Target Acq and Imaging Bright Object Constraints COS or STIS?

1. 170 mm COS Training Series III. Optimizing Observations (part 2) --- Soderblom, Friedman, Keyes --- 22 February 2007 • Target Acq and Imaging • Bright Object Constraints • COS or STIS?

2. 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

3. 170 mm COS Training Series III a.: Acquisitions and NUV Imaging --- Dave Soderblom --- (Scott Friedman, Brittany Shaw) 22 February 2007 • COS or STIS?

4. 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.

5. 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.

6. COS optical schematics

7. What is COS Acquiring? 1450 Å, at PSA: 95% throughput (R. Makidon)

8. Corrected image at MAMA FWHM about 2.4 pix 10% of light in center pixel

9. Centering for throughput Precise centering not important for throughput; leeway of 0.5 arcsec

10. Centering for wavelength Precise centering is critical for wavelength accuracy: One NUV resel = 3 pixels = 0.075 arcsec One NUV pixel = 1/40 arcsec = 0.025 arcsec Centering 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.

11. 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. • 4 x 4 arcsec sub-array on MAMA read out (about 150 pixels square) and saved. • 9 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.

12. 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

13. 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

14. Dynamic range for ACQ/IMAGE

15. Optical performance

16. 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 = 40. • Quality of centering probably 0.1 to 0.2 arcsec.

17. 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

18. 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.

19. 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.

20. The spiral search • Note default (and correct) STEP-SIZE = 1.767 arcsec • Initial point offset by half step if SCAN-SIZE = 2 or 4.

21. ACQs in Phase II To carry out an imaging acquisition, use • Mode=ACQ/IMAGE, Config=COS/NUV • Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB • There is one Optional Parameter: STRIPE [= DEF, SHORT, MEDIUM, LONG] • The “length” refers to wavelength. • No need to specify ordinarily, DEF=MEDIUM. • G230L is an exception. • Use ETC to calculate exposure time for S/N = 40.

22. ACQs in Phase II (2) To carry out an imaging search phase, use • Mode=ACQ/SEARCH, Config=COS/NUV • Aperture=PSA or BOA; SpecEl = MIRRORA or MIRRORB • 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. • CENTER specifies the algorithm • FLUX-WT is default and 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.

23. ACQs in Phase II (3) To carry out a dispersed-light acquisition, use first • Mode=ACQ/SEARCH, Config=COS/FUV (or NUV) • Aperture=PSA or BOA; SpecEl = grating • Choose SCAN-SIZE and STEP-SIZE. • CENTER specifies the algorithm; same as above. • Then use ACQ/PEAKXD to center in cross-dispersion direction • No Optional Parameters • Then use ACQ/PEAKD to center along wavelength. • NUM-POS (linear), STEP-SIZE, and CENTER as above.

24. Extended sources (NUV)

25. In this talk … • I covered: • Basic imaging acquisitions for point sources • Dispersed-light acquisitions for point sources • Mainly PSA and MIRRORA • Obtaining NUV images with COS • I did not cover: • Extended objects • High background situations • Off-center acquisitions (e.g., to dither) • Structured or multiple sources

26. 170 mm COS Training Series III b.: Bright Object Issues --- Scott Friedman --- 22 February 2007

27. 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

28. 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.

29. FUV Overlight Limit Checks • HV power supply overcurrent limits • HVAI, HVBI, and AUXI monitored for magnitude and persistence • Triggered if current exceeds Imax for time > tmin • 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

30. 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  106 counts sec-1 due to electronics limitations • Local Rate Check • Performed over localized area prior to each exposure

31. 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  106 counts sec-1 global limit

32. Count Rate Limits

33. Count Rate Screening Limits

34. Local and Global Flux Limits 1 FEFU = 10-15 erg cm-2 sec-1Å-1

35. 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

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

37. 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 Å) and NUV bands (p = 2297 Å) • Very useful for clearing objects

38. APT Listing of Objects in FOV QSO with nearby field star

40. PSA zone

41. PSA zone BOA zone

43. GALEX AIS Sky Coverage Tiles of All-Sky Imaging Survey (AIS)

44. 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

45. 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)

46. Pulse Height Distribution A few more words…

47. Microchannel Plates Wiza, 1979

48. Microchannel Plates

49. Pulse Heights (FUV only) Modal Gain Threshold FUSE • Pulse height thresholding can be used to screen photons • Default thresholding will be determined during SMOV

50. 170 mm COS Training Series III c.: Instrument Introduction (continued) --- Tony Keyes --- 22 February 2007 • COS or STIS?