SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
CTE of STIS CCD: External Tests
Paul Goudfrooij
STIS team @ STScI
• Overview of External CTE Tests for STIS CCD
• Inter-Comparison of Results of Different External CTE Tests
• Linking Externally Measured CTE to CR Tail Intensity
• Suggestions For Future Monitoring of CTE of STIS CCD
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
Overview of External Tests on CTE of STIS CCD
(Will mostly consider CTI = 1 − CTE)
1. (Apparent) “Non-Linearity Method”, most apparent for faint sources. First
results on M 67: Gilliland, Goudfrooij & Kimble (1999, PASP, 111, 1009).
Parametrized in terms of Stetson’s (1998, PASP, 110, 1448) phenomenological
model (used to correct WFPC2 data for CTI & “long vs. short” kind of loss):
1024 − Y
1024
sky = log10 (BKG) − 1
∆Y =
obj = log10 (CTS) − 4
Y−CTE = c1 + c2 × obj + c3 × sky
C = 0.01 × (c4 + ∆Y ) × exp (Y−CTE)
CTSCORR = (1 + C) × CTS
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
Figure 1: Best-fitting correction factors (parameter C of preceding set of equations) as a function of the
extracted spectrum count level (per 7-pixel extraction box, in e−) for three different y positions on the STIS
CCD. Overplotted symbols denote the observations; the solid lines are the best ‘Stetson-model’ fits. The largest fractional deviation from the fit is 2.5σ. Figure taken from Gilliland, Goudfrooij, & Kimble (1999).
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
2. “Bi-directional Clocking Method”, using observations of sparse fields
– Slope of AmpD/AmpB intensity ratio vs. AXIS2 yields parallel CTI. An
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
example is shown in the figure below, for two different intensity ranges.
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
Inter−Comparisons of Results of External CTE Tests
• “Non-Linearity Method” (NLM) so far applied to:
– Ron Gilliland’s deep imaging (gain = 4) GO data on 47 Tuc: Jul 4−11, 1999
– Slitless spectra (gain = 1) on NGC 346 (in nebulosity): Sep 2, 1999
– Slitless spectra (gain = 1) on M 67 (exact repeat of 1997 data): Nov 6, 1999
– Imaging (gain = 1) of NGC 6752 (in CVZ): Sep 15, 1999
• “Bi-directional Clocking Method” (BCM) so far applied to:
– Imaging (gain = 1) of ω Cen (2 different exposure times): Jun 4, 1999
– Imaging (gain = 1) of NGC 6752 (in CVZ), same as above; Sep 15, 1999
• Results not always obviously consistent with one another ... cf. Figure on next
page which shows the CTI vs. object intensity for the different datasets. The
symbols depict BCM measurements, while the solid lines depict the best-fit
NLM fits to the datasets, shown in the legend.
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
7
SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
• Best-fit NLM coefficients (e.g., the position-independent c4) rather jump
around across the fits to the different datasets. There is no clear evolution in
the coefficients, cf. the table below.
Observing
date
c1
c4
c2 (object term)
c3 (sky term)
(Y-independent term)
−0.798 +/− 0.344
−1.758 +/− 0.122
−
−
Jul 7, 1999
(47 Tuc)
1.019 +/− 0.110
−1.045 +/− 0.078
−
0.461 +/− 0.071
Sep 2, 1999
(NGC 346)
1.040 +/− 0.352
−0.680 +/− 0.125
−0.934 +/− 0.320
−0.145 +/− 0.081
Sep 15, 1999
(NGC 6752)
0.071 +/− 0.138
−1.685 +/− 0.053
−0.305 +/− 0.152
−0.289 +/− 0.011
Nov 6, 1999
(M 67)
−1.092 +/− 0.175
−2.093 +/− 0.060
−
Dec 27, 1997
(M 67)
(basic Y dependence)
−
– Crucial to cover large range in intensity (and sky) for trustworthy functionality. Gain = 1 saturates at ~30,000 e− -> use gain = 4 for NLM.
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
Externally Measured CTE vs. CR Tail Intensity
• STIS CTI-induced CR charge tails: Strong, Linear Growth with Time:
:
Y−CTI = 2.64893 + 0.032219 ∗ (MJD − 50509.6)
• So far, external CTI tests do not show such a quick growth with time for 3pixel radius aperture photometry (but: limited time coverage yet), cf. figure.
• Using CTI values from pre-flight internal sparse field test & Fe55 test (cf.
Kimble’s talk) as reference CTI value at on-orbit time = 0:
Source
Intensity
Functional dependence of CTI on time (∗ 10−5)
~ 100
∼1.5 + (3.5 +/− 2.9) ∗ (MJD − 50509.6)
~ 1000
(1.5 +/− 0.1) + (1.8 +/− 2.8) ∗ (MJD − 50509.6)
~ 10,000
(0.9 +/− 0.1) + (1.2 +/− 0.4) ∗ (MJD − 50509.6)
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SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
10
SPACE TELESCOPE SCIENCE INSTITUTE
Jan 31 − Feb 1, 2000
CTE Workshop @ STScI
STIS CTE: External Tests
Paul Goudfrooij
11
Suggestions for Future CTE Monitoring for STIS CCD
• Obtain some more external data (BCM, NLM) to pin down “effective” slope
of CTI vs. time for different source intensities & sky backgrounds.
• Effect of source structure (point vs. extended source) to be explored − important for, e.g., estimating CTI effects on absorption line profiles in galaxy spectra (galaxy dynamics represent a major fraction of STIS CCD science).
• Use “internal” method(s) [CR tails, Internal Sparse Field Test, EPER, perhaps
FPR (?)] for baseline CTI monitoring.