Early indications of performance
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Transcript of Early indications of performance
Early indications of performance
Max Voronkov
Software Scientist – ASKAP
13th May 2009
Performance tests - Main questions
• Typical delay scatter in vis plot and its stability• Wide bandwidth allows to estimate delays better
• May see weak effects we have never seen before
• Flux scale calibration• Bandpass shape, spectral indices and software can affect it
• Using 1934-638 at 7mm?
• Astrometric accuracy• Systematic effects (e.g. baseline solution)?
• Accuracy of the pointing solution• Greater sensitivity may improve the solution
• Bandpass shape and spectral indices can make it worse
• Could we use weaker sources?
Delays in vis plot
Delays in vis plot (cont’d)
Flux scale calibration
• We didn’t have a chance to study this in detail• More information will probably come from the calibrator monitoring
• Be prepared to see systematic effects
• Wide bandwidth effects are more serious for CX-band (3 and 6 cm).
• Probably the uncertainty is not worse than a few per cent for CX-band, 10% for 12mm, a few tens per cents for 7mm and 3mm bands.
• Paddle measurement at 3mm• Frequency-dependent Tsys measurement
• Does not converge amplitudes in vis.
• Can 1934-638 be used instead of the planet at 7mm?• The source is easily detectable at the level of about 0.3 Jy
• There is a factor of 1.8 difference in fluxes if calibrated on Uranus and 1934-638!
• The difference is present for narrow bandwidth too. Flux model?
• Previous such experiments with the old system at 36 GHz gave a factor of about 1.3.
Interference spikes
Things improved a lot since this picture was taken!
Observations of G19.61-0.23 at 7mm
Recombination lines in G19.61-0.23 at 7mm
H53
42951.47 MHz
Pointing accuracy
• Important for mm bands where wide band effects are small (for general users)• Pointing solution is done at 3/6cm after reconfig (staff)• Wide-band effects (spectral index and primary beam dependence on frequency) are not taken into account• Iterative solution for pointing should converge
• Did a number of pointing scans with a 10’’ offset in positions• Compared solutions with 10’’
Pointing accuracy (contd.)
• Antennas 3 (technical problems) and 6 were excluded from the analysis• Found 3.9 arcsec accuracy• This is similar to the accuracy obtained with the old system • Strangely enough, the pointing procedure always underestimates the offset• It would be interesting to repeat the same experiment in C/X and/or with narrower tvch
It is advisable to use the old strategy to choose pointing calibrators.
W33 - combining old and new data
• CABB data completely outweigh the data from the old system
• Blue contours - old system, 6km array• Red contours - the same 6km array data combined with the new CABB data (using H168)• Grayscale - VLA ammonia data (Keto & Ho, 1989)
Summary
• Usually delays are spread by less than 10 ps or so• Baseline errors can give a residual delay of the order of tens of
picoseconds (especially 6km baselines at 7mm). This should not be a problem for a typical science experiment
• The weather and other effects we don’t yet understand usually give a smaller delay
• Flux scale calibration and ATCA astrometry need further study• Calibration on 1934-638 and Uranus at 7mm gives different result
• There are interference spikes and lines in the spectra (flagging)• Always observe bandpass calibrator for every frequency setup• Pointing accuracy is no better than before
• Use the same strategy as before (1 Jy or brighter source close to the target)
• Merging old correlator data with CABB data is non-trivial
Contact UsPhone: 1300 363 400 or +61 3 9545 2176
Email: [email protected] Web: www.csiro.au
Thank you
Australia Telescope National FacilityMax VoronkovSoftware Scientist (ASKAP)
Phone: 02 9372 4427Email: [email protected]: http://www.narrabri.atnf.csiro.au/~vor010