Observing Modes

In the following, we give some brief descriptions about the observing modes currently available at both the SMT and 12m telescopes. More detailed information on these observing modes can be found in the various User's Manuals.

Relative Position Switching

Position switching, called the PS mode, is the most common and reliable observing mode for general spectral line observations. It involves considerable overhead in telescope movement and requires that equal time be spent in the ON and OFF source positions, but the data quality is usually good. In this mode, the telescope moves between the ON position and a relative OFF position, which may be specified in either azimuth and elevation, or right ascension and declination offsets. Usually the offset is in azimuth, so that the ON and OFF positions are taken at about the same airmass. The best rejection of the atmosphere and the best spectral baselines are achieved with small angular switches. Choose the smallest switch possible, so long as you are confident that the OFF position is free of emission. PS data recorded on disk is a final spectrum formed from the ratio (ON-OFF)/OFF, where the ON and OFF data are total power samples. In contrast to the total power observing modes TPON and TPOFF, discussed below, the ON and OFF samples are not saved as separate scans for independent processing. Although the PS mode offers less flexibility in processing data than do the total power modes, it also reduces the total volume of data and makes processing easier. To reduce telescope movement and provide the best compensation for linear drifts in atmospheric emission, choose the number of OFF-ON pairs to be a multiple of 2. The observing cycle will then be repeats of an OFF - ON - ON - OFF pattern. Each ON or OFF is called a sample and each OFF - ON pair is called a repeat. The observer must tell the operator how long to integrate for each sample (the default is 30 seconds) and how many repeats per scan, or alternatively, the total length of the scan in minutes. A typical scan might be 6 minutes long, with 30 second samples (meaning 6 repeats). You can, of course, vary the length of the scan to suit your own needs. The operator can issue the command to take scans one at a time, or can set the system into an automatic data-taking loop.

Absolute Position Switching

Absolute position switching, called the APS mode, is useful when observing in complex emission regions where it is difficult to find an emission-free reference position. In such cases, position switching with (Az,El) offsets can be dangerous because rotation of the parallactic angle as the source is tracked across the sky may cause emission to rotate into the reference beam. You will want to search for an emission-free position as close to the source position as possible, and use this as the reference position. If you wish, you can compute the (RA,DEC) offsets to this position and use ordinary position switching. Most observers find it most convenient, particularly for future observations, to enter the absolute (RA,DEC) coordinates of the reference position and use the absolute position switching observing mode. APS is identical to ordinary position switching except that the switching is done between two positions absolutely specified by their celestial coordinates. The reference (OFF) position should be given a different name from the signal (ON) position and is best placed in a different source catalog from the signal position. Data taking and calibration options are the same as for ordinary position switching.

Beam Switching

Spectral line beam switching can be useful when observing small angular diameter sources and when the best possible baselines are needed. This observing mode involves the nutation (chopping) of the subreflector and a positional movement of the telescope and is thus called the BSP (beam switching plus position switching) mode. The technique is much the same as that used for continuum ON/OFF's. With the subreflector nutating at a rate of typically 1.25 Hz, the telescope is moved to place the source first in one of the beam positions and then in the other. The beam position which, for a positive source signal, produces a positive response in the spectrometer is called the ``positive beam'' and a sample taken in this position is called an ``ON''. Conversely, the beam position which produces a negative response in the spectrometer is called the ``negative beam'' and a sample taken there is an ``OFF''. A BSP scan always consists of four samples taken in the order OFF - ON - ON - OFF. The samples are taken in this order to get the best atmospheric rejection, the best baselines, and to reduce telescope movement. The integration time of one of the individual ON or OFF samples controls the total integration time of the scan (sample length times 4). The beam switching mode usually produces very good spectral baselines. The subreflector switch rate is such that atmospheric changes and filter bank anomalies are most often subtracted out. The primary restriction for beam switching is that the source angular diameter must be smaller than the subreflector throw. The subreflector throw can be varied between 0 and 6 arc min. The default switching rate is 1.25 Hz. Switch rates of 2.5 and 5.0 Hz are also available. The observing efficiencies are poorer at the faster rates but the cancellation of atmospheric drifts may be better. You must decide upon the following parameters in a beam switched observation and give them to the operator:

  • The subreflector throw. Changes in the throw must be made manually; the computer must be updated (manually) as to the new value of the throw.
  • The switch rate of the subreflector.
  • The integration time per sample (ON or OFF). The total length of the scan is the sample time x 4.

