Spotting the life of stars

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Spotting the life of starsPi of the Sky Project

• Katarzyna Kwiecinska
• UKSW-SNS
• on behalf of the Pi of the Sky collaboration

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Pi of the Sky goal is

• to investigate objects of the variability
  timescale from seconds to years. Especially
• to look for optical counterparts of GRB
  afterglows
• GRB are short (0.01-100s) impulses
• They are emitted by extragalactic sources
• They can originate from supernovae explosions
  (collapse to a black hole), neutron stars
  collisions, etc
• Nowadays satellites record 2-3 GRB per month and
  send alerts via GRB Coordinate Network

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Pi of the Sky

• Taking so many exposures we can observe many
  other astronomical phenomena like
• Variable stars
• Meteors, etc.

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Pi of the Sky apparatus
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Data flow
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Data flow

• 2 cams x 3000 exposures x 8 MB50 GB/night
• First and Second Level Triggers implemented
• 20 000 stars per image (30 000 on co added
  images)
• 120 000 000 photometric measurements/night
• Raw data accessible for 5 days
• After reduction 2 GB/night
• Photometric data flown home every 3 months

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An astronomical pipeline

• Obtaining a light curve of a star requires
  several steps of analysis on each frame.

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Reduction. Its goal is to reduce an apparatus
effects. In general it consists of two steps

• A dark frame subtraction. It allows to reduce
  effect of a dark current accumulating in CCD
  pixels. The dark frame is obtained by exposing a
  chip with a closed shutter.

• A flat-field frame division. It permits to
  correct a lens optics. In general the flat-field
  frame is obtained by exposing the chip to a
  source of homogeneous light for example by
  taking sky pictures at dusk. Unfortunately, it is
  not a good method for large FOV because of a
  brightness gradient. So our flat-field is
  obtained from a median of frames taken at the
  same night with a fixed mount.

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Photometry

• It creates a list of stars occurring on the
  frame at coordinates (x,y) and calculates their
  instrumental brightness. The instrumental
  brightness is simply a sum of pixel values in a
  certain pre-defined neighborhood of a star,
  called aperture, minus the background level.

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Astrometry

• It compares the list of stars in instrumental
  coordinates (x,y) witha star catalogue and finds
  transformations of instrumental coordinates into
  the physical coordinates on the sky. Then it
  calculates the celestial coordinates of each
  star on the frame.

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Cataloguing

• It calibrates the instrumental brightness by
  comparing results for certain number of reference
  stars, and determines the physical brightness, or
  magnitudo, of each star on the frame.

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Visualization - Data Base
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Sigma vs. magnitudo
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Effective FOV
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Effective FOV

• ?

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Effective FOV

• 33 x 33 CCD
• 29 x 29 taking into account the mount drift
  and the mask
• 28 x 28 if we want to compare two nights

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Light curves
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Conclusions

1. Because of the mount drift and the shift between
   succeeding nights, effective FOV is about 5 deg
   smaller from each side of a frame.
2. Imperfect cataloguing procedure makes that mean
   magnitudo depends significantly on time. It is of
   importance especially when we talk about
   long-period stars.
3. If we want to distinguish a physical variability
   from the imperfect cataloguing effect, it is
   necessary to make the photometry much more
   stable.

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• The end.