Preparing for Calibration

A description of the range of prior information necessary to solve for calibration

There is a range of a priori information that may need to be initialized or estimated before calibration solving is carried out.  This includes establishing prior information about the data within the MS:

  • weight initialization --- if desired, initialization of spectral weights, using initweight (by default, unchannelized weight accounting is used, and no special action is required)
  • flux density models --- establish the flux density scale using "standard" calibrator sources, with models for resolved calibrators, using setjy as well as deriving various prior calibration quanitities using various modes of gencal
  • gain curves --- the antenna gain-elevation dependence
  • atmospheric optical depth --- attenuation of the signal by the atmosphere, including correcting for its elevation dependence
  • antenna position errors --- offsets in the positions of antennas assumed during correlation
  • ionosphere --- dispersive delay and Faraday effects arising from signal transmission through the magnetized plasma of the ionosphere
  • switched power (EVLA) --- electronic gains monitored by the EVLA online system
  • system temperature (ALMA) --- turn correlation coefficient into correlated flux density (necessary for some telescopes)
  • generic cal factors --- antenna-based amp, phase, delay

These are all pre-determined effects and should be applied (if known) as priors when solving for other calibration terms, and included in the final application of all calibration.  If unknown, then they will be solved for or subsumed in other calibration such as bandpass or gains.

Each of these will now be described in turn.

Weight Initialization

See the section on weights for a more complete description of weight accounting in CASA.

CASA 4.3 introduced initial experimental support for spectral weights.  At this time, this is mainly relevant to ALMA processing for which spectral $T_{sys}$ corrections, which faithfully reflect spectral sensitivity, are available.  In most other cases, sensitivity is, to a very good approximation, channel-independent after bandpass calibration (and often also before), except perhaps at the very edges of spectral windows (and for which analytic expressions of the sensitivity loss are generally unavailable).  Averaging of data with channel-dependent flagging which varies on sufficiently short timescales will also generate channel-dependent net weights (see split or mstransform for more details).

By default, CASA's weight accounting scheme maintains unchannelized weight information that is appropriately updated when calibration is applied.  In the case of spectral calibrations ($T_{sys}$ and bandpass), an appropriate spectral average is used for the weight update.  This spectral average is formally correct for weight update by bandpass.  For $T_{sys}$, traditional treatments used a single measurement per spectral window; ALMA has implemented spectral $T_{sys}$ to better track sensitivity as a function of channel, and so should benefit from spectral weight accounting as described here, especially where atmospheric emmission lines occur.  If spectral weight accounting is desired, users must re-initialize the spectral weights using the initweights task:

 

initweights(vis='mydata.ms', wtmode='nyq', dowtsp=True)

In this task, the wtmode parameter controls the weight initialization convention.  Usually, when initializing the weight information for a raw dataset, one should choose wtmode='nyq' so that the channel bandwidth and integration time information are used to initialize the weight information.  The dowtsp parameter controls whether or not (True or False) the spectral weights (the WEIGHT_SPECTRUM column) are initialized.  The default is dowtsp=False, wherein only the non-spectral weights (the WEIGHT column) will be initialized.  If the spectral weights have been initialized, then downstream processing that supports spectral weights will use and update them.  In CASA 4.3 and later, this includes applycal, clean, and split/mstransform; use of spectral weights in calibration solving (e.g., gaincal and other solve tasks) is scheduled for the CASA 5.0 release.

Note that importasdm currently initializes the non-spectral weights using channel bandwidth and integration time information (equivalent to using dospwt=False in the above example.  In general, it only makes sense to run initweights on a raw dataset which has not yet been calibrated, and it should only be necessary if the filled weights are inappropriate, or if spectral weight accounting is desired in subsequent processing. It is usually not necessary to re-initialize the weight information when redoing calibration from scratch (the raw weight information is preserved in the SIGMA/SIGMA_SPECTRUM columns).  (Re-)initializing the weight information for data that has already been calibrated (with calwt=True, presumably) is formally incorrect and is not recommended.