Spectral Line Mapping

The control system offers four modes of spectral line mapping: mapping by manual offsets, automatic mapping of rectangular grids in either the total power or position-switched modes, automatic absolute position- or frequency-switched mapping using catalog generation routines, and on-the-fly mapping. Mapping with manual offsets is appropriate for small maps or maps with unevenly spaced points. For most rectangular grid mapping, we recommend the automatic position-switched total power mode. With the total power mode, you can choose to observe several ONs per OFF and thereby increase your observing efficiency. The other alternative for grid mapping is through automatic catalog generation. With this method, you build a catalog of positions and step through them, either automatically or through manual selection by the operator. This method works in either the automatic position switched (APM) mode or the frequency switched mode. On-the-fly mapping is discussed in the separate manual On The Fly Observing.

Total Power Mapping of Rectangular Grids

In the total power mapping mode, called TPM, you can define a rectangular RA-DEC grid with different grid spacing in the RA and DEC coordinates. You can choose to observe several map positions (ONs) for each reference (OFF) position, and several OFFs for each vane calibration scan. After defining the grid, you can choose to map a subset by specifying the beginning and ending row numbers, and the beginning and ending column numbers. The map is scanned row-wise, starting at the negative-most RA offset and the negative-most DEC offset. The TPM mapping procedure insists upon placing a map point on the center position of the map. This is regardless of whether there are an even number of rows or columns. To use the TPM procedure, give the operator the following information:

  • The catalog and name of the source (map center position).
  • Standard setup parameters, including the (Az, El) pointing offsets, the pointing tolerance, and the focus setting.
  • The reference position offset (relative to the map center) in either the AZ-EL frame or the RA-DEC frame. The maximum RA-DEC offset is 2 degrees 48 arcminutes from the source position.
  • The RA grid spacing in seconds of arc (real angle).
  • The DEC grid spacing in seconds of arc.
  • The number of rows (displaced in DEC) in the map.
  • The number of columns (displaced in RA) in the map.
  • The integration time per ON or OFF (both must be the same) in seconds.
  • The number of map positions (ONs) to be observed for each reference position (OFF).
  • The number of OFFs for each vane CALIBRATE.
  • The beginning and ending rows of the map. (optional)
  • The beginning and ending columns of the map. (optional)

Position Switched Mapping

The position-switched mapping routine PSM will generate a position-switched RA-DEC grid map using relative offsets for its reference position. The reference offset can be specified in either the AZ-EL or RA-DEC frames. If you specify an RA-DEC reference offset, the map is equivalent to an absolute position-switched map since the reference position is always the same point on the sky. The map is done on a rectangular grid that can have different X and Y cell sizes. You can specify that only a subset of the originally specified grid be mapped. Each point of a PSM grid map is taken as a position switched scan using the PS observing algorithm, \ie\ the telescope switches on and off source in the pattern OFF - ON - ON - OFF ...., where each OFF - ON pair is called a repeat. You can also set the integration time of individual ON and OFF samples. The operator will need the following information to perform a PSM map:

  • The X (= RA) cell size in arc seconds;
  • The Y (= DEC) cell size in arc seconds;
  • The number of rows to map;
  • The number of columns to map;
  • The number of scans per vane calibrate;
  • The number of seconds of integration for each ON or OFF sample;
  • The number of ON - OFF pairs per mapping point.

When the map is executed, the RA step size in time measurement includes the cosine declination correction, so that all mapping grid offsets represent real angle on the sky. One can also observe a subset of the map by specifying a starting and ending row or column. As a general rule-of-thumb, it is best to use OTF mapping instead of the step-and-integrate mapping described in this section when your map field is larger than about 6 arcminutes in either RA or DEC and your target spectral line is expected to be reasonably strong. For those rare cases when OTF is not suitable, one must decide which step-and-integrate mapping technique to use. There is no hard rule to determine whether one should use PSM or TPM, the total power mapping mode. TPM is more efficient in the sense that you can use one OFF scan with several ONs. However, PSM may produce better baselines because of the switching pattern. As a general recommendation, we suggest that you use TPM for maps of strong lines and large mapping grids; use PSM if the lines are weak and baseline stability is critical. APM is also an alternative to PSM: it uses the same OFF - ON - ON - OFF ... pattern but the mapping positions are taken from discrete catalog entries rather than a grid built from offsets from a single central position. The PSM mapping procedure (like TPM) insists upon placing a mapping point on the center position of the map.

