TIPS: STIS Report

1. 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 • Full story available under http://www.stsci.edu/hst/stis/training/team/activities/lectures.html • Also STIS ISR 2003-03 • New “Pseudo-Apertures” (if time available)

2. 1 2 1 2 3 3   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 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.

3. Amp D Nominal Readout Direction Axis2 (Y) Nominal Clocking Direction 1 2 (fluxD / fluxB) Y CTI = Parallel (virtual) overscan Amp C Amp A Amp B Axis1 (X) Serial overscan Serial overscan Correcting CCD Spectroscopy for CTE Loss (with R. Bohlin) STIS CCD: • 4 Readout Amps (1 / corner) • Bi-directional Clocking yields CTI  1 – CTE: Sensitive Region (1024x1024 pix) Measured using “Sparse Field Tests”

4. (i) “Internal” Sparse Field Test “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: • 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)

5. “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

6. Spectroscopy Imaging CCD Column Number CCD Row Number CTI Parametrization:Imaging vs. Spectroscopy • Dependence on signal & background levels to be done separately for imaging and spectroscopy

7.  bck signal External Sparse Field Test: Imaging CTIAnalysis 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  exp –

8. 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 CCD Readout flush CCD

9. 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

10. 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 points from Tom Brown 120 180 500 3400

11. 0.17 bg GROSS 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) • 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.

12. Quality of CTI fit CTI Correction good to 7%  Spectrophotometry good to 1%

13. 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