ACA Calibration Issues
Last Update: JeffMangum - 02 Mar 2006
Scientific Benefits of ALMA/ACA Cross Correlation
Thanks for the combine-array memo. There is no doubt that Combined array observations will be very important for a significant fraction of the ALMA operations, and that the technical problems (in hardware and software) associated with combined operations have been solved. There may be some differences among the several reasons and priorities for using the Combined array or the separate arrays, but these are minimal. In the end, it will take experience with ALMA (unexpected calibration problems, the nature of mm-radio emission) to decide on the optimum strategy concerning independent or combine array use.
I have a few suggestions and comments:
What are the scientific disadvantages? I suppose there are some, and these should be mentioned.
- There is no doubt that the main goal of the ACA is to provide the short spacings (when necessary) that are missing from the 12-m array alone. My question is that when a scientific program needs this (u-v) range of observations, why not do them always with the combined array mode rather than coordinated mode? The advantages are:
- Better signal to noise, mainly through the 12x7m baselines.
- Natural cross-calibration between the ACA and 12m arrays.
- Easier to schedule.
- Easier to reduce.
Cheers, EdFomalont - 02 Mar 2006
- The Combined array usage ONLY to increase the SNR (the short (u-v) spacings are not needed) is not so strong a motivation. The estimate is that about 10% of the time ALMA projects require a well-defined high sensitivity that is not limited by dynamic range, is probably about right. For other projects where the needed rms noise is not known very well, Combined array usage is probably not the optimum use of the ACA.
- ACA calibration. There are some calibrations which are signal to noise limited that will need the participation of the Combine array. But, many calibrations are not SNR limited and can be done using strong sources by the ACA alone. Hence the 4 to 9 times in the observing time factor is not correct for all calibrations since it takes only a few seconds of observing to reach the accuracy needed for a calibration scan The two most crucial calibrations are the bandpass (plus sideband) calibration and the temporal complex gain calibration and you discuss these. Again, until we start observing and determine some of the instrumental, temporal and spatial properties of the array in general, optimum calibration strategies and combinations are now estimates.
- This is a guess, but I'll bet that at high frequencies the optimum resolution for some sources will be the ACA plus the part of the 12-m array which is closest to the ACA. Should this be mentioned? In more general terms, a more arbitrary splitting between the ACA and the 12-m array may be scientifically useful.
Discussion of Combined ACA-ALMA versus Coordinated ACA-ALMA Observations
Email from EdFomalont regarding the need to cross-correlate ACA and main ALMA array for calibration.
- Cross-correlation between ALMA and ACA for imaging? Nearly optimum SNR can be obtained by feathering the ACA image with the ALMA image (where image is a single pointing or a large mosaic). In addition, the added u-v coverage with ALMA-ACA baselines is in the 'middle' of the u-v plane, where coverage is not really needed. Hence, the ALMA+ACA beam would be worse than the ALMA beam alone, and reweighting would lose sensitivity.
- Cross-correlation between ALMA and ACA for calibrations? The set of calibrators available to the ACA will be less dense in the sky than those for ALMA because of its higher sensitivity. If ALMA and the ACA are cross-correlated (at least for the calibrator observations), then the larger set of phase calibrators is also available to the ACA.
- Phase referencing: If the closest detectable calibrator is generally used for phase referencing, then an independent ALMA and ACA observation may use different calibrators. (The use of the ACA calibrator for the ALMA observations may degrade the ALMA image because of the increased target-calibrator separation.) The use of separate calibrators will lead to a position shift and an amplitude scale difference between the two images. These shifts can be determined by including in the ALMA observations, some scans of the ACA calibrator. This bootstrapping is already done on the VLBA. Such bootstrapping requires the ALMA and ACA observations be made within a few days of each other, the limitation associated with source variability time scales. We do not yet know the relationship between the quality of phase calibration as a function of the calibrator-target separation, and this will depend on the WVR corrections. If the ACA image accuracy is significantly improved by using a closer calibrator, which can only be detected by cross-correlating with ALMA, then cross-correlation could be useful. But, my guess is that significant improvement is unlikely in the ACA image, say between a calibrator four degrees compared with two degrees away. Furthermore, for high dynamic range images, the phase reference calibration is only the first step in overall calibration to make 'reasonable' images. Further gains in dynamic range are made by self-calibration techniques which are not very dependent on the quality of the phase-referencing calibrations.
- General calibrations, These calibrations, like antenna locations and primary beam mapping, can be made on relatively strong calibrators over the sky; hence the ACA sensitivity alone is sufficient to obtain accurate results. The only possible exception may be the temporal and spatial change of bandpass or reference pointing, so that a calibrator near the target is desired.
Cross-correlation between ALMA and ACA is not needed. There will be little loss in image sensitivity by adding the separate images, compared with that from full correlation. For phase referencing and general calibrations, the density of sources available to the ACA is sufficient for accurate calibrations. If ALMA and the ACA observe a
target using a different calibrator, the extra observations needed to register the main ALMA calibrator with the main ACA calibrator are minimal, although this boot-strapping needs more investigation. The observations should be made within a few days of each other.
The only real gain in cross-correlation is if the ACA image quality is significantly improved using a very close calibrator that can only be detected with ALMA-correlations. However, most of the improvements in image quality for the ACA may be made using self-calibration techniques.
- 13 Oct 2005
Cross-correlation between ALMA-B antennas and the ACA is planned. An increase of 30% in observing time may be gained from combining the two arrays.
- 10 Nov 2005