STIS Design
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Transcript of STIS Design
STIS Design
Charles Proffitt
STIS Presentation 2
Outline of Topics
Introduction Basic Structure of STIS Detectors MSM optical elements & observing modes Slits and Apertures Target Acquistions Lamps and Wavecals
STIS Presentation 3
Introduction to STIS
Space Telescope Imaging Spectrograph Highly versatile spectrograph 3 detectors (can use only one at a time)
FUV MAMA ~ 1150 - 1700 Å, 1024 x 1024, ~0.025” pixels NUV MAMA ~ 1600 - 3200 Å , 1024 x 1024, ~0.025” pixels CCD ~ 1650 - 11,000 Å , 1024 x 1024, ~ 0.05” pixels
Long slit 1st order spectra High dispersion UV echelle spectra
• Resolution up to ~ 100,000
• Also slit-less and imaging modes
STIS Presentation 4
STIS Operational History
Replaced GHRS in Axial Bay 1 on Feb 14, 1997 during SM2 STIS Side 1 failed on May 16, 2001
4.25 years and ~ 42,000 hours of operation Probable short in tantalum capacitor - completely disabled side 1 Failure inaccessible without removing STIS
STIS Side 2 failed on August 3, 2004 3.25 years and ~ 27,000 hours of operation Failure in Interpoint converter that supplies power to move
mechanisms STIS other wise appears to be healthy, but no way to move the
mechanisms or get light to the detector STIS now turned-off (except for heaters) to avoid applying power
to bad component Repair planned for SM4; will replace LVPS2 board
STIS Presentation 5
STIS Optical Bench
STIS Presentation 6
STIS Presentation 7
STIS Mechanisms
Corrector/Focus mechanism Separately adjustable in tip, tilt, and focus Last used during SMOV2 in 1997 Will adjust during SMOV4 only if necessary
Echelle Blocker Keeps scattered light from unused detector’s echelles
Mode Isolation Mechanism Blocks direct light from MSM to MAMA unless desired
Calibration Insert Mechanism (CIM) Slit Wheel Mode Select Mechanism (MSM)
STIS Presentation 8
STIS Slit Wheel• Slit wheel is in a image plane • 77 standard slit wheel positions defined in aperture table
• 45 distinct clear slits and apertures• 7 ND slits• 13 imaging filters• 12 alternate rotations for barred apers
• Only a subset available for GOs
• For STIS the APERTURE keyword value refers to a unique slit wheel pos• Some physical slits have multiple slit wheel APERTURE positions defined (e.g., barred and regular long-slit pos) • Alternate pointings of HST at the same slit wheel position are captured in PROPAPER keyword (e.g., E1 aperture positions vs. regular positions).
• Sometimes called pseudo-aperture positions• Usage differs from other instruments.
• In STIS slits & filters are not “optical elements” as far as most ops data bases are concerned.
