One of the main calibration activities for OMC is devoted to build the CCD
pixel-to-pixel sensitivity matrix, also known as the flat-field correction.
To this purpose, two LED light sources within the optical cavity were foreseen
to provide "flat-field" illumination of the CCD. Unfortunately, the OMC CCD
suffered the deposition of some contaminants on its surface, especially during
the first months of operation, which modified somewhat the properties of the
anti-reflection coating. As a consequence, the LEDs alone can not be used to
calibrate the pixel-to-pixel sensitivity because the CCD response to the light
coming from the LEDs is different to that coming from the celestial objects.
The OMC team developed a new method to perform this calibration by combining
data from LED illuminated images, long exposure sky images and data obtained
during the special Earth occultation observations (see also INTEGRAL POMs of
March 2006,
November 2013,
February,
December 2014 and
February 2016).
The sky background in long exposure images is uniform enough to be used as a
"flat-field" illumination, but a rather high number of images is needed because
of low signal-to-noise ratio and high number of sky objects whose contribution
must be removed.
In order to facilitate the removal of sky objects, recently a narrow 3x3 dither
(off-pointings in steps of 2 arcminutes) has been implemented into the OMC calibration
observation within the ISOC planning system. Long exposure sky images acquired
in this way allow to derive an accurate pixel-to-pixel sensitivity matrix, if data
from several calibration observations are combined. A preliminary pixel-to-pixel
sensitivity matrix has been derived (left panel) by combining data from the first
two observations performed with the narrow 3x3 dither in revolutions
1746
and
1763.
One of the long exposure images used in the calibration is shown in the right
panel for reference. It can be seen the large number of detected sources whose
contribution has to be eliminated, making the calibration process very complex.
Credits: A. Domingo (INTEGRAL/OMC team, CAB-CSIC/INTA)
