SAX (MECS) commissioning

The SAX commissioning phase D (Payload commissioning) is terminated.
The MECS instrument switch on started on Thursday 6 June 1996 and has currently completed phase D3 (initial checkout) and phase D4 (verification of all NFI together) started on Saturday 22 June 1996.

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.

High voltage checkout

MECS unit 2 has been switched on (only photomultiplier tube at minimum voltage) for 2 minutes during pass 518, using a LED stimulation.
The instrument behaves nominally.

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 523 the same sequence was started for unit 3, with the LED stimulation at minimum value. Again all is nominal.

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.

During passes 534 to 537 the same sequence of activities was performed, last but not least, for MECS unit 1, which again showed results similar to the other two units.

Background measurements

On June 8, during passes 546 to 549 (from 8:00 to 13:00 approx.) detector M3 has been kept on for 3 consecutive orbits, detector M2 for 2 orbits, and detector M1 for just one, including management of the South Atlantic Anomaly passage via time-tagged commands.

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.

Further 4 orbits of background data were obtained on Jun 10, with nearly continuous coverage (i.e. excluding the South Atlantic Anomaly) and one more orbit on Jun 11 before AOCS activities.

Telescopes are open

The mirror unit shutters have been opened on June 15.

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.

First light

The first-light pointing (on Cyg X-1) was obtained on June 22-24, during passes 745 to 748 (see further below for a second, better pointing).
Due to an unfortunate combination of Murphy's law with the predictions of prophet Jonah the satellite was pointed some 10 arcmin off the source in one direction, positioning the target exactly above the intersection of the 600 micron strongback structures supporting the Berillium window.

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.

Our first 5 million photons

A further pointing on Cyg X-1 was obtained on June 25, during 3 orbits. This time the source was located on axis. The intensity of the source is very strong, in some 7000 seconds we collected more than 5 million photons in the 3 detectors, and the image allowed to verify the complete compatibility of the Point Spread Function with the one measured on ground.
We consider therefore that the mirror units and detectors are working very satisfacotrily.

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.

More to come ...

After Cyg X-1, a further pointing was made (data have not yet been analysed) and the Science Verification Phase will begin on July 1 as planned.
By the way this is not the comet Hyakutake but guess what ...