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Spotting the life of stars „Pi of the Sky” Pro ject. Katarzyna Kwieci ńska UKSW- SNŚ on behalf of the Pi of the Sky collab oration. „Pi of the Sky” goal is. to i nvestigat e objects of the variability timescale from seconds to years. Especially:
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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.
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.
Reduction. Its goal is to reduce an apparatus effects. In general it consists of two steps: • 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. • 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.
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.
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.
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.
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
Conclusions • Because of the mount drift and the shift between succeeding nights, effective FOV is about 5 deg smaller from each side of a frame. • Imperfect cataloguing procedure makes that mean magnitudo depends significantly on time. It is of importance especially when we talk about long-period stars. • If we want to distinguish a physical variability from the imperfect cataloguing effect, it is necessary to make the photometry much more stable.
