./guide
getbs.pro

Last modification date:

Wed Sep 28 13:27:08 2016

getbs

procedure getbs, scan, [ifnum=integer], [intnum=integer], [plnum=integer], [trackfdnum=integer], [sampler=string], [bswitch=integer], [tau=float], [tsys=float], [ap_eff=float], [smthoff=integer], [units=string], [/eqweight], [tcal=float], [/quiet], [/keepints], [useflag=boolean or string], [skipflag=boolean or string], [instance=integer], [file=string], [timestamp=string], [status=variable]

Procedure getbs retrieves and calibrates a beam switched Nod scan pair.

Beam switched modes are not recommended on the GBT. Total power Nod should be used instead. To process total power nod data use getnod instead of this routine. This procedure is provided only to accommodate old data.

The getbs routine produces a spectrum calibrated in Ta (K). Other recognized unit are Ta* and Jy.

Summary

• Data are selected using scan, ifnum, intnum and plnum or, alternatively, sampler and intnum if you know the specific sampler name (e.g. "A10"). The other scan in the scan pair is found using scan (see comments below).
• In beam switched data, there are two beams in each scan, some of that data comes from the signal phase, some of it from the reference phase and in each of those there is cal on and cal off data. So, there are 4 switching states in this data.
• Individual integrations are processed separately with the same integration number in the two scans processed together. Both scans must have the same number of integrations. Each integration is processed using donod
• The integrations are calibrated in Ta (K). If units of Ta* or Jy are requested via the units keyword, then dcsetunits is used to convert to the desired units.
• Within each integration, there are spectra identified as coming from beam 1 and other spectra identified as combing from beam 2. Some of those in each scan come from the signal switching phase and some come from the reference switching phase. The signal data from each integration is processed together using donod and the reference data from each integration is processed together also using donod. The reference data is multiplied by -1 before it is averaged with the signal data for that integration. The bswitch keyword can be used to limit which data is processed.
• Averaging of individual integrations is then done using dcaccum By default, integrations are weighted as described in dcaccum. If the eqweight keyword is set, then integrations are averaged with an equal weight.
• The final average is left in the primary data container (buffer 0), and a summary line is printed. The printing of the summary line can be suppressed by setting the quiet keyword. The first Tys displayed is that of the result. The other 4 Tsys values displayed are weighted averages of the Tsys values from each of the 4 spectra that make up each integration (see donod for more details). In the case of bswitch=0, the signal and reference tsys values are averaged together. This can be useful in assessing the quality of the parts of the data that make up the final result.
• The individual integration results can be saved to the currently opened output file by setting the keepints keyword. The final average is still produced in that case. In the case of bswitch=0, all 8 spectra comprising each integration are kept.

Parameters

The scan number is required. Either scan in the "Nod" scan pair can be given. Arguments to identify the IF number, polarization number, and feed number are optional. The default tracking feed number, trackfdnum, (0) is the lowest numbered FEED found in the data. This feed number is interpreted as the tracking (source) feed for the first scan in the signal phase. Specify trackfdnum=1 to identify the second FEED as the tracking feed. Tracking feed in this context means that the source signal was in that beam during the source phase of the first scan of the two "Nod" scans.

If ifnum, trackfdnum, or plnum are not supplied then the lowest values for each of those where data exists (all combinations may not have data) will be used, after using any user-supplied values. The value of ifnum is determined first, followed by trackfdnum and finally plnum. If a combination with data can not be found then showiftab is used to show the user what the set of valid combinations are. The summary line includes the ifnum, trackfdnum, and plnum used.

Tsys and Available Units

The procedure calculates Tsys based on the Tcal values and the data in the non-source feeds (both scans). The user can override this calculation by entering a zenith system temperature. The procedure will then correct the user-supplied Tsys for the observed elevation. If the data are calibrated to Ta* or Jy, additional parameters are used. A zenith opacity (tau) may be specified, and an aperture efficiency may be specified. The user is strongly encouraged to enter values for these calibration parameters, but they will be estimated if none are provided. The user can also supply a mean Tcal using the tcal keyword. That will override the tcal found in the data.

Smoothing the Reference Spectra

A parameter called smthoff can be used to smooth the reference spectra prior to calibration. In certain cases this can improve the signal to noise ratio, but it may degrade baseline shapes and artificially emphasize spectrometer glitches. Use with care. A value of smthoff=16 is often a good choice.

The Special "bswitch" Keyword

A parameter called bswitch allows the user to select between using all of the data (the detault, bswitch=0) or using only data when the beam switch is in the "cross" (reference) or "thru" (signal) position. Data taken in beam switched mode should be examined carefully in both cross and thru positions.

Weighting of Integrations in Scan Average

By default, internal averaging of integrations is weighted using tsys, exposure and frequency_resolution as described in the dcaccum documentation. To give all integrations equal weight instead of the default weighting based on Tsys, use the /eqweight keyword.

Summary Information

The scan number printed in the status line is that of the first scan in the "Nod" pair - no matter which scan number you actually provided as the scan parameter here. The values of ifnum, trackfdnum, and plnum are shown after the scan number. The first Tsys printed is the tsys value in the result. This is a weighted average of the Tsys values in the reference beams from both scans as described in donod The other 4 Tsys values printed on the status line are the 4 Tsys values calculated by donod and averaged (using the appropriate weighting scheme) over all integrations and beam switch positions.

