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VLA SKY SURVEY

March 4-6 2015 VLASS Review

VERY LARGE ARRAY SKY SURVEY Survey Design & Technical Summary Steven T. Myers (NRAO Socorro)

Galactic Center (Survey) Multiwavelength Image Credit: X-ray: NASA/UMass/D.Wang et al., Radio: N RAO/AUI/NSF/NRL/N.Kassim, Mid-Infrared: MSX

VLA SKY SURVEY


VLASS Technical Implementation Plan • Describes the:

•  basic assumptions and calculations for survey sensitivity •  basic and enhanced data products •  top-level observing schedule (by configuration) •  requirements and plans for processing and storage •  test & development program •  resourcing (personnel, processing, storage) •  risk assessment on all the above

•  Note: the TIP is intended to be a living document, and is intended to be revised, enhanced, and updated as survey testing, planning, and implementation progress

VLA SKY SURVEY


Survey Design Overview • Sensitivity of the JVLA for large-area surveys

•  Effective (RFI-free) bandwidth, point source sensitivity •  Spectral Index and Polarization sensitivity •  Sensitivity to extended sources

• Angular resolution of the VLASS •  Natural vs. Briggs Robust weighting

• Mosaicking strategies •  Pointed hex-pattern mosaics for DEEP •  On-the-fly mosaciking (OTFM) for fast scanning ALL-SKY •  OTFM Scanning Rate Limitations

• Calibration/slew overheads for JVLA surveys

VLA SKY SURVEY


Point Source Sensitivity •  1500MHz RFI-free bandwidth (JVLA Observing Guide)

•  risk: may be as low as 1200MHz at some Dec (Stripe-82?)

• Effective survey speed at image rms σ :

•  using VLA Exposure Calculator, close to what we find in Stripe-82

• Signal-to-noise needed for spectral index measurement •  S/N ~ 51 for σ(α) = 0.1

• Signal-to-noise needed for polarization characterization •  see talk by Rudnick

SS =16.55 σ100µJy!

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deg2 hr−125 Antennas Dual Pol Natural Weight 1500MHz BW

VLA SKY SURVEY


Sensitivity to Extended Sources •  Extended sources of total flux density S and size φF resolved by

synthesized beam (PSF) θF < φF will have reduced peak brightness :

•  e.g. a φF=3’’.1 galaxy in DEEP (θF=0’’.8) Sp/S = 0.0624 •  Convolving the image to φF (matched filtering, or uv tapering) will in

principle capture all the flux density S but only increase the noise by

•  e.g. for our galaxy in DEEP σ/σc = 0.25 (improved!)

Sp = SθF2

θF2 +φF

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σ c =σθF2 +φF

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θF2

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This is what was used in the proposal to calculate source yield from S3 counts

Using (developing?) a scale-sensitive source finder will improve the VLASS yield

VLA SKY SURVEY


Angular Resolution •  The sensitivities calculated and quoted in the proposal are

for naturally weighted images •  snapshot PSF for natural weighting is ≈ 2’’.9 (B) and 0’’.9 (A)

• Due to strong snapshot sidelobes in PSF, it is advantageous to use robust weighting (Briggs thesis) •  usually produces better images, lower rms than natural weighting

when sources present •  Briggs weighting r=0.5-1 seems reasonable in tests •  PSF resolution using Briggs thesis scaling: 2’’.5 (B) and 0’’.8 (A)

•  These are the angular resolutions quoted for the VLASS tiers •  cost: reduced sensitivity, rms ~1.11x higher than natural weighting •  optimizing this part of the Test & Development Plan

VLA SKY SURVEY


Pointed Mosaics The standard mode for mosaicking using a discrete (minimal) set of pointed observations uses a “hexagonal” packed pattern. For this pattern, the survey speed is given by :

assuming a conservative slew/settle overhead of 5s per point giving tint>25s (25% max overhead) and a 3GHz primary beam θFWHM=15’. To cover the ALL-SKY area 33885 deg2, this will take a minimum time

TAS > 6644 hours Faster surveys will require OTFM

See https://science.nrao.edu/facilities/vla/docs/manuals/obsguide/modes/mosaicking

SS = 0.4330θFWHM2

tint< 5.1deg2 hr−1

In future (2018+?) new ACUs should reduce settle times to ~3s or better, allowing 2x this SS.

