SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Point-Source CCD Photometry with STIS:
Correcting for CTE loss
Paul Goudfrooij
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
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. 532.
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
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STIS CCD Architecture;
Measurement Method
Amp
C
STIS CCD:
Nominal Readout Direction
Amp
D
• 4 Readout Amps (1 / corner)
Nominal Clocking Direction
• Bi-directional Clocking yields
CTI ≡ 1 – CTE:
1 d(fluxD / fluxB)
CTI =
dY
2
Measured using
“Sparse Field Tests”
STScI Calibration Workshop 2002
Axis2 (Y)
• Nominal Amp: D (lowest RN)
Parallel (virtual) overscan
Amp
A
Serial
overscan
Oct 17-18, 2002
Axis1 (X)
Serial
overscan
Paul Goudfrooij
Amp
B
3
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)
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
4
Sparse Field Tests
•
(ii) “External” sparse field test (annually)
– A. Imaging:
ÿ
ÿ
ÿ
–
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
ÿ CVZ target
ÿ Slitless; 3 exp. times; G430L
ÿ [O II] l3727, Hb, [O III] l5007 lines in
nebulosity provide three convenient,
~constant “sky” levels per spectrum
(Spectroscopy discussed by Ralph Bohlin)
ÿ
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
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Visual effects of CTE loss:
Dependence on Y position
“External” Sparse Field Test: Outer Field in NGC 6752 (Oct 2001)
Exp. time = 20 s
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
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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
STScI Calibration Workshop 2002
Exp. time = 100 s
Oct 17-18, 2002
Paul Goudfrooij
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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.
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
8
External Sparse Field Test:
CTI Analysis
3.0 e–
d fluxD
fluxB
CTI =
2 dY
Obvious
dependence
on flux
STScI Calibration Workshop 2002
3.0 e–
Oct 17-18, 2002
Paul Goudfrooij
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External Sparse Field Test:
CTI Analysis
No significant
dependence on
aperture size
fi
Can use small-aperture
photometry without CTIrelated headache
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
10
External Sparse Field Test:
CTI Analysis
Clear dependence on background level (“sky”)
• 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 a
µ exp –
counts
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
11
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) / 365.25
lcts = ln(counts) – 8.5
bg = sqrt(sky_)
lbg = ln(sqrt(sky_+1)) – 2
•
Functional form producing best fit to the data:
•
Similar (but not identical) to Dolphin’s (2000, PASP, 112, 1397)
functional form to correct WFPC2 CTE
bg h
CTI = (1 + a yr) * b exp(-c lcts) * [d exp (-e lbg) + f exp (-g
)]
counts
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
12
CTI Correction Formula
bg h
CTI = (1 + a yr) * b exp(-c lcts) * [d exp (-e lbg) + f exp (-g
)]
counts
a = 0.108 ± 0.034
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
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
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Quality of CTI fits
CTI Correction good to £ 7% fi Photometry good to £ 1%.
Cycle 8
Cycle 9
Cycle 10
50%
20
10
5
2
1%
Black: sky = 2
Red: sky = 4
STScI Calibration Workshop 2002
Blue: sky = 15
Oct 17-18, 2002
Magenta: sky = 40
Paul Goudfrooij
14
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.
STScI Calibration Workshop 2002
Oct 17-18, 2002
Paul Goudfrooij
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