On The Fly Mapping

A somewhat out of date manual for OTf mapping is available here.

A spreadsheet to calculate map parameters is available here: ODS format / Excel format.

Switched or Total Power ON/OFF's

There are available several observing procedures for making ON/OFF measurements, ie., measurements of the difference between the output powers at a defined point (ON) and a nearby reference position (OFF). These procedures work with the subreflector nutating or not. The ON/OFF procedures are often used for measuring the flux density of weak point sources. The standard point source ON/OFF observing procedure is called a sequence in the control system terminology. Using this procedure, the telescope moves between the ON and OFF source positions in the pattern OFF - ON - ON - OFF. A sequence is made up of one or more repeats of this basic cycle. This order of samples eliminates the effects on the measurements of linear drifts in atmospheric noise or receiver gain. You can specify integration time per position. The standard set-up for an ON - OFF sequence in beam switched mode is double beam switching (DBS), ie., with the ON and OFF positions separated by the subreflector throw. In this way the radio source to be studied can be cycled between the positive and negative beams by the movement of the telescope. For the DBS mode, the output power difference between the ON and OFF phases is proportional to twice the source flux density. The advantage of the ON - OFF approach is that, to first order, it cancels imbalances between the two beams. One can, of course, select the OFF position to be different from the position of the -BEAM (or the ON position different from the +BEAM). This is a single beam (SBS) observation. The most frequent use of the SBS mode is to determine the position of the +BEAM and the -BEAM independently, and thereby determine the subreflector throw and orientation. To perform an ON - OFF sequence, you must give the operator the following setup parameters:

  • The length of time to be spent on each ON or OFF sample and the number of repeats of the OFF - ON - ON - OFF cycle. The sample length is usually chosen to be 5 or 10 seconds. This length is usually a good compromise between maximum switching efficiency of the telescope and the best compensation for atmospheric drifts (the switching pattern corrects for linear drifts and will approximate higher order drifts if the sample time is sufficiently short). The total integration time of a sequence is (4 * seconds * repeats). Thus, if you request 5 seconds per sample and 6 repeats, the sequence takes 120 seconds.
  • Whether the sequence will be made in beam switched mode or position switched (PS) mode. If the observations are in the beam switched mode, indicate whether the Dual Beam (DBS) or Single Beam (SBS) option is in effect (the DBS mode is the normal case).
  • The required telescope tracking tolerance (TOL), and the optimum pointing positions of the ON and OFF beams (consult the pointing charts at the observer's console).

Continuum Mapping

Two options are available for mapping extended continuum sources: a grid-mapping procedure in which the telescope steps through a rectangular grid in the azimuth/elevation frame and On-The-Fly (OTF) mapping.

Grid Mapping

In continuum grid mapping, a field is mapped in a rectangular grid in azimuth and elevation relative to the field center. This observing procedure has been developed for use with the dual-beam restoration algorithm of Emerson, Klein, & Haslam (1979 Astr. Ap., 76, 92). It will, however, work satisfactorily for either fixed beam (total-power) mapping or the mapping of fields that are smaller than the subreflector throw via the +BEAM. The telescope builds a two-dimensional grid of observations by scanning rows at constant elevation relative to the source position. The telescope moves along a row in discrete steps, performing an integration at each position. The grid points are separated in azimuth by real angle. If a map has M columns and N rows the requested field center will lie at the central grid point INT[(M+1)/2], INT[(N+1)/2)] if N and M are odd numbers, where INT denotes an integer truncation. If N and M are even, the requested field center will fall at grid point [(M/2)+1, (N/2)+1]. A scan represents a row of the map in this observing mode. The mean sidereal time of each point is stored in the scan array. Analysis programs have been prepared to combine the scans into two-dimensional maps. You can process these maps using the dual beam restoration algorithm, then transform them into celestial coordinates and stack them. Unfortunately, we do not currently have an implementation of this analysis system at the telescope. To observe such a map, proceed as follows:

  • Tell the operator whether the data will be taken in total power, fixed beam (total power) or beam switched mode.
  • Give the operator the azimuth and elevation cell sizes, the number of rows and columns, and whether the map will be acquired in dual-beam or single-beam. In a dual beam map, the telescope is positioned so that the azimuth of the center of the map is at the mid-point of the two beams. In a single-beam map, the map is referenced to the +BEAM in azimuth rather than the mid-point between the +BEAM and -BEAM.