STIS Presentation 9
Mode Select Mechanism
• Tilted cylinders allow MSM to rotate and “wobble” to allow tip/tilt of elements
• Rotation of large wheels more precise than tip-tilt of small actuators
• Repeatability still not perfect• 21 optical elements in MSM
• 16 gratings & 1 PRISM• 4 imaging mirrors
• 10 elements have order sorter filters• For the 4 echelle modes, the cross-disperser is
the element in the MSM• Each optical element is intended for use with
only 1 optical path & 1 detector• Exceptions for echelle cross dispersers, but exceptions not
used on-orbit
STIS Detectors
STIS Presentation 11
MAMA Detectors
MAMA = Multi-Anode Microchannel Array Photocathode produces electron when hit by individual photon Microchannel plate turns electron into charge cloud (4 x 105 e-) 1K x 1K anode array detects and centroids charge cloud 1024 x 1024 pixels ~ 0.0245” pixel size - 25”x25” FOV
Can be subsampled to 2048 x 2048 Advantages of MAMA detectors
Good FUV and NUV sensitivity Photon counting
no read-noise Time-tag mode possible
Less sensitive to cosmic rays (just one more dark current count at worst) Low dark current More resistant to radiation damage than CCDs
(but maybe not completely immune to rad. damage) No charge transfers (no CTI losses or tails) High spatial resolution
STIS Presentation 12
MAMA Detectors
Dis-advantages of MAMA detectors Subject to damage if over-illuminated Cannot operate during SAA (high count rate) Difficult to manufacture
STIS MAMA peculiarities STIS MAMAs have optical isolators that scintillate from cosmic rays
This forces STIS MAMA low voltage to be turned off during SAA Prevents STIS MAMAs from observing in any SAA impacted orbit HV only on for one ~ 5 - 6 orbit block per day
STIS NUV MAMA has high dark current due to long phosphorescent window glow excited by charged particle impacts
STIS Presentation 13
FUV MAMA Detector
STIS Presentation 14
MAMA Anode Array
Pulse location positions are centroided using anode grid
Data routinely sub-sampled to 2048 x 2048 grid, but flat fielding issues prevent extra resolution from being useful
Amount of charge, number of “folds”, and location used to choose “valid” events.
STIS Presentation 15
MAMA Detectors
Micro channel plate consists of bundles of curved glass tubes1. Photon hits photocathode, ejects electrons
2. Voltage across plate accelerates electrons down tubes
3. Electrons collide with walls, eject more electrons
4. Average gain of 4 x 105 (electrons out per photon event)
5. Size and location of charge cloud used to distinguish valid events.
STIS Presentation 16
Differences Between MAMAs
FUV MAMA CsI photocathode on Micro-channel Plate Strongest response 1150-2000 Å Field electrode & repeller wire between window and photocathode
NUV MAMA CsTe2 photocathode on inside of detector window
Slight defocus from lateral drift of electrons No repeller wire or field electrode
Strongest response 1700-3200 Å Significant sensitivity down to 1150 Å Intended as backup for FUV MAMA
• Unused backup modes to replicate FUV abilities
• much lower FUV throughput
STIS Presentation 17
MAMA Detectors
MAMAs detect individual photons
One event recorded per photon Invalid events are discarded
ACCUM and TIME-TAG differ mostly in what gets saved In ACCUM mode, each event increments memory location for
that pixel. Only final accumulated image saved. Pixel locations shifted on-orbit for orbital doppler correction
In TIME-TAG mode, position and time of each valid event are saved. Doppler correction done later.
If HV is on, MAMA tubes continue to operate even when exposures are not in progress - events just aren’t recorded
STIS Presentation 18
MAMA Bright Object Limits
MAMAs can be damaged by excessive illumination Extracting too much charge too quickly, could limit future charge
extraction, cause localized gain sag, decrease effective sensitivity Very large count rates can produce gas in tube, perhaps leading
