SUSIM ATLAS Instrument

SUSIM ATLAS Instrument


The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument was designed and fabricated at the Naval Research Laboratory (NRL), Washington, DC under support from NASA. It flew on its first Space Shuttle flight, the OSS-1 Mission, in 1982. The instrument has reflown on Spacelab-2 in August 19852, ATLAS-1 in March 19923, ATLAS-2 in April 1993, and ATLAS-3 Missions in November 1994. Calibrations, updates, refurbishment were performed afer each of its flights. These solved problems, overcame operational limitations in calibration or flight, and implemented improvements discovered during the previous Space Shuttle missions. The launch of a sister instrument on the Upper Atmospheric Research Satellite (UARS) September 19914,5 made some of the improvements possible when the spare flight hardware from the UARS SUSIM instrument was no longer needed. Most of the changes were made specifically to decrease uncertainty in the absolute irradiance of the data. In the following paragraphs, the instrument as it flew on ATLAS-3 is described.

The SUSIM instrument diagram, has two identical double-dispersion scanning spectrometers, seven detectors, entrance and exit slits, filters, and gratings enclosed within a case. Doors seal the case when not making measurements to prevent external contamination from entering the optical area. All materials are specially chosen to minimize any contamination to the optical elements. The electronic modules and a microprocessor for command, control and telemetry surround the instrument are shown in the instrument photograph . SUSIM covers the wavelength range 110-410 nm with 0.15 and 5.0 nm resolution. The instrument parameters are given in Table I.

The SUSIM experiment has overcome the difficulty of making high accuracy measurements in the ultraviolet by using contamination control techniques to minimize changes. Remaining changes are tracked with an on board deuterium lamp (D2). Only one of the two spectrometers is used for solar measurements. The sensitometric changes of both spectrometers are monitored with the D2 lamp to make calibration and characterization measurements of all the optical elements. The solar spectrometer is compared to the calibration spectrometer to fully determine its solar UV induced sensitometric changes

The instrument has been described in detail in Ref. 1 and 2. The five major improvements made prior to the SUSIM ATLAS-3 pre-flight calibration will be discussed individually below. In addition to the improvements in accuracy provided by these changes, some comments on how these changes affect the operation of the instrument are included.

A. Wavelength encoder. A new electronic module was installed which doubled the measurement resolution of the incremental encoder which is attached to the grating drive arms. This encoder determines the position and eventually the wavelength of the spectrometer measurements. The new module gives 0.013 nm readout precision; a factor of ten more than the 0.15 nm spectral resolution. In addition to the improvement in the encoder electronics, a new technique of determining an absolute position for setting the encoder zero at turn-on was implemented. This new technique was patterned after the UARS SUSIM instrument and consisted of using a photo diode and light emitting diode pair to determine the position when a mechanical flag attached to the grating arms became centered between them. Measurement of the transition of both sides of the flag during its movement up and down the encoder scale prevents electronic or optical changes from shifting the center determination and hence the zero position for the wavelength scale. As will be seen later, these changes improved the 2 sigma wavelength measurement accuracy to 0.035 nm. The previous flights of the SUSIM instruments had a 2 wavelength accuracy of 0.2 nm. In addition to improvements in the accuracy, spectrometer scans can be made over correct wavelength range without large margins made necessary when there are errors in zeroing. This saving of time is especially important in the flight environment.

B. Photo diode Electrometer. The original SUSIM detector wheel had a high gain electrometer with six gain ranges with the five photo diodes multiplexed to one electrometer due to space limitations. In the UARS SUSIM individual electrometer of. Several SUSIM UARS flight spares with an improved design having three gains were available. One was chosen for its stability, time constant and noise characteristic. It was then modified to fit into the SUSIM ATLAS detector wheel. Three gain unit has greater overlap of the ranges than the six gain unit. The improvement in the signal to noise ratio was nearly a factor of two and the stability and time constant improvements allowed the spectral scans to be completed nearly 20% faster. This was an important time savings in flight operations since data sets could be completed during a single orbit. The new electrometer is responsible for much of the reduction in spectral irradiance uncertainty from 4-8% above 130 nm to 3-6% for ATLAS-3. The improved performance of the electrometer and photo diode system is important during calibration where ratios between gains, detectors, filters, spectrometers and polarization angles are made. Reducing the error in determining the ratio of optical elements and the UV induced degradation during the calibration process substantially improves the overall accuracy.

C. Filter Wheel. A fixed MgF2 entrance window which required instrument disassembly in order to measure its transmission before and after flight was removed and a new filter wheel was designed and installed behind the entrance slits. This wheel can position each of the entrance filters listed in Table I behind either of the two spectrometers thus providing a method for periodically measuring each filter's spectral characteristic. During calibration, the intense short wavelength radiation in the beam at the Synchrotron Ultraviolet Radiation Facility (SURF)6 at the National Institute of Standards and Technology (NIST) facility in Gaithersburg, Md., causes extensive aging of the SUSIM spectrographs. By using a "working" filter during much of the SURF calibration, the magnitude of the aging correction of the "flight" filters between pre-flight and post-flight calibration can be reduced. The UV degradation is largest on the MgF2 filters which are used below 250 nm. Three MgF2 filters were put in the filter wheel to provide a filter for periodic comparison to the flight and work filters to give an additional measure of their aging.

