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Data Flow System

IR Camera

Calibrating VISTA Data
Astronomy Unit Queen Mary University of London

Jim Emerson Simon Hodgkin, Peter Bunclark, Mike Irwin, Jim Lewis

Cambridge Astronomical Survey Unit, Institute of Astronomy, Cambridge

The United Kingdom Astronomy Technology Centre on the site of the Royal Observatory Edinburgh


VISTA
Data Flow System

· 4-m Survey Telescope · 1.65 deg diameter FOV · Near-IR Camera
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 2 of 21


Telescope
Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 3 of 21


only one moving part, the filter wheel Z,Y,J,H,Ks (0.9-2.5micron)

Camera

Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 4 of 21


Focal Plane
Data Flow System

·16 Raytheon VIRGO 2k x 2k · 4 x 4 sparse array · spacings 90% & 42% of detector · 0.34"pixels

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 5 of 21


Focal Plane
1.65 deg diagonal
Data Flow System

0.6 sq deg detector `pawprint'
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 6 of 21


Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 7 of 21


Types of Calibration
Data Flow System

calibrations characterize: 1. Transfer function (image in, DN out) of end-to-end system so that instrumental effects can be removed from the data.
· VISTA has a wide field of view, so particular attention must be paid to variations across the field ­ illumination, etc etc

2. astrometric distortions of the images 3. photometric zero points and extinction coefficients 4. generate Quality-Control measures (see Riello's talk).

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 8 of 21


Calibration Pipeline
Data Flow System

· Removes instrumental artefacts · Combine pawprints component exposures offset by small jitters · Calibrates each pawprint photometrically and astrometrically · Provides Quality Control measures · See Jim Lewis's talk

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 9 of 21


VISTA/WFCAM Similarity
Data Flow System

VISTA & WFCAM have similar data
WFCAM Telescope 2x2k Detectors Pixel size 4-m (UKIRT) 4 x Hawaii 0.4 arcsec VISTA 4-m 16 x VIRGO 0.34 arcsec

·How to mitigate risks in properly handling VISTA data (and archive volumes)? ·data flow system (developed in UK) designed to first handle already available WFCAM data ·Have learnt from this experience
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 10 of 21


Cross-Talk
Data Flow System

·WFCAM has Cross talk from saturated images =>`bumps' symmetrically above and below brightest stars. · WFCAM mostly (but not perfectly) correctable. ·VISTA???
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 11 of 21


Persistence
Data Flow System

· On a sequence of (monthly) dates choose a fairly empty field with a nearly saturated star. · Take an exposure and then a sequence of dark frames to measure the characteristic decay time.

WFCAM hard to correct. VISTA???

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 12 of 21


Dome `flat' screen
Data Flow System

· Not used for flatfielding · For Monitoring
­ instrument performance ­ image structure ­ confidence maps

· Linearisation: Take series of differently timed dome screen observations under constant illumination. · with pixel timing => true linear value for each pixel & badpixel maps for each detector
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 13 of 21


Twilight (flat) fields
Data Flow System

· Used to remove multiplicative instrumental signatures
­ ­ ­ ­ pixel-to-pixel gain variations instrumental vignetting profile. gain correction between the 16 detectors gain correction between the 16 read out channels of each detector

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 14 of 21


Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 15 of 21


Illumination Correction
Data Flow System

· Flat-field should remove all pixel-to-pixel gain differences as well as any large-scale variations due (generally) to vignetting within the focal plane. · BUT any scattered light within the camera may lead to large scale background variations which cannot easily be modeled and removed, as level depends critically on the ambient flux. · The illumination correction can be measured in three ways.
1. secondary photometric standard fields (100-200 objects/detector) & look at the variation of zero-point across each detector. => map of spatial systematics across each detector 2. `mesostep' sequence of exposures of a sparse field of relatively bright stars on a regular grid of offsets to completely sample across the face of the detectors in medium-sized steps => monitor residual systematics in photometry 3. Stacked zero point differences from 2MASS objects in each pawprint
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 16 of 21


Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 17 of 21


Calibration: Astrometric
Data Flow System

· based on 2MASS point source catalog
­ globally to ~100mas ­ internally to <~50mas.

· 2MASS tied to TYCHO 2 and in ICRS system. · 2MASS errors systematic <100 mas, RMS <~100 mas for S/N >10 point sources · distortion terms from on-sky observations. · strongest term in optical-distortion model is cubic radial term.

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 18 of 21


Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 19 of 21


Data Flow System

radial scale distortion also has an impact on photometric measurements, inducing an error up to 3.5% in the corners of the field, compared to the centre, if uncorrected.

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 20 of 21


Calibration: Photometric-0
Data Flow System

· ·

Goal 2% calibration accuracy Two independent methods:
i. from 2MASS all-sky point source catalogue. ii. from routine observations of standard star fields

· · · ·

Zeropoints derived for each image allows monitoring of effective Zero Points at ~few % level. Subsequent inter-detector comparisons enable residual errors in the gain correction to be detected and calibrated. Offline analysis => measure of median zeropoint for the night, associated error (and scatter), indicative of photometric quality
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 21 of 21


Calibration: Photometry-1
Data Flow System

· Photometry-1: based on 2MASS · Initial photometric calibration for all filters based on 2MASS photometric system which is globally consistent to ~1% (Nikolaev et al. 2000). · colour equations to convert 2MASS to VISTA instrumental system (with some colour s/n cuts) · enables each detector image to be calibrated directly from 2MASS stars that fall within field of view. · Analysis of WFCAM data wrt UKIRT standards => 2MASS calibration delivers product frame-by-frame photometric zero-points at the +/-2% level (with factored-in extinction tracking).
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 22 of 21


Calibration: Photometry-2
Data Flow System

· Photometry-2: based on a network of standard star fields
­ Network of Secondary Standard photometric fields, every 2 hours in RA will be set up with VISTA -2MASS `Touchstone' fields and or UKIRT faint standard fields ­ ~100 stars/detector J<18, s<16 to avoid long exposures will characterize systematic position dependent photometric effects ­ encompass broad spread in colour to derive colour terms robustly

· observe every two hours elapsed time throughout each night · enables an independent calibration to be made on a nightly basis. · Touchstone fields provide information on the stability · used to measure illumination correction.
VISTA Calibration Jim Emerson, Astronomy Unit, Queen Mary University of London Cal 07@ESO Slide 23 of 21


Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 24 of 21


Extinction
Data Flow System

Extinction monitored · from zeropoints of the 2MASS stars in each pawprint · from zeropoints in individual Touchstone fields · through each (photometric) night assuming a fixed zero point and measuring Touchstone fields over a range of airmass.

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 25 of 21


The end
Data Flow System

VISTA Calibration

Jim Emerson, Astronomy Unit, Queen Mary University of London

Cal 07@ESO Slide 26 of 21