= = = ASCA  EMERGENT  OPERATION = = =

Dear ASCA users,

After the first-pass operation on April 24 (06:52-07:03 UT), it was found 
that ASCA was not pointed to the aimed target (4U 1907+09). In between the 
1st and 2nd passes, next maneuver to M82 was done from the offset position 
and thus it was a little amplifiered since the planned tracking stars were 
not identified in the STT-A&-B tracking areas. In the following 3rd-5th 
passes we determined current attitude and fixed the satellite pointing 
direction to the current position, which was about 2 degree offset from M82.

In the last two days, we follow the history and investigated the cause of 
this malfunction. Among the two suspicious reasons, i.e., some failure in 
the star tracker system or malfunction in the Attitude Control Sytem (ACS), 
e.g., degradation of currently operating gyroscopes, we got fairly confidence 
that the latter is the case. At present we doubt degradation of the SLEW-2 
gyro (we are now operating ASCA by the combination of X-, Y- and S2 gyros). 
The clear identification and quantitative analysis of this trouble is now 
undergoing by tracing the operation history and performing some test 
operations during the contact passes of ASCA.

Meantime we cannot perform any large angle maneuver and moreover we need 
especially careful operation for maneuver. It needs two step, i.e., 
coarse maneuver and fine tuning maneuver, operations,  and for one maneuver 
sequence we must use 4-5 passes.  Hence, at present the change of attitude 
is possible only during the contact passes at KSC.

We are now considering restructure of the ASCA ACS system at the time 
when we complete analysis of the failure and confirm the procedure for 
restructuring the ACS system.  Tentatively we schedule the ACS restructure 
operation during the period of May 8th to May 11th . If the SLEW-2 gyro is 
really degraded, as we doubt now, we will replace it by a spare gyro.  
Hence, I expect ASCA will  return to its regular operation after May 11th.
The targets cancelled during this emergent operation will be reinstalled
either in this season or the next season half a year later. 

I regret this inconvenience for all the ASCA observers. However, I wish to 
take first priority importance to the safety maintenance of ASCA, meantime 
till we fix the ACS system trouble and return to regular operation. I 
hope those who have cancelled time critical targets will contact to 
the persons concerned and negociate possible alternatives.

Thank you for your sympathetic cooperation.
Fumiaki Nagase



-----------------------------------------------------------------------------
A S C A     WEEKLY  OBSERVATION   PLAN
-----------------------------------------------------------------------------
April  23 - May 11	 EMERGENT OPERATION  (96/04/26)
(ISAS contact scientist:                                 )
-----------------------------------------------------------------------------
Date	MNV	Target 		Tag. 	Exp.	Observation	PI or 
	start	name  		Cat.	(ks)	Cat.  Pri./TC	 ( PPI/Co-PI)
-----------------------------------------------------------------------------
4/23	23:00	4U 1907+09 #1	Bin.	10	US	1/Y	P. Michelson
		(Blank-sky observation due to failure of pointing)
4/25	10:25	M82 #5		AGN	10	JPN	2/n	T. Tsuru

4/26	~4:00	  (Coarse and Trim Maneuvers)

4/26	~11:00	0806+20		CG	40	JPN	1/n    K. Hashimotodani

4/27	~5:00	  (Coarse and Trim Maneuvers)

4/27	~11:00	AR UMa		CV	40	US	2/n	P. Szkody

4/29	~3:00	  (Coarse and Trim Maneuvers)

4/29	~9:00	Abell 697	CG	60	MJU	1/n	S. Miyoshi
								/J. Hughes
5/ 1	~2:00	  (Coarse and Trim Maneuvers)

5/ 1	~8:00	Abell 990	CG	40	ESJ	1/n	I. McHardy

5/ 3	~1:00	  (Coarse and Trim Maneuvers)

5/ 3	~8:00	AD Leo		Star	80	JUS	2/Y	T. Watanabe

5/ 5	~23:00	  (Coarse and Trim Maneuvers)

5/ 6	~6:00	PG 1114+445	AGN	50	US	1/n	I. George

5/ 7	~22:00	  (Operation for Attitude Control System Restructure)

5/11	~6:00	  (Return to normal schedule)
-----------------------------------------------------------------------------

(Note) Obs. Cat. JPN: Japan time, US: US time, JUS: Japan-US Collaboration, 
USJ: US-Japan Collaboration,  MJU: Merged Japan-US (PPI=Japan), 
MUJ: Merged US-Japan (PPI=US),  ESJ: ESA-Japan Program, 
MOT: Manager's observatory time (Calibration and maintenance)