Preliminary Observer's Guide for the GBT Ultrawideband Receiver

Introduction

The GBT ultrawideband receiver (UWBR) operates from 0.7 - 4 GHz and is optimized for high-precision pulsar timing, spectro-temporal studies of fast radio bursts and other fast transients, and the study of regions rich in molecular lines (e.g. radio recombination lines and astrochemistry). The receiver has a bandwidth ratio of close to 6:1, which is uniquely high among GBT receivers. UWBR makes use of the VEGAS backend, which has been expanded to process the full bandwidth with coherent dedispersion for certain pulsar modes. Pulsar modes using incoherent dedispersion, as well as spectral line modes, operate in a nearly identical way as for other GBT receivers.

This guide provides information on the expanded VEGAS modes, example configurations and observing scripts and information on the tools used to monitor data taking for the expanded VEGAS. Not all observing modes will need to make use of these enhanced tools, in which case standard GBT/VEGAS observing tools can still be used.

Data Mappings for Expanded VEGAS Modes

Prior to the enhancements for UWBR, VEGAS could process up to 10 GHz of usable instantaneous bandwidth in spectral line and incoherent dedispersion pulsar modes (but note that most GBT receivers do not provide this much bandwidth), but only 1.25 GHz of instantaneous bandwidth in coherent dedispersion pulsar modes. This is because VEGAS was equipped with eight digital samplers and eight computing nodes. In spectral line and incoherent dedispersion pulsar modes, the data from each sampler was processed by one corresponding compute node, so all eight samplers could be used. However, in coherent dedispersion pulsar mode all eight compute nodes have to bused to process the output of one sampler.

The expanded VEGAS system still has eight samplers, but now has 24 compute nodes, making it possible to use up to three samplers in coherent dedispersion pulsar mode (the data from each sampler is still processed by eight compute nodes). Data mappings from samplers to compute nodes are shown in the figure below. The ROACH2's are the samplers and the Banks are the compute nodes.

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Pulsar observers will generally want to use three samplers/24 compute nodes to process the full receiver bandwidth.

Example Obsering Scripts

General Considerations

In most ways UWBR is configured like other single-pixel GBT receivers, but there are some Config Tool keywords that take on restricted or slightly different values.

• receiver: Rcvr_2500 is the the name for UWBR used by the GBT Config Tool.
• pol: This must be Linear. UWBR cannot produce native circular polarization products. Circular polarization can still be derived from the cross product of the linear polarization channels.
• noisecal: hi-cal is used for the high-power noise diode state, and lo-cal is used for the low-power noise diode state. For other GBT receivers the values are typically just hi and lo. off is still used to turn the noise diodes off.

In addition observers should always perform an AutoPeakFocus to correct the telescope pointing and focus at the start of their observing session.

Coherent Dedispersion Pulsar Modes

UWBR can be used with all existing VEGAS coherent dedispersion pulsar modes (see the GBT Observer's Guide or VEGAS Pulsar Mode Wiki for details). The use of additional samplers/compute nodes is controlled simply be specifying additional rest frequencies in the configuration portion of a scheduling block. Most observers should use the rest frequencies specified below to optimally cover the full receiver bandwidth, but in principle other rest frequencies could be used. Note that the rest frequencies specified below provide overlap between different samplers to ensure that there are no gaps in the spectrum, so the effective bandwidth per sampler will be lower than the sampled bandwidth.

