Background information for users of stis
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Background information for users of STIS. Charles R. Proffitt. Outline of Topics. More on Bright and Faint limits with STIS Calibration Lamps Wavelength Calibration Target Acquisitions CCD Operations and Characteristics MAMA Characteristics Time Resolved Observations Observing Overheads

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Background information for users of STIS

Charles R. Proffitt

Outline of Topics

  • More on Bright and Faint limits with STIS

  • Calibration Lamps

  • Wavelength Calibration

  • Target Acquisitions

  • CCD Operations and Characteristics

  • MAMA Characteristics

  • Time Resolved Observations

  • Observing Overheads

  • Summary of Data Products

  • Spatial Undersampling of STIS Data

  • A few selected data artifacts

Bright Object Limits

  • STIS IHB gives tables of worst-case limiting magnitude as a function of grating and source spectrum.

  • Normalization can vary enormously, depending on grating, aperture, source SED, reddening, etc.

  • Cool stars especially tricky

    • NUV flux very sensitive to all stellar parameters esp. metallicity

    • FUV flux often dominated by chromospheric emission

      • Not included in Kurucz or other photospheric models

      • Strongly affected by stellar activity

Epsilon Eri: Kurucz model vs. observed spectrum

STIS Spectroscopic BOP Limits

  • Limit for CENWAVE with highest countrate

  • Assumes slitless 1st order; 0.2X0.2 for echelles

Bright and Faint Limits - example

  • Example: bright and faint limits for an A0 star

    • Faint limit defined as S/N=10 in one hour

    • For CCD bright limit will saturate CCD in 0.1 s @gain=4

    • For MAMA bright limit determined by local or global BOP limits

Approximate Bright and Faint Limiting Mag for A0V star at a single wavelength using typical clear apertures(don’t take exact numbers too seriously)

Calibration Lamps

STIS Calibration Lamps

  • Cal Insert Platform

    • Flatfielding lamps

      • Tungsten (4 lamps)

      • Krypton (130 - 170 nm)

      • Deuterium (165 - 310 nm)

    • Echelle wavelength cal

      • PtCr/Ne (LINE)

  • Cal insert mechanism (CIM) blocks external light & acts as additional external shutter

  • Hole in the Mirror (HITM)

    • PtCr/Ne (HITM1/2)

      • 1st order wavecals

      • Locate aperture during target ACQ

Flat fielding Lamps

For small scale pixel-to-pixel flat fielding

  • Krypton for FUV

  • Deuterium for NUV

  • Tungsten for CCD

    • Also used for IR fringe flats for G750L & G750M at  > 7500 Å

LINE and HITM lamps spectra

  • Low dispersion STIS spectra of LINE and HITM1 lamps

Wavelength Calibration

Wavelength calibration

  • Causes of Wavelength mis-alignments

    • MSM positioning does not repeat exactly.

      • Projection of target/aperture shifts by a few pixels

    • Thermal flexure of STIS bench can also shift projection on detector by a couple of pixels

    • Any drift/mis-centering of target in aperture will cause corresponding shift in wavelength scale

Wavelength calibration

  • Wavecal observations must be adjacent to science

    • No intervening MSM motions because of non-repeatability

  • Recommend repeating wavecals every 40 minutes

  • Slit-to-slit alignment & repeatability is good

    • No need to use same slit for science and wavecal

  • For best alignment do ACQ/PEAK in small aperture

G430L Wavecal with 52X0.2 Aperture

- Aperture bars allow offsets in cross dispersion direction to also be determined.

E140M Echelle Wavecal

Wavelength calibration

  • AUTO-Wavecals meet needed requirements

    • May not always schedule at most efficient time

  • AUTO-WAVECALS may be turned off for visit

    • GO-WAVECALS may then be specified by observer

    • No automatic enforcement of timing requirements for GO-WAVECALs

Target Acquisitions

Need for STIS Onboard Acquistions

  • With GSC1 typical rms pointing errors were ~ 1”

    • GSC2 is more accurate - 0.1”-0.3” accuracy expected

  • Many STIS apertures smaller than this

  • Pointing errors along dispersion direction, translate directly to wavelength errors

  • Basic STIS ACQ procedure desiged to centroid to ~1/5 CCD pixel or about 0.01”

Target ACQ exposures

ACQ procedure does the following:

  • Images target using 5”x5” subarray1

  • For point source ACQ, use flux weighted centroid around brightest 3x3 checkbox (extend source algorithm also available).

