Point-Source CCD Photometry with STIS: Correcting for CTE loss

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1 SPACE TELESCOPE SCIENCE INSTITUTE Operated for NASA by AURA Point-Source CCD Photometry with STIS: Correcting for CTE loss Space Telescope Science Institute Methods to measure CTE of STIS CCD (Visual) effects of CTE loss CTE correction formula for point source photometry Plans for the (near) future

2 A Brief History of STIS CTE Measurements CTE: Fraction of charge transferred per pixel transfer (which is < 1 due to traps in the CCD silicon) Typically, one measures CTI 1 CTE. Significant parallel CTI discovered on STIS in 1998 (Gilliland, Goudfrooij & Kimble,1999, PASP, 111, 1009). Apparent non-linearity, most significant for faint sources. Parametrized in terms of Stetson s (1998, PASP, 110, 1448) phenomenological model. Review of various effects of radiation damage on the STIS CCD including CTI: Kimble, Goudfrooij & Gilliland, 2000, Proc. SPIE, vol. 4013, p

3 STIS CCD Architecture; Measurement Method STIS CCD: Amp C Nominal Readout Direction Amp D 4 Readout Amps (1 / corner) Nominal Amp: D (lowest RN) Bi-directional Clocking yields CTI 1 CTE: CTI = 1 2 d(flux D / flux B ) dy Axis2 (Y) Nominal Clocking Direction Measured using Sparse Field Tests Amp A Serial overscan Parallel (virtual) overscan Axis1 (X) Serial overscan Amp B 3

4 Sparse Field Tests Sparse fields to ensure that sources do not overlap, in which case (e.g.) PSF wings could fill traps for sources along the readout direction Two varieties: (i) Internal Sparse Field Test Lamp images along narrow, (cross-)dispersion slits, projected at 5 positions along columns (or rows) Representative of worst case point source spectroscopy (essentially no background to fill traps) 4

5 Sparse Field Tests (ii) External sparse field test (annually) A. Imaging: Sparse outer field in NGC 6752 CVZ target ( cheap orbits; yields range of backgrounds) 3 exposure times; 50CCD B. Spectroscopy: Young open cluster NGC 346, in nebulosity CVZ target Slitless; 3 exp. times; G430L [O II] l3727, Hb, [O III] l5007 lines in nebulosity provide three convenient, ~constant sky levels per spectrum (Spectroscopy discussed by Ralph Bohlin) 5

6 Visual effects of CTE loss: Dependence on Y position External Sparse Field Test: Outer Field in NGC 6752 (Oct 2001) Exp. time = 20 s 6

7 Visual effects of CTE loss: Effects of Intensity and Sky External Sparse Field Test: Outer Field in NGC 6752 (Oct 2001) Exp. time = 20 s Exp. time = 100 s 7

8 Visual effects of CTE loss: Centroid shifts Internal Sparse Field Test Charge trailing and centroid shift measurable; most significant at low signal. Impacts shape measurements (surface photometry), especially for faint objects. 8

9 External Sparse Field Test: CTI Analysis 3.0 e CTI = d flux D flux B 2 dy Obvious dependence on flux 3.0 e 9

10 External Sparse Field Test: CTI Analysis No significant dependence on aperture size fi Can use small-aperture photometry without CTIrelated headache 10

11 Slope systematically flatter with increasing flux Sky presumably fills traps in bottoms of potential wells, mostly affecting transfer of small charge packets. Suggests CTI bckgr µ exp counts External Sparse Field Test: CTI Analysis Clear dependence on background level ( sky ) a 11

12 Preparing for CTE Correction Formula (for imaging) Currently three epochs analyzed (Oct 1999, 2000, 2001) Define count, background (sky) and epoch parameters relative to their (rough) averages: yr = (MJD 51831) / lcts = ln(counts) 8.5 bg = sqrt(sky_) lbg = ln(sqrt(sky_+1)) 2 Functional form producing best fit to the data: bg counts CTI = (1 + a yr) * b exp(-c lcts) * [d exp (-e lbg) + f exp (-g )] Similar (but not identical) to Dolphin s (2000, PASP, 112, 1397) functional form to correct WFPC2 CTE h 12

13 CTI Correction Formula bg counts CTI = (1 + a yr) * b exp(-c lcts) * [d exp (-e lbg) + f exp (-g )] a = ± b = (9.32 ± 0.09) 10 5 c = 0.37 ± 0.01 d = 0.23 ± 0.02 e = 0.60 ± 0.05 f = 0.48 ± 0.01 g = 1.80 ± 0.10 h = 0.40 ± 0.04 h 13

14 Quality of CTI fits CTI Correction good to 7% fi Photometry good to 1%. Cycle 8 Cycle 9 Cycle 10 50% % Black: sky = 2 Red: sky = 4 Blue: sky = 15 Magenta: sky = 40 14

15 Future Plans / Enhancements Analysis of Oct 2002 sparse field data to improve/solidify CTI correction formula (Dec 2002) Finalize correction formula for spectroscopic data Write IRAF (post-observation) tool to correct for CTI Analysis of extended source CTI measurements (Spectroscopy & imaging; Spring 2003) After 2003, internal measurements (internal sparse field, Cosmic Ray Tail intensity trending) should be sufficient to monitor CTI evolution See poster of Paul Bristow: Analytical modelling of STIS CTE as part of ST-ECF Calibration Enhancement project. When fully successful, it would enable CTE correction within the pipeline. 15