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GBT

  1. Mosaic
    1. ( Ref 1; Section 7.7)
      1. A RALongMap can take up to 25 seconds to start; scans <1 min have significant overhead
      2. Fields of Views larger than 10 - 20 beam FWHM should use RALongMap or DecLatMap; smaller FOV should Daisy Scan
      3. Scan lengths should be under 15 minutes
      4. There should be four switching periods per integration
    2. Raster scan lengths or petal lengths should be at least 30s so reduce stress on telescope (ref 1; Section 6.4.3)
  2. Front-end/BackEnd
    1. DCR has minimum integration time of 100ms

    2. If high frequency and if doing position switched observations and if the source is not extended, then use the Astrid Nod() procedure to observe (Ref 1; Section 6.4.2)

    3. VEGAS
      1. Table 8.2 (ref 1) has minimum recommended switching periods with VEGAS observations that use a noise diode; Table 8.3 has the switching periods that do not use the noise diode.
      2. Switching period, frequency switch, integration time must be the same for all banks; the banks can have different spectral resolutions, bandwidth and number of subbands (spectral windows). The subbands in a single VEGAS bank must be identical except for the central frequency.
  3. Calibration
    1. Table 7.1 of (Ref 1) gives recommended Point/Focus Spacing

    2. (Ref 1; Section 7.4) Flux density calibration : Peak/Focus on calibrator, followed by observation in same spectral line setup as program sources.

    3. Observing Strong Continuum Sources (ref 1; section 7.9)
      1. Might want BalanceOnOff() here instead of Balance()

    4. When there is a large difference in power received by the GBT between two positions on the sky, it is advantageous to balance the IF system power levels to be at the mid-point of the two power levels. Typically this is needed when the source position is a strong continuum source. One should consider using it when the system temperature on and off source differ by a factor of two or more. (ref 1; section 6.4.1.10)

    5. Nod should only be used with mutli-beam recievers (ref 1; section 6.4.2.5)
    6. AutoOOF Strategy (ref 1; section 7.2)
      1. recommended for f > 40 GHz
      2. only available for Rcvf26_40 (Ka-band), rRcvr40_52 (Q-band), Rcvr68_92 (4mm Reciever or Wband) RcvrArray75_115 (Argus), and Rcvr_PAR (MUSTANG)
      3. Choose a bright calibrator: preferably at least 7 K in the observed band (4 Jy at Q-band, for example)
      4. Allow ~25 minutes for AutoOOf
      5. AutoOOf is not necessary for extended source – if the science is not impacted by the primary beam variations
      6. Solutions are valid for:
        1. If at least 2 hours after sunset, solutions are good for 4 hours (nighttime)
        2. daytime is less predicable and depends on Sun/cloud cover: 30 minutes to 4 hours
        3. periodically examine peak scans for problems (not predicable)
    7. Balancing Strategies (ref 1; section 7.3)
      1. "There are not any set-in-stone" rules" – great.
      2. Balance IF system after performing a configuration
      3. minimize the number of times you balance when observing (oh my)
      4. you may balance when a chance in source equals a change in system_temp + t+source > 3 dB
      5. avoid balancing while making maps
      6. don't balance between signal and reference observations (on/off)
      7. don't balance between observations of targets and calibrators

VLA

  1. Mapping (https://science.nrao.edu/facilities/vla/docs/manuals/obsguide/modes/mosaicking#section-2)
    1. Angular spacing along rows of a discrete mosaic – what scalar for what mapping?
      1. 1.2 (Nyquist)
      2. sqrt(2)
      3. sqrt(3)
    2. If less the integration time is less than 15 seconds, we use OTF?
    3. the phasecenter cannot change faster than 0.6, 0.5 seconds?
    4. the scan rate cannot be greater than 3 ? 10 arcminutes/s? (https://science.nrao.edu/facilities/vla/docs/manuals/opt-manual/observation-preparation-tool/scan-modes-obs-modes/on-the-fly-mosaicking)
    5. How large of an extent in RA can be tolerated for OTF mapping?
    6. Should the highest part of the band be used to determine mapping parameters or should the center frequency? (depends on science)
  2. Calibrator and Wrap
    1. Another inconvenient wrapping situation appears when observations near zenith (elevations larger than about 80°) switch between sources on different azimuth wraps. This happens when a target and calibrator are close to, but on opposite sides of 34.1° (34°04′42.497′′) declination (latitude of the VLA). The OPT will show slew times much longer than expected for the angular source separation, and indicate an azimuth wrap (CW or CCW) on only one of the sources in the reports of the scan list. So even if a calibrator may be a bit further away or less strong, it may be the better option when it is on the same side of zenith. Experiment with different LST start times to investigate the effect for a particular SB if no suitable calibrator can be found on the same side of zenith. (https://science.nrao.edu/facilities/vla/docs/manuals/obsguide/dynsched/antenna-wraps )

VLBA

  1. Reliable calibrations can be obtained for sources observed with an elevation above 20 degrees for most VLBA antennas. JIVE (https://planobs.jive.eu/) uses 10 degs as a hard cut-off for elevation for any given station. It is possible to observe less than that cut-off however. Generally, at lower frequencies, there is "spill-over", which is emission from the ground. At high frequencies, there is more atmosphere along the line of sight, which increases the difficulty in calibrating. The RFI is also more severe at lower elevations.

Reflections

  1. For GBT observing, there are less predicable "stressors" on the time estimate. For example, AutoOOF solutions during the daytime are good for 30 minutes to 4 hours, depending on the sun, cloud cover, etc. Would it be useful to give a range, Delta(total time requested), or a best case /worst case time ranges for Overhead?

Reference

  1. GBTog PDF
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