Basic concepts of radio interferometric (VLBI) observations

Hiroshi ImaiDepartment of Physics and Astronomy

Graduate School of Science and EngineeringKagoshima University

• Proposing an interferometric observation• Preparing for the observation• During the observation and signal correlation• Preparation for the data reduction

• Data reduction see in the next week⇒• Feedback to further observations

Proposing an interferometric observation

• Scientific goals– Is a new radio observation necessary?• Is the radio emission detectable?• Is the identification essential?

– Is radio interferometry necessary for the observation?• Is the source compact enough to yield new finding?• Is interferometry helpful for increasing sensitivity?

– How to observe the sources?• Imaging, non-imaging?• special technique (spectroscopy, astrometry, polarimetry)?

– Is the interferometric observation technically feasible?• Is the target are is smaller than the FoV of interferometer?• Is the target detectable in the available freq. bandwidth?• Is the requested machine time realistic?

Field of view limited by integration time and bandwidth

• Wider band width• higher recording bit rate• Larger number of frequency band channels

• Avoiding integration smearing in correlation• Shorter integration time, but higher data throughput rate • Larger number of phase (delay-tracking) centers

Preparing for the interferometric observation

• You can visit only one of the stations (or do not have to).– But operators are necessary in all the stations

• You have to tell everything to the operators– Observatory managers know, but operators might not.– Not only procedure files but also extra requests

• Once the observation starts, your mistakes in the procedures may not be recovered.

• Your mistake might be found after everything finished (observations and correlation) and the original recorded data have gone one month after the observation.

Preparing for the interferometric observation• Observation procedure file(s) .drg (PC-SCHED), .key (NRAO SCHED) .vex (e.g. VERA)– Global control parameters

observation code, contact person, shipping address– Source list

name, coordinates (equinox, proper motion), velocity (e.g. LSR)– Station listname, station code, coordinates (plate motion), antenna mount, sensitivity (degree per flux unit=DPFU), recorder type – Frequency setup

used receiver, observed sky frequencies, bandwidths, calibration signals (e.g., phase-cal)

– Recording setup recording rate, (quantization bits)– Scan time table: see next– Correlation mode

number of spectral channels, integration time, (precise) source coordinates

Preparing for the interferometric observation• Observed sources in radio interferometry– science target sources

as short as 1/5-1/2 of total allocated time

– test source(s) (for spectroscopy and polarimetry)– fringe finder(s)• detectable within an accumulation period

– (group) delay calibrator(s)• close to the targets• Observation frequency (~every hour) dependent of weather

and stability of instrumental delays– amplitude calibrator(s)• giving flux density scale• monitoring gain variation

– reference sources for advanced technique• Phase-referencing (astrometry), polarimetry

During the observation and signal correlation

• You can go home if– your procedure files are reliable,– operators can do the observation without you.

• You have to wake up if– you request some special operation mode,– observatory requests to decide operation cancellation,– you believe that your mistake can be recovered during

the observation.• Quickly check after signal correlation– whether all the (detectable) sources are detected, – input parameters are correct.

Preparation for the data reduction

Have you already got?• Raw visibility data – FITS-IDI (flexible image transport system- interferometry data interchange)

format (VLBI)– Measurement sets (JVLA, ALMA)

• Supplemental data tables– antenna gain– system temperature– phase (and group delay) calibration– data flagging log (due to bad pointing, high Tsys, etc.)

• Pipeline script and logs– useful for first inspection

Feedback to further observations• Can you modify the procedure files?– Yes!station coordinates ….. as accurate as possiblescan pattern ….. as realistic as possible

(antenna slewing, nearer calibrators)– No! (unless the previous value was completely wrong)coordinates of targets (and their phase-reference sources)frequency setup and/or calibrators that will cause significant change in measurement conditions

• Can re-correlation be requested?– Yes if it has been already predicted (e.g. multi FoVs)– No if the raw recording medias have gone or have to

be soon recycled. – No with connected arrays (e.g. ALMA)

Difficulty in planning advanced observations1. scan patterns

• Multiple mission• Campaign program among multiple observatories

KaVA ESTEMA (Expanded Study on Stellar Masers)

Difficulty in planning advanced observations2. band allocation

KaVA ESTEMA (Expanded Study on Stellar Masers)

• Multiple mission

• Multiple calibration schemes

Important advices

• Never propose interferometric observations unless they are really scientifically and technically justified and you seriously serve the proposed research or think paper publication!– They are so expensive (running cost and manpower)– Heavy responsibility for compliance to tax payers.

• Proposing interferometric observations is the most ambitious and challenging activity in research!– Unique and advancing– The only way to become an independent (tenure track)

researcher