Long-term stability

It appears that the effective area has not varied significantly since launch. Observations of the sources HZ 43, Vega, A 2256, and N132D at several epochs yield non-varying count rates:

Source Date of observation Count rate (counts s )

A 2256* 1990 July 1 0.61 0.01

August 3, 1992 0.62 0.01

HZ 43 1990 July 2 12.3 0.2

1991 December 11 12.0 0.2

1992 June 9 11.91 0.04

Vega 1991 March 20 0.10 0.01

1992 March 21 0.11 0.01

1992 September 18 0.10 0.01

N132D 1990 July 3 4.41 0.13

1991 February 9 4.77 0.02

1992 September 18 4.89 0.06

1993 April 17 4.61 0.04

1994 January 10 4.51 0.04

1994 July 15 4.65 0.05

1995 February 11 4.54 0.04

1995 March 11 4.68 0.04

* The count rate for A 2256 is determined from the net emission

within a 6' radius.

Table 5.4: Stability of the HRI quantum efficiency

Source	Date of observation	Count rate (counts s )
A 2256*	1990 July 1	0.61 0.01
	August 3, 1992	0.62 0.01
HZ 43	1990 July 2	12.3 0.2
	1991 December 11	12.0 0.2
	1992 June 9	11.91 0.04
Vega	1991 March 20	0.10 0.01
	1992 March 21	0.11 0.01
	1992 September 18	0.10 0.01
N132D	1990 July 3	4.41 0.13
	1991 February 9	4.77 0.02
	1992 September 18	4.89 0.06
	1993 April 17	4.61 0.04
	1994 January 10	4.51 0.04
	1994 July 15	4.65 0.05
	1995 February 11	4.54 0.04
	1995 March 11	4.68 0.04
* The count rate for A 2256 is determined from the net emission
within a 6' radius.