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KYA 307

How to control the 26-metre antenna

You must first have permission from the observatory manager or the next most responsible person on-site before starting the antenna motors or moving the antenna. Be aware that it is not always safe to move the antenna.

The most straightforward way of controlling the antenna is with the vdesk program. You will be shown how to start and use it. Listed here are the commands that you will find most useful. The normal procedure for moving the antenna is

  1. Use drvon to power up the motors and release the brakes.
  2. Specify coorect coordinate mode, coord b1950
  3. Specify right ascension (RA) and declination (Dec) coordinates with longit and latit.
  4. Use slew to move the antenna to the source.
  5. Once the antenna is on-source use the scanning parameters and commands to scan across the source.

Useful commands:

drvonswitch ON the antenna drives
parkstow the antenna and turn the drives off
longit hh mm ssspecify the right ascension (RA) of the radio source
latit dd mm ssspecify the declination (Dec) of the radio source
coord b1950specify the coordinate mode for RA/Dec (could also be j2000 or date)
slewmove antenna to the specified RA and Dec and track
leftlength d.dspecify length (deg) of RA scan (choose 1.0–2.0)
rightlength d.dspecify length (deg) of Dec scan (choose 1.0–2.0)
leftrate d.dspecify rate (deg/min) of RA scan (choose 0.5–3.0)
rightrate d.dspecify rate (deg/min) of Dec scan (choose 0.5–3.0)
leftscanexecute a scan in RA using parameters specified in leftlength and leftrate
rightscanexecute a scan in Dec using parameters specified in rightlength and rightrate

Getting started

  1. Start vdesk and make sure the antenna is stowed (X=−2 deg, Y=+5 deg). If not, in vdesk:
    • drvon
    • park
  2. Move the S-X receiver to the focus position (X=−256.3, Y=248.7, Z=18, i.e. receiver 1) using the OTTER interface.
  3. Use the Configurable Backend to select S/X SRCP as Channel 1, SX XRCP as channel 2.
  4. (Optional: You may find that this is not necessary given the 16 MHz bandpass filtering (see below) but it may help if interference is causing the data to change amplitude quickly.) Put in a high pass filter with notches plus the 225 MHz low pass in S-band.
  5. Adjust attenuators at top of rack 2 to put Square Law Detector in the −4 to −5 range. You may also need to adjust the DAS Attenuation in the Configurable Backend to bring the levels within range.
  6. NOTE that the signal going into the square-law detectors is limited to 16 MHz bandwidth. This is necessary at S-band to reduce interference but it is possible to bypass the filters at X-band to provide more sensitivity if necessary. Talk to Jim if you want to try this.
  7. Make sure you have control of the noise diodes. At the terminal prompt:
    • calu -m sam26m
Then test that you do by putting the diodes into an on/off switching mode:
  • calu -m test1
CTRL-C when you’ve verified it’s OK.

Now go to a source using vdesk:

  • drvon
  • coord b1950
  • longit hh mm ss
  • latit dd mm ss
  • slew

Set up for a scan. Example:

  • leftlength 2.0
  • rightlength 2.0
  • leftrate 1.0
  • rightrate 1.0

Start collecting data using SamTest. e.g.:

  • samtest -c 0 -f 847 -i 0 -n 2 -s 300 -o /scratch/test3.oct sam26m

Note that the data from samtest will need some post-processing before it can be used. The samplers have significant quantisation so binning and averaging the data is required to improve the signal to noise. This example perl script takes two-column ASCII data and bins it but data can also be processed in Octave, Matlab or other software of your choice.

Then start a scan in vdesk:

  • leftscan

Watch the coordinates change at top of rack 1, and at the end, toggle the noise diodes:

  • calu -m test1

CTRL-C after a few cycles. Then a scan in Dec:

  • rightscan

Watch the coordinates change at top of rack 1, and at the end, toggle the noise diodes: calu -m test1 CTRL-C after a few cycles. Then put in 1 dB of attenuation at top of rack 2 for ~ 10 sec, then take it out again

Note that the antenna will stop at the position it reaches at the end of a scan and not go back on-source. To move back on source, use the slew command.

Consider repeating this sequence several times for the same source and consider faster or slower slew rates. Note you will need to bin the data somewhat before being able to see weaker sources.

Calibration sources

To get a list of useful calibration sources and their flux densities, use the flux_cal.pl script on newsmerd:

  • flux_cal.pl -frequency=[freq in GHz]

e.g. for X-band:

  • flux_cal.pl -frequency=8.5

Choose the brightest object that’s up at the time. Sources 10 Jy or more should be easy to see. For weaker ones, consider making multiple observations.

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Page last modified on April 17, 2014, at 02:56 AM