Heiles, et.al. 2001 PASP, preprint, and also available as AOTM 2000-05, available at http://www.naic.edu/$\sim$astro/aotms/performance.htm). The output is as follows:

• First Line: Source name, date, and starting scan number
• Second Line: Receiver name, board number, and center frequency (in MHz)
• Third Line: Azimuth and zenith angle pointing errors, in arcsec, for the A and B polarizations
• Fourth Line: System and source temperatures
• Fifth Line: Percent by which the source is linearly polarized; Position Angle of the polarization vector (North through East)
• Sixth Line: Percent by which the source is measured to be circularly polarized
• Seventh Line: The half-power beam width in the azimuth and zenith angle directions (in $^\prime$)
• Eight Line: Coma intensity and position angle
• Ninth Line: Telescope gain, in K/Jy, determined using the source flux density at the observing frequency
• Tenth Line: Efficiency for the Main Beam + First Sidelobe

Note that the data which results from the "calibrate" button goes into a different file than all other data taken with the gui. That is, rather than going into a file named ``corfile. $<today's\:date>.<proj>.<incremental\;number>$'', it goes into a files named ``calfile. $<today's\:date>.<proj>.<incremental\;number>$''.