The only data taken so far where sky is easily measured are the Ryan exposures on 15Jan2007, where the exposure time is 240 sec and
the V filter was used.

Note that V-Vega is about equal to V-AB, so I'm going to ignore the difference for now. The data on 15Jan2007 were taken with no moon
and no clouds.

In lbcb.20070116.075519, the background in 240s is about 10500 ADU. Subtracting 900 ADU for bias, gives 9600 ADU/pixel in 240 sec.
All of the Ryan exposures had about the same background.

1 pixel is .23"x.23" = .0529 arcsec^2

So 1 arcsec^2 = 1/0.0529 = 18.90 pixels

If I adopt the zero point in the LBC ETC for V (28.08 for 1 sec)
I get the sky is

-2.5 log (9600/240*18.9) + 28.08 = 20.88 magn/square arcsec.

By contrast, the Taylor, Jansen & Windhorst paper on the sky brightness
at the Vatican Telescope quotes the sky in V = 21.7 magn/arcsec^2.

The ETC says that in 240 sec, the number of sky counts is 4627 ADU/pixel.

This number of ADU corresponds to
-2.5log(4627.9/240*18.9) + 28.08 = 21.7 magn/arcsec^2
in good agreement with the measured sky at the Vatt.

So to check the zero point, we looked at exposure lbcb.20070116.023440.fits, which is a two second exposure in V of the L95
photometric standard field. We verified the exposure time by comparing the counts in stars
in lbcb.20070116.023345 and lbcb.20070116.023440 which are both V
exposures of L95 in the OB called L95_BV.

Star L95-115 has V=14.68 and 393,856 net ADU above sky in 2 seconds.

So the zero point from this star is
2.5log(393,856/2)+14.68 = 27.92. which pretty much agrees with the
zeropoint of 28.05 used in the ETC.

So...my conclusion is that the average sky in V is a factor of
about 2 larger in the images we're getting, compared to what we
expect from the sky in V at the Vatt, KPNO, MMT and La Silla.

This background is different from the "scattered light"
feature we mentioned earlier, and is the background everywhere in
the FOV.

-- Main.jill - 17 Jan 2007
Topic revision: r1 - 18 Jan 2007, jill
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