Other Scan Modes

by Gustaaf Van Moorsel last modified Jan 17, 2018 by Lorant Sjouwerman

Other scan mode options besides "Standard Observing" include "Interferometric Pointing" and "Tipping" scans, described in this section. These are special observing modes for calibration, typically applied when observing at high frequencies (above ~ 15 GHz).

The other two modes are "Holography", which is used to measure antenna beam response and is for internal NRAO use only, and "On The Fly Mosaicking" mode, described in Chapter 5 of this manual.


Pointing Scan

(IP) may be needed at frequencies of about 15 GHz and higher (K, Ka and Q band). 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 20 degrees in Azimuth or Elevation from the pointing position. Therefore, typically one 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" for your convenience. You may want to copy the resources and pointing sources you wish to use from the standard catalogs to your personal catalog. Do not forget to select "Interferometric Pointing" for the scan mode and an "on source" time of at least 2.5 (LST) minutes. You want to start a block of high frequency observations with a pointing scan, and tick the "apply reference pointing" in the first tab-page of the scans in this block thereafter. This tick-box will actually 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 very first scan may be a pointing scan, but as you don't know in what Azimuth the array starts, you want to allow for ample slewing time or anticipate a worst case scenario using the Azimuth starting conditions on the SB page.

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 however included in the observations, be it that you need special switches to load the data in your data reduction package. You may study this data for reference, but the real-time corrections are already applied and cannot be undone.


Tipping Scan

(TIP) may be needed if you are concerned about calibrating the absolute flux density of your target source(s). The atmosphere absorbs some of the radiation, and the fraction of the absorbed radiation depends on the opacity, the transparency of the atmosphere. It is mainly dependent on the content of water vapor between the target source and the antenna(s), and can be derived from a series of system temperature measurements at various elevations. One would redetermine the opacity on the time scale in which significant changes are expected, i.e., the time scale in which the water vapor content of the atmosphere above the telescopes changes. This is a strong function of baseline length and actual weather and no real guideline on time scales is available. Use common sense in the trade-off between overhead and usefulness of the scans in post-processing. Note that currently there is no suggested path to apply the results of tipping scans to the data in either CASA or AIPS.

Figure 4.5: Web browser screen shots of the SB scan listing page, top portion.


Tipping scans are performed toward an Azimuth direction close to your sources at about the observing frequency. The scan samples elevations between about 20 and 60 degrees for a system temperature and can be directed from top to bottom (down) or from bottom to top (up). When you select an Azimuth for your tipping scan, be aware that shadowing may occur, especially in C and D array configurations. Avoid the Azimuth directions of the arms, i.e., avoid measuring tips close to the Azimuths of -5, 56, 115, 175, 236, 295, 355 and 416 degrees.

You have to select "Tipping" for the observing mode to expose the tipping scan tab pages. Tipping scans are set up using one of your resources and probably are best done with the widest bandwidth available; make a new resource if you need it. You do not need a physical source. The "on source" time required for a tip is 1m50s (on source LST) because it takes this much time to complete your tipping scan (in one direction, up or down). At the bottom of the page you will have to set the Azimuth and direction; do not forget this as otherwise you will be slewing to the default Azimuth of 0 degrees (North) and may hit a wrap constraint. It can consume half an hour of your observing time to return to your science observing. Always set the Azimuth.

You may place any number of tipping scans anywhere in your schedule as you feel fit to monitor the opacity during your observations, although you may want to do this close to your block(s) of high frequency observations. Your very first scan may be a tipping scan, but as you don't know in what Azimuth the array starts, you want to allow for ample slewing time or anticipate a worst case scenario using the Azimuth starting conditions on the SB page.

If the tipping scan has not finished by the stop-time of this scan, the data will contain those elevation samples that were completed. If it has completed the tip before the stop-time, it simply will continue with the next scan until the regular stop-time for that scan - this scan may be used to buffer the difference, e.g., absorb the extra time on your bandpass calibrator.

A tipping scan is for off-line calibration and may or may not yield useful data for your project. The data is included in the observations and you need special switches to load the data in you data reduction package. A "tip" would allow you to determine the opacity of the atmosphere during the tipping scan (i.e., during your observation), and you can use that value to correct for the atmospheric absorption in your data. Read the manual of your data reduction package on how to obtain and apply tipping scan data corrections.