Here you'd see the MECS
first dark,
first sky,
first light and
second light
images.
In the former images a circle indicates the radius field of view of the
Berillium window (this is masked in correspondance of the two
calibration sources).
In the first light image the strongback supporting the window
is also shown.
Further 5 minutes of data have been acquired during pass 519, with the LED on and the PMT at nominal voltage.
The plasma suppression grids have been switched on during pass 520
In pass 521 the gas cell high voltages were also switched on
at the minimum value. Since the telescope shutters are still closed,
we accumulated a few minutes of counts due to the cosmic background
and to the calibration sources.
In pass 522 the gas cell voltages were brought to nominal values,
confirming the above results.
We obtained therefore our first
"dark" background image
at nominal voltages.
In pass 524 the PMT voltage was raised to nominal.
Activity resumed in the morning of June 7.
In pass 532 the cell was switched on at minimum voltage and
in pass 533 at nominal voltage, with results similar to the
ones of the previous detector.
A 15 % modulation of the background with the orbit can be clearly seen.
The hardware discrimination is quite effective, reducing the ratemeter
count of all events which trigger the detector from ~35 cts/s,
to ~10 cts/s in the scientific packets.
Most of these counts
are in the outer part of the cell (outside the field of view), and/or can be
effectively rejected by burst length discrimination, so that the actual
background is of the order of 3 x 10**-6 cts/pixel/s, i.e.
approximately twice the one measured on the ground.
On June 20, during passes 714 to 717 (from 9:20 to 14:30 approx.)
detector M3 has been kept on for 3 consecutive orbits,
detector M2 for 2 orbits, and detector M1 for just one, with the
satellite in Default Pointing Mode (North Pole), allowing to take
the first sky image.
We see no significant increase of the background due to possible
plasma contamination in the ratemeters, while the rate in the Field
of View, now inclusive of cosmic background, is about 5 x 10**-6 cts/pixel/s.
One further orbit of data was obtained in indirect mode (not analysed).
On June 21 two short exposures (during visibility only) were obtained while the satellite was pointed at different angles with its velocity vector, in order to assess the effect of the plasma contamination. No significant differences with the DPM case were noticed.
The image shows an overlay of the strongback and window structure in yellow, while the avoidance region of the calibration sources is shown in red. The intensity scale is logarithmic in order to enhance the low level parts of the Point Spread Function, inclusive of the single mirror reflections (on the right hand side) and of the obscurations due to the spiders supporting the mirror.
There are no evidences of damage to the UV-ion shield filters.
Since these observations the gain of the MECS has been re-adjusted to reach 10.6 keV in 239 channels all over the field of view.
A very preliminary spectral analysis, giving results comparable with the recent ASCA measurements, will be reported in a forthcoming IAU Circular together with results of other SAX instruments.
