**[IEH]{.underline}**

The IEH-3 payload is the third in a series of five flights dedicated to
the investigation of the absolute solar extreme ultraviolet (EUV) and
Far Ultraviolet (FUV) flux emitted by the plasma torus system around
Jupiter and stellar objects. The payloads also study the Earth\'s
thermosphere, ionosphere, and mesophere.

The IEH-3 payload consists of five prime experiments plus two GAS
canisters containing educational experiments:

a.  Solar Extreme Ultraviolet Hitchhiker (SEH), managed by University of
    Southern California

b.  Ultraviolet Spectrograph Telescope for Astronomical Research
    (UVSTAR), managed by University of Arizona

c.  Space Telescope for Astronomical Research (STAR-LITE), managed by
    the University of Arizona

d.  Solar Constant Experiment (SOLCON), managed by the Royal
    Meteorological Institute of Belgium

e.  Petite Amateur Navy Satellite (PANSAT), managed by the Department of
    Defense Space Test Program

f.  Get-Away Special (GAS) G-238 and G-764 Payloads

These experiments were supported by the Hitchhiker carrier avionics unit
and were mounted on the standard HH-M bridge in the payload bay of the
Orbiter.

The IEH-3 mission was by far the most successful flight of the IEH
initiative, as evidenced by the unprecedented achievements of the
UVSTAR, SEH and SOLCON experiments.

In addition to the scientific objectives of the mounted experiments, the
IEH-3 mission included the PANSAT satellite, a small, non-recoverable,
satellite developed by the Naval Postgraduate School (NPS) in Monterey,
Calif. PANSAT is both an educational tool for the officer students at
NPS and a digital communications satellite that will provide spread
spectrum communications for use by the amateur radio community.

The STS-95 executed a picture perfect deployment of the PANSAT satellite
on FD02, adding yet another success to the list of IEH mission
achievements.

**IEH-3/SEH -** The SEH extreme ultraviolet instrumentation has produced
excellent full disk absolute solar flux data throughout the mission. All
of our primary objectives were met or exceeded. The primary mission
objective was to measure the absolute solar EUV flux incident upon the
Jovian system. By measuring the absolute solar EUV flux, SEH provides
important data points for the UVSTAR experiment, which is investigating
the EUV emissions of both Jupiter and the Io plasma torus. UVSTAR
requires knowledge of the EUV flux input to the Juptier/Io system to
fully understand the EUV dynamics of the plasma torus. By alternating
the Shuttle orientation between the Io plasma torus and the sun, SEH has
provided near real-time measurements of the solar EUV flux during UVSTAR
Io plasma torus observations.

During the STS-95 mission the solar EUV/soft x-ray background flux
varied by more that 30%, with flare activity increasing the flux by more
than an order of magnitude! In response to this there is clear evidence
of variability in the Jovian and Io torus emissions observed by the
UVSTAR IEH instrument provided by the University of Arizona. Thus, the
major objectives of the IEH mission have been met, demonstrating the
tight coupling between solar EUV input and planetary system output.

The observations consisted of eighteen solar data sets, including seven
at sunset, eight at sunrise and two data sets running from sunrise
through sunset. Observing time was approximately fourteen hours per
instrument. Six instruments were flown giving a total observing time of
about 84 hours.

The SEH instrumentation consists of :

-   A Ti coated Si photodiode plus an Al free standing film for
    observing the full solar disk shortward of 200 A (Angstroms).

-   An Al coated Si photodiode plus an Al free standing film to observe
    the full solar disk in the wavelength region shortwards of 800 A.

-   An uncoated diode to monitor the visible solar radiation background.

-   A helium double ionization cell measuring the photo-ionization rate
    of helium.

-   A neon ionization cell which measures the absolute solar flux at
    wavelengths shortwards of 575 A.

-   A normal incidence vacuum ultraviolet spectrometer that measures the
    solar spectrum from 250 -1700 A.

Examples of the high quality raw data obtained in real time during the
flight are given in the following figures.

![](media/image1.wmf){width="5.217361111111111in"
height="3.567361111111111in"}Figure 1. Raw Solar EUV spectrum without a
spectral filter. The large peak is HI Lyman-alpha at 1216 A. The small
peak left of 350 A is the He II 304 A emission.

Figure 2. Al coated Si photodiode data obtained during the sunrise solar
occultation on November 2, 1998.

![](media/image4.wmf){width="4.932638888888889in"
height="3.372916666666667in"}

The very short term dynamic nature of the solar flux in the extreme
ultraviolet (EUV) can be seen in Figure 2. The clear eruption of a solar
flare event was captured by SEH during the STS-95 mission.

