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Prof. Kirschbaum presented methods to measure
the brightness of the night sky |
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Lux meter |
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Solar cell |
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CCD camera |
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Why not a normal digital photo camera? |
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These cameras are widespread, cheap and easy to
handle |
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Can these camera’s be used for reliable
measurements of the night sky brightness? |
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Costs about 150 euro |
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3 megapixel |
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Manual setting |
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Maximum exposure time of 8 seconds |
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Raw exposures |
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F2.8 aperture |
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Minimal magnitude of stars of about 7 |
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Magnitude 1 stars saturated |
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Pixel counts of sky about 20 |
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Possible to make images and measurements of sky
in different locations |
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Problem 1: always use the same camera and
results are not comparable |
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Problem 2: Colour sensitivity of camera and eye
are different |
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To make the measurements objective and
comparable |
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Use the brightness of the stars to scale the
method |
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Use a specific filter to make responses of the
camera in visual, specific wavelengths |
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Falchi and Cinzano, CCD measurements of night
sky brightness: |
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Make images of various parts of the sky with
known stars with different altitudes |
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Determine the Extinction coefficient k |
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Determine the Photometric scale factor C |
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Determine the sky brightness at the location |
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Bought |
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Johnson V filter (100 euro) |
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A lot of memory (tiff files are 6 Mb) |
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No moon |
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8 seconds |
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Tele (about 10 cm lens) |
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Aperture F2.8 |
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Focus 5 meter to obtain not saturated stars |
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Dark and flat frames |
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Extinction coefficient k= between 0,2 and 0,6 per airmass |
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Photometric scale factor C= 11,1 ±0,2 magnitude |
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The photometric scale factor C gives the correction factor to make
the camera a real measuring device |
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The formula to make a pixel count P into a
magnitude: |
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M=C-2,5*log(P/1168) |
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(1 pixel= 1168 square second) |
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Results not reliable with dark(er) skies |
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Try to find collaborators |
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Present results in Paris |
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Write an article about the results in Dutch |
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Try to get a camera with longer exposure time |
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Take this winter measurements in different towns
in Holland |
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Take measurements in one sport uring a night |
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?? |
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Poznámky