• Target Acq and ImagingTarget Acq and Imaging• Bright Object ConstraintsBright Object Constraints• COS or STIS?COS or STIS?

COS Training SeriesIII. Optimizing Observations (part 2)--- Soderblom, Friedman, Keyes ---

22 February 2007

170 mm

Today’s talks

Part 1 (Dave Soderblom): Part 1 (Dave Soderblom): – Acquisitions with COSAcquisitions with COS– Using COS/NUV for imagingUsing COS/NUV for imaging

Part 2 (Scott Friedman): Part 2 (Scott Friedman): – Bright object constraintsBright object constraints

Part 3 (Tony Keyes): Part 3 (Tony Keyes): – Comparing COS and STISComparing COS and STIS

• COS or STIS?COS or STIS?

COS Training SeriesIII a.: Acquisitions and NUV Imaging

--- Dave Soderblom ---(Scott Friedman, Brittany Shaw)

22 February 2007

170 mm

The story so far …

COS is an new ultraviolet spectrograph for HST to be installed in SM4, built by a team in Boulder (J. Green, P.I., plus Ball Aerospace)

The FUV channel is optimized for spectra of faint point sources at moderate resolution and signal-to-noise. The FUV detector is an open-faced XDL device, good for about 1100 to 1850 Å.

The NUV channel was added by NASA to provide some additional capabilities. This now includes imaging and imaging acquisitions. The NUV detector is a STIS spare MAMA, good from about 1800 to 3200 Å.

Both are used in TIME-TAG mode (also ACCUM) which includes real-time wavelength calibration.

COS Acquisitions

COS is a small-aperture instrument; acquisitions are critical for success of the observation and for data quality.

The quickest and most effective way to acquire – by far – is with an imaging acquisition.

As part of its initial design, COS flight S/W includes the means to acquire and center targets using dispersed light.

COS optical schematics

What is COS Acquiring?

1450 Å, at PSA: 95% throughput (R. Makidon)

Corrected image at MAMA

FWHM about 2.4 pixFWHM about 2.4 pix10% of light in center pixel10% of light in center pixel

Centering for throughput

Precise centering not important for throughput; leeway of 0.5 arcsec

Centering for wavelength

Precise centering is critical for wavelength accuracy:One NUV resel = 3 pixels = 0.075 arcsecOne NUV pixel = 1/40 arcsec = 0.025 arcsecCentering goal should be about 0.01 to 0.02 arcsec

Note that FUV resels are 6 pixels wide = 0.132 arcsec; again about 0.01 to 0.02 arcsec acquisition is desired.

Imaging acquisition steps

NUV ACQ/IMAGE is recommended for most cases:• Quick• Accurate• Minimal overhead (~2 min) to switch to a grating of choice

1. Pt-Ne lamp exposed.2. WCA image location implies PSA location (to be checked in SMOV).3. Shutter opened, target image taken (TIME-TAG) for user-selected exposure time.4. 4 x 4 arcsec sub-array on MAMA read out (about 150 pixels square) and saved.5. 9 x 9 pixel checkbox array passed over image. Pixel with most counts determined.6. The 9 x 9 array is centered on brightest pixel, and a flux-weighted centering algorithm used

to calculate target position.7. HST moved to this pointing and a verification image is taken and saved.

Imaging acquisition examples

1. Old (inactive) G dwarf, V = 13:• Exposure time for S/N = 40 is 40 sec (ETC)• Total time is (2 x 40 sec) + 7 min = ~9 min

2. QSO (flat spectrum), needs 10 orbits to get FUV spectrum to S/N = 20 at 1300 Å:• Target flux is 1.3 FEFU• G130M; Exposure time for S/N = 40 is 4.3 sec

3. QSO (flat spectrum), needs 10 orbits to get NUV spectrum to S/N = 20 at 1850 Å:• Target flux is 3.7 FEFU• G185M; Exposure time for S/N = 40 is 1.5 sec

• FEFU = Femto-erg flux unit = 10–15 ergs cm–2 s–1 Å–1

Imaging acquisition limits

Imaging mode is very sensitive. Local count rate screening limit is 80 per pixel per sec. For flat spectrum, what flux hits this limit?

• PSA + MIRRORA: 2 FEFU• BOA + MIRRORA: 400 FEFU• PSA + MIRRORB: 30 FEFU• BOA + MIRRORB: 6,000 FEFU

Dynamic range for ACQ/IMAGE

Optical performance

Dispersed-light acquisitions 1

Some targets will be too bright for an imaging acquisition, even with the BOA and MIRRORB. But if an object is safe to get a spectrum of, it can always be acquired in dispersed light.

