The Bright Object Tool

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The Bright Object Tool

• Ron Downes
• August 2007

2
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)

3
Documentation

• can be found at
• http//apst.stsci.edu/apt/apt-bright-objects/ind
  ex.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/S
  BC.html

4
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.

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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

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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

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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 Vlt12
  objects). For COS, BOT checks for objects in both
  apertures.

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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.

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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.

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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).

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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-Vlt0.1 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.

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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.

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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

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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

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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

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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)

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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/SB
  C.html
• You run the tool just like any other APT tool
  (select the exposure(s) in the Tree Editor and
  select BOT)

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Sample Outputs - Tabular
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Prior to Processing
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Post-processing
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Details Page
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Sample Outputs - Graphical
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Post-processing
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GSC2 in background
25
POS TARG
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Parallel
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Search Sizes
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Trigger Values
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Trigger Values cont.
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Trigger Values cont.
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Trigger Values cont.
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Trigger Values cont.
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Trigger Values cont.
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Trigger Values cont.
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Trigger Values cont.