# **IEH-3/SOLCON -** The SOLCON dual-channeled radiometer viewed the sun during 11 dedicated solar periods and during 7 non-dedicated solar periods, acquiring over 17 hours of data. The radiometer unit consists of two channels through which solar radiation may be sensed. Each channel contains a radiation sensor and has two apertures, the first of which is protected by independent shutters. Each shutter seals out solar radiation from the radiation sensor when closed and allows the sensor to receive solar radiation when open. An opening and closing outer cover on the radiometer unit provides protection from contamination.

The SOLCON instrument is designed to accurately measure the solar
constant and identify variations in its value during a solar cycle. This
is performed by determining the power difference required to bring two
cavities into thermal balance when one is open to the Sun and the other
is closed. This data ensures the continuity of the solar constant
measurement levels obtained by instruments that are mounted on
free-flyers and susceptible to aging and drifts. This data also provides
a means to monitor the solar cycle's variability and multidecadal
possible drifts at climate scale, as solar energy is the only external
energy source for the Earth and thus a main climate driver.

Most SOLCON measurements have been performed with the left channel in
its nominal measurement mode. The measurements during three of the
periods were performed with the right channel in its default measurement
mode. The right channel is systematically exposed less to the sun than
the left channel to allow us to detect eventual aging of the channel
that is dependent on solar exposure.

Detailed analysis has been performed on over half of the SOLCON data.
Successful observations were facilitated by the Orbiter's flawless solar
pointing throughout the mission, within 0.1 degrees of the sun. The
instrument has obtained very consistent results between the left and
right channel measurements. The SOLCON data also maintains very good
consistency with the DIARAD/VIRGO measurements obtained from the SOHO
satellite. These findings indicate that the scientific goals of this
SOLCON mission have been successfully met. It will now be possible to
utilize the SOLCON data to calibrate and verify the effects of aging on
long term solar observing radiometers such as ERBS and ACRIM II.

**IEH-3/STAR-LITE -** The objective of the Space Telescope for
Astronomical Research (STAR-LITE) project was to build and fly a novel
telescope and imaging spectrometer fabricated from an Al/C metal matrix
composite material and to observe diffuse sources of UV radiation in the
wavelength band 1150 to 900 .

STAR-LITE suffered two major difficulties during the STS-95 mission that
limited the planned program of observations. First, the azimuth drive of
the scan platform became stuck at an azimuth of approximately 87°. This
restricted the telescope to motion only in the elevation axis. With only
one degree of freedom, locating suitable targets at different shuttle
attitudes became a major task during the mission. The second problem was
the failure of the star tracker and finder cameras. This left STAR-LITE
without fine pointing information and with knowledge of the pointing
limited to the accuracy of the potentiometers on the scan platform axes:
approximately ±3°. In spite of these difficulties, the telescope and
spectrometer appear to have functioned very much as planned. A number of
stars were observed as they passed through or near the field of view of
the spectrometer.

The primary achievement of this mission was a critical examination of
the instrument's performance in the space environment. Operation in the
flight environment is the only reliable method of determining geocoronal
Lyman  scattering within the instrument, outgassing, detector noise
effects and many other concerns necessary to bring a sensitive
instrument into operation. The mechanical and electronic failures in the
pointing and tracking subsystems had no effect on our ability to make
this evaluation of the optical and detector systems.

The STAR-LITE instrument was turned on MET 1/18:24, after allowing time
for outgassing of the spectrometer chamber. Throughout the mission, the
detector performed very much as expected. In Figure 1 we show a
relatively featureless \'dark count frame\' obtained with the telescope
in the stow position, pointed at the front wall of the shuttle bay. This
is a night time exposure, so that there is only a minimal residual
geocoronal Lyman α emission entering the instrument and the counts in
the spectrum can largely be considered representative of instrumental
dark counts. This spectral image indicates that the detector was
operating in the photon-counting regime while in orbit. The low-level
spectral structure indicated that about 20% of these counts are from
diffuse emission reflected from the shuttle bay surface. In Figure 2 we
show a typical nighttime sky background image observed by STAR-LITE near
zenith. The spectral components of this image are as follows: a gradient
of grating-scattered geocoronal Lyman α plus the terrestrial Lyman β
emission. A preliminary analysis of the spectral data indicates that up
to 30 % of the signal is likely the result of indirectly scattered Lyman
 within the spectrometer indicating the need for further internal
baffling. The precise levels of detector noise, which is a primarily a
function of the level of residual gas in the spectrometer, and scattered
Lyman α, will be studied with the data in hand.

