Mike Garrett

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Mike Garrett Mike Garrett Joint Institute for VLBI in Europe Joint Institute for VLBI in Europe EVN Consortium Member EVN Consortium Member Dwingeloo, The Netherlands Dwingeloo, The Netherlands 27 September 2001 VLBI Observing Facilities oposing, scheduling, observing, correlation, analysis & suppo

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VLBI Observing Facilities. [Proposing, scheduling, observing, correlation, analysis & support]. Mike Garrett. Joint Institute for VLBI in Europe EVN Consortium Member Dwingeloo, The Netherlands. 27 September 2001. Overview of this lecture…. - PowerPoint PPT Presentation

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Page 1: Mike Garrett

Mike GarrettMike Garrett

Joint Institute for VLBI in EuropeJoint Institute for VLBI in Europe

EVN Consortium MemberEVN Consortium Member

Dwingeloo, The NetherlandsDwingeloo, The Netherlands

27 September 2001

VLBI Observing Facilities[Proposing, scheduling, observing, correlation, analysis & support]

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Overview of this lecture….

• Brief review of VLBI facilities open to all astronomers • Focus on the EVN, VLBA and Global VLBI • Some examples of “typical” VLBI Science • EVN Support for non-expert VLBI users• What you need to think about before submitting a VLBI proposal• Proposal writing, submission and feedback from the EVN and

NRAO PC• Scheduling a VLBI “experiment”• Absentee observing (let someone else take the strain…)• Absentee Correlation and Correlation set-up parameters • Data analysis options

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VLBA

CMVA VSOP

Australian LBA & APT VERA

VLBI Facilities “open” to all Astronomers

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• 10 identical, 25 meter telescopes, full time array, operated by a single entity – NRAO.

• Recording system – VLBA terminals/recorders. • Baselines of 200 - 8600 km – spread across continental USA [60

km baselines if VLA included] • Extensive frequency coverage: 0.3, 0.6,1.3-1.7, 2.3, 5, 8, 12-15,

22-24, 41-45, 80-96 GHz.• Resolution range: 20 milliarcsecond (0.3 GHz), 5 mas (1.7 GHz);

0.5 mas (15 GHz); 0.2 mas (43 GHz). • 86 GHz capability on 6 telescopes. Co-observes with CMVA.• Co-observes with EVN – Global VLBI.

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More on the VLBA…

• Good receivers, excellent polarisation characteristics.• Frequency switching on time scales of a few minutes.• Capable and efficient correlator (PSR gating; reasonable data

o/p rates but soon to be surpassed….).• Dynamic scheduling, Target of Opportunity observations

“easy”.• Can be augmented by the phased-VLA and Effelsberg 100-m

(and soon GBT) for extra sensitivity. • Recording data rates are limited to 128 Mbits/sec (sustained)

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The European VLBI Network

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• 15 non-identical telescopes operated and managed by a consortium of 11 different institutes.

• Part-time array: 3 x 4 weeks/yr. • Routinely includes the LARGEST telescopes in the world.

• Uses MkIV recording system. Can observe at extremely high data rates – 512 Mbits/sec SUSTAINED, => VERY SENSITIVE.

• Frequency coverage tends to focus around UHF, 1.2-1.7, 5, 6.7 and 8 GHz.

• Excellent uv-coverage; Baseline lengths range from 200 – 2200 km in Europe. Inclusion of UK MERLIN array adds even shorter baselines. Extension of baselines to 9000 km via Chinese telescopes.

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Resolution and Sensitivity

• Resolution of the EVN (in milliarcseconds)….

Freq EVN+MERLIN

Full EVN Global VLBI

1.6 GHz 150-15 mas 5 mas 3.5 mas

5 GHz 50-4 mas 2 mas 1.2 mas

At 6 cm, the typical sensitivity of the full EVN is around 20 microJy/beam in ~10 hours.

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• EVN is continually changing its capabilities through network wide and local institute upgrades:

• Frequency switching capability not yet proven. • Polarisation characteristics sometimes poor but improving (this

and other effects may lead to very bright sources being quickly dynamic range limited).

• EVN is perhaps inclined to look favourably on “speculative” proposals (experiments) that push VLBI or the science to its limits.

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• New MkIV Correlator at JIVE – superb spectral resolution; capable of producing fantastic data output rates…

• Combination of the EVN, MERLIN and VLBA (Global VLBI) provides highest resolution, best uv-coverage and superb sensitivity.

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A flavour of VLBI Science Drivers

•Radio jet & Black hole physics

•Radio source evolution

•Astrometry

•Galactic and extra-galactic Masers

•Gravitational Lenses

•SNR and GRB studies

•Nearby and distant starburst galaxies

•Nature of faint radio source population

•HI absorption studies in AGN…

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Before Proposing to use the EVN

You are inspired! You are thinking of submitting a VLBI proposal…. What next ? First consider the nature and feasibility of the VLBI observations you are proposing. Some key questions to ponder…

1. Is the target source visible (for the EVN this implies declinations > -25 degrees) ?

2. Radio Source Structure: Is the target source compact and strong enough to detect using standard VLBI self-calibration techniques (=> > 10 mJy on majority of baselines) ?

3. If not (2) then is the target source located nearby a bright reference source I can use as a phase-calibrator ? Phase-referencing can allow detections of sub-mJy radio sources.

4. Has the source been observed before (see “List of Sources Observed by VLBI” – EVN WWW), should it be re-observed anyway ?

5. What resolution/frequency is required in order to meet the scientific goals of the observation ?

6. Is astrometric, polarisation or spectral index information required ?

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7. Is more than one source to be observed e.g. “Snapshot” survey => the more telescopes the better to ensure good uv-coverage.

