SB Validation

by Tony Perreault last modified Mar 20, 2017 by Heidi Medlin

In this section we provide practical considerations to ensure that your VLA scheduling block (SB) adheres to the discrete requirements for successful observations. Please follow this list during the preparation of your SB and before submitting it through the Observation Preparation Tool (OPT). If you wish to check the validity of the correlator setup of your observations, please visit the Instrument Configuration Verification page.

It is essential that one checks their SBs carefully prior to submitting. Although the OPT has several features to deal with logistical observing issues, it will not catch everything. The same goes for the SB verification that NRAO staff performs after submission; SBs are checked only for common errors. Items like source positions, Doppler-setting frequencies, choice of calibrators, etc., and anything particular that refers to the science goal of the observations cannot be verified by the staff. Remember that the final responsibility for a correct observing schedule remains with the observer.

SB Information Tab

  1. Appropriate wind and API constraints: Important because most of VLA observing is dynamic.
  2. Be aware of selecting "avoid sunrise/sunset": In most cases this is not necessary and can decrease the chances of observing. For more details, refer to the Low Frequency and Very Low Frequency strategy guides.
  3. Duration of the SB: This is highly dependent on the observing priority of the SB. Please see the optimal duration question in the Frequently Asked Questions section. (Note, any length of SB is permitted, i.e., 33min, 1hr12min, etc..)
  4. Count: Should the SB be observed more than once?

SB Reports Tab

  1. Setup scans: These are needed for observations with both the 8-bit and 3-bit samplers. Please see the 8/3-Bit Attenuation and Setup Scans section for more details (note there are special instructions for S- and P-band observations).
  2. Start-up time: This is typically 10-12 minutes at the start of the SB. It is primarily for the initial slew to the first source in the SB and for setup scan(s). These are followed by the first calibrator-intent scan for low frequency observations or the first reference pointing scan for high frequency observations.
  3. Scans in Duration (LST) only: In this mode, scans include both slew and on-source time.
  4. Antenna Wrap Requests: While checking the elevation, make sure to set the LST start range to avoid antenna wrap issues and request an antenna wrap when appropriate. Note, not all SBs will require an antenna wrap request.
  5. Elevation Limits: Ensure that all scans (for all LST start times) have appropriate elevation limits.
    • High frequency (> 15 GHz) observing: avoid elevations < 30° (if possible) and > 80° (required).
    • Low frequency (< 15 GHz) observing: avoid elevations < 10° (recommended) and > 85°.
    • S-band (2-4 GHz) observing: avoid elevations < 20° (recommended to avoid ground spill-over).
    • The VLA's hard lower elevation limit is 8°.
    • Step through the LST start range to validate elevation limits for all scans.
  6. Double check the science target source coordinates are correct and are using the J2000 epoch.
  7. Include all necessary calibrators: complex gain, flux density, bandpass (and delay) and, if applicable, polarization.
    • Bracket the science target scans with the complex gain calibrator and use the appropriate cycle time: This is especially important for high frequency observing. An exception may be when the target can be self-calibrated. The final decision, however, is up to the observer.
    • If observing more than one frequency (LO/IF) setting - this being within a single receiver band or being through multiple receiver bands - and switching between them in the SB, there is no guarantee to return to the same phase state upon returning back to a given frequency setting. Therefore, in such cases it is extremely important to bracket the target with complex gain calibrator scans that use the same frequency setting as the target to avoid experiencing any phase jumps.
    • All scans have appropriate scan intents: If you wish to use the VLA Calibration Pipeline, a flux density and bandpass calibrator must be defined in the SB. (Note, do not use Determine Autophase or apply last Phase & Delay cal; these are reserved only for VLBI observations.)
  8. Use and apply reference pointing: Interferometric reference pointing is needed during high frequency (> 15 GHz) observing with the solutions applied to the proceeding scans. For more details, see the Antenna Reference Pointing Calibration section in the Calibration guide.
    • Repeat reference pointing scans when the difference from the last pointing position is at least 20° on the sky in AZ or EL.
  9. Minimum on source time: For regular (standard observing) scans, no less than 20 seconds on source after slewing (for flux density calibrator no less than 30 seconds on source after slewing); for reference pointing no less than 2.5 minutes on source after slewing. [On the Reports tab of the SB, step through the LST start range to verify sufficient on source time.]
  10. Maximum total scan length: regular (standard observing) scans may be up to 10 minutes in duration; interferometric pointing (reference pointing) scans and OTF mosaicking scans may be greater than 10 minutes in duration.

Do not observe too close to the Sun, especially at low frequencies. This is not related to selecting the "avoid sunrise/sunset" option within an SB. For more information, refer to the Avoiding the Sun section.

If you would like more help with validating your SB(s) (i.e., instrument configuration or appropriate calibrators), please submit your questions to the NRAO Helpdesk.