config_uwbr_fold = """
receiver = 'Rcvr_2500' # UWBR receiver name is 'Rcvr_2500'
pol = 'Linear' # UWBR can only be used with native linear polarization
restfreq = 1225.0,2350.0,3475.0 # These rest frequencies cover the full receiver bandwidth
nwin = 3 # Must match number of rest frequencies
if0freq = 6875.0 # This is needed to set the proper LO/IF frequencies
deltafreq = 0.0
noisecal = 'off' 
swmode = 'tp_nocal'
swtype = 'none'
swper = 0.04
swfreq = 0.0
vframe = 'topo'
vdef = 'Radio'
vlow = 0.0
vhigh = 0.0
obstype = 'Pulsar'
backend = 'VEGAS'
bandwidth = 1500 # Bandwidth per sampler
tint = 16*1024/1500*1e-6 # For coherent dedispersion the recommended tint is 16*vegas.numchan/bandwidth
vegas.outbits = 8
vegas.numchan = 1024 # Can be any supported VEGAS coherent dedispersion mode using 1500 MHz sampled bandwidth
vegas.scale = 1000
vegas.obsmode = 'coherent_fold'
vegas.polnmode = 'full_stokes'
vegas.fold_parfile = '/home/gpu/tzpar/B1937+21.par' # Must be changed to match the pulsar being observed!!!
vegas.fold_dumptime = 10.0
vegas.fold_bins = 2048
"""

Note that the above script is appropriate for coherent dedispersion fold-mode. Here is a complete script that uses other online processing modes.

all_psrs = Catalog(pulsars_all_GBT)
source = "B1937+21"
cal_scan_length = 95.0 # seconds
pulsar_scan_length = 305.0 # seconds

config_uwbr = """
receiver = 'Rcvr_2500' # UWBR receiver name is 'Rcvr_2500'
pol = 'Linear' # UWBR can only be used with native linear polarization
restfreq = 1225.0,2350.0,3475.0 # These rest frequencies cover the full receiver bandwidth
nwin = 3 # Must match number of rest frequencies
if0freq = 6875.0 # This is needed to set the proper LO/IF frequencies
deltafreq = 0.0
swtype = 'none'
swper = 0.04
swfreq = 0.0
vframe = 'topo'
vdef = 'Radio'
vlow = 0.0
vhigh = 0.0
obstype = 'Pulsar'
backend = 'VEGAS'
bandwidth = 1500 # Bandwidth per sampler
tint = 16*1024/1500*1e-6 # For coherent dedispersion the recommended tint is 16*vegas.numchan/bandwidth
vegas.outbits = 8
vegas.numchan = 1024 # Can be any supported VEGAS coherent dedispersion mode using 1500 MHz sampled bandwidth
vegas.scale = 1000
vegas.polnmode = 'full_stokes'
"""

config_cal = """
noisecal = 'hi-cal'
swmode = 'tp'
vegas.obsmode = 'coherent_cal'
vegas.fold_dumptime = 10.0
vegas.fold_bins = 2048
"""

config_fold = """
noisecal = 'off'
swmode = 'tp_nocal'
vegas.obsmode = 'coherent_fold'
vegas.fold_parfile = '/home/gpu/tzpar/{source}.par' # Use string substitution to ensure we are using the right parfile for this source.  parfile still has to exist!!
vegas.fold_dumptime = 10.0
vegas.fold_bins = 2048
""".format(source=source)

config_search = """
noisecal = 'off'
swmode = 'tp_nocal'
vegas.obsmode = 'coherent_search'
vegas.dm = 71.01515 # Must be appropriate for this source!
"""

AutoPeakFocus(location=source) # Correct telescope pointing and focus using a source automatically chosen near our pulsar

Slew(source) # Move telescope to source position

Configure(config_uwbr + config_cal) # Configure for a calibration scan
Balance() # Balance all electronics
Track(source, None, cal_scan_length) # Take a calibration scan

Configure(config_uwbr + config_fold) # Configure for a fold-mode scan
Track(source, None, pulsar_scan_length) # Take a fold-mode scan

Configure(config_uwbr + config_search) # Configure for a search-mode scan
Track(source, None, pulsar_scan_length)

Note that this script takes both a fold-mode and search-mode scan. In practice observers will typically choose one or the other.

