Technical Aspects of the Space Telescope Imaging

Technical Aspects of the Space Telescope Imaging Spectrograph Repair (STIS-R) S. A. Rinehart1*a, J. Domberb, T. Faulknerc, T. Gulla, R. Kimblea, M. Klappenbergerd, D. Leckronea, M. Niednera, C. Proffitte, H. Smitha, B. Woodgatea a NASA’s Goddard Space Flight Center, Greenbelt, MD 20771 b Ball Aerospace, Boulder, CO 80301 c SGT, Greenbelt, MD 20771 d Jackson & Tull, Greenbelt, MD 20771 e STScI/Computer Science Corporation , Baltimore, MD 21218 ABSTRACT In August 2004, the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) ceased operation due to a failure of the 5V mechanism power converter in the Side 2 Low Voltage Power Supply (LVPS2). The failure precluded movement of any STIS mechanism and, because of the earlier (2001) loss of the Side 1 electronics chain, left the instrument shuttered and in safe mode after 7.5 years of science operations. A team was assembled to analyze the fault and to determine if STIS repair (STIS-R) was feasible. The team conclusively pinpointed the Side 2 failure to the 5V mechanism converter, and began studying EVA techniques for opening STIS during Servicing Mission 4 (SM4) to replace the failed LVPS2 board. The restoration of STIS functionality via surgical repair by astronauts has by now reached a mature and final design state, and will, along with a similar repair procedure for the Advanced Camera for Surveys (ACS), represent a first for Hubble servicing. STIS-R will restore full scientific functionality of the spectrograph on Side 2, while Side 1 will remain inoperative. Because of the high degree of complementarity between STIS and the new Cosmic Origins Spectrograph (COS, to be installed during SM4)), successful repair of the older spectrograph is an important scientific objective. In this presentation, we focus on the technical aspects associated with STIS-R. Keywords: Hubble Space Telescope, Servicing Mission 4

1. INTRODUCTION The Space Telescope Imaging Spectrograph (STIS) was developed to replace the Goddard High Resolution Spectrometer (GHRS) on the Hubble Space Telescope (HST). This 2nd generation spectrograph was installed during Servicing Mission 2 in February 1997. Prior to the anomaly that left STIS nonoperational, it accounted for fully 30% of allocated observing time for Hubble. 1.1 The Instrument The Space Telescope Imaging Spectrograph (STIS) was designed and built by a team of scientists and engineers from the Goddard Space Flight Center and Ball Aerospace (Woodgate, et al. 1998). STIS is primarily a spectrograph, allowing spatially resolved spectroscopy over a wide range of wavelengths. The instrument makes use of three separate detectors to provide multiple operational modes over four different bands (covering 115-1000 nm). The operational modes include a camera mode (used for target acquisition and deep imaging), long-slit low and medium spectral resolution modes in first order, medium and high-resolution echelle modes, and a very low-resolution prism mode. These provide spectral resolving powers between 26 and 200,000. For the optical and near-infrared bands, STIS

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Further author information: Send correspondence to S.A.R. E-mail: [email protected]; Phone: 1 301 286 4591; Address: NASA’s Goddard Space Flight Center, Mail Code 665, Greenbelt, MD 20771. Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter, edited by Jacobus M. Oschmann, Jr., Mattheus W. M. de Graauw, Howard A. MacEwen, Proc. of SPIE Vol. 7010, 70104R, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.789248 Proc. of SPIE Vol. 7010 70104R-1

2008 SPIE Digital Library -- Subscriber Archive Copy

uses a CCD array; the near ultraviolet band uses a Cs2Te Multianode Microchannel Array (MAMA); the far ultraviolet band uses a CsI MAMA. The wide range of capabilities makes STIS a highly versatile instrument. In particular, the long-slit spectral imaging modes and the high-resolution echelle mode are capabilities unique to STIS. These modes are complementary to the capabilities of the Cosmic Origins Spectrograph (COS) to be installed on HST during Servicing Mission 4 (SM4). COS was designed for the primary purpose of measuring very faint levels of ultraviolet light from distant sources. STIS will provide spatially resolved spectra of extended objects, spectra in the visible and near-infrared wavelengths, and very high spectral resolution spectroscopy. A complete discussion of the design of STIS can be found in Woodgate, et al. (1998). The on-orbit performance of STIS is discussed in Kimble, et al. (1998). 1.2 The Science During the 7.5 years of STIS operation, it contributed to advances over a wide array of important astronomical questions, including studies of black holes (e.g. Devereux et al 2003; Olling et al. 2001), stellar evolution (e.g. Hillier et al. 2001, Nielson, et al. 2007; Gull et al. 2005), cosmology (Pettini & Bowen 2001), extrasolar planets (e.g., Brown et al. 2001; Charbonneau et al. 2002; Vidal-Madjar, et al. 2004), and protoplanetary disks (Grady et al. 2005). The repaired instrument, in combination with the new Cosmic Origins Spectrograph (COS), will provide a full set of spectroscopic tools that will enable still greater scientific discovery. With these instruments, HST will be able to continue its powerful studies into black holes, galaxies, stellar evolution, and the formation of planets.

