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COS Training Series II. Optimizing Observations --- David Sahnow --- 14 February 2007 170 mm
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COS Training Schedule Session 2: Optimizing COS Observations I – Quick Review of COS – Detectors types and characteristics – How the detectors work and how we operate them – BUFFER-TIME and buffer management – Internal Calibrations – Detector background – Pulse-heights, lifetime – Known anomalies
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Performance Specifications
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Microchannel Plates Wiza, 1979
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Microchannel Plates http://hea-www.harvard.edu/HRC/mcp/mcp.html
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Microchannel Plates Array of millions of glass channels, each ~10 – 25 µm in diameter. High gain electron multiplication via photoelectic effect Efficiency increased by proper choice of photocathode. Gains of ~10 5 – 10 8 Fast response time Low background High spatial resolution
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FUV Detector FUV: Cross Delay Line (XDL) detector – Windowless*, CsI photocathode, XDL anode – Two electrically independent 85 mm × 10 mm active area segments with ~9 mm (14-18 Å in M modes) gap – Curved MCPs (826 mm radius) – Analog ‘pixels’, with event position digitized to 2 × 16,384 × 1024 pixels; 6 m × 24 m pixel size (0.023 × 0.092 arcsec) – 6 pixels per resolution element (resel) along dispersion; 10 pixels per resel perpendicular to dispersion; (0.136 × 0.92 arcsec per resel) – Electronic “stim pulses” to characterize stretching and shifting in both coordinates – QE and ion repeller grids – Pulse height information available – High gain (~10 7 )
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XDL Anode UCB
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XDL Anode UCB
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FUV Detector Top View 170 mm
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FUV QE Grid
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FUV Detector Format Remember: FUV detector has two segments (A and B)
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XDL Flat Field Vallerga et al, SPIE 2001
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NUV Detector NUV: Multi-Anode Microchannel Array (MAMA) – STIS NUV flight spare – Sealed tube – CsTe photocathode on a MgF2 window – 25 mm × 25 mm detector format (constrains optical design) – 1024 × 1024 pixels; 25 m × 25 m pixel size (0.024 × 0.024 arcsec) ; no subarrays – 3 × 3 pixels per resel (0.072 × 0.072 arcsec per resel) – Curved-channel, flat MCPs with lower gain (~7x10 5 ) – No pulse height information – opto-isolator problem fixed
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MAMA Anode Array Pulse location positions are centroided using anode grid Amount of charge, number of “folds”, and location used to choose “valid” events.
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COS Detectors – NUV MAMA in the enclosure
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COS Physical Characteristics Summary ** MAMA dark limits quoted are one-fourth of STIS values; actual dark rates TBD on-orbit **
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Detector Backgrounds Table lists the dark count rates measured in ground tests. These values will be reevaluated during SMOV and as part of the COS calibration plan. Dark rate in the FUV detector is very small, about 1 count resel –1 in six hours.
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TTAG vs. ACCUM TTAG – Preferred mode – Entire detector read out – 32 msec time stamps – More flexibility for post-processing – Maximum count rate limited to 21,000 cps from entire detector – Doppler correction done on the ground – Full pulse height information (FUV only) ACCUM – Each photon event increments a memory location – Only part of detector read out. – Use for count rates > 30,000 cps – Doppler correction done onboard – Global pulse height only (FUV)
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Buffer Times BUFFER-TIME must be specified for all external TIME- TAG observations – – Used to establish the pattern and timing of memory dumps during an exposure – – BUFFER-TIME is the minimum time to collect 2.35×10 6 events (9 MB) – – Data is recorded in one of two buffers. After BUFFER-TIME, recording switches to the second while the first is read out – – Incorrectly specifying BUFFER-TIME may result in loss of data! (counts arriving when the buffer is full will be lost) – – Recommend scaling by 2/3 to provide a margin of error – – ETC will provide estimates (but not the 2/3 scaling factor) – – Minimum value is 80 seconds (~30,000 cps)
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Pulse Heights (FUV only) Pulse height thresholding can be used to screen photons Default thresholding will be determined during SMOV Threshold Modal Gain
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Internal Calibrations Wavelength Calibration Lamps – Pt-Ne hollow cathode lamps used with WCA – Routinely used with TAGFLASH and AUTO wavecal exposures. – Always done as TIME-TAG Flat Fields – D 2 hollow cathode lamps used with FCA > Calibration Programs only > Always done as TIME-TAG > May be difficult to get required S/N in FUV – Also done with external targets
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CBA CBA PtNeWavecalExternalScienceNUVMAMA FUV MCP (1 of 2 segments) External Science Internal PtNe Wavecal COS Spectral Layout for Simultaneous Internal Wavecals and Science Spectra
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Detector Lifetime FUV Lifetime requirement: ≤ 1% loss in QE after 10 9 events mm –2. Estimates of COS usage show that the total number of events detected in the FUV channel over a seven-year mission would be a few times this value. Spectrum can be moved in the cross dispersion direction onto a previously-unused portion of the detector by offsetting the aperture mechanism. This can be done up to four times.
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Anomalies: Bursts
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Anomalies: HV Current Transients
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DOOR Ion Pumps GSE Port Backplate Motor FUSE FL01
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