to catastrophic short circuit or gas venting to aft-shroud CARD (Constraints and Restrictions Document) Limits
Global > 1.5e6 counts for 1 second Local > 500 counts/lo-res-pixel/s over 4x4 area for > 30 s
Detectors become non-linear at > 300,000 counts/s Science calibration difficult
Lower screening limits set for operations Global limit 200,000 counts/sec for most modes 30,000 counts/sec for 1st order point sources Local limit 75 (spec) or 100 (imaging) counts/lo-res-pixel/s
• Brightest pixel Lower global limits (80,000/12,000) for irregularly variable objects
STIS Presentation 19
Automatic BOP Mechanisms
Bright Scene Detector (BSD) Every 32nd anode wire monitored by special circuit Unaffected by high count rates that may saturate normal counting electronics Detector safed if this monitor is triggered Trigger equivalent to uniform ~ 2x106 cnt/s global rate Sparse coverage - bright source could fall between monitored rows Spectrum at right location could safe detector with only ~ 120,000 counts/s
Software Global Monitor (SGM) Uses event counters giving global rate to detect overlight Monitors all counts (valid or invalid) above set threshold voltage Triggers at 1x106 c/s (equivalent to ~ 580,000 valid c/s) Affected by non-linearity at high counts rates (> 4x106 c/s) Can shut down detector within 0.1 s
Local Rate Check Image (LRC) 300 ms image taken before each MAMA observation Rebinned 2x2 and 4x4 and brightest pixels compared to limits Failure of LRC causes detector to be shuttered & and any lamps to be turned off
Only shuttered image lost Check only done at start of observation
STIS Presentation 20
STIS MAMA Detectors
STIS MAMA Detector Performance Characteristics
Characteristic FUV-MAMA Performance NUV-MAMA Performance-----------------------------------------------------------------------------------------------------------Photocathode CsI Cs2TeWavelength range 1150-1700 Å 1600-3100 ÅPixel format 1024 x 1024 1024 x 1024Pixel size 25 x 25 µm 25 x 25 µmImage mode pixel plate scale ~0.0245” x 0.0247” ~0.0245” x 0.0248”Field of view 25.1 x 25.3” 25.1 x 25.4”Quantum efficiency25% @ 1216 Å 10% @ 2537 ÅDark count 5 x 10-6 to 1 x 10-4 c/s/pixel 8 x 10-4 to 1.7 x 10-3 c/s/pixelGlobal count-rate linearity limit1 285,000 counts/s 285,000 counts/sLocal count-rate linearity limit1 ~220 counts/s/pixel ~340 counts/s/pixel-----------------------------------------------------------------------------------------------------------1Rate at which counting shows 10% deviation from linearity. These count rates are well above the bright-object limit.
STIS Presentation 21
STIS CCD
STIS CCD: 1024 x 1024 thinned backside illuminated SITe CCD Thermal electric cooler (TEC) to allow CCD to operate at -83 C. Includes overscan region to ease bias removal CCD accumulates charge in each pixel and then is readout by
transferring charge row by row to readout register and then pixel by pixel to amplifier
STIS Presentation 22
CCD Characteristics
ACCUM mode only - image read out after exposure Can read out subarrays Cosmic ray hits can affect numerous pixels
Often need to CR-SPLIT images Some charge can lag during readout.
Charge Transfer Inefficiency (CTI) Gets worse as CCD accumulates radiation damage on-orbit Affects fluxes and causes tails in images
Thinned CCD has good UV sensitivity, but too transparent in red - photons scatter in chip Interference fringing in IR Red light halo
STIS Presentation 23
STIS CCD
STIS CCD Detector Performance Characteristics
Characteristic CCD Performance------------------------------------------------------------------------------------------------Architecture Thinned, backside illuminatedWavelength range ~1600-11,000 ÅPixel format 1024 x 1024 illuminated pixelsField of view 52 x 52 arcsecondsPixel size 21x 21 µmPixel plate scale 0.05071 arcsecondsQuantum efficiency ~ 20% @3000 Å
~ 67% @6000 Å~ 29% @ 9000 Å
