Transcript Prezentacja programu PowerPoint

Spotting the life of stars
„Pi of the Sky” Project
Katarzyna Kwiecińska
UKSW-SNŚ
on behalf of the Pi of the Sky collaboration
„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
„Pi of the Sky”
Taking so many exposures we can
observe many other astronomical
phenomena like:
• Variable stars
• Meteors, etc.
„Pi of the Sky” apparatus
Data flow
Data flow
• 2 cams x 3000 exposures x 8 MB~50
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
An astronomical pipeline
Obtaining a light curve of a star
requires several steps of analysis
on each frame.
1. Reduction. Its goal is to reduce an
apparatus effects. In general it consists of
two steps:
a. A flat-field
dark frame
subtraction.
b.
frame
division. It
It allows
permits
reduce effect
a dark
to correct
a lens of
optics.
Incurrent
general
accumulating
in CCD
The
the
flat-field frame
is pixels.
obtained
by
dark frame
is chip
obtained
by exposing
exposing
the
to a source
of
a chip with a closed
homogeneous
light; shutter.
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.
2. 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.
3. Astrometry
It compares the list of stars in
instrumental coordinates (x,y) with
a 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.
4. 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.
Visualization - Data Base
Sigma vs. magnitudo
Effective FOV
Effective FOV

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
Light curves
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.
The end.