D. D2 lamp System. A program to produce a stable and repeatable D2 lamp was undertaken during the design and fabrication of the UARS SUSIM instrument. It was hoped that a new D2 lamp could be developed that would not have the tendency to periodically jump to new level of output. These jumps of up to 8% prevented detailed monitoring of the instruments spectroradiometric condition throughout the pre- to post-calibration period. The United Kingdom's National Physical Laboratory (NPL) together with its industry partners made several changes to the end-on, MgF2 lensed lamp used on the earlier SUSIM flights. They felt that stabilizing the source of electron generation on the filament could make the arc more stable and repeatable. First they supported the lamp filament with a ceramic rod so that its could withstand the vibration levels of launch and landing for the ATLAS instrument. In addition to mechanically improving the stability of the filament, a new power supply with increased accuracy in its 250 milliwatt current limited, greater stability and the controls to accommodate a new operating technique was developed. By reducing the filament current after the lamp was operating rather than turning the filament off, the source of the electrons would be more distributed. With the filament off, the source would tend to be a single spot and this spot could shift to a new location on the filament if the cathode material became aged. NPL also found a new operating technique. This technique was refined at NRL and became the standard operating procedure for the SUSIM D2 lamps. The lamp is operated for 20 minutes and then cooled for one hour to condition the lamp. This short conditioning and then again warming the lamp for 20 minutes prior to collecting data improves the accuracy of the irradiance output. The initial 20 minutes on and one hour off conditioning is done twice whenever the lamp has been shipped or stored for longer than a day. Repeated striking, burn-in, aging and launch level vibration testing was done then two sets of four flight lamps for the UARS SUSIM and two lamps for the SUSIM ATLAS were selected. A new power supply was developed for the SUSIM ATLAS instrument using the UARS design. The accuracy of the lamp selected for their high stability and repeatability has been demonstrated by the UARS SUSIM7 and SUSIM ATLAS-3 flight D2 lamps.

E. Microprocessor. The instrument command and control microprocessor used for the first four flights had become had to maintain. A number of the components such as the UV proms had become unavailability since the unit had been built in 1979. Therefore a new unit was purchased and flight qualified for the ATLAS-3 Mission. New software was developed which incorporated the new command and telemetry mentioned in the wavelength section above and to incorporate the changes in control required by the encoder, electrometer, filter wheel, and D2 power supply. A by-product of software effort was an addition of stored commands for the instrument so that canned procedures could be up-linked to the instrument and executed. These stored command sequences provided new opportunities for operations in flight. D2 conditioning and calibration operations could be started at the preprogramed times even when the space shuttle was out of ground command contacts. Calibrations at NRL and NIST went much faster and the calibration team could concentrate on the data quality rather than the need to operate the instrument.

REFERENCES

1. M. E. VanHoosier, J-D. F. Bartoe, G. E. Brueckner, D, K. Prinz, and J. W. Cook, "A high precision solar ultraviolet spectral irradiance monitor for the wavelength region 120-400 nm", Solar Physics, 74, 521-30, 1981.
2. M. E. VanHoosier, J-D. F. Bartoe, G. E. Brueckner, and D. K. Prinz, "Absolute solar spectral irradiance 120 nm-400 nm (results from the Solar Ultraviolet Spectral Irradiance Monitor - SUSIM - experiment on board Spacelab 2), Astr. Phys. Lett. Commun., 27, 164-168, 1988.
3. R. P. Cebula, G. O. Thuillier, M. E. VanHoosier, E. Hilsenrath, M. Here, G. E. Brueckner, and P. C. Simon, "Observation of the solar irradiance in the 200-360 nm interval during the ATLAS-1 Mission: A comparison among three sets of measurements - SSBUV SOLSPEC, and SUSIM", Geo. Res. Letters, In Press, 1996.
4. M. E. VanHoosier, "Absolute UV irradiance of the solar UV spectral Irradiance Monitor (SUSIM) instruments", SPIE Proc., 932, 1988.
5. G. E. Brueckner, K. L. Edlow, L. E. Floyd, J. L. Lean and M. E. VanHoosier, "The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on Board the Upper Atmospheric Research Satellite (UARS)", J. Geophys. Res, 98, 10695-10711, 1993.
6. E. B. Saloman, S. C. Ebner, and L. R. Hughey, "Vacuum ultraviolet and extreme ultraviolet radiometry using synchrotron radiation at the National Bureau of Standards", Opt. Eng., 21, 951, 1982.
7. D. K. Prinz, L. E. Floyd, L. C. Herring, and G. E. Brueckner, "On-orbit performance of deuterium lamps during 4-years of SUSIM operation on the UARS", SPIE Proc., 2831, 1996.


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Last Updated 08 October 1996