When combining datasets from different epochs, it is generally preferable to have used the same version of CASA (most recent is best), and with the same weight information conventions and calwt settings in calibration tasks.  Doing so will minimize the likelihood of arbitrary weight imbalances that might lead to net loss of sensitivity, and maximize the likelihood that real differences in per-epoch sensitivity (e.g., due to different weather conditions and instrumental setups) will be properly accounted for.  Modern instruments support more variety in bandwidth and integration time settings, and so use of these parameters in weight initialization is preferred (c.f. use of simple unit weight initialization, which has often been the traditional practice).

Alert: Full and proper weight accounting for the EVLA formally depends on the veracity of the switched power calibration scheme.  As of mid-2015, use of the EVLA switched power is not yet recommended for general use, and otherwise uniform weights are carried through the calibration process.  As such, spectral weight accounting is not yet meaningful.  Facilities for post-calibration estimation of spectral weights are rudimentarily supported in statwt.

Flux Density Models

It is necessary to be sure calibrators have appropriate models set for them before solving for calibration.  Please see the task documentation for setjy and ft for more information on setting non-trivial model information in the MS.   Also, information about setting models for flux density calibrators can be found here.   Fields in the MS for which no model has been explicitly set will be rendered as unpolarized unit flux density (1 Jy) point sources in calibration solving.

 

Antenna Gain-Elevation Curve Calibration

Large antennas (such as the 25-meter antennas used in the VLA and VLBA) have a forward gain and efficiency that changes with elevation. Gain curve calibration involves compensating for the effects of elevation on the amplitude of the received signals at each antenna.  Antennas are not absolutely rigid, and so their effective collecting area and net surface accuracy vary with elevation as gravity deforms the surface.  This calibration is especially important at higher frequencies where the deformations represent a greater fraction of the observing wavelength.  By design, this effect is usually minimized (i.e., gain maximized) for elevations between 45 and 60 degrees, with the gain decreasing at higher and lower elevations.  Gain curves are most often described as 2nd- or 3rd-order polynomials in zenith angle.

Gain curve calibration has been implemented in CASA for the modern VLA and old VLA (only), with gain curve polynomial coefficients available directly from the CASA data repository.  To make gain curve and antenna efficiency corrections for VLA data, use gencal:

gencal(vis='mydata.ms', caltable='gaincurve.cal', caltype='gceff')

Use of caltype='gceff' generates a caltable that corrects for both the elevation dependence and an antenna-based efficiency unit conversion that will render the data in units of approximate Jy (NB: this is generally not a good substitute for proper flux density calibration, using fluxscale!).  Use of caltype='gc' or caltype='eff' can be used to introduce these corrections separately.

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

Alert: If you are not using VLA data, do not use gaincurve corrections.  A general mechanism for incorporating gaincurve information for other arrays will be made available in future releases.  The gain-curve information available for the VLA is time-dependent (on timescales of months to years, at least for the higher frequencies), and CASA will automatically select the date-appropriate gain curve information.  Note, however, that the time-dependence was poorly sampled prior to 2001, and so gain curve corrections prior to this time should be considered with caution.

Atmospheric Optical Depth Correction

The troposphere is not completely transparent.  At high radio frequencies ($>$15 GHz), water vapor and molecular oxygen begin to have a substantial effect on radio observations. According to the physics of radiative transmission, the effect is threefold.  First, radio waves from astronomical sources are absorbed (and therefore attenuated) before reaching the antenna.  Second, since a good absorber is also a good emitter, significant noise-like power will be added to the overall system noise, and thus further decreasing the fraction of correlated signal from astrophysical sources.  Finally, the optical path length through the troposphere introduces a time-dependent phase error.  In all cases, the effects become worse at lower elevations due to the increased air mass through which the antenna is looking.  In CASA, the opacity correction described here compensates only for the first of these effects, tropospheric attenuation, using a plane-parallel approximation for the troposphere to estimate the elevation dependence.  (Gain solutions solved for later will account for the other two effects.)

To make opacity corrections in CASA, an estimate of the zenith opacity is required (see observatory-specific chapters for how to measure zenith opacity).  This is then supplied to the caltype='opac' parameter in gencal which creates a calibration table that will introduce the elevation-dependent correction when applied in later operaions. E.g. for data with two spectral windows:

gencal(vis='mydatas.ms',
       caltable='opacity.cal',
       caltype='opac',
       spw='0,1',
       parameter=[0.0399,0.037])

If you do not have an externally supplied value for opacity, for example from a VLA tip procedure, then you should either use an average value for the telescope, or omit this cal table and let your gain calibration compensate as best it can (e.g. that your calibrator is at the same elevation as your target at approximately the same time). As noted above, there are no facilities yet to estimate this from the data (e.g. by plotting $T_{sys}$ vs. elevation).

