1. Max Voronkov Software Scientist – ASKAP 28th September 2010
Observing strategies
Max Voronkov
Software Scientist – ASKAP
28th September 2010

2. Motivation In legalese:
No student or students, or person or persons acting on the direction or suggestion or supervision of the student or students may try, or attempt to try or make an attempt to try to use the facility without basic understanding of the technique, except as authorized by the head of the science operations.
The offence against this regulation is an offence of strict liability.
The penalty is 1 proposal grade unit.
In plain English:
You can do a better job if you’re know what you’re doing

3. ATCA and CABB Practical issues for an ATCA user
May be handy for other telescopes as well

4. Writing your proposal
There are a number of questions about your observations which must be answered at the proposal preparation stage

5. Proposal preparation - 1
What to observe?
When?
For how long?
Frequency?
Array
configuration?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
What to observe largely depends on your science!
You may want to narrow down the selection of targets to maximize the science return thinking about
How long the source can be observed with ATCA?
Shadowing?
Synthesized beam quality (bad for equatorial sources)
Overheads (e.g. bad for sources transiting through zenith)
Other issues like confusion and dynamic range

6. Proposal preparation - 2
What to observe?
When?
For how long?
Frequency?
Array
configuration?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
For a variable source science may dictate when to observe!
Even if variability is not important, it is often worth to consider whether the target is visible during the day or at night
Better atmosphere at night for mm-wavelength observations
Less solar (and probably man-made as well) interference at night (for low frequency observations)
Afternoon thunderstorms in summer

7. Proposal preparation - 3
What to observe?
When?
For how long?
Frequency?
Array
configuration?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
Check the sensitivity calculator!
Wait for lecture on sensitivity if you want the math
Effective bandwidth and spectral resolution
uv-coverage: spread short integration across the range of hour angles (note, overheads are increased!)
Overheads (understanding comes with experience…)

8. Proposal preparation - 4
What to observe?
When?
For how long?
Frequency?
Array
configuration?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
Usually the selection of frequency is driven by science!
If you have a choice consider how the telescope performs at different frequencies

9. Proposal preparation - 5
What to observe?
When?
For how long?
Frequency?
Array
configuration?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
Required resolution

10. Proposal preparation - 6
What to observe?
When?
For how long?
Frequency?
Array
configuration?
How big is the object / area of interest?
Single pointing: field of view is ~ /D (primary beam)
Mosaicing: stitching several pointings together
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
Image (rotated Cen A): Cornwell & Feain
uv-coverage per pointing
overheads
single dish data?
Tasks mosgen and atmos in MIRIAD will help to setup

11. Proposal preparation - 7
What to observe?
When?
For how long?
Frequency?
Array
configuration?
How big is the object / area of interest?
Single pointing: field of view is ~ /D (primary beam)
Mosaicing: stitching several pointings together
Spectral line or continuum?
Image: Kim et al.
Image: White & Duncan
Image: Saripalli et al.
Zoom

12. CABB - range of modes
Two independent 2 GHz wide spectral windows (or IFs as they’re often called)
Plus optional zoom windows (now up to 4 zoom windows per 2 GHz window)
Coarse resolution of the wide (2 GHz) window = bandwidth of 1 zoom window
Optional stitching of zoom windows is possible

13. Actual observations
Consider the simplest case: one band from 4.5 to 6.5 GHz
Go to (flux calibrator) (it is strong enough to do all the required calibration)
Initial array calibration (delays, phases and amplitudes)
Close the file (junk data)
Track for bandpass and flux calibration, e.g. 10 min
Then, start the loop with program source(s):
Nearby calibrator (<10o) for e.g. 2 min
Target for e.g. 20 min
………
If you worry about bandpass variations, observe bandpass calibrator several times (e.g. at regular intervals). Note, phase calibrator may be good enough!

