SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
STIS Status Report
Paul Goudfrooij
• Group News / Reorganization
• Unusual Target ACQ Failure
• Items for upcoming Phase II Update
– Planned New Apertures
– Corrections for CTE loss
Group Reorganization
•
Two IS’es on well-deserved sabbatical:
–
•
Jeff Valenti reassigned to JWST / NIRspec (Jan 03)
–
•
But will finish a few STIS assignments
Scott Friedman (COS) helping out with a few tasks
–
–
–
•
Nolan Walborn (Sep 02) and Kailash Sahu (Mar 03)
CCD Dispersion Solutions
CCD & MAMA Spectroscopic Sensitivity Monitors
SM4 Backup to Tom Brown
Claus Leitherer (COS) to help out with
–
Phase-2 reviews of Cycle 12 MAMA programs (if more than
we can reasonably handle in Spectrographs Branch)
TIPS Presentation
March 20, 2003
Paul Goudfrooij
2
Group Reorganization
•
“New” ESA Instrument Scientist: Jesús Maíz-Appelániz
–
–
•
•
•
User Support Lead IS
ETC & APT oversight
New Pipeline Lead IS: Charles Proffitt
New Calibration Lead IS: Linda Dressel
New Information Lead IS: Bahram Mobasher
TIPS Presentation
March 20, 2003
Paul Goudfrooij
3
Unusual Target ACQ Failure
•
Target ACQ of Spectroscopic Sensitivity Monitor Visit on March 6
failed: No Flux in the Lamp Image
1
2
1
2
3
3

•
•
•
•
•
•

 direct measurement to measure current across lamp
– Hint of lower OCLP30VC, but not statistically significant
Slit wheel / MSM resolver counts identical to other ACQs
Shutter closed-open-closed sequence occurred nominally
All other mechanisms in their nominal positions
Preliminary conclusion: Lamp did not fire (HSTAR closed)
ACQs after the problematic one were all OK to date
TIPS Presentation
March 20, 2003
Paul Goudfrooij
4
New “Pseudo-Apertures”
•
•
•
•
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
off-center in slits  0.2 arcsec wide
New WEDGEA0.6 position for 50CORON
Will provide POS TARGs to GOs for cycle 12;
Apertures to be implemented in next APT build.
TIPS Presentation
March 20, 2003
nominal
new
Paul Goudfrooij
5
Correcting CCD Spectroscopy
for CTE Loss
Amp
C
STIS CCD:
Nominal Readout Direction
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
Axis2 (Y)
• Nominal Amp: D (lowest RN)
Parallel (virtual) overscan
Amp
A
Serial
overscan
March 20, 2003
Axis1 (X)
Serial
overscan
Paul Goudfrooij
Amp
B
6
“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
–
–
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
March 20, 2003
Paul Goudfrooij
7
“Sparse Field” Tests
•
(ii) “External” sparse field test (annually)
– A. Imaging:



–
Sparse outer field in NGC 6752
CVZ target (‘cheap’ observing time;
yields range of backgrounds)
3 exposure times; 50CCD mode
B. Spectroscopy:




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
March 20, 2003
Paul Goudfrooij
8
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
March 20, 2003
Paul Goudfrooij
9
The Strong Effect of Background:
Gain=1 vs. Gain=4
•
Background level in spectroscopy mode typically low,
dominated by dark current
–
Need to account for spurious charge of the STIS CCD
TIPS Presentation
March 20, 2003
Paul Goudfrooij
10
Functional Dependence on Signal
and Background Levels
•
To be done separately for imaging and spectroscopy
Spectroscopy
Imaging
CCD Column Number
•
•
CCD Row Number
Spectroscopy:
– ISF; ESF in slitless mode
– CCD Sensitivity monitor data
Imaging (cf. Cal. Workshop ‘02):
– ESF & Full-field sens. monitor
TIPS Presentation
March 20, 2003
Paul Goudfrooij
11
Functional Dependence on Signal
and Background Levels
•
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

G230LB data allow suitable cross-comparison with MAMA G230L
AGK+81D266,
G230LB
TIPS Presentation
March 20, 2003
Paul Goudfrooij
12
Time Constant of CTI Evolution
•
•
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
60 e–
CTI = CTI0 + { 1 + 0.243 [± 0.016] (t – t0) }
(with t in yr)
CTI data from Tom Brown
TIPS Presentation
March 20, 2003
Paul Goudfrooij
13
Final CTI Correction Formula
(For Point-Source Spectroscopy)
•
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
•
Functional form producing best fit to the data:
CTI = 0.0467 GROSS – 0.720  exp –3.85
(
•
bg
GROSS
0.17
) (1 + 0.243 yr)
Implementation into the pipeline:


Formula parameters into CCD table reference file (new columns)
1-D extraction step (x1d) to correct for CTE by default for CCD data
(CTE correction step switchable)
• For Cycle 12 Phase II, provide downloadable IRAF script to
calculate correction factor for a given net & background level.
TIPS Presentation
March 20, 2003
Paul Goudfrooij
14
Quality of CTI fit
CTI Correction good to  7%  Spectrophotometry good to  1%
@ 2000.6
TIPS Presentation
March 20, 2003
Paul Goudfrooij
15
The Strong Effect of Background:
Gain=1 vs. Gain=4
Complex behavior at low signal levels
–
–
–
CTE-like behavior obvious, but details not quite understood
Notice somewhat different behavior for B vs. D amps
Renders low-signal CTI values somewhat uncertain
TIPS Presentation
March 20, 2003
Paul Goudfrooij
16