Solar Observing

The VLA is able to observe the Sun but it poses a number of challenges: the Sun is a powerful source, it has a complex brightness distribution, it is variable in time – due to solar rotation and due to intrinsic variability (e.g., flares)– and, as a solar system object it displays significant apparent motion on the sky. For these reasons, solar observations require special hardware modifications, different observing procedures, and special calibration software.

The main difference between solar observing and observing sidereal sources from a user perspective is the need to provide one or more ephemerides to ensure that the solar target or targets of interest are tracked. In this respect, solar observing is similar to observing other solar system objects such as planets and comets. Most of the details related to the hardware changes required for solar observing such as switching in the 20 dB attenuators, applying delay corrections, setting stepped attenuator levels, and referring to solar signal to solar Pcal signals are done automatically behind the scenes and are therefore largely invisible to the user.

The OPT is used to produce scheduling blocks (SBs) for solar observations. The basic pattern used for VLA observations applies to solar observations: observation of a bandpass calibrator, and interleaving observations of one or more solar targets with those of one or more gain calibrators. It is assumed here that users are familiar with the OPT and only steps needed to observe a solar source are detailed.

Special Considerations

Array Configurations: The Sun is large, time variable, and can have a complex brightness distribution. It is not advisable to use the A and B-configurations to observe the Sun in general because the uv coverage is simply too dilute. In addition, scattering in the solar corona on density inhomogeneities limits the useful angular resolution with which one can image the Sun. Hence, for most programs, the C and D-configurations are recommended.

Frequency Bands: Solar observations with the VLA are currently available in the L, S, C, X, and Ku bands for which switched-power flux calibration is implemented. They are also possible in P band (230-470 MHz). While P band employed the same 20 dB attenuators to observe the Sun as L, S, and C bands (T302 module) P band does not currently have the special Tcals needed for switched-power flux calibration. Users should be aware that accurate flux calibration is therefore not possible in P band.

Time Sharing vs Subarrays: Solar observers often wish to observe their target in more than one frequency band. It is not possible to do so over a number of days or weeks because solar targets evolve relatively quickly — on time scales of seconds (flares) to hours (active regions). Observers need to carefully consider whether their scientific objectives require observation in more than one band simultaneously, in which case subarrays should be used; or whether they can sample multiple bands in sequence, in which case time sharing is sufficient.

Mosaicking: The field of view of VLA antennas, taken to be roughly the FWHM width of the primary beam, is given as ϑ≈1.5λ arcmin, where λ is the wavelength in cm. For example, while the full disk of the Sun can be mapped with a single pointing for λ=20 cm, one must resort to mosaicking techniques to map the full disk at shorter wavelengths. The current time required per pointing is currently 40-50sec: 20sec to slew, 10sec for setup, and 10-20sec to integrate on source. Hence, users must again weigh desirability of imaging a larger angular domain against the time evolution of emission within the domain over time.

Short Correlator Integration Time (T_int): The Sun produces numerous transient phenomena: jets, flares, radio bursts. For such phenomena, the availability of short time integrations is desirable in order to resolve time scales of interest. The VLA can be used with very short integration times — down to tens of milliseconds. However, the use of short integrations comes at the cost of high data rates and large data volumes. They should be used with caution.

Ultra-high Brightness Sources: At frequencies less than 2-3 GHz, coherent radio bursts from the Sun become increasingly common. They can be highly polarized, show rapid variability (10s of ms), and complex spectral variability. The brightest bursts can exceed 10⁵ solar flux units, or 10⁹ Jy! The VLA 1-2 GHz band has a special signal path (the HNA or "reverse coupler" path) that allows such bursts to be observed without saturating the system. It has not yet been fully commissioned and is therefore not yet available to users. It is anticipated that it will be in the next two years.

Hardware Modifications

The Sun is an extremely intense source of radio emission. To ensure that that the system maintains adequate linearity the solar signal must be reduced to a level that no element along the IF/LO signal chain saturates. Solar observing is currently supported in five of the Cassegrain bands (L, S, C, X, and Ku) and one prime focus band (P band). In the case of the P, L, S, and C bands this is achieved by introducing a switchable 20 dB attenuators into the signal path of each antenna following the first LNA. The attenuator is switched into the signal path in the LSC frequency converter (T302 module). In the case of the X and Ku bands, the 20 dB attenuator is switched into the signal path after the first LNA and postamp. For all bands, the signal is further conditioned in the frequency downconverter which uses stepped input and output attenuators to set optimum signal levels (T304 module).

While the Sun can be observed when the 20 dB attenuators are switched into the signal path, calibrator sources cannot. Hence, the 20 dB attenuators must be switched out of the signal path when observing a calibrator source. The 20 dB attenuators introduce delay into the signal. This delay has been measured for each attenuator in each band and polarization (the attenuator is the same for the L, S, and C bands) and the delay correction is handled online. The stepped attenuators in the frequency downconverter are first optimized on the Sun. If they were allowed to re-optimize on a calibrator source, an uncalibrated phase error would be introduced. Therefore, the stepped attenuators settings are “set and remembered” on the Sun for use during calibrator scans.

Flux calibration is performed using the VLA switched power system (see Perley 2010). Under normal observing conditions a small, stable, and known calibration signal (Pcal) is periodically injected into the signal path following the polarizer at a rate of 20 Hz with a 50% duty cycle. At the point after the signal has been digitally subdivided into subbands, but before it is requantized and correlatated, the system power is synchronously detected when Pcal is switched on and when it is switched off, P_on and P_off. From these, P_dif = P_on - P_off and P_sum = P_on + P_off are formed from which the system gain and system temperature can be inferred. From the system gain and the so-called requantizer gain, the cross-power may be calibrated and from thence, the visibility amplitudes.

