Target Acq and Imaging Target Acq and Imaging Bright Object Constraints Bright Object Constraints COS or STIS? COS or STIS? COS Training Series III. Optimizing Observations (part 2) --- Soderblom, Friedman, Keyes February mm

Today’s talks  Part 1 (Dave Soderblom): – Acquisitions with COS – Using COS/NUV for imaging  Part 2 (Scott Friedman): – Bright object constraints  Part 3 (Tony Keyes): – Comparing COS and STIS

COS or STIS? COS or STIS? COS Training Series III a.: Acquisitions and NUV Imaging --- Dave Soderblom --- (Scott Friedman, Brittany Shaw) 22 February 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

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 = arcsec One NUV pixel = 1/40 arcsec = arcsec Centering goal should be about 0.01 to 0.02 arcsec Note that FUV resels are 6 pixels wide = 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 Pt-Ne lamp exposed WCA image location implies PSA location (to be checked in SMOV) Shutter opened, target image taken (TIME-TAG) for user-selected exposure time x 4 arcsec sub-array on MAMA read out (about 150 pixels square) and saved x 9 pixel checkbox array passed over image. Pixel with most counts determined The 9 x 9 array is centered on brightest pixel, and a flux-weighted centering algorithm used to calculate target position 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: Pt-Ne lamp is exposed to locate aperture using known offset Spiral search (ACQ/SEARCH), with SCAN-SIZE = 2, 3, 4, or 5 per side FUV acq’s use sub-arrays to avoid airglow lines STEP-SIZE is also a choice, but default (1.767) recommended Several algorithms to find source; CENTER=FLUX-WT, FLUX-WT-FLR, or BRIGHTEST Exposure time about 40 sec for 1 FEFU flat source with G130M or G160M to get the recommended S/N = 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 obtained Mean location in x-d direction computed Known offset applied Telescope moved to centroid Good to 0.03 to 0.04 arcsec. PEAKD for along-dispersion direction: Like ACQ/SEARCH, but linear Telescope moved, exposed, centroid computed Can choose 3, 5 (=DEF), 7, or 9 steps, plus STEP-SIZE Centering options as for ACQ/SEARCH Individual 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) STEP-SIZE = arcsec  Initial point offset by half step if SCAN-SIZE = 2 or 4.

ACQs in Phase II To carry out an imaging acquisition, use Mode=ACQ/IMAGE, Config=COS/NUV Mode=ACQ/IMAGE, Config=COS/NUV Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB There is one Optional Parameter: STRIPE [= DEF, SHORT, MEDIUM, LONG] There is one Optional Parameter: STRIPE [= DEF, SHORT, MEDIUM, 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, use Mode=ACQ/SEARCH, Config=COS/NUV Mode=ACQ/SEARCH, Config=COS/NUV Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB As noted, SCAN-SIZE=2 is recommended unless coordinates are believed inferior. As noted, SCAN-SIZE=2 is recommended unless coordinates are believed inferior. STEP-SIZE can be specified, but DEF is recommended as it exactly fills sky. STEP-SIZE can be specified, but DEF is recommended as it exactly fills sky. CENTER specifies the algorithm CENTER specifies the algorithm FLUX-WT is default and recommended. FLUX-WT is default and recommended. BRIGHTEST returns to brightest pixel and is not recommended. BRIGHTEST returns to brightest pixel and is not recommended. FLUX-WT-FLR subtracts the “floor” value from all points and is recommended for NUV (higher background) and is default for STEP-SIZE = 3, 4, or 5. FLUX-WT-FLR subtracts the “floor” value from all points and is recommended for NUV (higher background) and is default for STEP-SIZE = 3, 4, or 5.

ACQs in Phase II (3) To 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 = grating Aperture=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 direction Then use ACQ/PEAKXD to center in cross-dispersion direction No Optional Parameters No 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)

COS Training Series III b.: Bright Object Issues --- Scott Friedman February mm

Agenda  Bright object concerns  Limit checks  Count rate limits  Bright object mitigation strategies  APT is your friend  A special bonus…if there is time - Pulse Height Distribution

Bright Object Concerns  Excessive count rates can damage any microchannel plate detector  All microchannel plate detectors on HST have bright object limits – STIS (FUV MAMA, NUV MAMA), ACS (SBC)  Both COS detectors (FUV XDL, NUV MAMA) subject to bright object limits  After SM4 there will be 5 working MCP detectors on HST  Every science target and all nearby field targets for every COS observation will have to be cleared for safety. This is a large but necessary burden for observers, PCs, and CSs.

FUV Overlight Limit Checks  HV power supply overcurrent limits – HVAI, HVBI, and AUXI monitored for magnitude and persistence – Triggered if current exceeds I max for time > t min  Global Rate Monitor – Local limit reached before global limit – Value set to limit dead-time induced non-linearity  Local Rate Check – Performed over localized area prior to each exposure

NUV Overlight Limit Checks  Bright Scene Detection – Monitors pairs of anode rows with 32 row spacing – Applicable to extended objects more than point sources  Software Global Monitor – 0.1 second sampling time fastest of all checks – Ineffective above 4  10 6 counts sec -1 due to electronics limitations  Local Rate Check – Performed over localized area prior to each exposure

CARD Count Rate Limits (Constraints and Requirements Document)  FUV Detector – 1500 counts sec -1 resel -1 local limit – No global safety limit set because local limit more restrictive > 21,000 counts sec -1 segment -1 for no data loss  NUV Detector – 4500 counts sec -1 resel -1 local limit – 1.5  10 6 counts sec -1 global limit

Count Rate Limits

Count Rate Screening Limits

Local and Global Flux Limits 1 FEFU = erg cm -2 sec -1 Å -1

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

Bright Object Mitigation Strategies  Use MIRRORB – Attenuates by a factor of ~25 (3.5 magnitudes) – Forms double image

Bright Object Checking in APT  Target and field objects must be checked for safety  PSA and BOA displayed on DSS image – Aperture transmissions separately correct  GALEX catalog information can be imported into APT – AIS has FUV ( p = 1524 ) and NUV bands ( p = 2297 ) – AIS has FUV ( p = 1524 Å) and NUV bands ( p = 2297 Å) – Very useful for clearing objects

APT Listing of Objects in FOV QSO with nearby field star

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 sky – ~60% when DR4 released next month – ~75% when complete  Areas not covered: – Galactic plane – Large and Small Magellanic Clouds

Bright Object Issues - Summary  Both COS MCP detectors are subject to damage if subject to overlight conditions  All science targets and field objects must be checked for safety  The BOA (for all observing modes) and MIRRORB (for NUV imaging only) can be used to attenuate light  Bright Object Tool in APT has many useful features – GALEX data can accurately clear objects (or not!) – ORIENT specification can be used to avoid bright field objects (but use this sparingly)

Pulse Height Distribution  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 Series III c.: Instrument Introduction (continued) --- Tony Keyes February mm

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, portions 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 R R source size G140L G130M G230L point " diameter " diameter " diameter " diameter " diameter

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.