ASCA operation

[Fumiaki NAGASE <nagase@astro.isas.ac.jp>]

Dear ASCA Team members and ASCA users:

We again experienced malfunction of ASCA attitude control system 
on November 18 and found that the malfunction happened at the 
time of maneuver to NGC3516 at 23:40 (UT) on Nov. 17 when the 
ASCA star tracker did not catch the target stars in the tracking 
areas at the end of maneuver and the optical axis was about 3 
degrees offset. During the contact passes on Nov. 18 we examined 
and solved the problem. The phenomena was similar with the 
trouble on September 7 when ASCA transferred into the Safe Hold 
Mode.

At preset, we have been operating ASCA by using X- Y- and S1-
gyroscopes (Z-gyro being stand-by for emergent transfer to a safe 
hold mode). HK data for urrent monitor indicated that the motor 
current of one of the three gyros (S1) had become unstable since a 
few days before the time of malfunction. It seems that the rotation 
speed of the S1- gyro became unstable from the time,which caused 
the star tracker system failed to catch up the target stars in the 
tracking field at the end of the maneuver from 3C390.3 to 
NGC3516.  This is suspected to be due to the small change of pulse 
weight from the setted parameter (during pointing or small 
maneuver, the small error in pulse weight do not affect to the 
control but  the error in pulse-weight is accumulated during the 
long maneuver).

When we turned off and restarted the motor of S1 gyro, the ACS 
system have started to function properly. We started regular GO 
observations after the contact passes on Nov. 20 from PSR00437-
4715 along the weekly schedule.  We are now investigating 
methods to improve the ACS system for operating ASCA in more 
stable. We are now planning to take a few days for restructuring 
the ACS system in early December. After then we will tune the 
system along with the GO observations. 

Because of this events, we were not able to observe NGC3516, 
A1722, A1704 and half of PSR00437-4715. The new weekly 
schedules  will be announced seperately including the recovery of 
those lost observations and the ASCA maintenance. The ASCA 
long-term timeline will be also updated.

Regards,
Fumiaki Nagase