COS Training Series II. Optimizing Observations --- David Sahnow February mm

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

Performance Specifications

Microchannel Plates Wiza, 1979

Microchannel Plates

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

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 × 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 )

XDL Anode UCB

XDL Anode UCB

FUV Detector Top View 170 mm

FUV QE Grid

FUV Detector Format   Remember: FUV detector has two segments (A and B)

XDL Flat Field Vallerga et al, SPIE 2001

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 × arcsec) ; no subarrays – 3 × 3 pixels per resel (0.072 × arcsec per resel) – Curved-channel, flat MCPs with lower gain (~7x10 5 ) – No pulse height information – opto-isolator problem fixed

MAMA Anode Array   Pulse location positions are centroided using anode grid   Amount of charge, number of “folds”, and location used to choose “valid” events.

COS Detectors – NUV MAMA in the enclosure

COS Physical Characteristics Summary ** MAMA dark limits quoted are one-fourth of STIS values; actual dark rates TBD on-orbit **

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.

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)

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)

Pulse Heights (FUV only)   Pulse height thresholding can be used to screen photons   Default thresholding will be determined during SMOV Threshold Modal Gain

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

CBA CBA PtNeWavecalExternalScienceNUVMAMA FUV MCP (1 of 2 segments) External Science Internal PtNe Wavecal COS Spectral Layout for Simultaneous Internal Wavecals and Science Spectra

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.

Anomalies: Bursts

Anomalies: HV Current Transients

DOOR Ion Pumps GSE Port Backplate Motor FUSE FL01