# Backend operations¶

The backend to be used can be manually selected as follows:

> chooseBackend=BACKENDS/[bckname]


where bckname is the name of the backend. The currently available choices are:

• TotalPower
• Xarcos (for the XARCOS spectrometer)
• Sardara (for SARDARA, the ROACH-based spectrometer)

## Total Power - focus selector¶

Warning

The TotalPower backend works as a “focus selector”, sending the signal from the wanted receiver to any other backend. For this reason, it must be set up even when acquisitions take place using another backend. This is accomplished simply using the overall setup commands (such as setupCCB, etc…).

Bandwidth and sampling rate can be changed using:

> setSection=[sect],[startFreq],[bw],[feed],[mode],[sampleRate],[bins]


where:

• [sect] is an integer specifying the section number
• [startFreq] is the initial frequency for the section (not applicable for TPB)
• [bw] is a double for the bandwidth (MHz)
• [feed] is the number of the feed connected to the section (not applicable for TPB)
• [mode] is the polarisation mode (not applicable for TPB)
• [sampleRate] is also given in MHz
• [bins] is the number of frequency bins for the given section (not applicable for TPB)

To leave a parameter at its previously set value, or equivalently skip it when it is not applicable, use an asterisk. For the TPB, in particular, always use:

> setSection=[sect],*,[bw],*,*,[sampleRate],*


where bw can be chosen from a restricted range of options (MHz):

• 300.0
• 730.0
• 1250.0
• 2000.0

These values do not correspond to the true observed bandwidth, for the reason discussed in Initial Setup. When accumulations of the data “dumps” are required before integrating them to the output file, it is possible to set the integration time as follows:

> integration=[N]


where N is given in milliseconds.

Each section is provided with an attenuator, to manage the signal intensity. To set the desired attenuation:

> setAttenuation=[sect],[att]


where sect is the section number and att can vary, for the TPB, form 0 dB to 15 dB, with a 1 dB step.

If a backend re-initialization is needed, use:

> initialize=[code]


(code can be CCB, KKG, etc)

This command resets the backend only, setting it to the default values foreseen for the specified focus/receiver. After the frontend/backend configuration is changed, it is necessary to update the value for the beamsize (HPBW), which is computed as a function of the actually observed band and is used – during the observations – to evaluate the pointing accuracy. This is accomplished using the command:

> device=[sect]


which uploads to the system the parameters relative to section number [sect] (you can generally use 0, which exists for all the receivers, unless you need to observe only with a different feed).

## XARCOS¶

This spectrometer is fully integrated in DISCOS, which means that DISCOS can command the device, receive data from it and then write these data using a standard output (FITS format).

In order to configure the spectrometer, select it with the chooseBackend command and use one of the following commands in the operatorInput:

> initialize=XK77

This is to use the full K-band MF; each digital sample has a 6-bit representation. Full-Stokes sections are recorded, each having a 62.5 MHz bandwidth and 2048(x4) channels

> initialize=XK03

It exploits the K-band feeds 0 and 3. Each feed produces two full-Stokes sections respectively having bandwidths of 62.5 MHz and 4 MHz and 2048(x4) channels. Each digital sample has an 8-bit representation.

> initialize=XK06

It enables the K-band feeds 0 and 6. Each feed produces two full-Stokes sections respectively having bandwidths of 62.5 MHz and 4 MHz and 2048(x4) channels. Each digital sample has an 8-bit representation.

> initialize=XK00

This configuration is for the usage of the K-band central feed. It produces four full-Stokes sections respectively with bandwidths of 62.5 MHz, 8 MHz, 2 MHz and 0.5 MHz, each having 2048(x4) channels. Each digital sample has an 8-bit representation.

> initialize=XC00

This configuration is C-band usage. It produces four full-Stokes sections respectively with bandwidths of 62.5 MHz, 8 MHz, 2 MHz and 0.5 MHz, each having 2048(x4) channels. Each digital sample has an 8-bit representation.

Ideally, configuration details can be changed using the setSection command:

> setSection=[sect],[startFreq],[bw],[feed],[mode],[sampleRate],[bins]


where:

• [sect] is an integer specifying the section number
• [startFreq] is the initial frequency for the section
• [bw] is a double for the bandwidth
• [feed] is the number of the feed connected to the section
• [mode] is the polarisation mode
• [sampleRate] is also given in MHz
• [bins] is the number of frequency bins for the given section

However, the present implementation allows the user to change only part of these parameters, in particular:

• [startFreq] – initial frequency, it must be in the 125-250 MHz range. This value, added to the LO frequency, gives the lowest sky frequency observed by the section. Different sections can have different start frequencies (contrarily to what happens for the TPB);
• [bw] – bandwidth must be chosen from the following values (all are expressed in MHz): 125.0, 62.5, 31.25, 15.625, 7.8125, 3.90625, 1.953125, 0.9765625, 0.48828125;
• [sampleRate] – its value (MHz) must be twice the bandwidth.

Users must not change the feed, mode and bins parameters, which are fixed as described in the configuration defaults.

A valid example of setup and setSection usage, then, is:

> chooseBackend=BACKENDS/Xarcos
> initialize=XC00
> setSection=0,155.0,31.25,*,*,62.5,*


where asterisks indicate which parameters are to be set according to default values.

Warning

At present, integration time is equal to 10 seconds. Shorter integrations might be available in the future. Data transfer requires about 2 seconds for each integration; take this overhead into consideration when estimating how long your schedules are going to last.

## SARDARA¶

This is a ROACH-based spectrometer, now fully integrated in DISCOS. In order to configure it, select it with the chooseBackend command as described above and use one of the following commands in the operatorInput:

> initialize=SP00

This is to use P-band receiver in L/R mode

> initialize=SP00S

This is to use P-band receiver in full Stokes mode

> initialize=SL00

This is to use L-band receiver in L/R mode

> initialize=SL00S

This is to use L-band receiver in full Stokes mode

> initialize=SC00

This is to use C-band receiver in L/R

> initialize=SC00S

This is to use C-band receiver in full Stokes mode

> initialize=SK00

This is to use K-band receiver (central feed only) in L/R

> initialize=SK00S

This is to use K-band receiver (central feed only) in full Stokes mode

> initialize=SK03

This is to use K-band receiver (feeds number 0 and 3) in L/R

> initialize=SK03S

This is to use K-band receiver (feeds number 0 and 3) in full Stokes mode

> initialize=SK06

This is to use K-band receiver (feeds number 0 and 6) in L/R

> initialize=SK06S

This is to use K-band receiver (feeds number 0 and 6) in full Stokes mode

> initialize=SK77

This is to use K-band receiver (all the 7 feeds) in L/R

> initialize=SK77S

This is to use K-band receiver (all the 7 feeds) in full Stokes mode

By default, the spectral bin number is set to 1024 and the filter bandwidth to 1500 MHz (except for the RP00 configuration, which uses a 500 MHz bandwidth). The actually observed bandwidth and frequency range are reported in the FITS files (see the dedicated documentation).

Users can change the frequency bin number to 16384, using:

> setSection=[sect],*,*,*,*,*,16384


Where sect is the section number. Users can also change the integration time:

> integration=[N]


where N is given in milliseconds. Attenuations can be handled with the command:

> setAttenuation=[sect],[att]


where att can vary form 0 dB to 15 dB, with a 1 dB step. This attenuation is actually applied at the focus selector level.