Observing strategies

Max VoronkovSoftware Scientist – ASKAP

28th September 2010

Motivation

In plain English:

You can do a better job if you’re know what you’re doing

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.

ATCA and CABB

Practical issues for an ATCA userMay be handy for other telescopes as well

Writing your proposal

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

Proposal preparation - 1

• What to observe?• When?

• For how long?

• Frequency?

• Array

configuration?

Image: Saripalli et al.Image: White & DuncanImage: Kim 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

Proposal preparation - 2

• What to observe?• When?

• For how long?

• Frequency?

• Array

configuration?

Image: Saripalli et al.Image: White & DuncanImage: Kim 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

Proposal preparation - 3

• What to observe?• When?

• For how long?

• Frequency?

• Array

configuration?

Image: Saripalli et al.Image: White & DuncanImage: Kim et al.

Check the sensitivity calculator!

• 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…)

http://www.atnf.csiro.au/observers/docs/at_sens

Wait for lecture on sensitivity if you want the math

Proposal preparation - 4

• What to observe?• When?

• For how long?

• Frequency?

• Array

configuration?

Image: Saripalli et al.Image: White & DuncanImage: Kim et al.

Usually the selection of frequency is driven by science! If you have a choice consider how the telescope performs at different frequencies

Proposal preparation - 5

• What to observe?• When?

• For how long?

• Frequency?

• Array

configuration?

Image: Saripalli et al.Image: White & DuncanImage: Kim et al.

Required resolution

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: Saripalli et al.Image: White & DuncanImage: Kim 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

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: Saripalli et al.Image: White & DuncanImage: Kim et al.

Zoom

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

Actual observations

• Go to 1934-638 (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 1934-638 for bandpass and flux calibration, e.g. 10 min

Consider the simplest case: one band from 4.5 to 6.5 GHz

Then, start the loop with program source(s):

Nearby calibrator (<10o) for e.g. 2 minTarget 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!

Initial calibration (dcal,pcal,acal)

vis display

Delay calibration (“corr dcal a”)

Phase calibration (“corr pcal a”) Amplitude calibration (“corr acal a”)

Why bother?

Delay = phase slope

Frequency average leads to decorrelation

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 4.5-6.5 GHz and 8.0-10.0 GHz)

• 1934-638 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 1253-055, 1921-293 and 0537-441)

• 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 1934-638 at least once (to fix the flux scale)

mm-wavelength observations

• 1934-638 is definitely too weak for bandpass at mm-wavelength• Use mm-wavelength bandpass calibrators 1253-055, 1921-293 and 0537-441

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)• 1934-638 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

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

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 15-20 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

Weather

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

Typical 12mm or 7mm observation

• 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)

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

Typical 3mm observation

It may be handy to create a separate schedule for setup sources (but same approach as before can also be used)

Special considerations for line observers

We put sky frequencies intoATCA 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!

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

1934-638 has <0.2% polarisation, phase calibrator may be good enough

Rely on absolute XY phase measurement using noise diode, otherwisewe would need an additional bright and strongly polarised calibrator

Additional challenges: direction-dependent polarisation (primary beam), variation of instrumental polarisation within 2 GHz band

Need good bandpass calibration.

Further information

• Check the users guide• http://www.narrabri.atnf.csiro.au/observing/users_guide/html/atug.html

• Check the ATCA forum / post questions• http://atcaforum.freeforums.org/

• Come to observe and learn from the experience• Talk to your Duty Astronomer and experienced observers• Check current issues

• http://www.narrabri.atnf.csiro.au/observing/CurrentIssues.html

• Check CABB pages• http://www.narrabri.atnf.csiro.au/observing/CABB.html

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: enquiries@csiro.au Web: www.csiro.au

Thank you

Australia Telescope National FacilityMax VoronkovSoftware Scientist (ASKAP)

Phone: 02 9372 4427Email: maxim.voronkov@csiro.auWeb: http://www.narrabri.atnf.csiro.au/~vor010