The sky brightness in K'
at the OAN-SPM
22 May 2006
Michael
Richer
To measure
the sky brightness, observations of the sky at different airmasses are used and
extrapolated to an airmass of unity.
The method is slightly different in K' than in J and H. In K', it is necessary to account for
the emission from the instrument and telescope, an effect that is absent in J
and H. In K', the separation of
instrumental and sky backgrounds depends upon the fact that the instrument’s
background contribution is constant while that from the sky varies according to
the mass of air along the line of sight.
As a result, by fitting a straight line to the sky brightness as a
function of airmass, the intercept provides a measure of the instrumental
background while the slope measures the sky brightness at the zenith. In J and H, the process is the same,
but the fit is forced through the origin, equivalent to assuming no instrumental
contribution. The sky background
in instrumental units (ADU/s/pixel) may be converted to sky brightnesses in magnitudes
per square arcsecond by first dividing by the area of a pixel and then by the photometric
zero point (the number of counts from a star of magnitude 0.0 at the
zenith). Converting this result to magnitudes
gives the sky brightness in mag/c". Note that the optics that were used from March to August
2005 produced a different
plate scale from the usual one. Also, the two sets of optics have
different photometric
zero points.
The
following graph presents the data for the night of 24 March 2005 as an example. The points represent the sky brightness
measured at one place on the detector (really, they are the median of a 10x10
pixel box), but at four different airmasses. The fit indicates an instrumental background of 340±28
ADU/s/pixel and a sky background of 148±19 ADU/s/pixel/c, where c denotes a unit airmass. To convert to mag/c", it is first necessary to divide by the
pixel area (the temporary optics used then had a plate scale of 0.75"/pixel
in K'), implying a sky brightness of 263 ADU/s/c"/c. That
night, observations of standard stars indicated a photometric zero point of 1.54x108 ADU/s for a star of K' = 0.0 mag at the zenith. Dividing by this zero point and
converting to magnitudes, one obtains a sky brightness of 14.4 mag/c".
The table presents
the data for all of the nights available.
In addition to the instrumental and sky backgrounds in instrumental
units, the table includes the photometric zero point measured and the sky
background in mag/c". (For the nights in March and April 2005 when the zero point
was not measured, the average of nearby nights was used.)
Both the
instrumental background and the sky brightness vary from night to night, which
is expected. The instrumental
background is a result of the temperature balance set by the thermal load due
to the ambient air in the dome and the instrument’s cooling system (circulation
of alcohol in 2005; of liquid nitrogen in 2006). The sky’s variation (on time scales of minutes) is due to
the variable contributions of both blackbody thermal emission (K' band) and the
line emission from OH molecules (J, H, and K' bands). Clearly the instrumental background depends sensitively upon
its operating temperature, as the data from April 2005 demonstrate. The sky brightness in K' band in May
2006 is greater than that in March and April 2005, presumably due to the annual
variation in the atmosphere’s temperature, an effect that would not be expected
in J and H bands.
Sky brightness in K'
|
Date |
Filter |
Instrumental background |
Temp |
Sky background |
Zero point |
Sky brightness |
|
|
|
(ADU/s/pixel) |
(°C) |
(ADU/s/píxel/c) |
(107 ADU/s) |
(mag/c") |
|
21 March
2005 |
K' |
392±61 |
|
155±33 |
15.6 |
14.4 |
|
24 March
2005 |
K' |
272±19 |
|
94±11 |
15.6 |
14.9 |
|
26 March
2005 |
K' |
375±99 |
|
131±61 |
15.6 |
14.6 |
|
19 April
2005 |
K' |
875±10 |
+6 |
81±6 |
15.6 |
15.1 |
|
20 April
2005 |
K' |
459±13 |
-22 |
88±8 |
15.5 |
15.0 |
|
21 April
2005 |
K' |
395±30 |
-22 |
103±17 |
15.9 |
14.8 |
|
25 April
2005 |
K' |
340±28 |
-25 |
148±19 |
15.4 |
14.4 |
|
17 May
2006 |
K' |
1113±77 |
|
232±48 |
9.68 |
13.9 |
|
18 May
2006 |
K' |
1200±19 |
|
308±11 |
9.78 |
13.6 |
|
19 May
2006 |
K' |
1520±55 |
|
209±31 |
9.28 |
14.0 |
|
21 April
2005 |
J |
0 |
-22 |
26±1 |
27.8 |
16.9 |
|
25 April
2005 |
J |
0 |
-25 |
58±7 |
29.3 |
16.1 |
|
25 April
2005 |
H |
0 |
-25 |
258±10 |
20.1 |
14.1 |
Note that
a) Temp is
the temperature of Camila’s optical bench (the “carnicero” thermometer)
b) c = airmass