Calibration Issues Raised by the ALMA Science Advisory Committee (ASAC)
Last Update: JeffMangum - 12 Nov 2005
Calibration Issues Raised by ASAC in their 2005/10 Report
The October 2005 ASAC Report
contains several comments and questions related to ALMA calibration:
- Section 1, Executive Summary and Recommendations, Page 1, Science Requirements: "We recommend that the Board adopt the Science Requirements document after some clarification, especially in the area of calibration and solar observations."
- Section 4, Charge 2: ALMA Science Requirements Document, Page 5:
- "In reviewing the Science Requirements document, the ASAC continues to be concerned that the requirements relating to ALMA calibration are not laid out with sufficient clarity. In particular, we have emphasized in the past that the repeatability of ALMA calibration needs to be significantly better than the accuracy of ALMA's absolute calibration, which is currently set at 3% below 300 GHz and 5% above (see September 2003 ASAC Report for a detailed discussion). the difference in requirements between absolute and relative calibration as well as repeatability is not sufficiently clear in the current document. Additional clarification is also needed for the calibration requirements relating to polarization and for the requirements on the primary beam calibration accuracy."
- "Another area of concern relates to imaging the Sun. While the ASAC believes that ALMA will provide outstanding results in the field of solar observations, these observations will be both technically challenging and very different from other ALMA observations with the potential risk of damage to ALMA antennas and equipment during the observations. For ALMA, most secondary beams and far side lobes will look at the Sun, which will make interpretation of the data very difficult without specialized software. Advanced simulations accounting for far side lobes from each telescope will be needed to correctly establish the requirements on beam characterization and software developments when compared with those required by most DRSP projects."
The ASAC is asking for clarification of the wording in two areas of the ALMA Science Requirements document:
- Amplitude Calibration: Absolute, Relative, and Stability (Repeatability): What the ASAC is refering to is the need to distinguish between accuracy (i.e. how close a measured flux is to the true source flux) and precision (i.e. what the statistical error is on a measurement). In their September 2003 Report the ASAC recommended the following regarding amplitude calibration stability: "The Project should thus aim for the highest possible specification on repeatability, retaining the current goals of 1/3%". This should be added to the ALMA Science Requirements document.
- Polarization Calibration Requirements: In their September 2003 Report the ASAC recommended the following: "To measure polarised flux and polarisation angle requires an excellent, <<1%, and repeatable calibration between the polarisation channels [of a single band]". "In particular, to measure polarisation accurately in interferometric mode to 0.1% levels requires a differential gain stability between the two polarisation channels of better than 5X10(-4) in 5 minutes, the typical time between which calibration of instrumental polarisation can be performed." This should be added to the ALMA Science Requirements document.
- Primary Beam Calibration: I think that this comment was coupled to the Solar observation issue. Maybe the wording of the primary beam calibration statement in the ALMA Science Requirements document needs to be changed?
ACTION taken: Science Requirements No. 300 and were modified to read:
3.9 Amplitude Fluctuations The corrected visibility amplitude fluctuations on time scales of 1 second to 300 seconds shall not exceed 1% at frequencies less than 300 GHz, 3% at higher frequencies, considering :
- A. antenna gain stability under changing wind and gravity conditions;
- B. ability to measure and correct for atmospheric opacity and emission fluctuations.
- C. Instrumental gain fluctuations ,
to provide repeatable precision in amplitude measurements. Accurate relative calibration (the ability to measure a flux ratio between frequencies in two different bands) requires absolute calibration to accuracy of 5% or better at all frequencies. Sci Req 300
5.0 Calibration The final visibilities shall be on a calibrated flux density scale, accurate to within 5% at all frequencies. Sci Req 350 -- AlWootten
- 21 Dec 2005
- 12 Nov 2005
Calibration Issues Raised by ASAC in their 2004/05 Report
- Phase Stability Specification Review:
- Delay error specification of 22 fsec max rms change in 300 sec (drift) and 65 fsec rms deviation from 10 sec average (noise), proposed in the ALMA System Technical Requirements (ALMA-80.04.00.00-005-A-SPE), should be adopted.
- The relatively strict specifications being adopted for phase stability result in significant sensitivity loss ranging from 10% at frequencies < 275 GHz to 22% at 950 GHz. Further degradation of these requirements is not acceptable.
- Science IPT should carry-out simulations of the affects of this phase stability specification on high-resolution imaging.
- Total Power Stability Specification Review:
- Troubled by TP stability specs recommended in the ALMA System Technical Requirements (ALMA-80.04.00.00-005-A-SPE), which are:
- Gain stability of < 1e(-3) for all antennas over 0.05 to 0.5 second time scales (and at 100 seconds?).
- Gain stability of < 4e(-4) for four "total power antennas" over 0.05 to 0.5 second time scales.
- Problem is that this second specification for the four TP antennas is a relaxation of the recommended 1e(-4) level based on the analysis presented in ALMA Memo 490. Based on the ALMA Memo 490 results, ALMA will require 6-9 times more integration time to reach a given rms noise level in TP continuum imaging with this relaxed specification.
- It may be possible to trade gain stability for sensitivity in the four TP antenna systems. Robert Laing has proposed using GaAs amplifiers, an idea which should be investigated.
- Polarization Stability:
- Definition of a satisfactory specification are still incomplete.
- Calibration Issues:
- DSRP Questionaire Results:
- DSRP questionaire results suggest that for amplitude calibration:
- 1-3% absolute, relative, and repeatable accuracy for continuum measurements,
- 10% absolute, 5% relative, and 5-10% repeatable accuracy for spectral line measurements,
- 1-5% absolute, relative, and repeatable accuracy for polarization measurements.
- Study direct impact of calibration errors on derived physical quantities from most challenging Level 1 science goals.
- Amplitude Calibration
- Bring to completion tests of calibration devices.
- Develop a dedicated effort within the Project to study, develop, and maintain an absolute calibration measurement system.
- Phase Calibration
- Plan and execute phase calibration surveys at the ATF.
- Collaborate with parallel efforts at phase calibrator surveys at IRAM, CARMA, JCMT, and APEX.
I proposed some elements of a response to the ASAC Report, which I forwarded to Tom Wilson for comment. See Draft Response to ASAC
- 20 Jul 2004