Instrument Introduction 170 mm COS Training Series Instrument Introduction --- Tony Keyes --- 7 February 2007 Training Schedule COS Design and Components Spectral Ranges Observing Modes TAGFLASH COS or STIS? 1

COS Training Schedule Session 1: COS Instrument Introduction and Overview Design characteristics, spectral elements, apertures, observing modes, sensitivities, resolutions, COS vs STIS, “TAGFLASH” Session 2: Optimizing COS Observations I Detectors, faint limits, backgrounds, optical design, BUFFER-TIME and buffer dump management, read-out charcteristics, pulse-heights,  anomalies, internal calibrations Session 3: Optimizing COS Observations I Target acquisition, bright object issues Session 4: COS Post-Observation Pipeline, headers, keywords; calibration reference files and tables; reduced data products, formats

COS Science Design Requirements Moderate Resolution (R~20,000) point-source UV spectroscopy; 0.1 arcsec pointing; 15 km/sec absolute (5 km/sec relative) radial velocity accuracy Highest Possible Throughput Maximize wavelength coverage per exposure

COS Design Science Design Requirements met using a combination of HST capabilities large collecting area UV coatings excellent pointing stability superb image quality (after aberration correction); and FUV: single reflection system; fully corrected along dispersion, astigmatism perpendicular to dispersion large format solar-blind cross delay line (XDL) detector high-efficiency 1st-order holographic gratings NUV: FUV concept not available (no large format detectors) STIS flight spare MAMA, fully aberration-corrected system, enhance wavelength coverage through use of three camera optics

COS Apertures Apertures Primary Science Aperture (PSA): 2.5 arcsec circular field stop Bright Object Aperture (BOA): 2.5 arcsec + ND2 (~150x attenuation) circular field stop Wavelength Calibration Aperture (WCA) Flat Field Aperture (FFA)

COS Optical Elements Optical Elements (Gratings, Mirrors) FUV: G130M, G160M, G140L NUV: G185M, G225M, G285M, G230L, TA1 mirror (as MIRRORA and MIRRORB)

COS Detectors FUV: Cross Delay Line (XDL) detector windowless, CsI photocathode, MCPs feed XDL anode; photon counting two 85 mm (dispersion-direction) x 10 mm segments (~9 mm gap between segments) 2 x 16,384 x 1024 pixels; 6mm x 24mm pixel size (0.023 x 0.092 arcsec) 6 pixels per resolution element (resel) along dispersion; 10 pixels per resel perpendicular to dispersion; (0.136 x 0.92 arcsec per resel) Electronic “stim pulses” to characterize stretching and shifting in both coordinates (more robust than FUSE) NUV: MAMA (STIS flight spare) sealed CsTe photocathode; photon counting 25 mm x 25 mm detector format (constrains optical design) 1024 x 1024 pixels; 25mm x 25mm pixel size (0.024 x 0.024 arcsec) ; no subarrays 3 x 3 pixels per resel (0.072 x 0.072 arcsec per resel) opto-isolator problem fixed

COS Detectors – FUV XDL 170 mm Top View

COS Detectors – NUV MAMA in the enclosure

COS Mechanisms External Shutter (STIS heritage) Aperture mechanism (translational motion in x and y perpendicular to beam) Optic Select Mechanisms 1 and 2 FUV Detector Door

Optics Select Mechanisms (OSM) Full 360 Degree Rotation 101 arcsecond step size, with selectable step rate User can specify minimum one-step movement via FP-POS command OSM2 OSM1

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

COS Design at a Glance

COS Optical Layout Calibration Platform 4 lamps, 3 beam splitters NCM3a, 3b, 3c (Focusing mirrors) NUV Detector (MAMA) NCM2 (Collimating mirror) FUV Detector Head (DVA) External Shutter (not shown) Aperture Mechanism positions 1 of 2 science and 2 calibration Apertures 2 degrees of freedom (x & y translation) Calibration Fold Mirror OSM2 positions 1 of 5 optics 1 degree of freedom (rotation) OSM1 positions 1 of 4 optics 2 degrees of freedom (rotation, focus)

COS Optical Layout

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

COS Spectral Layout for Simultaneous Internal Wavecals and Science Spectra FUV MCP (1 of 2 segments) External Science Internal PtNe Wavecal C B A PtNe Wavecal External Science NUV MAMA

Sample FUV PtNe Wavecal Spectrum  Note: correct aspect ratio shown in lower panel --- graphic courtesy of COS IDT

Sample NUV PtNe Wavecal Spectra Single grating tilt yields 3 stripes G285M R ~ 20,000

COS Spectral Resolution (PSA*) and Bandpass Summary Spectral Element  Range (Å)  Coverage (Å per exp) Resolving Power (l / Dl) Dispersion (Å / pixel) G130M 1150–1450 300 20,000–24,000 ~0.0094 G160M 1405–1775 370 ~0.0118 G140L 1230–2050 >820 2500–3000 ~0.0865 G185M 1700–2100 3 x 35 16,000–20,000 ~0.0342 G225M 2100–2500 20,000–24,000 G285M 2500–3200 3 x 41 ~0.0400 G230L 1700–3200 (1 or 2) x 400 ~0.3887 For BOA, wedge in ND filter degrades resolution by factor of ~2.5 for FUV modes and ~4 for NUV modes.

