space telescope science institute operated for nasa by aura stis status report paul goudfrooij group...

SPACETELESCOPESCIENCEINSTITUTE

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

2March 20, 2003TIPS Presentation Paul Goudfrooij

Group Reorganization

• Two IS’es on well-deserved sabbatical: – Nolan Walborn (Sep 02) and Kailash Sahu (Mar 03)

• Jeff Valenti reassigned to JWST / NIRspec (Jan 03)– But will finish a few STIS assignments

• Scott Friedman (COS) helping out with a few tasks– 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)


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


Unusual Target ACQ Failure

• Target ACQ of Spectroscopic Sensitivity Monitor Visit on March 6 failed: No Flux in the Lamp ImageNo Flux in the Lamp Image

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

1 23

1 23


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.

nominal

new


Correcting CCD Spectroscopy for CTE Loss

• 4 Readout Amps (1 / corner)

• Nominal Amp: D (lowest RN)

• 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 Clocking D

irection

Nominal Readout Direction


“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)


“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


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 –


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

flush

CCD ReadoutCCD


Functional Dependence on Signal and Background Levels

• To be done separately for imaging and spectroscopy

• Spectroscopy: – ISF; ESF in slitless mode– CCD Sensitivity monitor data

• Imaging (cf. Cal. Workshop ‘02):– ESF & Full-field sens. monitor

Spectroscopy

Imaging

CCD Row NumberCCD Column Number


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


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 from Tom Brown

60 e–

120

180

500

3400


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) 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.


Quality of CTI fit

CTI Correction good to CTI Correction good to 7% 7% Spectrophotometry good to Spectrophotometry good to 1% 1%

@ 2000.6


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

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