8. How much observing time is required in order to meet your goals ? Note that image noise decreases as the square-root of on-source integration time and observing bandwidth.

THERE ARE MANY THINGS TO CONSIDER – but there are many sources of information, support and help:

at www.jive.nl/jive/evn

VLBA web pages: www.aoc.nrao.edu/VLBA

“Human” support at JIVE – EVN Support Group (e-mail [email protected]) can help with technical aspects of EVN proposal preparation for all new and inexperienced VLBI users.

EVN also actively encourages collaboration between EVN staff and outside

users.

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Proposal Submission

• Submit a proposal – first READ the “EVN Call for Proposals” appears 3 times per year – deadlines: 1 October, 1 February, 1 June. Text is available via the EVN WWW pages.

• Proposal comes in two parts – (i) a coversheet that summarises all aspects of the proposal including a technical summary and (ii) a Scientific justification (short and sweet, make it clear what your proposal can achieve) .

• Proposals can be submitted electronically (concatonated PS files of (i)+(ii). Global VLBI proposals are submitted to both the EVN and NRAO separately.

• Justify request for MERLIN, DSN participation, & multiple correlator passes. • The EVN PC meets face-to-face a month or so after the deadline and rates

your proposal. Later you will receive an email from the PC chair (Simon Garrington) informing you of your grade. Projects with grades < 1.5 are often scheduled.

• Don’t be disappointed if you are asked to re-submit (your idea may be ahead of its time!). Comments from the referees can be useful in improving the proposal and should certainly be addressed in any re-submission.

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Scheduling

• If you have been successful in getting time on the EVN and your project is scheduled, expect to receive PS file from the EVN scheduler – Dr. Rolf Schwartz (MPIfR). This file is known as the “block schedule”.

• The block schedule tells you when your project will be observed: day number, UT range, list of telescopes.

• You are now ready to produce a VLBI observing Schedule (see Bill Cotton demo). Both the EVN and VLBA use Sched (C. Walker) to generate schedules.

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Scheduling – first order points to consider…

• DO NOT PANIC – for new or outside users a JIVE Support Scientist will be assigned to your project and can help you produce the schedule.

• READ the “PI Instructions Sheet”- R.W. Porcas (available via the EVN WWW)

• Get your schedule in on time. Schedules are submitted to “vlbeer.ira.bo.cnr.it”.

• If you replace a schedule you MUST contact the EVN VLBI Friends and ensure that they have downloaded the new version of the observing file.

• Schedule a 5 minute scan on a “fringe-finder” at least once.• Even if you are observing a relatively bright but complex

source (> 20 mJy) I would recommend that you observe nearby calibrators (from JVAS) often (at least every 20 minutes, twice per 44 min tape pass).

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• Leave 10 minutes for tape changes (15 mins if both Jodrell Bank and Cambridge telescopes are involved)

• Don’t schedule scans which are shorter than 90 seconds without a good reason (some EVN telescopes are “slow” and for WSRT phased-array the correlator is reconfigured for each source change)

• Don’t switch frequencies more often than every 3 minutes.

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Observing

• Absentee observing.• Occasionally things go wrong – see EVN Feedback Pages

(EVN WWW).• Its possible for new or inexperienced users (if they wish)

to visit one of the telescopes during their observations – in order to get a “feel” for the “VLBI experience” (highly recommended!)

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Correlation at the EVN MkIV Correlator

• After your observations have been made the tapes are sent from the EVN telescopes to JIVE.

• Sometime after they arrive at JIVE you will get a message from JOPS (JIVE operations) asking you to confirm your correlation parameters (these will have already been specified in the Schedule – now is your last chance to check they are correct!).

• Major points of concern include: 1. Source Coordinates (e.g. Methanol masers, B1950…) 2. Spectral resolution (Number of channels e.g. 32

channels per 8 MHz filter band for continuum) 3. Time resolution (Integration dump time e.g. 2 seconds) 4. All cross-correlation products (LR,RL,LL,RR) ?

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Re-correlation ?

• After correlation the data will be inspected by a JIVE Support Scientist.

• Initially the data is in aips++ (MS) format. A summary of data quality plots is available via ftp.

• The default is that the tapes will be released after the JSS has “OK’d” the data.

• Alternatively you can review the data quality plots in parallel with the JSS. You have 4 weeks to raise an alarm and request re-correlation. Otherwise after 4 weeks the tapes are released (erased).

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Data Analysis

• The EVN MkIV correlator supplies the user with data in FITS format on DAT tapes.

• Basic calibration of the data is conducted in AIPS (amplitude calibration, fringe-fitting etc)

• Correctly formatted amplitude calibration data (ANTAB format) is generated automatically and can be retrieved via vlbeer.ira.bo.cnr.it (same place the you deposited the original schedule).

• Further analysis (e.g. mapping) is mostly performed in AIPS or difmap.

• EVN users are encouraged to visit JIVE in order to analyse their data (experienced users and new users). JIVE Support Scientists are directly assigned to assist new or inexperienced users.

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Support for outside EVN users

• All EVN projects (including globals) led by Europeans (EU + Associated State residents) can count on FULL financial support. This covers:

1. All costs associated with travelling and visiting JIVE in order to generated a VLBI schedule.

2. All costs associated with travelling and visiting the EVN observatories (for the “vlbi experience”).

3. All costs associated with travelling and visiting JIVE (or any of the EVN observatories) in order to analyse EVN data.

4. All costs associated with travelling and attending EVN User Meetings (held during EVN Symposia – next one in Bonn June 2002).