FUV Central Wavelengths and Wavelength Ranges per Segment Optic Central l (Å) Observed Wavelengths (Å) G130M 1291 1300 1309 1318 1327 1132-1274,1291-1433 1141-1283,1300-1442 1150-1292,1309-1451 1159-1301,1318-1460 1168-1310,1327-1469 G160M 1577 1589 1600 1611 1623 1382-1556,1577-1752 1394-1568,1589-1764 1405-1579,1600-1775 1416-1590,1611-1786 1428-1602,1623-1798 G140L 1105** 1230 <300-970 (not usable – zero-order),1105-2253 <300-1095,1230-2378 Yellow text indicates default setting ** always used with SEGMENT A only

NUV Central Wavelengths Spectral Central l (Å) G185M 1786, 1817, 1835, 1850, 1864 1882, 1890, 1900, 1913, 1921 1941, 1953, 1971, 1986, 2010 G225M 2186, 2217, 2233, 2250, 2268, 2283, 2306, 2325, 2339, 2357, 2373, 2390, 2410 G285M 2617, 2637, 2657, 2676, 2695 2709, 2719, 2739, 2850, 2952 2979, 2996, 3018, 3035, 3057 3074, 3094 G230L 2635, 2950, 3000, 3360 Yellow text indicates default setting

Offset from lc (OSM-steps) A Word About FP-POS COS provides optional parameter FP-POS to perform dispersion-direction or wavelength dithers One FP-POS step corresponds to the minimum movement (one step) of the OSM carrying the spectral element being used For an exposure with any grating and lc, the observer may specify FP-POS = 1, 2, 3, 4, or AUTO (default is FP-POS=3) AUTO corresponds to an automatic sequence of four separate exposures - that is, one at each FP-POS in the order FP-POS=1,2,3,4 The central wavelength for any grating corresponds to FP-POS=3 (default) for that grating; the table gives the offset from lc in OSM steps for each allowed FP-POS NOTE: For the FUV gratings, the lc are always separated by four OSM1 steps, so there is a continuous increment between FP-POS of adjacent central wavelength settings. This is not the case for the NUV gratings. FP-POS Offset from lc (OSM-steps) 1 -2 2 -1 3 4 +1

FUV Default Wavelength Ranges (FP-POS=3) Note: G140L segment A records signal in “FUSE band” (l < l 100 Å ) } G140L } G160M } G130M RED: Segment A YELLOW: Segment B

Covering the (FUV) Gap G130M and G160M: an 8 OSM1-step offset is required to bridge the segment A/B detector gap (any two non-adjacent lc settings at same FP-POS) G130M: gap is ~17 Å; one FP-POS step ~2.3 Å; step between successive lc (e.g., 1309 to 1318) is ~9 Å G160M: gap is ~21 Å; one FP-POS step ~2.9 Å; step between successive lc (e.g., 1600 to 1611) is ~11.6 Å

NUV Default Wavelength Ranges (FP-POS=3) G230L G285M G225M G185M A B C (this labeling for M modes only)

Field-of-View (arcsec2) COS Imaging Summary Optical Element  Range (Å) Plate Scale Field-of-View (arcsec2) FOV (Number of Pixels) TA1 1650–3200 0.024 arcsec/pixel 0.072 arcsec/resel 12.5 full diameter 4.9 un-vignetted 166 full diameter 100 un-vignetted

Optic Select Mechanism residual settling (“drift”) Spectral Drift Optic Select Mechanism residual settling (“drift”) Non-repeatable post-rotation motion measured in TV and ambient testing G160M #03285081037 Plateau G285M #03285123358 G140L #03285101309 Steep Rise Drift exceeds 1 resolution element Drift exceeds 1 resolution element

Spectral Drift Correction (TAGFLASH) Correction needed to meet design requirements Spectral resolution (and perhaps more importantly, line shape) Wavelength zero point TAGFLASH (FLASH=YES) will embed wavecals in science time TIME-TAG mode ONLY; no correction for ACCUM No overhead for wavecals at all CALCOS corrects for drifts

Spectrum Drift Correction (TAGFLASH) Analysis of thermal vac data shows image motion can be corrected to <0.25 resel/hr FLASH=YES (TAGFLASH) now implemented as default observing mode for COS Lamp is flashed at beginning and at intervals during every TIME-TAG exposure (based on time since last mechanism motion) GO does not specify flash intervals or durations Projected lamp usage sufficient to support COS over projected lifetime

Flash Characteristics and Monitoring Flash Frequency (after major OSM move) Always flash at start of exposure and at automatic intervals First orbit following OSM move (4 flashes) Flash at beginning exposure as reference Intermediate flash early (after 600 sec) to follow fast drift Intermediate flash after another ~1800 sec to follow change in drift slope Flash every 2400 sec thereafter If CVZ, then flash again after another 2400 sec, etc Flash durations typically 5-10 seconds SMOV and early Cycle 17 tests and monitoring planned Determine drift character on-orbit for suite of OSM motions Method allows changes for on-orbit understanding of drift Early pattern is conservative (more flashes)

