Notes

Outline

Solar Calibration Strategies Gordon Hurford UC Berkeley FASR Workshop           Green Bank              25 May 2002

Calibration Classes “Fixed” pre-observation calibrations Relative amplitudes, phases Baselines, pointing, etc Time-dependent “Daily” calibrations Time-dependent gain Time-dependent phases Interference survey? Post-observation calibrations Application of fixed calibrations, data editing, etc Self-calibration closure amplitudes and phases

Some FASR Calibration Requirements Requirements need to be science-driven (rough estimates -- to be used as ‘talking points’ only) Absolute flux scale ~5% Relative flux vs frequency ~1% for nearby frequencies ~3% for frequencies separated by an octave Absolute locations ~1 arcsecond at 20 GHz eases to ~1 arcmin at 100 MHz

Decimetric Calibration A different problem than at microwave frequencies Absolute flux scale is less important More reliance on system phase stability Reliance on self-calibration Location accuracy limited by ionosphere Colocated LOFAR could help with absolute locations

Microwave phase calibration - generalities Baseline redundancy May be useful By itself, cannot meet requirements Conventional approach Reasonable, but not heroic measures to stabilize system phase Add 1 or 2 large antennas for non-solar calibrations Interrupt solar observations every hour or two to recalibrate Comments on conventional approach Cost of large antennas is equivalent to many small antennas Interruptions degrade solar science

Some FASR - Specific Considerations Large number of small antennas has sensitivity comparable to a moderate sized antenna.  (Calibrator sensitivity ~ n * A) Wide bandwidth (Calibrator sensitivity ~ ÖBW) Even with large antennas, must rely heavily on self-calibration to achieve dynamic range goals. Will almost always have compact solar sources in FOV Frequency-dependence of phase variations with time can be simply parameterized (eg  terms that scale with f and f^-2.) Lots of time available for daily pre- and post-observation calibration Frequency coverage suggests that communications satellites might be useful as secondary calibrators.

An Extreme Phase Calibration Scenario Overnight calibrator observations used to update locations and motions of selected communications satellites Overnight calibrations used as starting point for full-disk mapping and establishment of positions of dominant solar compact sources Solar self-calibration used to refine antenna phases Will gradually increase uncertainty in location of solar sources. Periodically, a small subset of antennas briefly observe a pre-located communications satellite.  (Preserves continuity of solar coverage.) Returning subset is used to update absolute locations of compact solar sources. At end of day, long calibration on phase calibrator to confirm inferred antenna phases and compact source locations.