COS Training SeriesII. Optimizing Observations

--- David Sahnow ---14 February 2007

170 mm

COS Training Schedule

Session 2: Optimizing COS Observations ISession 2: Optimizing COS Observations I– Quick Review of COSQuick Review of COS– Detectors types and characteristicsDetectors types and characteristics– How the detectors work and how we operate themHow the detectors work and how we operate them– BUFFER-TIME and buffer managementBUFFER-TIME and buffer management– Internal CalibrationsInternal Calibrations– Detector backgroundDetector background– Pulse-heights,  lifetimePulse-heights,  lifetime– Known anomaliesKnown anomalies

Performance SpecificationsFUV Design Specifications

Item Detector Specification Wavelength range 1150 Å – 2050 Å Input active area (85 x 10 mm) x 2 segments Gap between segments <9 mm Active area curvature 0.826m radius Pixel format (16384 x 1024) x 2 segments Pixel size <(8 m x 15 m) Maximum data rate (Science Data Link) 83,333 cps total

FUV Performance Requirements

Item Performance Requirement SOR Ref Surface Figure Focal cylinder matching

<100 m 7.1.1

Image Quality Spatial resolution <(25 m x 50 m) FWHM over 80% of

active area 3.7

Integral non-linearity (>=1mm frequency) <+/- 100 m over 80% of active area 3.8.1 Differential non-linearity (<1mm frequency) Characterize to 3.2% RMS 3.8.2 Thermal spatial stability <3.3 pixels/°C (DVA), 1.7pixels/°C

(DEB) 3.15

Detection Efficiency Quantum efficiency (QDE) >=25% @ 1335Å 3.5 Visible light rejection QDE<10-6 @ 4000Å to 6500Å 3.5.2 Deadtime efficiency loss

10% QDE loss at 10kcps per segment 3.13

Event Rates Dark count rate (at I&T) <0.5 cps/cm2 3.9 Maximum global count rate >=40,000 cps/segment 3.11 Maximum local count rate 10 cps/pore over 103 m2 (81cps/103 m2) 3.12 MCP Fluence Life >109 events per mm2 3.14 FUV Detector performance requirements are specified in COS-08-0003 Statement of Requirements for the HST-COS FUV Detector. Performance requirements are verified by methods described in AV-02 Verification Procedures.

Microchannel Plates

Wiza, 1979

Microchannel Plates

http://hea-www.harvard.edu/HRC/mcp/mcp.html

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 ~105 – 108

Fast response time Low background High spatial resolution

FUV Detector

FUV: Cross Delay Line (XDL) detectorFUV: Cross Delay Line (XDL) detector– Windowless*, CsI photocathode, XDL anodeWindowless*, CsI photocathode, XDL anode– Two electrically independent 85 mm Two electrically independent 85 mm ×× 10 mm active area segments 10 mm active area segments

with ~9 mm (14-18 with ~9 mm (14-18 ÅÅ in M modes) gap in M modes) gap– Curved MCPs (826 mm radius)Curved MCPs (826 mm radius)– Analog ‘pixels’, with event position digitized to 2 Analog ‘pixels’, with event position digitized to 2 ×× 16,384 16,384 ×× 1024 1024

pixels; 6pixels; 6m m ×× 24 24m pixel size (0.023 m pixel size (0.023 ×× 0.092 arcsec) 0.092 arcsec)– 6 pixels per resolution element (resel) along dispersion; 10 pixels per 6 pixels per resolution element (resel) along dispersion; 10 pixels per

resel perpendicular to dispersion; (0.136 resel perpendicular to dispersion; (0.136 ×× 0.92 arcsec per resel) 0.92 arcsec per resel)– Electronic “stim pulses” to characterize stretching and shifting in both Electronic “stim pulses” to characterize stretching and shifting in both

coordinates coordinates – QE and ion repeller gridsQE and ion repeller grids– Pulse height information availablePulse height information available– High gain (~10High gain (~1077))

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)NUV: Multi-Anode Microchannel Array (MAMA)– STIS NUV flight spareSTIS NUV flight spare

– Sealed tubeSealed tube– CsTe photocathode on a MgF2 windowCsTe photocathode on a MgF2 window– 25 mm 25 mm ×× 25 mm detector format (constrains optical design) 25 mm detector format (constrains optical design)– 1024 1024 ×× 1024 pixels; 25 1024 pixels; 25m m ×× 25 25m pixel size (0.024 m pixel size (0.024 ×× 0.024 0.024

arcsec) ; no subarrays arcsec) ; no subarrays – 3 3 ×× 3 pixels per resel (0.072 3 pixels per resel (0.072 ×× 0.072 arcsec per resel) 0.072 arcsec per resel)– Curved-channel, flat MCPs with lower gain (~7x10Curved-channel, flat MCPs with lower gain (~7x1055))– No pulse height informationNo pulse height information– opto-isolator problem fixedopto-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

TTAGTTAG– Preferred modePreferred mode– Entire detector read outEntire detector read out– 32 msec time stamps32 msec time stamps– More flexibility for post-processing More flexibility for post-processing – Maximum count rate limited to 21,000 cps from entire detectorMaximum count rate limited to 21,000 cps from entire detector– Doppler correction done on the groundDoppler correction done on the ground– Full pulse height information (FUV only)Full pulse height information (FUV only)

ACCUMACCUM– Each photon event increments a memory locationEach photon event increments a memory location– Only part of detector read out.Only part of detector read out.– Use for count rates > 30,000 cpsUse for count rates > 30,000 cps– Doppler correction done onboardDoppler correction done onboard– Global pulse height only (FUV)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 2.35×106 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 LampsWavelength Calibration Lamps– Pt-Ne hollow cathode lamps used with WCAPt-Ne hollow cathode lamps used with WCA– Routinely used with TAGFLASH and AUTO wavecal exposures.Routinely used with TAGFLASH and AUTO wavecal exposures.– Always done as TIME-TAGAlways done as TIME-TAG

Flat FieldsFlat Fields– DD22 hollow cathode lamps used with FCA hollow cathode lamps used with FCA

> Calibration Programs onlyCalibration Programs only> Always done as TIME-TAGAlways done as TIME-TAG> May be difficult to get required S/N in FUVMay be difficult to get required S/N in FUV

– Also done with external targetsAlso done with external targets

CC BB AA CC BB AA

PtNePtNeWavecalWavecal

ExternalExternalScienceScience

NUVNUVMAMAMAMA

FUV MCP (1 of 2 segments)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 109 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