The first characteristics generally measured on astronomical images are the
"sky emission'' level (which includes telescope and instrument background in
the
band), and the noise on sky images and darks.
Variations in sky level are observed in all three bands. In i, they are
related to the distance to, and phase of, the Moon. In J, they are due to
variations in the hydroxyl radical's emission intensity (OH- Meinel
bands), connected to the passage of density and temperature perturbations
through the upper atmosphere (Ramsay et al. [1992]). In
they
come from the temperature variations. Also note that, in crowded fields, the
background value is set by the confusion level (background of faint
undistinguishable stars).
From these variations, the conversion factor between ADU (analog-to-digital
units) and electrons can be deduced. Table 1 compares the result
of these gain measurements with the expected values, calculated from the
electronics characteristics of the chips, the preamplifiers gains, and the
analog-to-digital conversions. Table 2 gives for each band the
median value of the read-out noise in electrons, the median and faintest values
of the sky level, in mag arcsec-2, and the median and minimal values of
the sky image noise in electrons, calculated from more than 2000 images taken
during the last year of observations (April 1998 to April 1999), and adopting zero-points
of 23.5, 21.3, and 19.2 in i, J, and ,
respectively (see Sect. 4 and
Table 5). The large values of sky and sky noise in
come
from the thermal background of the instrument, which does not include cool stop
optics. The "best'' value of the sky level in J is suspect.
Note that some care must be taken in applying the conversion factors and
zero-points to the whole DENIS survey: first, J and
conversion factors
seem to be slightly variable (11.4 to 15.3 in J, 51.6 to 57.0 in
), and
second, changes in the instrument have altered these values: the pre-amplifier
boards of J and
cameras were changed in June 1996, and a spare J
camera has been in use from April 3, 1998 to May 9, 1999. Old conversion
factors valid until June 1996 were 12.47 in J and 39.3 in
(Chevallier
[1996]). Zero-point variations will be analyzed in a future paper.
Gain | i | J | ![]() |
Expected | 3.08 | 18.4 | 58.6 |
Measured | 3.04 | 11.4 | 56.2 |
Std dev | 0.01 | 0.1 | 0.1 |
Band | RON | SKY | SKY noise | ||
median | best | median | min | ||
![]() |
mag arcsec-2 | mag arcsec-2 | ![]() |
![]() |
|
i | 7.3 | 19.2 | 20.2 | 15 | 11 |
J | 21 | 16.6 | 18.1? | 37 | 27 |
![]() |
40 | 11.2 | 12.0 | 311 | 222 |
Wavelength (![]() |
i | J | ![]() |
![]() |
0.802 | 1.248 | 2.152 |
![]() |
0.795 | 1.235 | 2.160 |
![]() |
0.788 | 1.221 | 2.144 |
![]() |
0.791 | 1.228 | 2.145 |
Band |
![]() |
![]() |
![]() |
![]() |
W/m2/![]() |
Jy | |
i | 0.791 |
![]() |
2499 |
J | 1.228 |
![]() |
1595 |
![]() |
2.145 |
![]() |
665 |
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