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The Bright Object Tool. Ron Downes August 2007. Outline. Documentation High-level assumptions Limitations How it works (the guts) Basic processing Low-level assumptions Use of other catalogs (GALEX, 2MASS) Sample output Table view Graphical (Aladin) view
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The Bright Object Tool Ron Downes August 2007
Outline • Documentation • High-level assumptions • Limitations • How it works (the guts) • Basic processing • Low-level assumptions • Use of other catalogs (GALEX, 2MASS) • Sample output • Table view • Graphical (Aladin) view • Search Sizes and Triggers (background info)
Documentation • can be found at: http://apst.stsci.edu/apt/apt-bright-objects/index.html • Includes: • New Bright Object Tool Documentation (Detector-specific) • New Bright Object Tool Testing (Detector-specific) • Original Bright Object Tool Documentation (general) • Movie can be found at: http://apst.stsci.edu/apt/external/help/movies/SBC.html
High-level Assumptions • the search area is a circle centered on the fiducial point of the aperture (i.e. where the prime target will be placed) that encompasses the entire aperture, padded to account for possible mispointings (padding values come from the instrument teams). • all objects are Main Sequence stars (with only 1 GSC2 color, this is the best we can do). • all objects are unreddened (with only 1 GSC2 color, this is the best we can do). This is clearly incorrect in the plane, but is not unreasonable at high galactic latitude (where many HST observation are obtained). Since reddening would decrease the ultraviolet flux, this is also the conservative assumption when doing health-and-safety checks. • low-level assumptions (detector specific) are noted in the detailed explanation of how to tool works.
Limitations • assumes all field stars are normal MS stars (use the ETC for prime target) • no extended targets are checked (not in GSC2) • variable targets only use magnitude at time of GSC2 • STIS/MAMA: M and H gratings only have the bluest and reddest central wavelength in the tool - when processing an intermediate settings, the tool selects the closest value • ACS/WFC and ACS/HRC: only 2 gains per detector are supported, and polarization is not supported • NICMOS: polarization is not supported • COS: only selected central wavelengths are in the tool • WFC3/IR: grisms are not supported • WFC3/UVIS: grisms and quad filters are not supported
How it works - Overview • obtain exposure information from the Phase II proposal • based on target coordinates and aperture, search GSC2 for all stars in the search area • convert GSC2 F and J to V and B-V • use B-V to determine spectral type • determine counts and count rates for each star based on ETC-generated lookup tables • compare with trigger limits • display results in both tabular and graphical output
How it works - Details • load APT file and select exposures/visits to process • select BOT tool and click on Update Display button to process • GSC2 is searched for all objects in the search area. The information returned is the name, coordinates, and magnitudes (F and/or J for faint objects, V and B-V from Tycho for bright [V<12] objects). For COS, BOT checks for objects in both apertures.
Details – cont. Magnitude Cases • Objects with F and J – processing continues • Objects with no F and no J - processing stops • Objects with J and no F - processing stops (except WFC3/IR) For WFC3/IR objects with only a J mag, an extended search is performed around the position and the F mag of the object is assumed to be equal to the faintest F mag detected. This conservative assumption allows us to process and clear many objects that would otherwise require "manual" processing.
Details – cont. • for objects with F and no J - processing stops (except for ACS/SBC, STIS/MAMA, WFC3/IR, and COS) For ACS/SBC, STIS/MAMA, and COS objects that have only an F mag, an extended search is performed around the position and the J mag of the object is assumed to be equal to the faintest J mag detected. This conservative assumption allows us to process and clear many objects that would otherwise require "manual" processing. For WFC3/IR objects that have only an F mag, assume the object is an M2V star (with appropriate J-F), and continue processing.
Details – cont. • for objects with F and J, but "not a star" - processing stops (except for ACS/SBC, STIS/MAMA, WFC3, and COS) For ACS/SBC, STIS/MAMA, WFC3, and COS, the GSC2 flag indicating the object is not a star is ignored for all faint (detector-specific cutoffs) objects. This is due to the fact that the reason most of these objects have the flag set due to the poor S/N in the PSF. While this issue is important in determining if the object can be used as a guide star, it is not relevant for bright object checking. Failure to do this results in a large number of "unknown" objects appearing in many fields, which then need to be "manually" cleared. Note that an additional step has been proposed to handle cases with only 1 magnitude (will discuss later).
Details – cont. • F and J are converted to V and B-V via standard conversions (with some sanity checks) • B-V is used to determine a spectral type (with adjustments due to unusually large photometric errors) For ACS/SBC, STIS/MAMA, and COS, all objects with B-V<+0.1 are assumed to be O5V stars. This assumption is necessary due to the near degeneracy in the spectral type-color relation at the blue end. For ACS/SBC, STIS/MAMA, and COS, all objects brighter than a Vcrit value (spectral element dependent) are assumed to be O5V stars. This assumption is necessary due to the lack of reddening information.
Details – cont. • adjust the V magnitude for ACS/SBC and COS observations to account for the deviations of real stars from the models (as is done in the ETC) • based on magnitude, spectral type, and exposure information, determine the count rate and total counts for each star in the search area via ETC-based lookup tables For COS and STIS spectroscopy, the aperture throughput is taken into account.
Details – cont. • for ACS/SBC, STIS/MAMA, and COS, also perform a global field check • compare the count rates and total counts against trigger values to determine the status of each object (trigger values for each detector included in handout) • present the results both a tabular and graphical format
Change to single mag processing • For ACS/SBC, STIS/MAMA, and COS • When only 1 magnitude is available, assume the object is O5V • Determine if object is safe • If safe, mark object as such • If not safe, continue with extended field search • ACS/STIS has signed off, need COS
Use of other catalogs • 2MASS for WFC3/IR • Search 2MASS catalog for field objects (ignore contaminated objects) • No spectral type determination needed • Use J or H magnitude, depending on filter, to determine count rates • Fall back to GSC2 if no 2MASS data
Use of other catalogs – cont. • GALEX problems • Incompleteness • Crowded fields (LMC, Galactic Plane) • UV bright objects (most O and B stars) • How to use fluxes to determine count rates • Spectral typing not useful (most objects are A stars) • Cross-matching between GSC2 and GALEX problematic • Decision: GALEX not used (should revisit once GALEX survey is more complete)
How to Run BOT • There is a training video that shows how to run the tool and explains the output: http://apst.stsci.edu/apt/external/help/movies/SBC.html • You run the tool just like any other APT tool (select the exposure(s) in the Tree Editor and select BOT)
