1. 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

2. 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

3. 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

4. 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

5. 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)

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

7. 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 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 arcsec) ; no subarrays
3 x 3 pixels per resel (0.072 x arcsec per resel)
opto-isolator problem fixed

8. COS Detectors – FUV XDL
170 mm
Top View

9. COS Detectors – NUV MAMA
in the enclosure

10. 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

11. 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

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

13. COS Design at a Glance

14. 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)

15. COS Optical Layout

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

17. 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

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

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

20. 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.

21. FUV Central Wavelengths and Wavelength Ranges per Segment
Optic
Central l
(Å)
Observed
Wavelengths (Å)
G130M
1291
1300
1309
1318
1327 , , , , ,
G160M
1577
1589
1600
1611
1623 , , , , ,
G140L
1105**
1230
< (not usable – zero-order),
< ,
Yellow text indicates default setting ** always used with SEGMENT A only

22. 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

23. 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

24. 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

25. 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

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

27. 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

28. 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 #
Plateau
G285M #
G140L #
Steep Rise
Drift exceeds
1 resolution element
Drift exceeds
1 resolution element

29. 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

30. 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

31. 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)

32. 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)

33. 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

34. 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

35. 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

36. 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.

37. 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.

38. Cosmic Origins Spectrograph Supplementary Material

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

41. 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

42. 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 Å 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