Avoiding the Sun

The Sun is a bright and variable radio source and can be a problem for VLA observations at all frequencies if it is near the target source. Phase fluctuations and elevated system temperatures will result when observing too close to the Sun. This section gives guidelines on how far sources should be from the Sun as a function of observing band at the VLA, as well as some links to tools to help in planning observations. Please note that these guidelines should not be confused with avoiding daytime observing for RFI reasons, or avoiding sunrise and sunset for phase stability reasons.

Currently, the OPT does not look for or flag SBs containing sources which could be observed too close to the Sun. The Observation Scheduling Tool (OST) will provide a Sun proximity message to the VLA operators to note in the observing log. However, this does not mean the operator will not proceed with a non-solar observation if the sources are too close or behind the Sun. It is the responsibility of the observer, to monitor both if and when their sources will be too close to the Sun and the activity of the Sun. The VLA Sun & Moon Distance Check Tool can help determine if and when a source will be too close to the Sun. This tool also gives the distance to the Moon, although typically the Moon is not a problem unless it is very nearby (within a few primary beam FWHMs).

Solar Activity Webpages

• NOAA Space Weather
• Long Wavelength Array (LWA) provides near real-time and archival solar activity (< 100) MHz all-sky observations.

Handling SBs with Sources Near the Sun

If any of the sources in an SB are too near the Sun (especially if it is particularly active), then there are a couple ways observers may address this:

• On the Information tab of an SB, you may set an earliest/latest UT start date/time to avoid dates when the sources will be too near the Sun. This field can be edited at any time, i.e., before/after SB submission and when the SB is schedulable (in the observing queue).
• You may also unsubmit the SB. If you choose to unsubmit, please be sure to resubmit when appropriate. We will not know why the SB was unsubmitted and will not preemptively re-submit when solar activity has subsided.

Recommended Observing Distances from the Sun

Table 2.5.1 shows the minimum distance from the Sun for 10° phase errors (Φ), assuming the longest baselines can be tolerated. Depending on the solar activity of the Sun, the acceptable observing distance will increase.

Note: We have noticed an increase of solar activity causing problems with phases in the data. Consider a more conservative distance from the Sun than what is list in the table. For example, L-band in D and C-configuration may want to consider observing ~10-20° from the Sun.

The numbers in table 2.5.1 were calculated via:

\[R_{deg}≈\left(\frac{7\lambda_{cm} * B^{0.29}_{km}}{\phi_{deg}}\right)^{0.71}\]

where B_km is the baseline in km, λ_cm is the frequency in cm, and Φ_deg is the phase error in degrees. Details on how this equation was derived are in VLA Test Memo 236, "How close to the Sun should we observe with the VLA?" Another useful memo is EVLA Memo 136, "EVLA Measurements Close to the Sun: Elevated System Temperatures."

More details about the effect of the Sun on VLA observations may be found in the Low Frequency and Very Low Frequency strategy guides.