Backends Available with CIMA

Backends available with CIMA

Different backends are supported by CIMA depending on the selected receiver and observation type (spectral line or pulsar). CIMA has traditionally supported two backends: the interim correlator for spectral line single-pixel receiver observations and the WAPPs for any kind of observation. The WAPPs behave quite differently in spectral line and in pulsar mode, with the differences being so big that the observer has to select which type of obsercvation already at start-up of CIMA. In spectral line mode, the data from all WAPPs are collected into a single CIMAFITS file with a dump rate not faster than 1 second, while in pulsar mode, each WAPP produces an individual data file written in the raw WAPP pulsar format which allows for high speed sampling. Also, other backends like the ASP and machines supported by the 30 MHz input can be run in parallel with the WAPPs. The GALFA spectrometer has not been supported from CIMA and to take GALFA data with CIMA, it has thus been necessary to take data with the WAPPs in parallel.

The newer versions of CIMA have expanded capabilities. The single-pixel receiver WAPP dual board observing mode was added in version 2.3 as well as the possibility to run observations with GALFA without the WAPPs, while version 3.0 adds the new Mock spectrometer for ALFA observations.

The interim correlator

The interim correlator can only be used for spectral line single-pixel receiver observations and it can not be used with any of the observing modes developed for ALFA. Most of the observing modes are using implementations in the 'datataking' program and once CIMA has set everything up for an observation, it hands over control to the 'datataking' program (called 'VX-works' in CIMA) and waits until it has completed the observation. The interim correlator can not be used in parallel with any other backend. It is the only backend used in the 'Calibration' window in CIMA.

The interim correlator is the only backend that can be run without CIMA, since most observing modes are implemented and executed by the 'datataking' program which also handles all telescope control and IF/LO set-ups.

The WAPPs

The WAPPs (Wideband Arecibo Pulsar Processors) are four boxes which each run two correlator boards capable of handling 100 MHz each. They can thus be used for up to 800 MHz of bandwidth coverage with single pixel receivers (in dual board mode) or 100 MHz per beam for ALFA. The possibility to use the dual board mode for single pixel receivers was added in CIMA version 2.3 - with older versions of CIMA, the WAPPs can only handle a maximum of 400 MHz. Originally built for pulsar observations, the WAPPs were later adapted also for spectral line observations. The history has left its traces though, since the WAPPs behave very differenly in pulsar mode and in spectral line mode, and this big difference is the reason why all CIMA observers have to specify whether they are going to run spectral line or pulsar observations already when launching CIMA. Pulsar mode and spectral line mode both have their own sets of observing modes in CIMA.

In pulsar mode data can be taken with high dump rates (writing up to 25 MB/sec for each WAPP). The file format is simple with a data header written just before the observation starts followed by data without any meta data. Dynamic information like telescope positions are thus not available in the data (except once in the header). The files have a maximum size of 2 GB and all files needed for a scan are preallocated before the scan is started. Each WAPP writes the data to their own disks one file at a time - dual board observations (including ALFA) thus have the two data streams interleaved. Each scan produce its own set of files.

In spectral line mode, data is dumped no faster than once a second (and not slower than once per 15 seconds). The data streams from the WAPPs are sent to WAPPDATA which merges them and writes the data to a single CIMAFITS-file in the /share/wappdata area. Data from many scans can be added to the same file. The file is changed either by a request from the observer or when the header table gets full.

Starting with CIMA version 3.1, the WAPPs can also be run in continuous data taking mode for spectral line observations. In this mode, the WAPPs are kept running and recording data also between the scans. It is possible to write an entire observation as one continuous data stream to one file or to create one file per observing loop.

The WAPPs can be run in parallel with the Mock spectrometer for ALFA observations and with ASP and 30 MHz input machines for single-pixel pulsar observations.

The Mock spectrometer

The Mock spectrometer is a versatile machine consisting of no less than 14 boxes which each run two correlator boards capable of handling 172 MHz each. The designed data rate for writing is 50 MB/sec per box. The Mock spectrometer can so far only be used for ALFA observations, and the 14 boxes are then divided up in two groups which apart from a few common parameters can be configured independently. The two groups can thus work almost as two independent spectrometers for ALFA. The Mock spectrometer is the third backend that is completely controlled from CIMA and it was added to CIMA with version 3.0.

The Mock spectrometer can be used for all observing modes supported by the WAPPs for ALFA and it can be run in parallel with the WAPPs. Data taking does not start at exactly the same time when run in parallel with the WAPPs, since the Mock spectrometer needs more time to start.

The data format is the new CIMAFITS-2 format which works both for slow spectral line data and high-speed pulsar data. One or more files is generated for each correlator board and each scan. The maximum file size is selectable. There is an alternative FITS-format for pulsar observations called PSRFITS, which soon will be available. Data can also be output in PDEV-format which was an early engineering format that is not recommended.

Starting with CIMA version 3.1, the Mock spectrometer can also be run in continuous data taking mode for spectral line observations. In this mode, the Mock spectrometer is kept running and recording data also between the scans. It is possible to write an entire observation as one continuous data stream to one set of files or to create one set of files per observing loop.

The GALFA spectrometer

The GALFA spectrometer can not be controlled from CIMA and does not interact with CIMA at all. It is started up independently and is then running in a continuous data taking mode. In CIMA version 2.2 and earlier, it was necessary to also run the WAPPs since they are telling CIMA when each individual scan is finished and when it is time to move on. Starting with CIMA version 2.3, there is no longer a need to run the WAPPs, since there is now a GALFA only option in CIMA. There is still no communication between CIMA and GALFA, but CIMA is now capable of self keeping track of time.

Other backends

When the WAPPs are used for single-pixel receiver observations in pulsar mode, CIMA has support for the ASP and machines supported by the 30 MHz input. However, they can't be run alone, unless You just set things up with CIMA and manually wait while they are taking data. There is no observing mode that can be run without the WAPPs or the Mock spectrometer.

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This page is administered by Prakash Atreya ( patreya (a) naic . edu ) and was last updated on 11 December 2008.