Observing in "Fast" Mode

Fast Mode Observing Setup

Purpose

The normal operating mode of EOVSA is to tune across the entire 1-18 GHz band (50 tunings) each second, which means taking data in a given frequency band for 20 ms (actually 19 ms after 1 ms of settling time) per second. Some solar phenomena, especially coherent emission, varies on a much shorter timescale, making it of interest to have an observing mode that repeats a given frequency with higher time resolution. Luckily, the frequency-agile nature of EOVSA allows this.

Implementation

We have implemented a mode that we call "Fast Mode" where the system dwells on one or more bands for a larger fraction of a second and records data at a 20 ms rate during that dwell time. This is subject to the system requirement of a 1-s repetition rate, but is otherwise quite unrestricted. For example, it is possible to sit on one band for 0.8 s and then tune to 10 other bands at a 0.02 s rate to finish out the 1-s cycle, with this cycle repeating every second. Many other combinations are possible.

Hittite LO Command File

The first requirement of the system is the local oscillator (LO) tuning itself, which is performed by a Hittite 40 GHz signal generator. An example control file (called a frequency sequence file, or FSEQ file, which is a text file with extention .fsq) is shown below:

LIST:DWELL

0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,

0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.8,

0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,

0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,

0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,0.01975,

0.01975,0.01975

LIST:SEQUENCE 1, 2, 5, 6, 9, 13, 19, 20, 26, 37, 51

The first command, LIST:DWELL, gives a list of 52 durations, one for each of our possible tuning bands. Most bands have a duration of 19.75 ms, slightly less than the target 20 ms in order to account for some overhead in tuning. Note that the 20th band has a duration of 0.8 s (I have highlighted it in bold). In the actual file, the LIST:DWELL command is on a single line, but here I have broken it into lines of 10 numbers each for clarity. The second command, LIST:SEQUENCE, specifies the bands to be used in the sequence, here a roughly logarithmically spaced set of bands over the whole range, but with band 20 (the dwell band) inserted. Note that the sum of durations for these 11 bands must add to 1 s (or slightly less due to the overhead).

Dwell Sequence Helper Routine

To create such DWELL sequences, a helper routine has been written in Python called make_flare_fseq() in the file make_fseq.py. The call sequence is

make_flare_fsq(dwellband, bandlist=None, acc=True, check=True, verbose=False)

where dwellband is the band number to dwell on and bandlist is a list of additional bands to sample. There is a default list of bands [5,6,8,11,14,18,24,31,40,52] that will be used if bandlist is None. A couple of examples follow:

make_flare_fsq(20)

is the simplest way to call the function, which will use the sequence [5,6,8,11,14,18,20,24,31,40,52] where band 20 is inserted into the list, and band 20 will be the DWELL band.

make_flare_fsq(20,[20])

would make a DWELL file with no frequency switching, remaining on band 20 only for the entire 1 s duration.