STIS Design Charles Proffitt
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STIS Design Charles Proffitt Outline of Topics Introduction Basic Structure of STIS Detectors MSM optical elements & observing modes Slits and Apertures Target Acquistions Lamps and Wavecals STIS Presentation 2 Introduction to STIS Space Telescope Imaging Spectrograph Highly versatile spectrograph 3 detectors (can use only one at a time) Long slit 1st order spectra High dispersion UV echelle spectra • • 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 Resolution up to ~ 100,000 Also slit-less and imaging modes STIS Presentation 3 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 4 STIS Optical Bench STIS Presentation 5 STIS Presentation 6 STIS Mechanisms Corrector/Focus mechanism Echelle Blocker Keeps scattered light from unused detector’s echelles Mode Isolation Mechanism Separately adjustable in tip, tilt, and focus Last used during SMOV2 in 1997 Will adjust during SMOV4 only if necessary Blocks direct light from MSM to MAMA unless desired Calibration Insert Mechanism (CIM) Slit Wheel Mode Select Mechanism (MSM) STIS Presentation 7 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 8 Mode Select Mechanism • Tilted cylinders allow MSM to rotate and “wobble” to allow tip/tilt of elements • • • • • • 21 optical elements in MSM • • 16 gratings & 1 PRISM 4 imaging mirrors 10 elements have order sorter filters For the 4 echelle modes, the crossdisperser is the element in the MSM Each optical element is intended for use with only 1 optical path & 1 detector • STIS Presentation Rotation of large wheels more precise than tip-tilt of small actuators Repeatability still not perfect Exceptions for echelle cross dispersers, but exceptions not used on-orbit 9 STIS Detectors 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 Less sensitive to cosmic rays (just one more dark current count at worst) Low dark current More resistant to radiation damage than CCDs no read-noise Time-tag mode possible (but maybe not completely immune to rad. damage) No charge transfers (no CTI losses or tails) High spatial resolution STIS Presentation 11 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 12 FUV MAMA Detector STIS Presentation 13 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 14 MAMA Detectors Micro channel plate consists of bundles of curved glass tubes 1. 2. 3. 4. 5. STIS Presentation Photon hits photocathode, ejects electrons Voltage across plate accelerates electrons down tubes Electrons collide with walls, eject more electrons Average gain of 4 x 105 (electrons out per photon event) Size and location of charge cloud used to distinguish valid events. 15 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 STIS Presentation Significant sensitivity down to 1150 Å Intended as backup for FUV MAMA • Unused backup modes to replicate FUV abilities • much lower FUV throughput 16 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 17 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 Detectors become non-linear at > 300,000 counts/s Global > 1.5e6 counts for 1 second Local > 500 counts/lo-res-pixel/s over 4x4 area for > 30 s Science calibration difficult Lower screening limits set for operations STIS Presentation 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 18 Automatic BOP Mechanisms Bright Scene Detector (BSD) Software Global Monitor (SGM) 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 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 STIS Presentation Only shuttered image lost Check only done at start of observation 19 STIS MAMA Detectors STIS MAMA Detector Performance Characteristics Characteristic FUV-MAMA Performance NUV-MAMA Performance ----------------------------------------------------------------------------------------------------------Photocathode CsI Cs2Te Wavelength range 1150-1700 Å 1600-3100 Å Pixel format 1024 x 1024 1024 x 1024 Pixel size 25 x 25 µm 25 x 25 µm Image 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 efficiency 25% @ 1216 Å 10% @ 2537 Å -6 -4 Dark count 5 x 10 to 1 x 10 c/s/pixel 8 x 10-4 to 1.7 x 10-3 c/s/pixel Global count-rate linearity limit1 285,000 counts/s 285,000 counts/s 1 Local count-rate linearity limit ~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 20 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 21 CCD Characteristics ACCUM mode only - image read out after exposure Can read out subarrays Cosmic ray hits can affect numerous pixels Some charge can lag during readout. Often need to CR-SPLIT images 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 22 STIS CCD STIS CCD Detector Performance Characteristics Characteristic CCD Performance -----------------------------------------------------------------------------------------------Architecture Thinned, backside illuminated Wavelength range ~1600-11,000 Å Pixel format 1024 x 1024 illuminated pixels Field of view 52 x 52 arcseconds Pixel size 21 x 21 µm Pixel plate scale 0.05071 arcseconds Quantum 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 detector Saturation limit 33,000 e- at GAIN=1 (16 bit A-to-D limit) 144,000 e- at GAIN=4 ------------------------------------------------------------------------------------------------ STIS Presentation 23 MSM Optical Elements and Observing Modes STIS Observing Modes Echelle Modes 1st Order and PRISM Modes E140M, E140H E230M, E230H 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 25 STIS Presentation 26 STIS Presentation 27 STIS Presentation 28 STIS Presentation 29 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 30 Scanning of gratings 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) 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 STIS Presentation 31 Dispersion, coverage, & throughput STIS Presentation 32 Dispersion, coverage, & throughput STIS Presentation 33 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 34 G230MB, G430M, G750M Cenwaves CCD M gratings, lower dispersion, but larger coverage than MAMA M gratings STIS Presentation 35 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 36 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 37 STIS Presentation 38 STIS Echelle Spectra E140M LINE lamp spectrum E140M Stellar Spectrum STIS Presentation 39 Apertures and Filters 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 names 0.05X29 0.09X29 0.2X29 0.05X31NDA 0.05X31NDB F28X50LP F28X50OII F28X50OIII 52X0.1B0.5 52X0.1B1.0 52X0.1B3.0 F25CIII F25CN182 F25CN270 F25LYA F25MGII F25ND3 F25ND5 F25NDQ F25QTZ F25SRF2 0.1X0.03 0.1X0.06 0.1X0.09 0.1X0.2 0.2X0.05ND 0.2X0.06 0.2X0.09 0.2X0.2 0.2X0.5 0.3X0.05ND 0.3X0.06 0.3X0.09 0.3X0.2 0.5X0.5 0.2X0.06FPA 0.2X0.06FPB 0.2X0.06FPC 0.2X0.06FPD 0.2X0.06FPE 0.2X0.2FPA 0.2X0.2FPB 0.2X0.2FPC 0.2X0.2FPD 0.2X0.2FPE 1X0.06 1X0.2 2X2 6X0.06 6X0.2 6X0.5 6X6 31X0.05NDA 31X0.05NDB 31X0.05NDC 36X0.05N45 36X0.05P45 36X0.6N45 36X0.6P45 50CCD 50CORON 52X0.05 52X0.05F1 52X0.05F2 52X0.1 52X0.1F1 52X0.1F2 52X0.2 52X0.2F1 52X0.2F2 52X0.5 52X0.5F1 52X0.5F2 52X2 52X2F1 52X2F2 Some wheel position “APERTURES” have multiple target positions defined for same slit wheel setting. These are not included in this list. STIS Presentation 41 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 42 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 43 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 44 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. Dispersion STIS Presentation 45 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 geocoronal lyman-alpha in slitless G140L images) STIS Presentation 46 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 shows the images of the inner circumstellar ring in [OI], Hα, [NII], and [SII]. Diffuse Hα emission 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 47 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 48 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 49 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. STIS Presentation 50