  • Move spacecraft to put target at reference location on CCD

  • Re-image target1 & centroid again

  • Image reference aperture using HITM1 lamp & locate aperture

  • Move spacecraft to put target at center of 0.2X0.2 reference aperture

First image

Second image

Lamp image of 0.2X0.2 aperture

1Each external ACQ image is actually made from 2 subarray images dithered by 3 pixels in x and y. They are shifted into alignment and then combined by taking the minimum value at each pixel to eliminate cosmic ray hits and hot pixels.

Selecting Target ACQ parameters

  • For point-source ACQ exposure S/N > 40:1 suggested

    • More is better. If 40:1 SN needs < 0.1 s minimum exposure time, see if 0.1 s exposure is unsaturated before switching to less sensitive setting.

    • But don’t let ACQ saturate. Allowing central pixel to overfill and bleed along columns may affect centroid in y direction.

  • If there are multiple close stars, be sure which one will be brightest in chosen ACQ filter.

  • Point source ACQ accurate to 1/5 pixel or 0.01”

  • Diffuse source ACQ algorithms also available

    • Larger checkboxes (up to 101x101 pixels or ~ 5”x5”)

    • Choice of flux Weighted or geometric centering

ACQ/Peak exposures

  • Peakups recommended for apertures ≤ 0.1” in size

  • Do after ACQ

  • Always done using CCD

  • Peakups measure flux through small aperture and move spacecraft to maximize flux

  • Need to peakup in both directions for small & short apertures (0.1X0.09)

  • Special procedures for 0.1X0.03 peakups

  • Peakups can use images or dispersed light

  • Accuracy ~ 5% of slit width

Fixing Orientation on Sky

  • STIS long slit can be oriented to put extended or multiple targets in aperture

  • Orient in APT should be (degrees east of N) + 45

  • Usually 180 degree alternative is just as good

STIS CCD Operations

CCD Operations


  • CCD includes physical and virtual overscan regions.

  • Four amps, but most science uses AMP D.


  • Bias and dark correction

    • Daily dark and bias observations and more intensive pre-and post anneal observations used to create weekly superbias and superdark images used for OTFR pipeline reduction.

    • Super-bias image subtracted from science image.

    • Serial and parallel overscan regions used to provide 2D correction to bias levels of image.

    • Superdark is subtracted from science image.

      • For side 2 data, superdark scaled for CCD housing temperature

CCD Operations

  • Science data also divided by pixel-to-pixel flat field images based on data collected in yearly campaigns.

  • Some models also have low order flat field images to correct for vignetting.

  • Monthly anneals warm CCD from ~-85 to ~ +5 C

    • Heals ~80% of transient hot pixels;

    • Increasing numbers of permanent ones accumulate.

CCD Dark Current & Hot Pixels

  • Initial dark current low: median value ~0.0015 e-/s

    • Extrapolation predicts 0.009 cnts/pixel/s for Cycle 17

  • Increased over time due to radiation damage

  • On side-2 no closed loop T control

    • CCD temperature & dark current varies with T

    • Use housing temperature to scale dark current before dark subtraction

      • Inexact scaling is an additional source of noise

    • Monthly anneal (warm from -85 C to + 5C) to heal hot pixels

CCD Read Noise

  • AMP D has always had lowest read-noise and is used for science

    • At Gain=1, read-noise initially ~ 4 e-

      • Increased to 4.5 e- after SMOV3a

      • After switch to STIS side-2, additional 15-18 kHz electronic noise increased read noise to ~ 5.5 e- (herring bone pattern)

        • careful Fourier filtering can sometimes remove this

From STIS ISR 2001-05

By Tom Brown

  • Gain=4 showed pick-up noise even on side-1

    • ~7.3 e- on side 1; ~ 7.7 e- on side 2

CCD Options

  • Gain: 1, 2, 4, or 8

    • Only Gains values of 1 and 4 supported for GO observations.

    • Gain=1 has lower read-noise, but amps saturate at ~33,000 e-.

    • Gain=4 has higher read-noise, but allows full well of CCD to be used (144,000 e- at center, ~ 120,000 e- at edges)

      • In saturated GAIN=4 images, electrons bleed to other pixels (perpendicular to dispersion direction), but are not lost. Total response remains linear, allowing very high S/N with special processing techniques.

  • Binnng at readout by 1, 2, or 4 in either axis or both

    • Binning data during read-out reduces read-noise and file size

    • Increases impact of bad pixels and cosmic rays.