COS can acquire targets using the spectrum itself:1. Pt-Ne lamp is exposed to locate aperture using known offset.2. Spiral search (ACQ/SEARCH), with SCAN-SIZE = 2, 3, 4, or 5 per side.3. FUV acq’s use sub-arrays to avoid airglow lines.4. STEP-SIZE is also a choice, but default (1.767) recommended.5. Several algorithms to find source; CENTER=FLUX-WT, FLUX-WT-FLR, or

BRIGHTEST.6. Exposure time about 40 sec for 1 FEFU flat source with G130M or G160M to get the

recommended S/N = 40.7. Quality of centering probably 0.1 to 0.2 arcsec.

Dispersed-light acquisitions 2

After the ACQ/SEARCH, can peak-up in both directions:PEAKXD for cross-dispersion direction:

TIME-TAG spectrum obtainedMean location in x-d direction computedKnown offset appliedTelescope moved to centroidGood to 0.03 to 0.04 arcsec.

PEAKD for along-dispersion direction:Like ACQ/SEARCH, but linearTelescope moved, exposed, centroid computedCan choose 3, 5 (=DEF), 7, or 9 steps, plus STEP-SIZECentering options as for ACQ/SEARCHIndividual spectra not saved, but total counts are

Dispersed-light examples

Flat-spectrum sources, same “faint QSO” that needs 10 orbits to reach S/N = 20:

• FUV: G130M, 1300 Å: 60 counts/sec from spectrum, 27 sec exposure• NUV: G185M, 1850 Å: 23 counts/sec from source, background = 16;

117 sec needed for S/N = 40

Note that these are exposure times per dwell point, plus 20 sec overhead per point; this is what makes D-L acq’s relatively slow.

Initial pointing and sky searches

COS has a very small aperture (2.5 arcsec), although the image plane “sees” slightly more of the sky than that.

How much sky should be searched (ACQ/SEARCH) to ensure a good acquisition every time without unduly wasting telescope time?

The quality of dead-reckoning pointings after SM4 is not yet known, but with the advent of the GSC2 coordinate system and with regular aperture location determinations, initial pointings with errors < 1 arcsec are anticipated.

Our Cycle 17 recommendation is that observers use a 2x2 spiral at the start of their acquisition sequence to guarantee acquiring the target. This may be eliminated in Cycle 18.

The spiral search

Note default (and correct) Note default (and correct) STEP-SIZE = 1.767 arcsecSTEP-SIZE = 1.767 arcsec

Initial point offset by half step if Initial point offset by half step if SCAN-SIZE = 2 or 4.SCAN-SIZE = 2 or 4.

ACQs in Phase II

To carry out an imaging acquisition, useTo carry out an imaging acquisition, use• Mode=ACQ/IMAGE, Config=COS/NUVMode=ACQ/IMAGE, Config=COS/NUV• Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORBAperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB• There is one Optional Parameter: STRIPE There is one Optional Parameter: STRIPE [= DEF, SHORT, MEDIUM, [= DEF, SHORT, MEDIUM,

LONG]LONG]• The “length” refers to wavelength.The “length” refers to wavelength.• No need to specify ordinarily, DEF=MEDIUM.No need to specify ordinarily, DEF=MEDIUM.• G230L is an exception.G230L is an exception.

• Use ETC to calculate exposure time for S/N = 40.Use ETC to calculate exposure time for S/N = 40.

ACQs in Phase II (2)

To carry out an imaging search phase, useTo carry out an imaging search phase, use• Mode=ACQ/SEARCH, Config=COS/NUVMode=ACQ/SEARCH, Config=COS/NUV• Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORBAperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB• As noted, SCAN-SIZE=2 is recommended unless coordinates are believed As noted, SCAN-SIZE=2 is recommended unless coordinates are believed

inferior.inferior.• STEP-SIZE can be specified, but DEF is recommended as it exactly fills STEP-SIZE can be specified, but DEF is recommended as it exactly fills

sky.sky.• CENTER specifies the algorithmCENTER specifies the algorithm

• FLUX-WT is default and recommended.FLUX-WT is default and recommended.• BRIGHTEST returns to brightest pixel and is BRIGHTEST returns to brightest pixel and is notnot recommended. recommended.• FLUX-WT-FLR subtracts the “floor” value from all points and is FLUX-WT-FLR subtracts the “floor” value from all points and is

recommended for NUV (higher background) and is default for STEP-SIZE = recommended for NUV (higher background) and is default for STEP-SIZE = 3, 4, or 5.3, 4, or 5.

