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.

The OPT and the Observation Scheduling Tool (OST) currently do not look for or flag SBs containing sources which will be observed too close to the Sun. It is the responsibility of the observer, therefore, 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. That 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). A useful web page providing information on solar activity is: NOAA Space Weather site. If any of the target sources in an SB are too near the Sun (especially if it is particularly active), then the SB should not be submitted or, if already submitted, should be unsubmitted. Once the sources are all far enough away from the Sun, the SB can be (re)submitted. Another useful web page is the near real-time and archival solar activity (< 100) MHz all-sky observations by the Long Wavelength Array (LWA), located adjacent to the VLA.

Table 2.4 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.

The numbers in table 2.4 were calculated via:

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

where Bkm is the baseline in km, λcm is the frequency in cm, and Φdeg 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."

Table 2.4: Minimum Observing Distance from the Sun

A (36km)

B (11km)
C (3km)
D (1km)
Q 0.7 3 3 3 3
Ka 1.0 3 3 3 3
K 1.3 3 3 3 3
Ku 2.0 3 3 3 3
X 3.5 4 3 3 3
C 6.2 6 5 4 3
S 10 8.3 7 5 4
L 21 14 11 8 7
P 90 40 31 30 30
Table 2.4: Short distances have been rounded up to 3°; distances less than this should always be avoided. P-band at C and D configurations have been rounded up to 30° to minimize the impact of the Sun on these observations. Therefore, distances less than these should always cause concern, and greater distances may be required depending on the observation and the activity of the Sun.

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