Very Large Array Sky Survey

Transcription

1 1 Very Large Array Sky Survey Technical Implementation & Test Plan Casey Law, UC Berkeley 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

2 2 How do we get the science? General Requirements RFI and Usable Frequencies Tsys and Time Estimate Calibration Sensitivity and Completeness

3 Challenge: Radio Interference 3 +4 h Worst case: Clarke belt (δ~-5 above 3.6GHz Hour Angle -5 h 2GHz Frequency 4GHz

4 2.0 GHz spw GHz 3.0 GHz 3.5 GHz spw GHz March VLASS Review 4 Usable Frequencies Bandwidth: 1.5 GHz (nominal), 1.35 GHz (aggressive) Effective central frequency varies with RFI, Tsys, spectral index RFI RFI α>=-0.7, RFI-free νc RFI-free νc nominal νc

5 5 Tsys and Time Estimates Tsys (via Rick Perley) Spectral and elevation structure Mean El => Dec in 3 hour transit Conservative (but an eyeball fit) Time estimate process BW=1.5 GHz, 25 antennas (as for Exposure Calculator) Natural visibility weighting Scale by Tsys scale factor at given Declination Scale by overhead factor (talk by S. Myers)

6 6 Sensitivity Brightness temperature

7 7 Resolution and Completeness One source seen in VLASS-like, FIRST, NVSS J from Mooley et al. Source counts are conservative VLASS-like FIRST NVSS

8 Resolution and Completeness (Mooley et al.) JVLA S-Band B-config (Hodge et al.) 8

9 9 How do we get the science? Tier1: All-Sky 3.3π sr B config S-band) 69 microjy/beam 3 epochs Requirements On-the-fly Mosaicking Wideband spec/polarimetry Robust pipeline

10 10 On-the-Fly Mosaicking Technique θp(defined for 3.6 GHz) = 12.5 θrow = 8.84 Sample each 0.1θp θrow low high θp/ 2 row spacing gives uniform sensitivity Joint deconvolution allows stacking of multiple passes Data rate limit is important constraint Data rate limit = 25 MB/s This defines min tdump = 0.45 s (nspw/16) This defines max slew rate = /s (16/nspw) This defines max survey depth = 122 µjy

11 11 OTFM Spectra and Polarimetry Octave spectral coverage (2-4 GHz) Effective time on sky grows as ν -2 Stripe spacing conservative No primary beam nulls! Polarimetric calibration On-axis polcal is easy (as done in prior work) Formally, beam leakage must be applied per integration Leakage measurements underway and test plan will study polarimetric bias

12 March VLASS Review Stripe82-S: OTFM Case Study Single epoch rms 92µJy over 270 deg2! Beats All-Sky spec in exact All-Sky mode Undersampled: 0.5s dumps, (~0.3 θp)

13 13 13 Stripe82-S: Pipeline Case Study Mooley et al used AIPS-LITE + CASA Completes in 6 hours with σn~1.1x ideal Pipeline stability vs sensitivity Flagged 5/16 spw by default Gain compression from RFI

14 14 All-Sky Calibration First need gain calibrator density of ~0.1/deg 2 Density up to 0.5/deg 2 from CLASS and VLBA Observations needed to confirm and fill in gaps Self-calibration Run during imaging loop VLASS all-sky needs 3 mjy sources ~10/deg 2 => once per minute External constraints (e.g., optical positions) inferior to radio!

15 15 How do we get the science? Tier 2: Deep 10 sq deg A config S-band) 1.5 microjy/beam Requirements Extreme sensitivity Control of systematics

16 16 Deep Imaging Deep tier is not *that* novel Deeper JVLA imaging ongoing Simulations effective Imaging algorithms in active development Pipelining Deep is novel Flagging and calibration Time variable sources Computing resources Rao et al, in prep

17 17 Deep and Systematics Imaging algorithm is iterative Start with a priori knowledge (Tsys, primary beam shape, etc.) Iterate solution for source properties Numerical effects AW projection w/mfs JVLA has no primary beam nulls! Rao et al, in prep

18 18 COSMOS: Deep Case Study 12B-158 (Smolcic et al.) Data will become part of test plan Bandwidth smearing Noise properties

19 19 How do we get the science? 3) General Challenges Optimal image cube compression Source finding Pipeline on demand Commensal observing V-LITE millisecond transients