ACQs in Phase II (3)

To carry out a dispersed-light acquisition, use firstTo carry out a dispersed-light acquisition, use first• Mode=ACQ/SEARCH, Config=COS/FUV (or NUV)Mode=ACQ/SEARCH, Config=COS/FUV (or NUV)• Aperture=PSA or BOA; SpecEl = gratingAperture=PSA or BOA; SpecEl = grating• Choose SCAN-SIZE and STEP-SIZE.Choose SCAN-SIZE and STEP-SIZE.• CENTER specifies the algorithm; same as above.CENTER specifies the algorithm; same as above.• Then use ACQ/PEAKXD to center in cross-dispersion directionThen use ACQ/PEAKXD to center in cross-dispersion direction

• No Optional ParametersNo Optional Parameters• Then use ACQ/PEAKD to center along wavelength.Then use ACQ/PEAKD to center along wavelength.

• NUM-POS (linear), STEP-SIZE, and CENTER as above.NUM-POS (linear), STEP-SIZE, and CENTER as above.

Extended sources (NUV)

In this talk …In this talk …

I covered:I covered:• Basic imaging acquisitions for point sourcesBasic imaging acquisitions for point sources• Dispersed-light acquisitions for point sourcesDispersed-light acquisitions for point sources• Mainly PSA and MIRRORAMainly PSA and MIRRORA• Obtaining NUV images with COSObtaining NUV images with COS

I did not cover:I did not cover:• Extended objectsExtended objects• High background situationsHigh background situations• Off-center acquisitions (e.g., to dither)Off-center acquisitions (e.g., to dither)• Structured or multiple sourcesStructured or multiple sources

COS Training SeriesIII b.: Bright Object Issues

--- Scott Friedman ---22 February 2007

170 mm

Agenda

Bright object concernsBright object concerns Limit checksLimit checks Count rate limitsCount rate limits Bright object mitigation strategiesBright object mitigation strategies APT is your friendAPT is your friend A special bonus…if there is time - Pulse Height DistributionA special bonus…if there is time - Pulse Height Distribution

Bright Object Concerns

Excessive count rates can damage any microchannel plate Excessive count rates can damage any microchannel plate detectordetector

All microchannel plate detectors on HST have bright object All microchannel plate detectors on HST have bright object limitslimits

– STIS (FUV MAMA, NUV MAMA), ACS (SBC)STIS (FUV MAMA, NUV MAMA), ACS (SBC) Both COS detectors (FUV XDL, NUV MAMA) subject to Both COS detectors (FUV XDL, NUV MAMA) subject to

bright object limitsbright object limits After SM4 there will be 5 working MCP detectors on HSTAfter SM4 there will be 5 working MCP detectors on HST Every science target and all nearby field targets Every science target and all nearby field targets for every COS for every COS

observationobservation will have to be cleared for safety. This is a large will have to be cleared for safety. This is a large but necessary burden for observers, PCs, and CSs.but necessary burden for observers, PCs, and CSs.

FUV Overlight Limit Checks

HV power supply overcurrent limitsHV power supply overcurrent limits– HVAI, HVBI, and AUXI monitored for magnitude and persistenceHVAI, HVBI, and AUXI monitored for magnitude and persistence– Triggered if current exceeds ITriggered if current exceeds Imaxmax for time > t for time > tminmin

Global Rate MonitorGlobal Rate Monitor– Local limit reached before global limitLocal limit reached before global limit– Value set to limit dead-time induced non-linearityValue set to limit dead-time induced non-linearity

Local Rate CheckLocal Rate Check– Performed over localized area prior to each exposurePerformed over localized area prior to each exposure

NUV Overlight Limit Checks

Bright Scene DetectionBright Scene Detection– Monitors pairs of anode rows with 32 row spacingMonitors pairs of anode rows with 32 row spacing– Applicable to extended objects more than point sourcesApplicable to extended objects more than point sources

Software Global MonitorSoftware Global Monitor– 0.1 second sampling time fastest of all checks0.1 second sampling time fastest of all checks– Ineffective above 4 Ineffective above 4 10 1066 counts sec counts sec-1-1 due to electronics due to electronics

limitationslimitations Local Rate CheckLocal Rate Check

– Performed over localized area prior to each exposurePerformed over localized area prior to each exposure

CARD Count Rate Limits(Constraints and Requirements Document)

FUV DetectorFUV Detector– 1500 counts sec1500 counts sec-1-1 resel resel-1-1 local limitlocal limit– No global safety limit set because local limit more restrictiveNo global safety limit set because local limit more restrictive

> 21,000 counts sec21,000 counts sec-1-1 segment segment-1-1 for no data loss for no data loss

NUV DetectorNUV Detector– 4500 counts sec4500 counts sec-1-1 resel resel-1-1 local limitlocal limit– 1.5 1.5 10 1066 counts sec counts sec-1-1 global limitglobal limit

