Our solar observations with the 32m Medicina radio telescope employ the Equatorial (RA-Dec) On-the-fly (OTF) single-dish mapping technique - taking advantage of the availability of the dual-feed receiver - acquiring data with the Total Power back-end.

Observing schedules are produced using a custom generator, in turn relying on solar ephemeris computed with the JPL Horizon web tool.

The maps are obtained setting the two feeds in different dynamic ranges, so as to coevally acquire optimised data on the bright solar disk and on the much fainter coronal structures. This is accomplished via a double layer of variable signal attenuators, i.e. the ones on board of the Total Power back-end, plus the so-called dimed device, which is placed upstream of the back-end.

In order to acquire spectral information, observations are carried out, as much as possible, at the boundaries of the available RF band: maps are acquired first at 18 GHz, then at 26 GHz. The frequency and bandwidth actually employed may slightly vary along time, according to the incidence of RFI. As concerns such polluting signals, it must be stressed how their population is fast growing in the K-band frequency range. For this reason, we aim at exploiting, as soon as it will be available, the ROACH-based digital spectrometer - at present being commissioned in Medicina - for our acquisitions. As this back-end is going to be fully integrated in the DISCOS control system (in charge of all the single-dish observations), the data format of its acquisitions will be compatible with our processing pipeline; using proper RFI-clipping algorithms, spectral acquisitions will be cleaned and integrated in total-power-like files, identical to the FITS files presently produced by the Total Power back-end. Our pipeline, though, can also fully exploit the spectro-polarimetric content of the original acquisitions.

The 18 and 26 GHz maps are obtained by scanning the desired area along Right Ascension and Declination, respectively. This choice reflects the need to reduce the total observing time, while preserving the opportunity to produce, if desired, a cross-scanned map by combining the initial two maps (of course, rebinning the higher-resolution map in order to match the features of the lower-resolution one), mainly for consistency checks.

Both maps are achieved with a total observing time of 2h and 30m.

Table 1 and 2 respectively describe the typical mapping parameters and setup configurations used for the receiver and back-end, in most of the observing sessions.

Flux density calibration is achieved through the observation of calibration sources, either in cross-scan or mapping mode. Mostly, Cas A is observed producing 40x40 arcmin maps, with 2.0 scan/beamsize, in order to obtain the conversion factor from arbitrary counts to flux density units (Jy). The two calibration maps, one for each of the observed frequencies, are completed in 30 minutes.

Skydip acquisitions, again one for each frequency, are employed to estimate the atmospheric opacity. They require additional 12 minutes and complete the dataset.

Taking into account setup time, slewing time and general overhead, each standard observing run requires a total time of 3h 30m. Such observing times have been assessed with the experience gained during the sessions performed for the pilot project.

TABLE 1: Mapping Parameters

Parameter Value

Map dimensions 80x80 arcmin

Scanning speed 6 arcmin/s

Scan interleave 4.0 scan/beamsize

Scan direction RA for 18 GHz map, Dec for 26 GHz map

Overall duration 2h 30m to acquire both maps

TABLE 2: Receiver and back-end configurations

Parameter 18-GHz map 26-GHz map

Observed band 18.20-18.45 GHz 25.70-25.95 GHz

Beamsize 2.1 arcmin 1.5 arcmin

Sampling interval 40 ms 40 ms

Solar disk map on Feed 1 Feed 1

Coronal map on Feed 0 Feed 0