SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
TIPS: STIS Report
Paul Goudfrooij
1. Unusual Target ACQ Failures: Update & Resolution
2. Calibration of CTE loss in Spectroscopic Modes
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Full story available under
http://www.stsci.edu/hst/stis/training/team/activities/lectures.html
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Also STIS ISR 2003-03
3. New “Pseudo-Apertures” (if time available)
Recent Target ACQ Failures
(with L. Dressel, R. Pitts, T. Wheeler)
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Two recent ACQs failed (March 2, April 6) due to No Flux in the Lamp Image
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All mechanisms show nominal telemetry
ACQ macro used 3.8 mA setting for HITM1 lamp, much below ‘default’ 10 mA
3.8 mA setting originally put in place to allow wavecals for the most
sensitive MAMA settings, and to save lamp life time
Contacted manufacturer + their consultant
– Sputtered material forms a ring inside glass envelope around cathode. If
set at (too) low current, electrons may flow to sputtered ring rather than to
cathode
Conclusion: Lamp did not fire
Since 5/12/03, ACQs use 10 mA setting. All ACQs taken so far are OK.
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TIPS Presentation
May 15, 2003
Paul Goudfrooij
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STIS CCD:
Amp
C
Nominal Readout Direction
Axis2 (Y)
Correcting CCD Spectroscopy
for CTE Loss (with R. Bohlin)
Sensitive Region
(1024x1024 pix)
Amp
D
• 4 Readout Amps (1 / corner)
• Bi-directional Clocking yields
CTI  1 – CTE:
1 (fluxD / fluxB)
CTI =
Y
2
Measured using
“Sparse Field Tests”
TIPS Presentation
Parallel (virtual) overscan
Amp
A
Serial
overscan
May 15, 2003
Axis1 (X)
Serial
overscan
Paul Goudfrooij
Amp
B
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“Sparse Field” Tests
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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
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Annual series of lamp images through narrow slits,
projected at 5 positions along columns (or rows)
Designed to represent “worst–case” point source
spectroscopy (should be no background to fill traps)
TIPS Presentation
May 15, 2003
Paul Goudfrooij
4
“Sparse Field” Tests
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(ii) “External” sparse field test (annually)
– A. Imaging:
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Sparse outer field in NGC 6752
CVZ target (‘cheap’ observing time;
yields range of backgrounds)
3 exposure times; 50CCD mode
B. Spectroscopy:
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Young open cluster NGC 346, in
nebulosity
CVZ target
Slitless; 3 exp. times; G430L
[O II] 3727, H, [O III] 5007 lines in
nebulosity provide three convenient,
~constant “sky” levels per spectrum
TIPS Presentation
May 15, 2003
Paul Goudfrooij
5
CTI Parametrization:
Imaging vs. Spectroscopy
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Dependence on signal & background levels to be done
separately for imaging and spectroscopy
Spectroscopy
Imaging
CCD Column Number
TIPS Presentation
CCD Row Number
May 15, 2003
Paul Goudfrooij
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External Sparse Field Test:
Imaging 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
bck 
 exp – signal
TIPS Presentation
May 15, 2003
Paul Goudfrooij
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The Strong Effect of Background:
Gain=1 vs. Gain=4
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Background level in spectroscopy mode typically low,
dominated by dark current
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Also need to account for spurious charge of the STIS CCD
TIPS Presentation
May 15, 2003
Paul Goudfrooij
8
Functional Dependence on Signal
and Background Levels
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Iterative Process for Spectroscopy
– Parameter space covered by ESF test at a given epoch is limited
– Sensitivity monitor: good coverage of signal levels, but not of sky
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G230LB data allow suitable cross-comparison with MAMA G230L
AGK+81D266,
G230LB
TIPS Presentation
May 15, 2003
Paul Goudfrooij
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Time Constant of CTI Evolution
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Need several datasets, each with same signal & background level
Need datasets covering long baseline in time  ISF data
– Have to correct for signal & background dependence prior to fitting
CTI = CTI0 + { 1 + 0.243 [± 0.016] (t – t0) }
(with t in yr)
CTI data points from Tom Brown
TIPS Presentation
May 15, 2003
Paul Goudfrooij
10
Final CTI Correction Formula
(For Point-Source Spectroscopy)
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Define background (sky) and epoch parameters:
yr = (MJD – 51765.25) / 365.25
(i.e., relative to 2000.6)
bg = max(BACKGROUND,0) + 0.5 for CCD Gain = 1
+ 5.0 for CCD Gain = 4
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Functional form producing best fit to the data:
CTI = 0.0467 GROSS – 0.720  exp –3.85
(
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bg
GROSS
0.17
) (1 + 0.243 yr)
Implementation into the pipeline:
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Formula parameters into CCD table reference file (new columns)
1-D extraction step (x1d) will correct for CTI by default for CCD data
(CTE correction step switchable)
• For Cycle 12 Phase II, provided downloadable IRAF script to
calculate correction factor for a given net & background level.
TIPS Presentation
May 15, 2003
Paul Goudfrooij
11
Quality of CTI fit
CTI Correction good to  7%  Spectrophotometry good to  1%
TIPS Presentation
May 15, 2003
Paul Goudfrooij
12
New “Pseudo-Apertures”
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FUV-MAMA first-order spectroscopy at detector
location with low dark
– ~ 2’’ above bottom of detector
– Reduction of dark current by factor of 5
– 52x0.05D1, …, F25QTZD1
Improvement of Fringe Flats at E1 positions
– Important to align fringes in flat with those in
target spectrum
– 52x0.1 slit (best for defringing) location is offset
in dispersion direction from wider slits
– New ‘E2’ positions will place target slightly offcenter in slits  0.2 arcsec wide
New WEDGEA0.6 position for 50CORON
Provide POS TARGs to GOs in Phase-II Update;
Apertures to be implemented in next APT build.
TIPS Presentation
May 15, 2003
nominal
new
Paul Goudfrooij
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