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