Dark count at -83° C 0.007 e-/s/pixel (but varies with detector T)Read noise (effective values) 5.4 e- rms at GAIN=1 (1 e- of which is pattern noise)
7.6 e- rms at GAIN=4 (0.2 e- of which is pattern noise)Full well 144,000 e- over the inner portion of the detector
120,000 e- over the outer portion of the detectorSaturation limit 33,000 e- at GAIN=1 (16 bit A-to-D limit)
144,000 e- at GAIN=4------------------------------------------------------------------------------------------------
MSM Optical Elementsand Observing Modes
STIS Presentation 25
STIS Observing Modes
Echelle Modes E140M, E140H E230M, E230H
1st Order and PRISM Modes G140L, G230L, G230LB, G430L, G750L G140M, G230M, G230LM, G430M, G750M NUV Prism
Imaging Modes Imaging mirror for each detector
2 for FUV - clear and filtered Acquistion modes
ACQ mode uses image to align target with standard slit
ACQ/Peak modes center target in small slit
STIS Presentation 26
STIS Presentation 27
STIS Presentation 28
STIS Presentation 29
STIS Presentation 30
1st Order vs Echelle
1st order gratings (m=1) Low blaze angle, fine ruling Large free spectral range Some use blocking filters to remove higher order
Echelle gratings (orders m = 66 - 747) High blaze angle, coarser ruling Use higher order spectra for much higher dispersion Smaller free spectral range per order~ /m Use cross-disperser to separate orders Can image many orders on detector at once Flux calibration more difficult
STIS Presentation 31
Medium resolution 1st order gratings, need to be scanned in dispersion direction to cover full spectral range
Echelle gratings, need to be scanned in cross-dispersion direction to cover all wavelengths
For STIS L-modes, whole spectral range fits onto detector G140L ~ 1150 - 1736 Å G230L ~ 1570 - 3180 Å G230LB ~ 1680 - 3065 Å G430L ~ 2900 - 5700 Å G750L ~ 5240 - 10,200 Å (larger contaminated by 2nd order light)
Scanning of gratings
STIS Presentation 32
Dispersion, coverage, & throughput
STIS Presentation 33
Dispersion, coverage, & throughput
STIS Presentation 34
G140M & G230M Central Settings
Only pre-defined grating tilts allowed Prime settings will scan whole range (~10% overlap) Secondary settings for special purposes
STIS Presentation 35
G230MB, G430M, G750M Cenwaves
CCD M gratings, lower dispersion, but larger coverage than MAMA M gratings
STIS Presentation 36
Use of 1st order mode with long slit G750M observation of M84 (Radio Galaxy) nucleus
Bower et al. (1998), ApJL, 492, L111
Long slit (52X0.2) at 6581 CENWAVE across nucleus Shows N II and S II emission lines from disk Gives rotation curve with high resolution ~ 0.05”/pixel
STIS Presentation 37
Echelle mode CENWAVE settings
Many secondary settings to allow flexibility. Used frequently by GOs
E140M covers ~ 1123 to 1710 Å with one setting
STIS Presentation 38
STIS Presentation 39
STIS Echelle Spectra
E140M LINE lamp spectrum
E140M Stellar Spectrum
Apertures and Filters
STIS Presentation 41
STIS Slit Wheel
Very large number of apertures Slit name convention, e.g, 52X2
“length in spatial direction” x “length along dispersion direction” F preceeds name of filtered apertures
Full list of slit wheel position names0.05X29 F25CIII 0.1X0.03 0.2X0.06FPA 1X0.06 52X0.050.09X29 F25CN182 0.1X0.06 0.2X0.06FPB 1X0.2 52X0.05F10.2X29 F25CN270 0.1X0.09 0.2X0.06FPC 2X2 52X0.05F20.05X31NDA F25LYA 0.1X0.2 0.2X0.06FPD 6X0.06 52X0.10.05X31NDB F25MGII 0.2X0.05ND 0.2X0.06FPE 6X0.2 52X0.1F1F28X50LP F25ND3 0.2X0.06 0.2X0.2FPA 6X0.5 52X0.1F2F28X50OII F25ND5 0.2X0.09 0.2X0.2FPB 6X6 52X0.2F28X50OIII F25NDQ 0.2X0.2 0.2X0.2FPC 31X0.05NDA 52X0.2F152X0.1B0.5 F25QTZ 0.2X0.5 0.2X0.2FPD 31X0.05NDB 52X0.2F252X0.1B1.0 F25SRF2 0.3X0.05ND 0.2X0.2FPE 31X0.05NDC 52X0.552X0.1B3.0 0.3X0.06 36X0.05N45 52X0.5F1 0.3X0.09 36X0.05P45 52X0.5F2 0.3X0.2 36X0.6N45 52X2 0.5X0.5 36X0.6P45 52X2F1 50CCD 52X2F2 50CORON
Some wheel position “APERTURES” have multiple target positions defined for same slit wheel setting. These are not included in this list.
