Observation and Data Reduction Strategy of WIRCam

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Observation and Data Reduction Strategy of WIRCam

• Chi-hung Yam
• Institute of Astronomy and Astrophysics, Academia
  Sinica

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Outlines

• Overview of WIRCam.
• Observation using WIRCam
• Basic Rule in IR observation
• Optimized exposure time fragment
• Dithering Patterns
• Data evaluation procedures
• Data reduction
• Post-run data pre-processing
• Image stacking
• Science with WIRCam
• Some images of CFHT and IAA projects (Engineering
  and science).

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Overview of WIRCam

• 4 2k-by-2k pixel Hawaii2-RG arrays. FoV is 20x20
  arcmin with resolution 0.3 arcsec per pixel.
• Filters
• Broad-band filters Y, J, H, Ks
• Narrow band filters Low OH-1 Low OH-2, CH4 On,
  CH4 Off, H2, K continuum.
• Most different features
• On-chip guiding
• Sub-pixel dithering
• First on-chip guiding IR array. This function
  works with image stabilization unit to provide
  sub-pixel dithering.
• The measured optical distortion of WIRCam is
  lt0.8 (maximal in the corners of the field) or
  20 pixels.
• Exposure overhead 5 seconds (expected), current
  engineering setup 10 seconds
• Shortest exposure 1 seconds (expected), current
  setup 5.6(readoutguiding) seconds, will be 2.2
  seconds in April.

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Observation using WIRCam

• Basic Rule in IR observation
• Optimize exposure time fragment
• Dithering Patterns
• Overhead
• Nodding
• Read-out

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Basic Rule in IR observation

• The sky variability in the near infrared is
  important. As a canonical rule, the level varies
  by 10 over a 10 minute timescale. The H band is
  notoriously variable by /-0.5 mag. In J and Ks,
  it /-0.3 mag.

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Optimized exposure time fragment

• The longest exposure time is limited by sky
  background.
• The shortest exposure time is limited by image
  read-out time guiding system.

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Dithering Patterns

• MD-Micro-dithering
• DP-Dithering pattern
• WDP-WIRCam dithering pattern
• NDP-Nodding pattern
• Amount of shift NDPgtWDPgtDPgtMD
• For target gt 10 or 30 of the mosaic, use NDP
  will get better sky background.

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WIRCam Dithering Pattern
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Nodding Pattern
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Micro-dithering

• The best seeing at Mauna Kea can reach to 0.50.6
  arc-second. This means images taken by WIRCam
  (0.3 arcsec/pixel) under extremely good seeing
  is under-sampled.
• Therefore, we apply micro-dithering to
  reconstruct under-sampled images. This method
  are used on HST before to enhance image
  resolution.
• The shift of each MD frame is set to 0.5 pixel.
  Each MD loop contains 4 sub-exposures. The
  maximum MD loops is 7, which means 28 frame in
  total.
• The performance of MD is depending on guiding
  system. We need to have guiding star, which is
  bright enough, on each chip to provide.

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Drizzling Algorithm
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Image Stabilize Unit
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Data evaluation Components (Real-time pipeline)

• Factor
• IQ
• Sky background
• Absorption
• Bash script
• Starting RT process as a demon in system.
• Perl scripts
• wircam_realtime_improc
• wircam_realtime_sextract
• C programs
• wcreduce image reduction.
• imextract extracting central region of image.
• seeingstat calculating seeing based on a catalog
  of detected stars.
• fhdrizzle combining micro-dithering/multi-exposur
  e images.
• imabs absorption calculating.
• libeir library for ELIXIR_IR system.
• Other tools
• SEXtractor (Terapix)
• Configuration files

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Image Reduction

• Raw images are reduced right after each exposure.
• NO SKY BACKGROUNG SUBTRACTING!!

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Yes
No
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Absorption

• The absorption is calculated by the following
  equations

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Data reduction

• Post-run pre-processing
• Images will be reduce in CFHT using Elixir-IR
  data pipeline.
• Current version of Elixir-IR is IDL version, not
  yet finalized.
• Image stacking
• The very accurate astrometry (WCS) is need for
  image stacking.
• Distortion of the WIRCam images should be
  considered.
• Terapix_at_fr will provide image stacking for WIRCam
  images
• We are also developing our own software package
  for WIRCam user in ASIAA.
• Data archiving

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Science with WIRCam

• Galactic Astrophysics
• Star formation
• The initial mass function
• Galactic structure and stellar populations
• Galaxy evolution
• Nearby galaxy
• Extragalactic Astronomy and Cosmology
• Galaxy counts
• Galaxy clusters
• Super nova event
• QSO

http//www.cfht.hawaii.edu/News/Projects/WIRCam/Ne
w/WSGroup/JLB_Science.pdf
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NGC 1976
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