    • With older, noisier detector, usually not worthwhile

CCD Options - cont

  • CCD Sub-arrays

    • Can save only part of image array on read-out

      • Reduces file size and number of buffer dumps required

      • Decreases readout time, allowing increased cadence.

    • For GOs only support reducing AXIS2 size (perpindicular to dispersion direction)

      • Discards virtual overscan in parallel direction, but retains physical overscan in serial direction to aid in bias removal

      • Lack of virtual overscan does make bias subraction more difficult

    • Reducing AXIS1 is an available-but-unsupported mode.

      • Reducing both discards all overscan regions, greatly increasing difficulty of accurate bias removal.

    • Different clocking patterns used by any CCD sub-arrays may introduce artifacts, and invalidate assumptions of empirical CTI corrections algorithms.

  • Cosmic Ray rejection is normally done by taking multiple images.

    • CRREJECT=2 is default



CCD Charge Transfer Inefficiency

  • During parallel transfers some electrons get trapped

  • Trapped e- be released later during read, causing extended “tail”.

  • Number of free traps depends on flux level that has moved through that pixel.

  • CTI gets worse with increasing radiation damage

  • No sufficient pixel based physical model, so need empirical corrections.

  • Loss increases with # of transfers (1-CTI)n

    • Putting target near readout amp reduces losses

    • E1 positions defined near row 900

    • Typical exposures of faint targets with the STIS CCD in cycle 17 might experience 20-30% CTI losses when target at center of the detector, but only 5-8% if at E1 near row 900.


FUV MAMA Dark Current

  • FUV MAMA initially had very low dark current (7x10-6 counts/lo-res-pixel/s), but occasionally showed enhanced glow.

  • Initially glow present only rarely

    • became more frequent over time

  • Lower edge & lower right hand corner remains mostly dark (near original 7 x 10-6 counts/lo-res-pixel/s).

  • Physical basis of FUV dark current glow is unclear

131 darks

Apr 1997

Aug 1998

178809 s

Mean 0.21

Glow 0.37

D.C. 0.161

x 10-5 c/p/s


2048 x 2048

126 darks

Aug 1998

Nov 1999

173880 s

Mean 0.45

Glow 1.08

D.C. 0.165

x 10-5 c/p/s

Glow region

Dark Corner

141 darks

Dec 1999

May 2001

194580 s

Mean 0.56

Glow 1.24

D.C. 0.161

x 10-5 c/p/s

125 darks

May 2003

Aug 2004

172500 s

Mean 0.65

Glow 1.65

D.C. 0.154

x 10-5 c/p/s

FUV MAMA Dark Current

  • FUV MAMA dark current increases ~ linearly with time since HV turn-on

  • Increases faster at higher T

  • Rate of increase has gone up over the years

  • Hot pixels also increasing

FUV MAMA Dark Current

  • Mitigation strategies

    • Use only first orbit of each SAA free period for observations that need low dark current. (only 1 orbit per day).

    • Keep FUV HVPS off when detector not in use (ops change).

    • Cool detector (NUV MAMA off).

    • Place target on darker part of detector.

      • New D1 aperture position defined near bottom edge of detector

The count rate summed in each column over a seven pixel high region of the mean dark image covering the period between May 2003 and August 2004. The dotted line gives the results for a region near the standard 1st order spectral location, and the solid line gives the results at the new D1 position located near the bottom edge of the detector.

NUV MAMA Dark Current

  • NUV MAMA dark current dominated by a different physical mechanism than the FUV MAMA

  • Meta-stable states with lifetimes of days to weeks are populated by high-energy particle impacts, leading to a phosphorescent window glow.

  • Long term trend depends on low-earth orbit radiation environment

NUV MAMA Dark Current

  • Effect of temperature changes on dark current is complex

  • Short term changes lead to a large increase in the dexcitation rate, leading to a large, but temporary, increase in the dark current, including daily cycling as MAMA warms up.

  • Over the long term, a smaller equilibrium number of populated states partially balances the higher excitation rate caused by higher average T.

  • If detector cold for long time, large but temporary increase until a new equilibrium is reached.

MAMA Pipeline Dark Images

  • Low dark rates require averaging hundreds of images to make useful dark image.

  • NUV darks semi-empirically scaled for time and temperature changes and subtracted in pipeline.

    • Secular changes are seen in shape of NUV dark current over time.

  • Unpredictable nature of FUV glow makes subtracting it in OTFR pipeline impractical

    • Only base dark current and hot pixels subtracted by pipeline - users need to to custom extraction of glow.

    • In background limited observations, FUV hot pixels should just be masked out because poor statistics makes subtraction difficult.