VLA SKY SURVEY


All-Sky: OTF Mosaicking •  “On-the-Fly” Mosaicking (OTFM) for efficient coverage of large areas

•  Scan telescopes in a deterministic way while taking data •  Lose no data while scanning instead of 3-7s per step in standard mode

•  Efficient when dwell times on-sky are <25s •  Produces highly uniform thermal noise sensitivity across entire area

•  High scanning rates (1-17’/s) and survey speeds (up to 24 deg2/hr) •  limited by data rates (25MB/s) for fast correlation dump times (2.8’/s =24 deg2/hr)

•  Image Processing support for OTF Mosaicking (in CASA) – in development and testing

See https://science.nrao.edu/facilities/vla/docs/manuals/obsguide/modes/mosaicking

× × × ×

VLA SKY SURVEY


Scanning Rate Limitations •  Fast scanning of the array (e.g. for OTFM) is limited by

the need for short system “dump” (visibility integration) times to avoid errors due to movement of the primary beam during the integration

• Conservative limit < 0.1*FWHM at highest frequency during integration •  e.g. FWHM 12’.5 at 3.6GHz, limit 1’.25/dump

• Data rate limit imposed on VLASS: 25MB/s •  0.45sec per dump for full band (1024 channels)

• Combined limit 2.78’/s • OTFM row spacing 8’.84 (FWHM/√2 at 3.6GHz)

•  Survey Speed SSmax = 24.6 deg2/hr •  Image rms at SSmax σmax = 122 µJy •  cannot go shallower at these data rates!

VLA SKY SURVEY


Overheads •  For a scheduling block (SB) to have self-contained

calibration it must include: •  startup slew (possibly from far away) to first target •  flux density calibrator •  polarization leakage and angle calibrator(s) •  nearby gain (amp and phase) calibrator interleaved with target

• Experience with CNSS Stripe-82 blocks (and others) let us build overhead model based on SB length for the various components: •  ALL-SKY 17% (long blocks, shared infrequent calibration) •  Elais-N1 21% (7.25 hour blocks) •  COSMOS, ECDFS 25% (7.5, 5 hour blocks)

•  These values were used for proposal

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Schedule & Observing •  The VLASS 7-year 6-cycle survey schedule • Observing considerations • A day in the life of the VLASS • A year in the life of the VLASS

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Observing Schedule •  7+ years spanning 6 cycles (88 mos. 2016-23)

•  Impact: 1480 hrs/cycle

Cycle Config. ALL-SKY COSMOS ECDFS Elais-N1 Total 1 B 906 906 1 A 60 325 188.5 573.5 2 B 906 906 2 A 60 335 188.5 573.5 3 B 906 906 3 A 60 325 188.5 573.5 4 B 906 906 4 A 60 325 188.5 573.5 5 B 906 906 5 A 60 325 188.5 573.5 6 B 906 906 6 A 0 335 188.5 523.5

Total 5436 300 1960 1131 8827

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Pressure •  Observing pressure for VLASS by LST:

•  B-config: high uniform “pressure” (37.75 per cycle) •  assume split in Declination so can be observed over full LST range

•  A-config: highest pressure in ECDFS (65.33 per cycle) •  requires around 65 passes in each cycle

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Observing • Observing considerations

•  where practical used “fixed LST blocks” as long as possible •  self-contained calibration (factored into overheads) •  will use Python script generation (e.g. as in CNSS Stripe-82) •  blocks interruptible (for TOO alert responses, overrides, etc.)

• A day in the life of the VLASS •  schedules queued (previously), observed •  QL calibration run, QA assessment (Observer of the Day+DAs) •  QL imaging run, QA assessment •  when competed & passed, stage cal data and images to archive

• A year in the life of the VLASS •  SD pipelines run on ALL-SKY and epoch accumulated DEEP •  more detailed assessment and processing, TBD

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Data Products and Storage • Raw Data – available from archive, no proprietary period • Basic Data Products (BDP)

•  promised and delivered by NRAO (w/community help where possible)

• Enhanced Data Products & Services (EDP,EDS) •  added value provided by community

•  The Archive & Archive Services

VLA SKY SURVEY


VLASS Basic Data Products • Deliverables by NRAO (with SSG collaboration where

possible):

Product Timescale Notes Raw Data immediate no proprietary period Calibrated Data 1 week same, served from archive Quick-Look Images 48 hrs. continuum only, simple QA Quick-Look Catalog w/QLI only basic image object finding Single-Epoch Images 6 mos. (T1)

