6.2 Retrieving Individual Records

For most users, the data retrieval and calibration procedures discussed in the previous section will be sufficient. Others may need to access data in its raw, uncalibrated form. There are two commands for accessing uncalibrated data, get and getrec. The getrec command is also useful for retrieving calibrated data from a keep file (a file which contains data already calibrated in GBTIDL and stored using the keep command).

get: The get procedure can be used to retrieve individual data records from the input data file based on the scan number, feed, IF, integration, polarization, cal state, and sig/ref state of the data. If these parameters are not sufficient to uniquely identify a single row in the SDFITS file, only the first matching row is returned and a warning message is printed. The get procedure might be used as follows to calculate a system temperature from an uncalibrated data file:

get, scan=10, pol='LL', ifnum=1, fdnum=1, int=1, sig='T', cal='T'  
calon = getdata()
get, scan=10, pol='LL', ifnum=1, fdnum=1, int=1, sig='T', cal='F'  
caloff = getdata()                                          
tcal = !g.s[0].mean_tcal                                   
tsys = caloff/(calon-caloff)*tcal                           
print,'Mean Tsys = ',mean(tsys)

A complete list of parameters for the get procedure is given below:

Parameter	Description
index	index number as shown in bold
project	project ID
file	SDFITS file names (only relevant if the input data set is specified with dirin rather than filein)
timestamp	scan timestamp as YYYY_MM_DD_HH:MM_SS
extension	SDFITS extension number
row	SDFITS row number
source	source name
procedure	procedure name
procseqn	procedure sequence number
scan	M&C scan number
polarization	polarization, e.g. 'LL', 'RR', 'XX' or 'YY'
plnum	polarization index, zero-based
ifnum	IF (i.e. spectral window) index number, zero-based
feed	feed name (e.g. B1)
fdnum	feed index number, zero-based
int	Integration number
numchn	number of channels in the spectrum
sig	'T' or 'F' to identify SIG state
cal	'T' or 'F' to identify cal-on or cal-off
sampler	backend sampler name
azimuth	antenna azimuth
elevation	antenna elevation
longitude	longitude-like axis, e.g. RA
latitude	latitude-like axis, e.g. DEC
lst	LST
centfreq	center frequency in Hz
restfreq	rest frequency in Hz
velocity	source velocity in km/s
freqres	frequency resolution in Hz
freqint	frequency interval (channel spacing) in Hz
dateobs	date-time value
bandwidth	bandwidth in Hz
exposure	exposure time
tsys	system temperature
nsave	nsave index
trgtlat	latitude coordinate of source
trgtlon	longitude coordinate of source
obsid	observation ID
subref	subreflector state (subref_state); 0=moving, 1=first position, -1=second position

getrec: To retrieve an individual record, use the getrec procedure. This procedure takes one parameter, the record number. The record number is equivalent to the row number in the SDFITS file. Like all indices in IDL, the record number is a zero-based index. So, for example, the fifth record can be retrieved and displayed as follows:

getrec,4