14. Initial calibration (dcal,pcal,acal)
vis display
Why bother?
Delay = phase slope
Frequency average leads to decorrelation
Phase calibration (“corr pcal a”)
Amplitude calibration (“corr acal a”)
Delay calibration (“corr dcal a”)

15. Variations - 1 A bit lower frequencies (i.e. near 1.4 GHz)
The second 2 GHz band (IF) is redundant
Only part of the band has signal (at this stage)
Many interference spikes
Need to choose the channel range for initial calibration (via “corr tvch”)
Calibrators are often confused (hard to find isolated point sources)
A bit higher frequencues (i.e simultaneous GHz and GHz)
may be too week for bandpass (about 2 Jy at 9 GHz, which is not too bad, but is getting low)
Use something else for bandpass and initial setup (i.e. mm-wavelength bandpass calibrators , and )
Have to give a flux estimate with the “corr acal” command (doesn’t need to be very accurate as the flux scale is corrected during the data reduction, but it is good to have the Tsys scale roughly right)
Still observe flux calibrator at least once (to fix the flux scale)

16. mm-wavelength observations
is definitely too weak for bandpass at mm-wavelength
Use mm-wavelength bandpass calibrators , and for bandpass/initial setup
Have to give a flux estimate with the “corr acal” command at 12mm and 7mm (3mm is special, no “corr acal” is needed, but need a paddle scan)
is still a flux calibrator at 12mm and 7mm. Observe a planet at 3mm (normally Uranus).
There is an additional step to adjust power (mm-attenuators)
You may need to play with them during setup (e.g. “set mm ca01 3 3”)
Setting is per polarisation (applies to both frequencies)
The goal is to move CABB attenuators away from the edge (0 or 15)
May be tricky if two 2-GHz IFs are well separated
Need a pointing scan as the global pointing model is not good enough
Observations of continuum calibrator with half-beam offsets
Close to the target (<15o). Don’t do pointing on a planet.
Roughly once an hour or if moving to a different part of the sky.
May need to adjust focus

17. Pointing scan Pointing scan offsets dishes around expected position
Options to choose suitable initial estimate: update, offpnt, refpnt
caobs options: point_antenna, point_ifflag and point_pattern

18. Paddle scan
Use paddle to measure the system temperature and correct for the atmosphere at 3mm
Kate’s talk for more details….
Paddle replaces the amplitude calibration (“corr acal” during setup)
Every min or if moving to a different part of the sky or change frequency
Do it near phase calibrator / target source
Also need paddle scans before flux and bandpass calibrator scans

19. Weather No data is often better than bad data!
Calibrator scans
No data is often better than bad data!
Atmosphere attenuates astronomical signal and adds noise!
In addition, the effect is not constant in time

20. Typical 12mm or 7mm observation
I often like to add setup sources (bandpass and flux calibratior + associated pointing scans) to the same schedule and delete them just before I start the loop
Can benefit from the “Master/Slave” feature
Can just copy the scans and ensure exactly the same parameters are used for your setup scans
Adding scans on calibrators is very easy (the scheduler knows everything about these sources, you just type the name)

21. Typical 3mm observation
It may be handy to create a separate schedule for setup sources (but same approach as before can also be used)

22. Special considerations for line observers
We put sky frequencies into
ATCA schedules
(in other words we work in the topocentric reference frame)
Science is usually done with the spectra in either LSR or barycentric reference frame
LSR = local standard of rest
(motion w.r.t. nearby stars)
Barycentric is w.r.t. barycentre of the Solar system
Conversion is position- and time-dependent
May be tricky for narrow-band observations of multiple sources!

23. Polarisation observations
CABB always measures XX, XY, YX and YY. But we need to know the phase offset between X and Y as well as the leakage between them to convert the measurement into something scientifically useful.
Need unpolarised/weakly polarised source observed at a number of hour angles to estimate instrumental polarisation
has <0.2% polarisation, phase calibrator may be good enough
Rely on absolute XY phase measurement using noise diode, otherwise
we would need an additional bright and strongly polarised calibrator
Need good bandpass calibration.
Additional challenges: direction-dependent polarisation (primary beam), variation of instrumental polarisation within 2 GHz band

24. Further information Check the users guide
Check the ATCA forum / post questions
Come to observe and learn from the experience
Talk to your Duty Astronomer and experienced observers
Check current issues
Check CABB pages

25. Thank you Australia Telescope National Facility Max Voronkov
Contact Us
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Australia Telescope National Facility
Max Voronkov
Software Scientist (ASKAP)
Phone:
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Thank you