MAMA Flat Fields

  • On-orbit lamp images used to provide MAMA pixel-to-pixel flats collected during occasional campaigns.

    • MAMA flats very stable once data binned to lo-res (1024x1024)

    • Can use same pixel-to-pixel flats for essentially all data.

    • Flat fielding of unbinned 2048x2048 hi-res images not repeatable - significant structure remains

      • hi res mostly useful for filtering out hot pixels.

  • Low order MAMA flatfields provided for selected modes (mostly FUV modes).

MAMA Observation Modes

  • ACCUM mode

    • Keeps track of how many events fall on each pixel.

    • For medium and high dispersion modes, the pixel locations are corrected for spacecraft doppler motion as image is accumulated.

    • STIS data buffer can hold

      • 1 hires (2048x2048) image or

      • up to 7 lowres MAMA + CCD full frame images or

      • 1 hires image + 3 lowres or CCD full frame images

    • Hires format default for MAMA science, lores for wavecals

MAMA Observation Modes

  • TIME-TAG mode

    • Records x-y location and time of each event with 125 micro-second resolution.

    • Corrections for spacecraft Doppler motion done on ground, not on spacecraft

    • STIS buffer divided into two sections for time-tag

      • Each half of buffer can hold 2 x 106 events.

      • One half of buffer can be dumped while other half is recording.

      • User must predict rate and specify buffer time so that buffer is dumped before one half fills, otherwise gaps will appear in sequence.

      • If global rate < 20,000 counts / s, continuous observations can be sustained for extended periods (up to 30 buffer dumps).

  • For some projects needing time resolved data, a series of ACCUM observations may be better than time-tag mode.

    • For CCD observations, the use of subarrays may increase cadence.

Time resolved STIS Observing

Other MAMA Constraints

  • STIS MAMAs cannot be used in any SAA impacted orbit

    • Optical isolators scintillate from cosmic rays and can cause random bit flips in MAMA electronics

      • STIS low & high voltage turned off during deepest SAA passages; not practical to turn on MAMA for only part of individual orbits.

    • Allows use during only one ~ 5 - 6 orbit block per day

    • Observers required to separate CCD and MAMA science observations into separate visits when practical

Summary of Overheads

STIS Data Products

Selected STIS data file types:

  • opppvvnnd_tag.fits - table of time tag events

  • opppvvnnd_raw.fits - 2d image of unproccesed data

  • opppvvnnd_flt.fits - flat fielded image

  • opppvvnnd_crj.fits - cosmic ray rejected image (CCD)

  • opppvvnnd_x1d.fits - fits table with 1D extracted spectra

  • opppvvnnd_sx1.fits - 1D spectra from summed images

  • opppvvnnd_x2d.fits - 2D spectral image (rectified and flux calibrated)

  • opppvvnnd_sx2.fits - 2D spectral image (rectified and flux calibrated) from summed images

1D spectral extraction

  • In 1D spectral extraction, an extraction box is centered on spectrum, and summed over cross dispersion direction at each pixel in dispersion direction.

  • Extracted spectrum is then background subtracted and flux calibrated

    • Corrections for aperture throughputs, time-dependant sensitivity changes and CTI losses (CCD only) are applied.

Geometry for extraction of 1st order STIS spectra

1D spectral extraction - cont

  • For echelle modes, a separate 1D extraction is done for each spectra order

  • Background subtraction is done using a special algorithm that models the scattered light (see STIS ISR 2002-001 by Valenti et al

2D spectral extraction

  • Image rectified so that wavelength and spatial scales are linear and aligned with x and y coordinates.

  • X2d image is flux calibrated (science images only)

    • Corrections for aperture width and time-dependant sensitivity changes are applied (no CTI correction).

Spatial Undersampling of STIS

  • Critical sampling of PSF requires about 2 pixels per PSF FWHM

    • STIS CCD spatial scale of ~0.051”/pixel undersampled by ~2x @ 5000 Å.

    • STIS MAMA spatial scale ~0.0245”/pixel undersampled by ~2x @ 2500 Å.

  • Undersampling can produce artifacts when extracting spectra at small spatial scales (affected by tilt of spectrum on detector)

  • Dithering along long slit to sub-sample spatial scale recommended if spatial structure is significant.

Selected Data Artifacts

CCD window reflections.

Brightest ring about 1% of flux

Airy rings produce

spectroscopic fringes

IR fringing due to multiple reflections in CCD. Need contemporaneous tungsten fringe flats to correct properly.

Some MAMA modes also

show imaging ghosts.

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