FCRAO Cookbook for Beam Switching Observations

S. Leon and M. Yun (February 2003)

This a short cookbook for conducting "beam switching" observations using the 4x4 pixels SEQUOIA array on the FCRAO 14-m telescope. Any arbitrary pair of pixels can be selected for a beam switching observation, but generally it makes a good sense to choose two adjacent pixels in AZ direction. In the example observation described below, pixels 11 and 27 are assumed to be the ON-SOURCE or the (0,0) position. The OFF position is taken to be the next adjacent pixels in azimuth, i.e. the pixels 7 and 23. The ON and OFF positions are beam-switched so that the source is always observed with either the ON or the OFF beam in both polarizations.

The SEQUOIA receivers have a very large instantaneous bandwidth (16 GHz) and tuning is done on the IF with the backend. So tuning is not required other than by specifying the line+sideband with the CONF command, and there is virtually no overhead for tuning.

Getting Started

Connect to the FCRAO computer by typing:
> ssh -l observer fcrao.astro.umass.edu
Contact Mark Heyer (heyer@astro.umass.edu) or one of the postdocs for the password. Once logged in, start a terminal emulator by typing
> mterm
You are now connected to the MODCOMP running the telescope. After providing some contact information, look for ?NET prompt. (You will also need another password in order to take command of the control program.) To refresh the screen, type
?NET REFR

?COM POINT

Update Telescope Pointing

Telescope pointing is most commonly done by observing SiO masers. Locate an appropriate source from the pointing source catalog. In this example, we are using RLEO.

MAIN NAME RLEO
IFTYPE 2
STAR QTUNE
CONF RESET
SAME REF AZOFF -.2
CONF 1 COR LINE SIO-4 LSB
TMAI 10 TREF 10 RPT 1
CBAND 50
APMODE 2
REF STAR CAL STAR APOINT

Pointing is stable over many hours. An update is recommended mainly after the sunrise and sunset. APOINT routine goes through one set of 5-point measurements, then the focus optimization, followed by another 5-point measurement. It takes a total of 10-15 minutes, depending on the convergence of the pointing measurements. To inspect the pointing progress, type SCRE 19. (SCRE 1 to return to the main screen) Pointing offsets are automatically adopted every time the 5-point pointing measurement converges.

Beam Switching Observation using Position Switching Mode

The example shown here is CO (1-0) observations of NGC 2798 using the QEF backend. For observations of Galactic sources, change the spectrometer setting to the digital correlator accordingly.

MAIN NAME NGC2798 RA 091722.8 DEC 415958.0 VLSR 1726
IFTYPE 1
CONF RESET
CONF 1 QEF LINE CO-1 USB
QSELECT 11
START QTUNE
TMAI 18 TREF 18 RPT 10
ABC = SAME REF AZOFF -.0248 START CAL START PS REPE 5
ABC

Each integration consists of 10 repeats of 18 seconds ON source and 18 seconds OFF source, with a total of 360 seconds. SAME REF AZOFF -.0248 defines the OFF beam to be the next pixel in azimuth, -0.0248 degree (89") away. START CAL START PS REPE 5 initiates 5 repeats of CAL (Tsys measurement) and PS mode integrations. One can define a new function as in "ABC = SAME REF..." and use this fuction call rather than typing long commands every time.

If you want to interrupt an integration, type
KEYBOARD
STOP PS (if you are doing a position switch PS)

Stopping the Observing Session

DROP (puts the telescope in the stow position)
HALT (Halts the telescope)
EXIT

For more information look at : http://donald.phast.umass.edu/~fcrao/library/manuals/

Data Reduction

If you do not know exactly what are the correct scan numbers, here is a short direction from Mark Heyer on how to do this usin SPA.

Signal from the target source is found in two ON pixels (11 and 27) and in two OFF pixels (7 and 23). Ultimately we want to sum all data from these four pixels together to obtain the best S/N spectrum. However, all OFF position pixels (7 and 23) have inverted spectra because the target source is in their OFF position. Furthermore, these OFF spectra cannot be summed together in a straight forward way because sky rotation causes the coordinate information on the OFF pixels to change with time. To reduce the data in CLASS, you should:
  1. invert all OFF spectra; and
  2. relax the coordinate match parameter to ignore differences in the header coordinate information (e.g. set match 200)

There is a macro called qef.fcrao which can be run within the FCRAO package (which is a local version of CLASS running on the fcrao computer). To start, type

>fcrao
fcrao> @qef first_scan last_scan v1 v2

This will dump all the scan in a directory and get the ON pixels (11+27) and the OFF pixels (7+23) corrected. The "v1" and "v2" are to set the line window for subtracting a DC offset and recording "rms" value for each spectrum. Then do the usual reduction in CLASS:

fcrao> sum
fcrao> set win v1 v2
fcrao> base 1
fcrao> gauss
fcrao> fit
fcrao> write

The output is a file called "temp.14m", which should be renamed to be saved.
Last modified by M. Yun (12/3/2003).