Instrument Science Report CAL-FOS-001

Laboratory Calibration of the Faint Object Spectrograph:

Absolute Sensitivity

(First Results for the Blue Side)

J.	Koornneef

R.	C. Bohlin and

R.	J. Harms


06/17/83

Summary: The present (03/25/83) peak Q. E. of the iilue tube (Flight
*7), as based on measurements of a deuterium lamp through the
F0Spectrograph, is calculated to be 14.2% between 2500 and 3200A. A
similar value (of 13.8%) was found by Ed Beaver on 04/12/83 by
directly illuminating the Digicon with a Mercury PENlamnp through an
interference filter (2537A). These results are to be compared with the
initial value of Q.E. 20% at 2537A as derived by M2C (07/08/81 &
09/14/81). In interpreting the the apparent loss in Q.E., it has not
been taken into account that the standard photocathode voltage has
been lowered from 22 to 18 KV since the initial 1981
measurements. This causes a drop in count rate in the blue of app. 15%
(Ed Beaver; priv. contm. 05/27/83).

A much larger temporal drop in Q.E. is found for the red end of the
sensitivity curve. This is clemonstrated in Table 4, where the Visible
Current Mode Quantum Efficiency measurements of 07/06/81 are compared
with the data of 03/25/83 as interpolated from the attached Tables 2
and 3. The lossfactor appears to turn up very steeply going towards
the red. [Note that it is not known whether the countrate loss caused
by the lower high voltage is wavelength dependent].