| Gordon Hurford | |
| UC Berkeley | |
| FASR Workshop Green Bank 25 May 2002 | |
| “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 | |||
| 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. | ||