Using or Ignoring Flags

Flags (set via flag) can be selectively applied or ignored using the useflag and skipflag keywords. Only one of those two keywords can be used at a time (it is an error to use both at the same time). Both can be either a boolean (/useflag or /skipflag) or an array of strings. The default is /useflag, meaning that all flag rules that have been previously set are applied when the data is fetched from disk, blanking any data as described by each rule. If /skipflag is set, then all of the flag rules associated with this data are ignored and no data will be blanked when fetched from disk (it may still contain blanked values if the actual values in the disk file have already been blanked by some other process). If useflag is a string or array of strings, then only those flag rules having the same idstring value are used to blank the data. If skipflag is a string or array of strings, then all flag rules except those with the same idstring value are used to blank the data.

Dealing With Duplicate Scan Numbers

There are 3 ways to attempt to resolve ambiguities when the same scan number appears in the data source. The instance keyword refers to the element of the returned array of scan_info structures that scan_info returns. So, if scan 23 appears 3 times then instance=1 refers to the second time that scan 23 appears as returned by scan_info. The file keyword is useful if a scan is unique to a specific file and multiple files have been accessed using dirin. If file is specified and instance is also specified, then instance refers to the instance of that scan just within that file (which may be different from its instance within all opened files when dirin is used). The timestamp keyword is another way to resolve ambiguous scan numbers. The timestamp here is a string used essentially as a label by the monitor and control system and is unique to each scan. The format of the timestamp string is "YYYY_MM_DD_HH:MM:SS". When timstamp is given, scan and instance are ignored. If more than one match is found, an error is printed and this procedure will not continue.

Once a unique match is found to the desired scan (using instance, file, or timestamp) then the scan paired with that scan necessary to finish this procedure is found. The match must be found within the same file as the desired scan. It must have the appropriate matching scan number (scan-1 if scan is the second scan in the procedure or scan+1 if scan is the first scan in the procedure). If those two rules are not sufficient to find a unique match, the matching scan with the closest timestamp in the appropriate direction (before or after depending on which procseqn is associate with scan) is used. Finally, the matched pair must have the appropriate procseqn given the procseqn that scan is.

Note: if you see the message "No data found, can not continue" the most likely explanation is that the IF numbers are confused, probably due to a bad configuration (e.g. both feeds do not have data from the same IF and polarization). Consequently, this calibration routine can not reduce that data. It is likely that all of the IFNUM values for this data are -1.

Examples

    ; average both polarizations from ifnum=1
    sclear
    getbs, 76, ifnum=1, plnum=0
    accum
    getbs, 76, ifnum=1, plnum=1
    accum
    ave
 

Uses

accumave accumclear calsummary check_calib_args data_free dcaccum dcscale dcsetunits donod find_paired_info find_scan_info get_calib_data set_data_container showiftab

Version

$Id$

Parameters
scan in, required integer	M&C scan number

Keywords
ifnum in, optional integer	IF number (starting with 0). Defaults to the lowest value associated with data taking into account any user-supplied values for trackfdnum, and plnum.
intnum in, optional integer	Integration number, defaults to all integrations.
plnum in, optional integer	Polarization number (starting with 0). Defaults to the lowest value with data after determining the values of ifnum and trackfdnum if not supplied by the user.
trackfdnum in, optional integer	Tracking feed number. Defaults to the lowest value with data after determining the value of ifnum if not supplied by the user and using any value of plnum supplied by the user.
sampler in, optional string	sampler name, this is an alternative way to specify ifnum and plnum. When sampler name is given, ifnum and plnum must not be given. Note that data from the associated switched sampler will also be used.
bswitch in, optional integer	determines which beamswitch positions are used. 0=both, 1=ref, 2=sig (default 0)
tau in, optional float	tau at zenith, if not supplied, it is estimated using get_tau tau is only used when the requested units are other than the default of Ta and when a user-supplied tsys value at zenith is to be used.
tsys in, optional float	Tsys at zenith, this is converted to a Tsys at the observed elevation. If not suppled, the Tsys for each integration is calculated as described elsewhere.
ap_eff in, optional float	Aperture efficiency, if not suppled, it is estimated using get_ap_eff ap_eff is only used when the requested units are Jy.
smthoff in, optional integer	smooth factor for reference spectrum, defaults to 1 (no smoothing).
units in, optional string	takes the value 'Jy', 'Ta', or 'Ta*', default 'Ta'.
eqweight in, optional boolean	When set, all integrations are averaged with equal weight (1.0). Default is unset.
tcal in, optional float	Cal temperature (K) to use in the Tsys calculation. If not supplied, the mean_tcal value from the header of the cal_off switching phase data in each integration is used. This must be a scalar, vector tcal is not yet supported. The resulting data container will have it's mean_tcal header value set to this keyword when it is set by the user.
quiet in, optional boolean	When set, the normal status message on successful completion is not printed. This will not have any effect on error messages. Default is unset.
keepints in, optional boolean	When set, the individual integrations are saved to the current output file (fileout). This is ignored if a specific integration is requested using the intnum keyword. Default is unset.
useflag in, optional boolean or string	Apply all or just some of the flag rules? Default is set.
skipflag in, optional boolean or string	Do not apply any or do not apply a few of the flag rules? Default is unset.
instance in, optional integer	Which occurrence of this scan should be used. Default is 0.
file in, optional string	When specified, limit the search for this scan (and instance) to this specific file. Default is all files currently opened.
timestamp in, optional string	The M&C timestamp associated with the desired scan. When supplied, scan and instance are ignored.
status out, optional variable	An output parameter to indicate whether the procedure finished as expected. A value of 1 means there were no problems, a value of -1 means there were problems with the arguments before any data was processed, and a value of 0 means that some of the individual integrations were processed (and possible saved to the output file if keepints was set) but there was a problem with the final average and buffer 0 likely contains just the result from the last integration processed. This keyword is primarily of use when using getbs within another procedure or function.