12 mos. (T2) better quality assurance

Single-Epoch Catalog w/SEI more object parameters Cumulative Images 12 mos. (T1)

16 mos. (T2) produced after each epoch after first

Cumulative Catalog w/CI more detailed

VLA SKY SURVEY


VLASS Basic Data Products • Deliverables by NRAO (with SSG collaboration where

possible):

Product Contains Raw Data full dataset Calibrated Data calibrated dataset (or raw+tables) Quick-Look Images I images (continuum only) Quick-Look Catalog flux density,position,size Single-Epoch Images IQU (continuum,spectral cubes*) Single-Epoch Catalog flux density,position,size,spectrum Cumulative Images IQU (continuum,spectral cubes*) Cumulative Catalog flux density,position,size,spectrum

* spectral cubes may be limited/compressed (see TIP)

VLA SKY SURVEY


Enhanced Data Products/Services A community led effort

•  Transient Object Catalogs & Alerts • Rotation Measure Images and Catalogs •  Light Curves (IQU) • Multi-Wavelength Catalogs for VLASS sources • Enhanced VLASS Archive with Image & Catalog Service

²  e.g., as currently available by IPAC/IRSA allowing for VLASS data to be integrated with Spitzer/Planck/WISE/Euclid/etc…

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Processing & Pipelines • Quick-Look (QL) pipeline

•  fast calibration & imaging (48hrs) •  CASA-based (fallback: AIPS-Lite)

• Standard Data (SD) pipeline •  used for single-epoch and cumulative products •  CASA-based (ALMA/VLA standard pipeline, possibly modified)

• Calibration •  Imaging • Analysis

•  source finding & catalog •  spectral index •  polarization

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Archive •  In long-term planning for the JVLA, we project to have

storage for 3300TB in 2017, and 5700 in 2020 •  In the TIP, we estimate VLASS usage as 1280 TB

•  Raw data = 794 TB •  Image products = 486 TB

•  This is 22% of the total archive in 2020 • Can be accomodated in to NRAO DM budget plans

•  higher image volumes or raw data rates would require significantly supplementing this budget

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Test & Development Program (TDP) •  JVLA for Surveys: current status

•  Caltech-NRAO Stripe-82 Survey (CNSS) •  COSMOS field

• Basic tests • OTFM tests • DEEP field tests • Algorithm & Software development and implementation • Processing and Data Service Challenges for the VLASS

•  These will be discussed in more detail tomorrow

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Readiness

•  The VLASS is a moderate extension of capabilities already proven on smaller-scale P.I. programs •  ALL-SKY: proven to similar depth on Stripe-82 (13B-370) for Quick-

Look. Cumulative processing (images & catalogs) underway. When combined Stripe-82 (12A-371,13B-370,15A-421) approach this combined depth and cadence.

•  DEEP: Smolcic et al. COSMOS (12B-158) to ~2µJy, processing and analysis still underway

•  Test & Development Plan: Mar 2015 – May 2016 (3.1 FTE) •  Plus community members (students,post-docs) to help!

• Operations Staffing: 4.3 FTE/yr (incl. 3 FTE/yr data analysts) • Algorithm/Software Development: >2 FTE-years?

•  These capabilities needed for JVLA anyway •  Most are current high priority dev targets

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Date Activity

2015 March 4 – 6 External Community Review (Socorro), project office opens

2015 March – 2016 April Test & Development Program carried out

2015 July 1 Call for proposals (2016A) issued (deadline Aug 3)

2015 August VLASS Preliminary Design Review (PDR)

2016 April VLASS Critical Design Review (CDR), final go/no-go

2016 May 27 VLASS Cycle 1 observations commence (2016A B-config)

2016 Sep 30 End of 2016A B-config (with a 1 month extension from 29Aug)

2017 Mar 09 End of 2016B A-config (1 month extension, 2 months total)

2017 Mar 31 Delivery of B-config Epoch 1 (6 months: ALL-SKY I only)

2017 Sep 30 Delivery of B-config Epoch 1 (12 months: Pol.)

2017 Sep VLASS Cycle 2 observations commence (B-config)

2018 Mar 09 Delivery of A-config Epoch 1 (12 months: DEEP)

VLASS Future Milestones Notional schedule (including TIP activities), assuming postitive review outcome

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