STIS Presentation 42
Slits for 1st order spectroscopy
First order observations usually use 52” long slits, 52X2, 52X0.5, 52X0.2, 52X0.1, 52X0.05
52X2 for best absolute photometry Smaller slits for cleaner LSF or extended
targets The 52” slits all have a pair of fiducial bars
Alternate slit rotations (rotate aperture wheel) are defined to bring either fiducial bar closer to center, and target behind bar
STIS Presentation 43
Barred aperture positions
Append F1 or F2 to aperture name (e.g. 52x0.2F1) Rotation of aperture wheel gives different slit angle Only 52X0.2F1 “supported”, but other bars “available”
STIS Presentation 44
Alternate Aperture Positions
For some apertures, multiple positions defined, but using same aperture wheel rotation
For STIS these are often referred to as pseudo-apertures. In image header in PROPAPER keyword
APERTURE keyword still set to name of physical aperture. E1 apertures for lower CTI D1 apertures for lower FUV dark current
STIS Presentation 45
STIS Aperture Selection, continued
Echelle observations usually use short slits 0.2X0.2 for best throughput & photometry Smaller slits matched to each grating for better LSF
0.2X0.06 for E140M & E230M; 0.2x0.09 for E140H & E230H Smallest slit 0.1X0.03 to maximize resolution Small ND slits 0.2X0.05ND (100X) and 0.3X0.05ND (1000X) FP-SPLIT slits (0.2X0.2FPA-E & 0.2X0.06FPA-E) to dither target
along dispersion direction - solve for fixed-pattern noise.D
ispersion
STIS Presentation 46
STIS Aperture Selection - cont
Most apertures can be used with most gratings NUV-PRISM and 1st order gratings often used slitless Can use long slits with echelle for spatially resolved
observations of emission line sources
Filters are also in aperture wheel and can be crossed with gratings, (e.g., use long-pass filter to block geo-coronal lyman-alpha in slitless G140L images)
STIS Presentation 47
Wide slits and extended sources
With wide slit, cannot separate spatial offsets in dispersion direction from wavelength shifts
Wide slit observations of extended objects degrade spectral resolution
For emission line sources can take advantage of this to do emission line images.
STIS G750M 6581 52 × 2 Spectral Image of SN1987A. This showsthe images of the inner circumstellar ring in [OI], Hα, [NII], and [SII]. Diffuse Hαemission from the LMC fills the 52 × 2 slit, and broad Hα emission from the SN is also visible. The continua of stars produce the horizontal bands. The image shown is a 950 × 450 subsection of the 1024 × 1024 image. (Figure courtesy of Jason Pun and George Sonneborn, see also Sonneborn et al. 1998, ApJ, 492, L139).
STIS Presentation 48
MAMA Imaging FUV Imaging
FUV imaging capabilities similar to ACS SBC Solar blind, but red-leak may be worse than specs
NUV Imaging Some sensitivity at FUV wavelengths At long , overlaps with ACS/HRC and WFC3/UVIS abilities
STIS Presentation 49
STIS CCD Imaging Modes
CCD Imaging Primarily needed for target ACQs, but also for science Between SM2 and SM3B (ACS install), the unfiltered STIS CCD
was the most sensitive imaging instrument on HST Only limited filters (OIII has significant red leak) Also ND filters
STIS Presentation 50
Imaging Coronagraphy
Coronagraphic Mask in filter wheel for use with CCD imaging Unfiltered coronagraphy only, cannot cross with other filters.
Predefined aperture locations, WEDGEA1.0, etc. Imaging mode mirror has Lyot stop, but secondary and spider not
aphodized.