Tipping (TIP)

Important Note: Currently tipping scans are discouraged since neither CASA or AIPS can apply the results of the tipping scans to the data.

Tipping scans may be needed if you are concerned about calibrating the absolute flux density of your science 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. An observer 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.

Tipping scans are performed toward an AZ 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 AZ for your tipping scan, be aware that shadowing may occur, especially in C and D-configurations. Avoid the AZ directions of the arms, i.e., avoid measuring tips close to the AZs of -5, 56, 115, 175, 236, 295, 355, and 416 degrees.

Setting Up a TIP Scan

To utilize the tipping scan mode, select Tipping from the drop-down menu under the scan mode section of a scan (Figure 4.22). 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 total duration (LST) of a scan should include the required on source time of 1min 50sec for a tip scan to complete (in one direction, up or down). At the bottom of the page you will have to set the AZ and direction; do not forget this as otherwise you will be slewing to the default AZ of 0d (North) and may hit an antenna wrap constraint/limit. Hitting an antenna wrap limit, can consume half an hour of your observing time to return to your science observing, so always set the AZ.

Figure 4.22: Example of a Tipping scan.

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. The first scan following the setup scan(s) may be a tipping scan, but as you don't know in what AZ the array starts, you want to allow for ample slewing time or anticipate a worst case scenario using the AZ starting conditions on the Reports tab.

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 post-observation 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 CASA or AIPS. 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.