Combining Bandpasses

Under construction

Sometimes the bandpass calibrator at the required frequency is contaminated with line absorption or emission. In particular, this is nuisance for observations close to the rest frequency of HI where Galactic HI is observed along almost any sightline.

A technique to still obtain a usable bandpass solution is to observe frequencies offset to the main frequency. These offset frequencies should be chosen clean from any line contamination. Ideally it is advisable to observe two offset frequencies, symmetric to the main observing frequency.

In the following we explain how to combine these offset frequencies for a common bandpass and how to apply it to the central frequency.

Three steps are needed:

1) gaincal: the offset frequencies typically also have different phases. Since the bandpass is obtained by 'vector averaging' the data, one needs to correct for the phase offsets. The best way to do this is to derive and apply a complex gain solution.

#  gaincal :: Determine temporal gains from calibrator observations
vis                 = '9_4_sb1190721_1_000.55268.03407332176.ms' #  Name of input visibility
                                        #   file
caltable            = 'corr.gcal'       #  Name of output gain calibration table
field               =         ''        #  Select field using field id(s) or field name(s)
spw                 =         ''        #  Select spectral window/channels
selectdata          =      False        #  Other data selection parameters
solint              =      'inf'        #  Solution interval: egs. 'inf', '60s' (see help)
combine             =         ''        #  Data axes which to combine for solve (scan, spw,
                                        #   and/or field)
preavg              =       -1.0        #  Pre-averaging interval (sec) (rarely needed)
refant              =         ''        #  Reference antenna name.  ' '= '0'
minblperant         =          4        #  Minimum baselines _per antenna_ required for solve
minsnr              =        0.0        #  Reject solutions below this SNR
solnorm             =      False        #  Normalize average solution amplitudes to 1.0 (G, T
                                        #   only)
gaintype            =        'G'        #  Type of gain solution (G, T, or GSPLINE)
calmode             =       'ap'        #  Type of solution" ('ap', 'p', 'a')
append              =      False        #  Append solutions to the (existing) table
gaintable           =       ['']        #  Gain calibration table(s) to apply on the fly
gainfield           =       ['']        #  Select a subset of calibrators from gaintable(s)
interp              =       ['']        #  Temporal interpolation for each gaintable (=linear)
spwmap              =         []        #  Spectral windows combinations to form for
                                        #   gaintables(s)
gaincurve           =      False        #  Apply internal VLA antenna gain curve correction
opacity             =        0.0        #  Opacity correction to apply on the fly (nepers)
parang              =      False        #  Apply parallactic angle correction on the fly
async               =      False        #  If true the taskname must be started using

# gaincal(...)

2) bandpass: now we combine the two offset frequency observations into a single bandpass solution by averaging.

3) applycal: The combined bandpass will, by definition, be based on the frequency of the first input spw. The third step is map this solution to the central spw.

--Juergen Ott 19 March 2010