Important Modes/Parameters TIME-TAG: position and time of each valid event are saved; pulse heights are recorded for FUV, but not NUV. Doppler correction is performed in the pipeline default, highly recommended, for all targets less than 21,000 cts/sec; OK to 30,000 cts/sec with some continuity breaks (flux cal OK); requires BUFFER-TIME (time to fill one-half buffer or 2.3 million counts) [use ETC to estimate – details in Session 2] ACCUM: each event increments memory location for recording pixel; only final accumulated image is saved. 16384x128 (or 1024x1024) x 16 bits; for all count-rates > 30,000; justify for any other target Pixel locations shifted on-board for orbital doppler correction Important optional parameters: FP-POS=1,2,3,4 or AUTO (either ACCUM or TIME-TAG) FLASH=YES (TIME-TAG and PSA only; so-called TAGFLASH)

Target Acquisition Modes ACQ/IMAGE preferred option, “faint” targets; precise centering ACQ/SEARCH spiral-search; “bright” targets or poorer initial coordinates; moderate centering ACQ/PEAKXD cross-dispersion peakup ACQ/PEAKD long-dispersion peakup (must follow ACQ/PEAKXD) Details in Session 3 Keyes (COS Training) – 7 February 2008 Slide 33 of xx

STIS or COS? COS is 10-30x faster than STIS in FUV at R=20,000 for point sources; even greater advantage at faint end due to low noise and pulse-height discrimination capability COS has quite degraded resolution for extended objects (see table below); for FUV, protions of objects closer than 1 arcsec apart will overlap; for NUV, spectrum stripes will partially overlap for objects more than 1 arcsec in spatial extent R source size   G140L G130M G230L point 3000 20000 2000 0.5" diameter 780 5200 300 1.0" diameter 390 2600 150 1.5" diameter 260 1733.3 100 2.0" diameter 195 1300 75 2.5" diameter 156 1040 60

COS or STIS? In NUV, COS M mode observing is inefficient for cases requiring large spectral coverage In NUV, COS background rate is expected to be 4x lower than STIS, but is TBD COS has no resolution higher than 20,000 and is a UV-only instrument COS FUV TIME-TAG mode includes the pulse-height for superior noise rejection STIS TIME-TAG has higher time-resolution and may be used on brighter targets The answer depends upon your application: refer to the IHBs and use the ETCs to evaluate your targets

Limiting Flux to achieve S/N=10 in 3600 sec exposures with uniform binning corresponding to R~20,000 (0.08 Å). COS PSA aperture used; STIS slit losses included.

Limiting Flux to achieve S/N=10 in 3600 sec exposures with uniform binning corresponding to R~20,000 (0.12 Å). COS PSA aperture used; STIS slit losses included.

Cosmic Origins Spectrograph Supplementary Material

COS Instrument Overview FUV Detector Electronics Box Cal Platform Main Electronics Box Remote Interface Unit FUV Detector 1150-1775A NUV Detector 1750-3200A Optics Select Mechanism–2 (G185M, G225M, G285M, G230L, TA1) Optics Select Mechanism–1 (G130, G140L, G160M, NCM1) Aperture Mechanism

Lamp Lifetime Reduced lamp flash durations On-orbit Usage G130M will be workhorse grating (longest lamp exposures) But other gratings might be used with more OSM motions e.g., extended NUV wavelength coverage Assume 1000 COS orbits per year Assume 4 flashes per orbit (4000 flashes per year, probably high) Assume 15 seconds per flash Assume 10 milli-amp current setting On-orbit Usage 8 amp hr lifetime budget per lamp (2 lamps available) In one year, use 4 x 1000 x 0.01 x 15 / 3600 = 0.17 amp hr /yr A 10 year mission => less than 2 amp hr of on-orbit usage => Lamp on-time should not be a lifetime issue

Central Wavelength Of Stripe B Stripe A Stripe B Stripe C (2nd order) 2635 Å 1334 – 1733 Å 2435 – 2834 Å (1768 – 1967 Å) 2950 Å 1650-2050 Å 2750-3150 Å (1900 – 2100 Å) 3000 Å 1700 – 2100 Å 2800 – 3200 Å (1950 – 2150 Å) 3360 Å 2059 – 2458 Å 3161 – 3560 Å (2nd order: 1580 – 1780 Å) (2164 – 2361 Å) Aperture Size (Arcseconds) (mm) PSA 2.5 (diam.) 0.700 (diam.) BOA 2.5 (Diam.) 0.700 (Diam.) WCA N/A 0.020 x 0.100 FCA 0.750 x 1.750 Detector Plate Scale (mm/arcsec) Active Area (mm) (pixels) Pixel Size Resel Size XDL (FUV) ~265 2 x (85 x 10) 2 x (~14160x~400) 6 x 24 6 x 10 MAMA (NUV) ~970 25.6 x 25.6 1024 x 1024 25 x 25 3 x 3