Count Rate Limits

Count Rate Screening Limits

Local and Global Flux Limits

1 FEFU = 10-15 erg cm-2 sec-1 Å-1

Bright Object Mitigation Strategies

Use Bright Object Aperture (BOA)Use Bright Object Aperture (BOA)– Available for all modes (spectroscopy, imaging, target acquisition)Available for all modes (spectroscopy, imaging, target acquisition)– Wedge in BOA degrades resolution by factor of 3-5Wedge in BOA degrades resolution by factor of 3-5

Bright Object Mitigation Strategies

Use MIRRORBUse MIRRORB– Attenuates by a factor of ~25 (3.5 magnitudes)Attenuates by a factor of ~25 (3.5 magnitudes)– Forms double imageForms double image

Bright Object Checking in APT

Target and field objects must be checked for safetyTarget and field objects must be checked for safety PSA and BOA displayed on DSS imagePSA and BOA displayed on DSS image

– Aperture transmissions separately correctAperture transmissions separately correct GALEX catalog information can be imported into APTGALEX catalog information can be imported into APT

– AIS has FUV (AIS has FUV (pp = 1524 = 1524 Å) and NUV bands () and NUV bands (pp = 2297 = 2297 Å) ) – Very useful for clearing objectsVery useful for clearing objects

APT Listing of Objects in FOV

QSO with nearby field star

PSA zone

PSA zone

BOA zone

GALEX AIS Sky Coverage

Tiles of All-Sky Imaging Survey (AIS)

GALEX All-Sky Imaging Survey

Covers only a fraction of the skyCovers only a fraction of the sky– ~60% when DR4 released next month~60% when DR4 released next month– ~75% when complete~75% when complete

Areas not covered:Areas not covered:– Galactic planeGalactic plane– Large and Small Magellanic CloudsLarge and Small Magellanic Clouds

Bright Object Issues - Summary

Both COS MCP detectors are subject to damage if subject Both COS MCP detectors are subject to damage if subject to overlight conditionsto overlight conditions

All science targets and field objects must be checked for All science targets and field objects must be checked for safetysafety

The BOA (for all observing modes) and MIRRORB (for The BOA (for all observing modes) and MIRRORB (for NUV imaging only) can be used to attenuate lightNUV imaging only) can be used to attenuate light

Bright Object Tool in APT has many useful featuresBright Object Tool in APT has many useful features– GALEX data can accurately clear objects (or not!)GALEX data can accurately clear objects (or not!)– ORIENT specification can be used to avoid bright field objects ORIENT specification can be used to avoid bright field objects

(but use this sparingly)(but use this sparingly)

Pulse Height Distribution

A few more words…A few more words…

Microchannel Plates

Wiza, 1979

Microchannel Plates

Pulse Heights (FUV only)

Pulse height thresholding can be used to screen photons Default thresholding will be determined during SMOV

Threshold

Modal Gain

FUSE

• COS or STIS?COS or STIS?

COS Training SeriesIII c.: Instrument Introduction (continued)

--- Tony Keyes ---22 February 2007

170 mm

STIS or COS? COS is 10-30x faster than STIS in FUV at R=20,000 for COS is 10-30x faster than STIS in FUV at R=20,000 for

point sources; even greater advantage at faint end due to point sources; even greater advantage at faint end due to low noise and pulse-height discrimination capabilitylow noise and pulse-height discrimination capability

COS has quite degraded resolution for extended objects COS has quite degraded resolution for extended objects (see table below); for FUV, portions of objects closer than (see table below); for FUV, portions of objects closer than 1 arcsec apart will overlap; for NUV, spectrum stripes will 1 arcsec apart will overlap; for NUV, spectrum stripes will partially overlap for objects more than 1 arcsec in spatial partially overlap for objects more than 1 arcsec in spatial extent extent R R R

source size G140L G130M G230L

point 3000 20000 20000.5" diameter 780 5200 3001.0" diameter 390 2600 1501.5" diameter 260 1733.3 1002.0" diameter 195 1300 752.5" diameter 156 1040 60

COS or STIS? In NUV, COS M mode observing is inefficient for cases requiring In NUV, COS M mode observing is inefficient for cases requiring

large spectral coveragelarge spectral coverage In NUV, COS background rate is expected to be 4x lower than STIS, In NUV, COS background rate is expected to be 4x lower than STIS,

but is TBDbut is TBD COS has no resolution higher than 20,000 and is a UV-only instrumentCOS has no resolution higher than 20,000 and is a UV-only instrument COS FUV TIME-TAG mode includes the pulse-height for superior COS FUV TIME-TAG mode includes the pulse-height for superior

noise rejectionnoise rejection STIS TIME-TAG has higher time-resolution and may be used on STIS TIME-TAG has higher time-resolution and may be used on

brighter targetsbrighter targets The answer depends upon your application: refer to the IHBs and use The answer depends upon your application: refer to the IHBs and use

the ETCs to evaluate your targetsthe 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.