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

Below, we give instructions for determining opacity values for Jansky VLA data from weather statistics and VLA observations where tip-curve data is available.  It is beyond the scope of this description to provide information for other telescopes.

Determining opacity corrections for modern VLA data

For the VLA site, weather statistics and/or seasonal models that average over many years of weather statistics prove to be reasonable good ways to estimate the opacity at the time of the observations. The task plotweather calculates the opacity as a mix of both actual weather data and seasonal model. It can be run as follows:

myTau=plotweather(vis='mydata.ms',doPlot=True)

The task plots the weather statistics if doPlot=T, generating a plot shown in the figure below. The bottom panel displays the calculated opacities for the run as well as a seasonal model. An additional parameter, seasonal_weight can be adjusted to calculate the opacities as a function of the weather data alone (seasonal_weight=0), only the seasonal model (seasonal_weight=1), or a mix of the two (values between 0 and 1). Calculated opacities are shown in the logger output, one for each spectral window.  Note that plotweather returns a python list of opacity values with length equal to the number of spectral windows in the MS, appropriate for use in gencal:

gencal(vis='mydata.ms', caltype='opac', spw='0,1', parameter=myTau)  

Note that the spw parameter is used non-trivially and explicitly here to indicate that the list of opacity values corresponds to the specified spectral windows.

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

Type Figure
ID plotwx
Caption The weather information for a MS as plotted by the task {\tt plotweather}.}

 

Determining opacity corrections for historical VLA data

For VLA data, zenith opacity can be measured at the frequency and during the time observations are made using a VLA tipping scan in the observe file.  Historical tipping data are available here.  Choose a year, and click Go to get a list of all tipping scans that have been made for that year.

If a tipping scan was made for your observation, then select the appropriate file.  Go to the bottom of the page and click on the button that says Press here to continue.  The results of the tipping scan will be displayed.  Go to the section called 'Overall Fit Summary' to find the fit quality and the fitted zenith opacity in percent.  If the zenith opacity is reported as 6%, then the actual zenith optical depth value is 0.060.  Use this value in gencal as described above.

If there were no tipping scans made for your observation, then look for others made in the same band around the same time and weather conditions.  If nothing is available here, then at K and Q bands you might consider using an average value (e.g. 6% in reasonable weather).  See the VLA memo here for more on the atmospheric optical depth correction at the VLA, including plots of the seasonal variations.

 

Antenna-position corrections

When antennas are moved, residual errors in the geographical coordinates of the antenna will cause time-dependent delay errors in the correlated data.  Normally, the observatory will solve for these offsets soon after the move and correct the correlator model, but sometimes science data is taken before the offsets are available, and thus the correction must be handled in post-processing. If the 3D position offsets for affected antennas are known, use gencal as follows:

gencal(vis='mydata.ms', caltable='antpos.cal', caltype='antpos', antenna='ea01',parameter=[0.01,0.02,0.005])

In this execution, the position offset for antenna ea01 is [1cm,2cm,0.5cm] in an Earth-centered right-handed coordinate system with the first axis on the prime meridian and third axis coincident with the Earth's axis.  Corrections for multiple antennas can be specified by listing all affected antennas and extending the parameter list with as many offset triples as needed. 

In general, it is difficut to know what position offsets to use, of course.  For the VLA, gencal will look up the required offests automatically, simply by omitting the antenna and parameter arguments:

gencal(vis='mydata.ms', caltable='antpos.cal', caltype='antpos')

For the historical VLA, the antenna position coordinate system was a local one translated from the Earth's center and rotated to the VLA's longitude.  Use caltype='antposvla' to force this coordiate system when processing old VLA data.

