tips: stis report
DESCRIPTION
SPACE TELESCOPE SCIENCE INSTITUTE. TIPS: STIS Report. Operated for NASA by AURA. Paul Goudfrooij. Unusual Target ACQ Failures: Update & Resolution Calibration of CTE loss in Spectroscopic Modes - PowerPoint PPT PresentationTRANSCRIPT
SPACETELESCOPESCIENCEINSTITUTE
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
• Full story available under http://www.stsci.edu/hst/stis/training/team/activities/lectures.html
• Also STIS ISR 2003-03
3. New “Pseudo-Apertures” (if time available)
2May 15, 2003TIPS Presentation Paul Goudfrooij
Recent Target ACQ Failures(with L. Dressel, R. Pitts, T. Wheeler)
• Two recent ACQs failed (March 2, April 6) due to No Flux in the Lamp ImageNo Flux in the Lamp Image
• 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.
1 23
1 23
3May 15, 2003TIPS Presentation Paul Goudfrooij
Correcting CCD Spectroscopy for CTE Loss (with R. Bohlin)
• 4 Readout Amps (1 / corner)
• Bi-directional Clocking yields CTI 1 – CTE:
CTI = (fluxD / fluxB)Y
1 2
STIS CCD:
Measured using “Sparse Field Tests” Serial
overscanAxis1 (X)
Axi
s2 (
Y)
Parallel (virtual) overscan
Serial overscan
Amp A
Amp C
Amp D
Amp B
Nom
inal C
locking Direction
Nominal Readout Direction
Sensitive Region (1024x1024 pix)
4May 15, 2003TIPS Presentation Paul Goudfrooij
“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 beshould be no background to fill traps)
5May 15, 2003TIPS Presentation Paul Goudfrooij
“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
6May 15, 2003TIPS Presentation Paul Goudfrooij
CTI Parametrization:Imaging vs. Spectroscopy
• Dependence on signal & background levels to be done separately for imaging and spectroscopy
Spectroscopy
Imaging
CCD Row NumberCCD Column Number
7May 15, 2003TIPS Presentation Paul Goudfrooij
External Sparse Field Test: Imaging CTI Analysis
• Slope systematically flatter with increasing flux
• “Sky” presumably fills traps in bottoms
of potential wells, mostly affecting transfer of small charge packets.
• Suggests CTI
Clear dependence on background level (“sky”)
bcksignal
exp –
8May 15, 2003TIPS Presentation Paul Goudfrooij
The Strong Effect of Background: Gain=1 vs. Gain=4
• Background level in spectroscopy mode typically low, dominated by dark current– Also need to account for spurious charge of the STIS CCD
flush
CCD ReadoutCCD
9May 15, 2003TIPS Presentation Paul Goudfrooij
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
10May 15, 2003TIPS Presentation Paul Goudfrooij
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
CTI = CTI0 + { 1 + 0.243 [± 0.016] (t – t0) }
(with t in yr)
CTI data points from Tom Brown
60 e
–
120
180
500
3400
11May 15, 2003TIPS Presentation Paul Goudfrooij
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 (1 + 0.243 yr)
bgGROSS
0.17
) (• Implementation into the pipeline:
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.
12May 15, 2003TIPS Presentation Paul Goudfrooij
Quality of CTI fit
CTI Correction good to CTI Correction good to 7% 7% Spectrophotometry good to Spectrophotometry good to 1% 1%
13May 15, 2003TIPS Presentation Paul Goudfrooij
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• Provide POS TARGs to GOs in Phase-II Update;
Apertures to be implemented in next APT build.
nominal
new