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 Creator

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. A different set of sampling frequencies can be specified using the bandlist parameter. If fewer than 10 frequencies are given in the bandlist parameter, the DWELL duration on the dwellband will be increased or decreased accordingly to make the total duration 1 s. It is possible to use only one frequency, e.g.,

make_flare_fsq(20,[20])

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

It is important to note that the name of the DWELL frequency sequence file is automatically generated with the string DWELL followed by the dwell band, a + symbol, and the number of additional frequencies in the sequence. Thus, the make_flare_seq(20) command would generate a file named DWELL20+10.fsq, written to the current directory. The FSEQ file must begin with the string DWELL or it will not trigger additional requirements discussed below. If the acc parameter is True (default), the file will also be automatically transferred to the ACC for use by the system. It is a good idea to set check=True, which checks the total number of "science channels" to be recorded does not exceed 512 (more on this below). Note that no distinction is made in the filename between one set of additional (non-dwell) frequencies and another, so if you create a sequence with DWELL band 20 and 10 other frequencies that is different from a previous one, the new file will overwrite the old one.

It is also possible to hand modify a file to dwell on 2 bands. In the above listed file, for example, could be modified to change 0.8 for band 20 to 0.4 and the 0.01975 to 0.4 for band 2, and the system will dwell for 0.4 s on both bands 2 and 20. (This should be verified.)