Without azimuth motion and the finder and tracker, it was difficult to
locate targets. We succeeded in observing stars crossing the
instrumental slit by using several strategies. While in inertial holds
we would point the spectrograph near a star and hope that the shuttle
limit cycle would carry the star thorough the slit. This worked on
several occasions. A second method of addressing this problem was to
allow a star to over run the slit when the shuttle was in non-inertial
attitudes. This method also worked in several instances. Obtaining
stellar spectra is important to establish the absolute calibration of
the instrument

In all, STAR-LITE slewed to over 80 targets between days 2 and 8 of the
mission. Nineteen observations have been flagged in quick look summaries
as those containing data of immediate interest and which merit further
attention. Approximately 4 Gbytes of data or 12,000 spectra were
obtained during the mission. When these data are assembled into
chronological order and systematically reduced, it is likely that much
more will become evident. In most instances where no sources were
immediately obvious, it is likely that we were not pointed at the
target. A certain amount of knowledge of the pointing can be determined
after the fact by determining exactly which stars were observed from sky
maps and the spectral character of the stars. In summary,

STAR-LITE accomplished about 20% of its observational scientific
objectives.

![](media/image5.png){width="4.991666666666666in"
height="3.545138888888889in"}

Fig 1 The 1024 (vertical) by 256 (horizontal) STAR-LITE image of the
nighttime shuttle bay surface. The spatial dimension of the image is 1.0
vertical and the wavelength dimension is horizontal covering
approximately 1116 Å to 940 Å.

Fig. 2. A spectral image of the night sky as observed by STAR-LITE. The
spectra and spatial dimensions are the same as in Fig.1a. The large
'feature ' to the left is scattered Lyman  and Lyman
.![](media/image6.png){width="4.991666666666666in"
height="3.9590277777777776in"}

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#### **IEH-3/UVSTAR -** This UVSTAR flight, third of a series of five, has been a complete success. All hardware has performed as expected, thus allowing all scientific objectives to be reached. UVSTAR has obtained a total of 54 series of spectral images: 17 of the Jupiter system, 37 of celestial targets. In the last set of data are also included several serendipitous spectra which were obtained during the sleep time of the astronauts or during other useful Discovery attitudes. 

Changes made in the spectrograph outgassing system and in the
pointing/tracking system have produced the expected improvements in the
instrument sensitivity. For one, the spectrograph detectors now operate
in a cleaner environment. In addition, the spectrograph detectors now
obtain a better spectral resolution (less than 3 arc sec) then in the
previous (STS-85) flight. As a result we have obtained clearer and
sharper spectral images of the Jupiter system (planet and Io Torus) in
both spectral channels, Far Ultraviolet (FUV) and Extreme Ultraviolet
(EUV), and more resolved celestial objects spectra in both channels.

The minor problems encountered early in the mission were solved during
the flight. After an initial failure, the autonomous pointing/tracking
system operated very smoothly even during the worst ambient conditions:
rapid movements of the Orbiter, water or fuel dumping, etc.
Unfortunately, due to this failure, were not able to acquire science
data of the first scheduled targets: 101, 104, 107 and 112. These are
the only data lost of this mission.

The calibration source, Eps CMa, was observed twice: with the first
series of images we were able to align the telescope and the FUV
spectrograph; with the second series we succeeded in aligning the EUV
channel with the telescope. This led us to acquire both the Jupiter
system and the Eps Cma EUV spectrum.

Jupiter spectra are particularly impressive because one sees the planet
spectrum and the emission line images of Io and the torus. The next
observation flights will be aimed at obtaining Jupiter system spectra
with higher sensitivity by re-coating the mirrors, gratings and
improving the quantum efficiency of the channel plate detectors.

Among the most important celestial targets that we obtained are the weak
Seyfert galaxy F9 and the Large Magellanic Cloud sources 30 Dor and S
Dor. Other important achievements are the FUV spectra of Eps CMA, Beta
Lyrae, and many others. Also the EUV spectrum of Eps Cma has been
acquired.

Other characteristics of UVSTAR are that it operates remotely without
requiring crew assistance. It has its own pointing/tracking capabilities
and its own on board recording system, which is used when medium rate
links are not available. It has an almost constant sensitivity curve
over the full 900 -- 1250 FUV range.

We believe the science material obtained so far is important for the
scientific success of the 3^rd^ UVSTAR flight. Also the instrument is
working very nicely and is reliable.

