Interferometric Pointing (IP)
Interferometric pointing (a.k.a., reference pointing) may be needed at frequencies of about 15 GHz and higher (K, Ka, Q-band, and most Ku-band setups). At these frequencies the antenna pointing accuracy (a few arcseconds) becomes a significant fraction of the primary beam. Observing with an inaccurate pointing thus may degrade the signal by a significant fraction. The antenna pointing is a function of the shape of the reflective surfaces and is influenced by, amongst other things, gravity and temperature. Therefore, observing at high frequencies may require regular pointing scans to determine offsets from the pointing model. These pointing offsets remain reliable for target sources within about 20d in AZ or EL from the pointing position. Therefore, typically an observation would redetermine pointing solutions when moving to a different portion of the sky, or roughly hourly when tracking a (group of nearby) target source(s).
Pointing scans are performed as a five-point raster observation on a strong (over 300 mJy) continuum calibrator, in first instance in X-band continuum. This primary reference pointing scan usually yields sufficiently accurate pointing offsets, but if more accurate solutions are required a secondary reference pointing may follow at the (standard) frequency of the observing band. Secondary pointing is also performed in continuum mode (to be as sensitive as possible to the continuum source) in an attempt to improve the antenna pointing in the band of interest. However, local lore is that although this might improve the pointing a bit toward the pointing source, subsequent slews and with time passing by, this secondary pointing in general does not yield a long lasting improvement on the primary pointing. In addition, there is the risk that for some antennas the secondary solution fails. The resources for secondary pointing scans are available, but it is debatable whether the extra time spent to perform a secondary pointing scan is worthwhile. Determining pointing solutions using spectral line sources, e.g., with SiO masers in Q-band, has not been tested.
Default pointing resources are included in the NRAO defaults catalog in the group Pointing setups. You may want to copy the X-point (formerly known as the X band pointing) resource and pointing sources you wish to use from the standard catalogs to your personal catalog.
Setting Up an IP Scan
To utilize the interferometric pointing scan mode, select Interferometric Pointing from the drop-down menu under the scan mode section of a scan (Figure 4.21). These scans should use a long enough scan duration (in LST) to include an on source time of at least 2min 30sec. It is advised to start a block of high frequency observations with a pointing scan, and tick the apply reference pointing for each scan thereafter. This tick-box will apply the offsets that were determined in a previous pointing scan; if you forget you will be using the (most likely less accurate) default pointing model. Your first calibrator scan (after the setup scans) may be a pointing scan, but as you don't know in what AZ the array starts, you should allow for ample slewing time or anticipate a worst case scenario using the AZ starting conditions on the Reports tab.
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Figure 4.21: Example of an Interferometric Pointing scan used in an SB. |
If the pointing scan has not finished by the stop-time of this scan, no valid solutions can be applied. If it has determined a pointing solution before the stop-time has been reached it will continue with another five-point raster, which may or may not yield new solutions (which will be averaged with the first raster solutions). For secondary reference pointing scans, apply the solutions of the preceding primary reference pointing scan.
A pointing scan is for real-time calibration and, while very useful for real-time calibration, usually does not yield useful data for your project. The data is included in the observations, however, you will need special input parameters to load the data in your data reduction package (CASA or AIPS). You may study this data for reference, but the real-time corrections are already applied and cannot be undone.
