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GHRS Instrument Science Report 042 1
TRENDS IN GHRS FAR­UV SENSITIVITY
Ronald L. Gilliland and Lisa E. Walter
The GHRS has an ongoing absolute sensitivity monitoring program that now
has a total baseline of over 1.5 years. The 2.0 arcsec LSA is used to observe
a standard star every few months at five wavelengths: 1200 š A, 1500 š A, 2000 š A,
2500 š A, and 3000 š A. A trend analysis is consistent with long­term stability at
all wavelengths. In particular the far UV (1200 š A) sensitivity relative to the
near UV (mean of 2500 š Aand 3000 š Ameasures) does not show any significant
trend with time. The GHRS data suggests that the OTA far UV response
is not degrading with time; independent measurements on two different stars
yield formal trends of ­2.8% per year and +0.7% per year for the far to near
UV response.
I. Introduction.
The GHRS sensitivity monitoring calibration program provides information on any long­
term drifts in the total system sensitivity (OTA + GHRS). We report here the measure­
ments conducted with the GHRS that may yield information on possible degradation
with time of the far UV OTA throughput.
The sensitivity monitoring program consists of two parts: 1) detailed observations of ¯
Col about once per year at a dense grid of wavelength settings sufficient to fully define
the grating sensitivities, and 2) quick observations of BD+28d4211 conducted a few times
per year as a spot check for any significant change. The observations of ¯Col occurred
at two epochs with a 1.14 year baseline, while the BD+28d4211 observations are at four
(more recent) epochs with a 0.64 year baseline.
Further details of the observations and reduction procedures may be found in GHRS
Instrument Science Reports No. 36 and 38. For current purposes it is sufficient to note
that data from different epochs was acquired and reduced with identical procedures.
II. Results for ¯ Col.
¯ Col was observed once in SV on 5 February 1991 and once in Cycle 1 on 28 March 1992
for the purpose of basic sensitivity calibrations. Table 1 shows the derived sensitivities at
five different wavelengths over three separate first order gratings: G160M at 1200 š Aand
1500 š A, G200M at 2000 š A, and G270M at 2500 š Aand 3000 š A. Dates are in year.day­of­year
form and sensitivities are standard (counts s \Gamma1 diode \Gamma1 )/(ergs s \Gamma1 cm \Gamma2 š A \Gamma1 ) calculated
in the obvious way. Focus is given in microns from nominal­optimal and is taken from an
April 1992 memo of Chris Burrows, but with an assumed desorption to ­19 ¯ in August
1992 and a refocus to ­5 ¯ by September 1992.
1 Copies of this report may be obtained from the Telescope and Instruments Branch, Space Telescope
Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.

Table 1. GHRS Sensitivities from ¯ Col
Date 1200 š A 1500 š A 2000 š A 2500 š A 3000 š A Focus
91.026 2.99e11 4.30e11 9.06e11 3.80e12 2.19e12 ­25
92.088 2.96e11 4.39e11 9.35e11 3.88e12 2.25e12 ­16
To minimize any minor mis­centering or OTA­focus affects we report only the statistic
of relative change of the 1200 š Asensitivity divided by the mean of the near UV responses
at 2500 š Aand 3000 š A. The direct affect of focus or centering errors should cancel to first
order for the ratio of sensitivity at different wavelengths. Defined in this way the far UV
to near UV sensitivity shows a drop of 2.8% per year from the ¯ Col calibrations. The
1200 š Abandpass extends from 1194.31 to 1203.85 š Aand is well isolated from the Lyman ff
line.
III. Results for BD+28d4211.
Observations of BD+28d4211 were initiated in Cycle 1 with the intention of having four
epochs distributed over the full cycle. The Side 1 problems early in the cycle resulted in
missing the first opportunities and we have had the four independent visits compressed
into a span of only 0.64 years as shown in Table 2.
Table 2. GHRS Sensitivities from BD+28d4211
Date 1200 š A 1500 š A 2000 š A 2500 š A 3000 š A Focus
92.013 3.13e11 4.14e11 9.11e11 4.00e12 2.24e12 ­13
92.128 3.03e11 4.21e11 8.79e11 3.94e12 2.19e12 ­17
92.182 3.07e11 4.19e11 8.86e11 3.93e12 2.16e12 ­18
92.245 3.22e11 4.32e11 9.29e11 4.10e12 2.31e12 ­5
A least­squares linear trend analysis for the far to near UV ratio for BD+28d4211 yields
a formal result of +0.7% per year.
The sensitivity changes from 92.182 to 92.245 of about +4.7% are generally independent
of wavelength and are consistent with expectations from the 13 ¯ focus adjustment be­
tween these observations. The far to near UV ratio of sensitivities is not sensitive to the
focus.
Summary
Calibration observations with the GHRS suggest the total system (OTA + GHRS) far
UV sensitivity is best represented as a constant with time. The long­term trending with
¯ Col from early 1991 to early 1992 could be interpreted as showing a small decline
of far UV sensitivity early in the mission. The long­term trending with BD+28d4211
conducted over an eight month time base in 1992 can be interpreted as consistent with
both total system throughput stability and with stability of the far to near UV response.
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