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

 

Ionospheric corrections

CASA 4.3 introduced initial support for on-axis ionospheric corrections, using time- and direction-dependent total electron content (TEC) information obtained from the internet.  The correction includes the dispersive delay ($\propto \nu^{-1}$) delay and Faraday rotation ($\propto \nu^{-2}$) terms.  These corrections are most relevant at observing frequencies less than $\sim$5 GHz.  When relevant, the ionosphere correction table should be generated at the beginning of a reduction along with other calibration priors (antenna position errors, gain curve, opacity, etc.), and carried through all subsequent calibration steps.  Formally, the idea is that the ionospheric effects (as a function of time and on-axis direction) will be nominally accounted for by this calibration table, and thus not spuriously leak into gain and bandpass solves, etc.  In practice, the quality of the ionospheric correction is limited by the relatively sparse sampling (in time and direction) of the available TEC information.  Especially active ionospheric conditions may not be corrected very well.  Also, direction-dependent (within the instantaneous field-of-view) ionosphere corrections are not yet supported.  Various improvements are under study for future releases.

To generate the ionosphere correction table, first import a helper function from the casapy recipes repository:

from recipes import tec_maps

Then, generate a TEC surface image:

tec_maps.create(vis='mydata.ms',doplot=T,imname='iono')

This function goes to the web to obtain TEC information for the observing date and location, and generates a time-dependent CASA image containing this information.  The string specified for imname is used as a prefix for two output images, with suffixes .IGS_TEC.im (the actual TEC image) and .IGS_RMS_TEC.im (a TEC error image).  If imname is unspecified, the MS name (from vis) will be used as the prefix.

The quality of the retrieved TEC information improves with time after the observing date, becoming optimal 1-2 weeks later.  Both images can be viewed as a movie in the CASA imag viewer.  If doplot=T, the above function will also produce a plot of the TEC as a function of time in a vertical direction over the observatory.

Finally, to generate the ionosphere correction caltable, pass the .IGS\_TEC.im image into gencal, using caltype='tecim':

gencal(vis='mydata.ms',caltable='tec.cal',caltype='tecim',infile='iono.IGS_TEC.im')

This iterates through the dataset and samples the zenith angle-dependent projected line-of-sight TEC for all times in the observation, storing the result in a standard CASA caltable.  Plotting this caltable will show how the TEC varies between observing directions for different fields and times, in particular how it changes as zenith angle changes, and including the nominal difference between science targets and calibrators.

This caltable should then be used as a prior in all subsequent calibration solves, and included in the final applycal.

A few warnings:

  • The TEC information obtained from the web is relatively poorly sampled in time and direction, and so will not always describe the details of the ionospheric corruption, especially during active periods.
  • For instrumental polarization calibration, it is recommended that an unpolarized calibrator be used; polarized calibrators may not yield as accurate a solution since the ionospheric corrections are not yet used properly in the source polarization portion of the polcal solve.

Special thanks are due to Jason Kooi (UIowa) for his contributions to ionospheric corrections in CASA.

 

Switched-power (EVLA)

The EVLA is equipped with noise diodes that synchronously inject a nominally constant and known power contribution appropriate for tracking electronic gain changes with time resolution as short as 1 second.  The total power in both the ON and OFF states of the noise diodes is continuously recorded, enabling a gain calibration derived from their difference (as a fraction of the mean total power), and scaled by a the approximately known contributed power (nominally in K).  Including this calibration will render the data in units of (nominal) K, and also calibrate the data weights to units of inverse K2.  To generate a switched-power calibration table for use in subsequent processing, run gencal as follows:

gencal(vis='myVLAdata.ms',caltable='VLAswitchedpower.cal',caltype='evlagain')                        

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

To ensure that the weight calibration by this table works correctly, it is important that the raw data weights are proprotional to integration time and channel bandwidth.  This can be guaranteed by use of initweights as described above.

 

System Temperature (ALMA)

ALMA routinely measures $T_{sys}$ while observing, and these measurements are used to reverse the online normalization of the correlation coefficients and render the data in units of nominal K.  To generate a $T_{sys}$ calibration table, run gencal as follows:

gencal(vis='myALMAdata.ms',caltable='ALMAtsys.cal',caltype='tsys')                                    

The resulting calibration table should then be used in all subsequent processing the requires the specification of prior calibration.

 

Miscellaneous ad hoc corrections

The gencal task supports generating ad hoc amp, phase, and delay corrections via appropriate settings of the caltype parameter.  Currently, such factors must be constant in time (gencal has no mechanism for specifying multiple timestamps for parameters), but sometimes such corrections can be useful.  See the general gencal task documenation for more information on this type of correction.