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Subsections

• General
• Interferometer Data
• Data Editing

Data Correction and Calibration

General

1.

Data should be selectable in terms of identification with a particular type of calibration observation

• This is unclear: we presume that it means that for example phase calibration observations must be distinguished from bandpass calibration observations. If so this is a far-reaching problem, extending back into the observing system.

2.

Calibration should be made as generic as possible, with telescope-specific methods kept to a minimum

3.

Both standard and user-defined models of data behavior should be usable in determining calibration information from data sets

4.

Instrumental behavior that affects calibration should be integrable in the calibration process through a mixture of parameterized functions and models in tabular form

5.

Data correction based upon standard and user-defined functions, with user supplied parameters, should be possible

6.

Calibration and correction of data should be reversible, with the capability to apply calibration/correction information either ``on-the-fly'' during processing, or ``once and for all'', creating new, calibrated data sets

• The current design has an additional column in the MeasurementSet for corrected coherences.

7.

Calibration/correction of data should be possible from derived tables of instrumental parameters (e.g., system temperature vs. time, gain vs. elevations), with derivation of such tables from calibration observations or from on-line measurements

8.

The calibration process should include flexible averaging of calibration data and application with interpolations or weighted averaging, all under control of the user

9.

Cross-calibration from different instruments should be possible (e.g. flux scale, pointing) particular when data from different arrays are to be combined

10.

Model fitting should be possible in both the image and u-v planes, and it should be possible to use the resultant models for further calibration and self-calibration

• Model fitting in the u-v plane is not part of the MEGI formalism. It must be performed outside the MEGI framework.

11.

There must be simulation programs for single dish, interferometer, and mosaicing data bases for both planning and comparison of data with models - with optional error generation for thermal noise, pointing errors, primary beam errors, atmosphere, antennas surface errors, beam-switching for total power, etc.

Interferometer Data

1.

Transfer of calibration matrices from one observation to another should be possible and easy.

2.

Redundancy in data (possibly including crossing points) should be used whenever possible as an additional constraint on calibration and self-calibration

3.

Determination of, and application of corrections for, closure errors should be possible with flexible averaging of input closure information

4.

Fringe fitting for a range of spectral channels and fringe rates should be possible by baseline, as well as globally by antenna

5.

Spectra calculation from complex summing of visibilities in each spectral channel for user-specified positions in the field of view

6.

Interferometric pointing, baseline, and beam pattern fitting and related analysis

• These lie somewhat out of the MEGI framework and should be regarded as operations on Jones matrices. For example, for antenna position fitting, one would presumably use the MEGI framework to derive a G-matrix object, and then one would fit the phases in that object to find antenna positions.

7.

Application and de-application of astrometric/geodetic correction factors with complete and reversible histories

8.

Calibration of data for effects of the ionosphere, utilizing data at multiple frequencies and/or external data on variations of electron content

9.

Self-Calibration for non-isoplanicity must be possible

• The MEGI framework allows it but as far as we know, there are no demonstrated algorithms

10.

Determination and correction for pointing errors, and errors in beam shape, using mosaic self-calibration techniques, will be important

11.

For spectral line sources one can do amplitude calibration with auto-correlation spectra plus calibration at one antenna

12.

Accurate Doppler correction for each spectral channel is essential

13.

For polarization calibration, one must be able to determine both source polarization structure and instrumental polarization (``D-term'' self-calibration)

14.

Full phase calibration is an iterative process involving limits set by: astrometry, geodesy, and weak source imaging/detection, therefore one needs:

(a)

very accurate geometric models, typically to at least 1/10 of a wavelength accuracy

• Possible eventually with the Measure system, currently being designed and implemented by Wim Brouw. Models accurate at this level will become available in late 1996.

(b)

knowledge of location of the Earth's pole and UT1, both of which are generally known only after astrometric/geodetic analysis

• Also possible with the Measure system. Available mid-1996

(c)

values of ionospheric delay as determined from measurements at simultaneous frequencies, or external measurements of ionospheric electron content

(d)

measurement of properties of troposphere dry terms (from surface meteorological measurements) and wet terms (Kalman filtering, GPS multi-frequency satellite measurements, WVR)

(e)

instrumental delays as determined from phase calibration signals

(f)

knowledge of non-rigidity of the earth due to earth tides and atmospheric loading

Data Editing

The requirements for data editing lie more in the domain of visualization.

1.

Data display and editing should be seen as generic tools applicable to single dish, interferometer, and other forms of data

2.

Data visualization for evaluation and editing purposes should be seen as an integral, or closely coupled, aspect of the data system

3.

It should be possible to do interactive editing based upon display, with ``zoom'' or magnification, and menu selection of editing options

4.

Various ``viewing strategies'' should be available

(a)

For interferometer data, baseline by baseline display (with magnification of local areas) and interactive editing (including multiple, simultaneous baselines) using both Intensity-time-baseline displays and Intensity displays in u-v plane

(b)

Displays of spectra and spectral cubes aggregated in various ways (spectra vs. time, averaging in time, averaging of channels)

(c)

Selection of data by specifying windows in space and/or time

(d)

Selection of arbitrary cuts through data (e.g. circular, radial, or a user-defined locus) through selected data coordinates

(e)

Display of expanded data aggregates (e.g., pointing and clicking on an average multi-channel region of data to show the component spectrum

(f)

Comparison displays of generic model data (from fitted components) with observed and/or processed data, including display of data with model subtracted or divided

5.

Data editing should be reversible, with the capability to store, apply, and un-do editing information

6.

Data editing should be possible on the basis of monitor/observing log data

7.

Editing should be possible from ``consistency check'' information, particularly where there is redundancy or (for interferometric data) where there are crossing points in the u-v plane

8.

It is desirable to have parameter-driven, automated flagging for large data sets

9.

Editing must be possible based upon difference between data and models generated during self-calibration

10.

Data editing based upon recognition of interference patterns in intensity-time-frequency data is very important, particularly for low frequency observations