SCUBA-2: on-sky calibration using submillimetre standard sources Dempsey et al. 2013, MN, 430, 2534 Reported by Y. Tamura (IoA, UTokyo) 2-Oct-2014 1
1. Introduction 2 1. Opacity / PWV The opacity in these bands can vary rapidly and markedly as a function of precipitable water vapour (PWV), particularly at shorter wavelengths. Rapid measurements of the line-of-sight PWV are required in order to compensate adequately for this attenuation in the observation. 2. flux conversion factor (FCF) The second parameter required to calibrate submillimetre continuum data is a measure of the optical throughput of both the telescope and instrument itself. This value is referred to as the flux conversion factor (FCF) which converts the measured power in picowatts (pw) into astronomical fluxes in janskys (Jy). Flux ( ) focus (+pointing) 2
2. SCUBA-2 / 2.1 Flat-fielding Flat-field = Responsivity (load-curve measurement) = di/dp See Holland+13, 3.4.2 & 3.5.1 Flat-field: Power (pw) (A) FCF: Flux density (Jy) Power (pw) + Optical response (on I-V plane) operation Pincident = (thermal coupling factor) x Pheater coupl. factor di/dp On-sky SQUID setup Dark flat-field (w/ shutter closed) shutter opened heater track (low S/N) (~pw) ( ) flag (10% at 450um, 20% at 850um) (non-linear) 3
3. Submm atmospheric transmission ( ) ( 850, 450um) FTS Imeasured = I0 e τa (I0: unattenuated flux, τ: zenith opacity, A: airmass) τ / σ(τ) = 20% 50-80% flux error (Archibald+02) 4
3.1 The water vapour monitor CSO tipper JCMT WVM 225 GHz opacity ( ) τ(225)wvm = 0.04 PWVzen + 0.017 (PWV mm) CSO tipper JCMT WVM Freq 225 GHz 183 GHz Direction EL at fixed AZ line-of-sight JCMT CSO 300m Rate per 15 min per 1.2 sec 5
3.1 The water vapour monitor WVM at JCMT Wiedner+01 Wiedner et al. (2001) (CSO ) H2O line at 183 GHz DSB (3 IF) flo = 91.655 GHz SCUBA-2 0.2x0.1m ( 7 ~ 3 Φ) 1 km 2.1m 16.6m Hot (100 ), warm (30 ) sky 0.4s on sky, 0.2s on each load, 0.2s for slew http://research.uleth.ca/irma/ 6
3.2 Tau relations (zenith opacity) τλ τλ = a(pwv b) flux (6 mon, 2011/5-10) least-square fit tau_850 = 0.179 (PWV + 0.337) tau_450 = 1.014 (PWV + 0.142) tau = airmass A tau_225 tau_850 = 4.6 (tau_225 0.0043) tau_450 = 26.0 (tau_225 0.012) 7
4. SCUBA-2 beam shape (< 5%) coadd 2 ( GMB, GS) Gs MB AZ/EL ( ) 450um eta_a 850um 30% cyan: az blue: el red: model 8
4. SCUBA-2 beam shape Gtotal = α Gmb + β Gs V = pi/(4 ln(2)) * (α * θmb 2 + β * θs 2 ) HPBW = 7.9 (450um), 13.0 (850um) (Te = 3 db) HPBW = 7.4 (450um), 12.75 (850um) large-scale ring D = 250 (850um), 183 (450um) f f < 1% (850), < 3% (450um) close to 1st peak of airy pattern of uniform illum main beam efficiency 9
5. Flux calibration FCF (pw -> Jy) FCF optical path (dish, beam, filter) Calibration sources ( 5.1, Table 2) Mars (Wright_76), Uranus (Moreno+10) -> 5% absolute err. CRL 618, CRL 2688 Observation method and data reduction ( 5.2) Chapin+13 bright source configuration makemap on SMURF 2011-2012 500 10
5. Flux calibration Aperture photometry ( 5.3) D < 60 w/ sky annulus D=90-120 Flux 60 450 4% 850 8% underestimate (Table 3) FCFarcsec = S / (I0 A) Jy pw 1 arcsec 2 [S: known flux density (Jy), I0: measured signal (pw), A: pixel area (arcsec 2 )] FCFpeak = S / I0 Jy/beam pw 1 11
5. Flux calibration, 5.4 Flux conversion factor FCF_arcsec ( ): FCF[850]arcsec = 2.34 ± 0.08 Jy pw 1 arcsec 2 FCF[450]arcsec = 4.71 ± 0.5 Jy pw 1 arcsec 2 FCF_peak (, ) FCF[850]peak = 537 ± 26 Jy pw 1 absolute acc. < 5% FCF[450]peak = 491 ± 67 Jy pw 1 absolute acc. < 10% Absolute accuracy sqrt(meas 2 +model 2 ) = 8% (850um), 12% (450um) FCFarcsec/FCFpeak (beam/arcsec 2 ): error beam ( 4 ) FCF_arcsec FCF_peak 12
6. Calibration source fluxes Integrated flux Peak flux secondary calibrator flux (Table 4) ( see text) CRL 618 (850um) (450um) 13