Incoherent Dedispersion Pulsar Modes

UWBR can be used with all existing VEGAS incoherent dedispersion pulsar modes (see the GBT Observer's Guide or VEGAS Pulsar Mode Wiki for details). The setup for incoherent dedispersion modes is very similar to that for coherent dedispersion modes. The only significant differences are

• vegas.obsmode takes on values of search, fold, or cal instead of coherent_search, coherent_fold, or coherent_cal.
• There are different recommended values of tint. See the VEGAS Pulsar Mode Wiki for details.
• There are different recommended values of vegas.scale. Contact your project friend for more details.

The values of these keywords in the observing scripts in the previous section can be modified for incoherent dedispersion. Other aspects of the observing scripts remain the same.

In most cases, coherent dedispersion pulsar modes will be more appropriate than incoherent dedispersion modes. Observers should only use incoherent dedispersion modes if their science case requires it (usually this will only be the case if higher spectral resolution is required than can be achieved with coherent dedispersion modes).

Spectral Line Modes

UWBR can be used with all existing VEGAS spectral line modes (see the GBT Observer's Guide for details). Other than the special keywords listed under the General Considerations section above, there are no UWBR-specific changes to spectral line setups. Nonetheless, we provide an example observing script to observe CH emission in the 3-4 GHz range.

sources = Catalog("""format = spherical
coordmode = J2000
head = name   ra   dec   velocity
W49N   19:10:13.20   +09:06:11.88   11.8""")

source = "W49N"
scan_length = 600.0 # seconds

config_spec = """
receiver = 'Rcvr_2500' # UWBR receiver name is 'Rcvr_2500'
pol = 'Linear' # UWBR can only be used with native linear polarization
restfreq = 3263.793447, 3335.479356, 3349.192556
nwin = 3 # Must match number of rest frequencies
if0freq = 4836.206553 # This is needed to set the proper LO/IF frequencies
noisecal = 'hi-cal' # Use high-power noise diode state
swmode = 'tp'
swtype = 'none'
swper = 2.0
swfreq = 0.0
vframe = 'lsrk'
vdef = 'Radio'
vlow = 0.0
vhigh = 0.0
obstype = 'Spectroscopy'
backend = 'VEGAS'
bandwidth = 23.44
tint = 2.0
nchan = 32768
vegas.subband = 1
vegas.vpol = 'self'
"""

AutoPeakFocus(location=source)
Slew(source)

Configure(config_spec)
Balance()
OnOff(source, Offset("AzEl",1.0,0.0,cosv=False), scan_length)

Observation Monitoring Tools

VEGAS

Spectral line and incoherent dedispersoin pulsar mode observers should continue to use the same observation monitoring tools.

In coherent dedispersion pulsar tools observers may wish to make use of a new CLEO screen that is specific to the expanded 24-compute node modes of VEGAS. To launch this applicatoin run [exact command TBD after software updates].

A screen shot is below. This shows the status of all 24 compute nodes, as well as some additional configuration information.

By clicking on one of the tabs at the top of the screen a user can see more information about a specific set of compute nodes.

Observers will also notice that the vpmHPCStatus tool and VEGAS auto-plotting webpage (www.gb.nrao.edu/vpm) will update to display information about the additional compute nodes when they are in use. The vpmHPCStatus tool is shown below.

The VPM auto-plotting webpage is shown below. Note that data from additional banks is off the page. Scroll down to see data from additional banks. As a reminder, we recommend using Firefox when viewing this webpage. In this example we see the square-wave pulse of the receiver's calibration noise diode.

The Receiver

A new CLEO screen exists for monitoring the state of UWBR itself. To launch this application run [exact command TBD after software updates]. A screenshot is shown below. Observers will usually not need to use this tool; it is intended primarily for engineers and other GBO staff. We show it here for completeness.

Accessing Data

As with other receivers, VEGAS data is written to /stor/gbtdata (/lustre/gbtdata is an alias for this location) in the appropriate sub-directories for a project and session. The only difference is that in coherent dedispersion mode there will be additional sub-directories for each compute node. Pulsar data will be "merged" as usual to combine data from different compute nodes. Observers should use the same data reduction procedures as they would for other receivers.

-- RyanLynch - 15 Feb 2023