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##### **Figure 2** 

# **IEH-3/UVSTAR Observations of Jupiter and the Io Torus:** The UVSTAR observational program was remarkably successful. The instrument worked well and provided Jovian data for 15 of 17 scheduled observations. One observation was missed because of GSFC/JSC communication problems. The other observation was changed to a stellar observation for alignment purposes. From the first half of the observations we were able to detect the Io Torus in the FUV spectrograph, but not in the EUV channel. It was set to observe in the 400 -- 900 Å region. Stars are not visible short of 900 Å, and we had no assurance that the slits of the two spectrographs were aligned. By shifting the wavelength to 950 Å, we were able to detect Eps Cma in the EUV spectrograph and moved the FUV spectrograph to coincide, confirming that the slits were misaligned. On the next observation of Jupiter, the Torus was detected in the EUV spectrograph. It was observed through the rest of the mission. We had a total of (20 × 15) 300 minutes of observations of the Jovian system.

A typical spectral image acquired in one observational period of 20
minutes is shown in Figure 1. This is the FUV channel. Noise in the EUV
channel is more complicated and difficult to handle with available
software. However, we have identified enhancements at the appropriate
wavelengths. The diameter of the Torus was measure at 12 Jupiter radii
with respect to the planet's center as expected.

The Spectral image in Figure 1 was scaled in three ways to show
different features. In the top image, (0-100) scale the faint Torus can
be seen all the way from the H Ly α to shorter wavelengths. The primary
emission features are all present in the UVSTAR spectral images. The
1020 Å enhancement is visible in Figure 1. Similar signatures were noted
in the EUV channel. The position and radial extents are correct. The
presence of H Ly alpha Torus emission can be seen in the upper image.
The weak airglow line of Oxygen at 989 angstroms characterizes the
spectrograph slit which is 30 pixels wide to allow all of the emissions
from the Torus to enter the spectrograph in a slitless manner and
consequently giving a monochromatic image of the Torus at each strong
line position.

The emissions associated with the planetary disk are clear. The
atmospheric H~2~ bands, studied by the Voyager UVS, are dispersed as a
line through the planet image. The spectrum can be obtained form this
line by processing as an intensity verses position plot.

The bottom image at a scale of (3000-11000) shows the geocorona Ly alpha
as a rectangle a pedestal of emission, super imposed is the image of
Jupiter in H Ly alpha emission. The planet image is elongated because
the pixels are summed by two in the vertical direction. The geocorona
reached a level of about 6000 analogue to digital units. The Jovian disk
reached 12000-13000 units, about equivalent to the geocorona intensity.

Through the examination of the real time data it was noted that the
intensity of the Torus emissions was variable. The SEH experiment
detected considerable variability in the solar EUV data. Although the
solar flux has not been considered a serious source of Torus excitation
the variability in emission seems curious and will be examined carefully
as our analysis proceeds.

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##### **Figure 1**

##### One "Spectral Image" at three scaling factors to show spectral features 

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**IEH-3/ Improvements and Lessons Learned**

-   During the IEH-1 and IEH-2 flights of UVSTAR the internal pressure
    in the spectrographs was too high and prevented us from getting to
    the low noise level required to record the Jupiter and Torus
    emissions. The changes made to the spectrographs included opening
    the pumping port by a factor of twenty and placing 50 watt heaters
    in the spectrograph bodies. The heaters were used for the first day,
    holding the temperature in the 20° C to 40° C range. When the
    heaters were turned off, the internal pressure dropped to \< 5 × x
    10^-7^ and remained there throughout the mission. Jupiter was
    acquired on the first opportunity and held in the center of the slit
    by the star tracker provided by the Italian collaborators.

-   The tracking facility provided by out Italian colleges was
    exceptionally good, allowing tracking to less than four seconds of
    arc. The effect of the shuttle limit cycle is minimized by working
    within a deadband of plus or minus one degree. The system is
    autonomous now allowing the target to be captured and tracked by
    time tagging the "start track" command. Little data was lost due to
    LOS periods.

-   Some data was lost due to an unexplained plasma effect. During each
    observation there was a period extending from Jupiter acquisition to
    several minutes after sunset on the orbiter when a very specifically
    patterned noise appeared in the images. It could be removed by
    changing the grid bias at the expense of detector efficiency. More
    potential gradients will be added for control of this problem in the
    spectrographs in the future.

-   Some increased frictional drag was noted in the azimuth drive
    indicated by loss of steps to the stepper motor/encoder combination.
    We will consider an improved design to replace the monoball bearing
    for future missions

-   For future missions aimed at Jupiter system spectra, we will need to
    improve the sensitivity of the telescope spectrograph system. This
    will include re-coating the mirrors, the gratings, and improving the
    quantum efficiency of the micro channel plate detectors. We believe
    an improvement of 5-10 is possible.