The VLA front ends have remarkable dynamic range and this arrangement is sufficient to observe the quiet Sun, active regions, and small flares. Unfortunately, the front end will saturate for large flares with the exception of the 1-2 GHz band (L band) for which special provisions have been made.

Source Information

Ephemerides

The user must provide the solar ephemeris or ephemerides needed to execute a solar observing program. A given ephemeris is used to track a particular feature of interest on the Sun, correcting for the Sun’s apparent motion on the sky and for the Sun’s differential rotation. Instances where more than one ephemeris is needed are:

Time sharing between multiple solar targets.
The use of subarrays to observe a solar target in more than one frequency band.
The use of mosaicking to map an angular domain on the Sun that is larger than the primary beam.

Observers may wish to generate their own ephemeris using the JPL Horizons website at http://ssd.jpl.nasa.gov/horizons.cgi (please see the Observing Guide regarding Moving Objects for details). Alternatively, a convenient solar ephemeris generator can be found at http://celestialscenes.com/alma/coords/CoordTool.html. Please read the user manual carefully before using. As its name implies, the ALMA Solar Ephemeris Generator was developed as a tool in support of solar observations with ALMA. However, it can be used to generate ephemerides for solar observations from other observatories, including the VLA.

The ALMA Solar Ephemeris Generator offers two interface choices: GUI or Text. The former is attractive because it allows the user to point and click on solar targets using a user-selected Solar Dynamics Observatory Atmospheric Imaging Assembly reference image or a user-provided reference image. The latter requires the user to specify the helioprojective coordinates of the target.

Note 1: Unlike ALMA solar observations, VLA solar observations use ephemerides that are referenced to the geocenter, not the location of the array. The reference is specified in the Location field. Click the Change default location (ALMA) box and use the pull-down menu to the right to select geocenter (not VLA!) as the location to which the ephemeris will be referenced.
Note 2: If using the GUI, do not use the Mosaic observation option in the Pointing field. It is designed for ALMA mosaicking. Instead, generate one ephemeris per VLA mosaic pointing as described in the previous section.

The remaining fields are largely self-explanatory. Upon generating a given ephemeris, the user can inspect the result. If it is satisfactory, it may be downloaded for import into the OPT.

SB Setup

Observers can structure their SBs in the usual way with the following exceptions:

For a solar SB the first scan must always slew to the Sun. This slew can be used to set attenuators for a dummy resource that uses the same receiver as the actual science resource.
Once on the sun, observers must use setup scans for each frequency band that will subsequently be used. This ensures that the stepped attenuators in the frequency downconverter are appropriate, i.e., set and remembered.
For each source thereafter, there should be a scan that slews to the source using the dummy resource, followed by a 10sec setup scan using the science resource to set the requantizer gain (use the setup intent), followed by an observation (calibrate complex gain, calibrate bandpass, calibrate delay, or observe target intents). That is, each source observed — calibrator or target — should be comprised of a triplet of scans that slews, sets up, and observes with the appropriate scan intent(s).
Note: In cases where a preceding scan uses a different receiver, the requantizer scan length should be 30sec long to account for sub-reflector rotation.

Example

05m00s SOL mode, slew to Sun with dummy resource [intent: setup intent]
01m00s SOL mode, attenuator setup scan on Sun with science resource [intent: setup intent]
02m00s STD mode, slew to phase cal with dummy resource [intent: setup intent]
00m10s STD mode, requantizer setup scan on phase cal with science resource [intent: setup intent]
03m00s STD mode, phase cal scan with science resource [intent: calibrate complex gain (A and P)]
02m00s SOL mode, slew to Sun with dummy resource [intent: setup intent]
00m10s SOL mode, requantizer setup scan on Sun with science resource [intent: setup intent]
Begin Loop (repeat scans n times)
 02m00s SOL mode, observe Sun with science resource [intent: observe target]
End Loop
02m00s STD mode, slew to phase cal with dummy resource [intent: setup intent]
00m10s STD mode, requantizer setup scan on phase cal with science resource [intent: scan intent]
01m20s STD mode, phase cal scan with science resource [intent: calibrate complex gain (A and P)]
04m00s STD mode, slew to flux cal with dummy resource [intent: setup intent]
00m10s STD mode, requantizer setup scan on flux cal with science resource [intent: setup intent]
03m00s STD mode, flux cal scan with science resource [intents: calibrate flux, calibrate bandpass]

Solar SBs, like those of other programs, can be complex and setting them up manually in the OPT can be tedious. It is often advantageous to import an SB or to import scans from an external text file. This is done through, e.g., FILE → IMPORT SCANS whereupon a window pops up prompting the user to Choose File and to Import. It is important to conform to the format expected for an SB or scan import as detailed in Section 5 of the OPT manual.

Calibration

As noted above, flux calibration of solar observations requires use of the switched power system described by Perley (2010). Flux calibration using the switched power system requires calibration in the Astronomical Image Processing System (AIPS). The decision to support flux calibration in AIPS was made as a matter of expediency – implementation in CASA will eventually take place but CASA development has a long lead time. Hence, the recommendation is for users to calibrate their data in AIPS, after which either AIPS or CASA may be used to image the calibrated data.

Calibration of solar data in AIPS proceeds in much the same way that it does for non-solar sources. The one exception is that instead of using the task TYAPL to apply switched power calibration to visibility data, solar observers must use the task SYSOL, which recognizes solar Tcal values for those antennas outfitted with solar Cal sources.