STIS, outside of HST_," -I

1L

net4SetL

.V2 Side Fig. 1. The drawing at left shows the layout of the STIS instrument within HST. The picture at right snows STIS, with the blowup indicating elements to be accessed during STIS Repair.

1.3 The Anomalies On 16 May, 2001, STIS Side 1 failed, after approximately 42000 hours of operation. This failure was most likely due to a shorted tantalum capacitor. After approximately 27000 hours of further operation, on 2 August 2004, STIS Side 2 failed, due to a failure of a 5V mechanism power converter on LVPS-2 in MEB 1. The failed unit delivered power to the mechanisms within STIS, including the aperture wheel, the Mode Select Mechanism, and the CCD shutter. This left the instrument fully shuttered, and it has remained in Safe mode since the failure, with the instrument and its on-board computer switched off. The instrument heaters have been left on in order to ensure a stable thermal environment. The Failure Review Board (FRB) which was assembled following the Side 2 failure included engineers from Goddard Space Flight Center (GSFC), the Space Telescope Science Institute (STScI), Ball Aerospace, and Interpoint. The board

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evaluated the STIS anomaly in order to identify the cause of the failure (including recommending additional testing, if needed) and to determine whether STIS could be returned to operation. The panel localized the failure to the Interpoint power converter, and determined that no work-around existed to restore STIS operations. To restore STIS operations requires the replacement of the LVPS-2 card within the instrument; this operation will be carried out during SM4.

2. STIS REPAIR During SM4, astronauts will attempt to repair STIS. The Side 1 electronics (in MEB 2) are inaccessible while STIS is in place on HST, but the SM4 crew can access the Side 2 electronics. The crew will access the instrument, opening it to remove the faulty card within MEB 1. A new board (LVPS-2R) will be installed in its place; this should restore full function of STIS on a single electronics chain. While replacing an electronics board is a relatively straightforward task, the complete STIS-R has a number of different elements. An ordered breakdown of STIS-R includes: • STIS Preparation • Open +V2 Doors • MEB Cover Removal Preparation o Disengage Clamp, Remove Handrail o Install Fastener Capture Plate (FCP) • MEB Cover Removal • Card Removal and Recovery • Install LVPS-2R • Install MEB Cover-R • Close +V2 Doors

Fig. 2. In order to remove the MEB cover, the EVA Handle and the MEB Cover Extension Clamp must be removed. New EVA tools have been developed to facilitate the removal of these components. At the left is the Handrail Removal Tool, and at right is the Clamp Removal Tool.

MEB Cover Removal Preparation: In order to access the malfunctioning LVPS-2 board, the MEB cover and cover extension must be removed (Figure 1). Before the cover can be removed, the MEB Cover Extension Clamp and the EVA Handle must be removed. Following preparation for STIS-R, these elements will be removed using two new tools, the Handrail Removal Tool (HRT) and the Clamp Removal Tool (CRT) (Figure 2). MEB Cover Removal: With the MEB Cover now accessible, it remains to remove 111 screws. These screws will be removed using the new Mini-Power Tool (MPT). In order to capture the large number of screws and washers, the MEB Fastener Capture Plate (FCP) was designed and built (Figure 3). The FCP is attached to the existing MEB Cover, and as the screws are released using the MPT, they are captured between the Cover and the FCP. When all of the screws are thus removed, the FCP and MEB Cover are removed as a single unit, and stored back on the Shuttle. The interior of the MEB is now accessible for board replacement (Figure 4).


1-I Fig. 3. The Fastener Capture Plate is attached to the front of the MEB Cover. This allows the astronauts to use the MiniPower Tool to remove and capture the 111 screws from the MEB cover. This is a critical component of STIS-R, as without this, the astronauts would have to try to capture the large number of screws with their gloved hands, an infeasible task given the limited duration of EVAs.

Card Removal and Recovery: The faulty LVPS-2 card is now removed from the MEB using the Card Extraction Insertion Tool (CEIT; Figure 5), and is then stowed for return. The new LVPS-2R card will then be installed using the CEIT. Mechanical design of the LVPS-2R card will allow for it to be guided into alignment (Figure 6). As a result of analysis, an “H-bracket” stiffener has been added to the board to increase stiffness and to provide additional protection during the stress of card insertion. STIS Closeout: The final step in STIS-R is installing the new MEB Cover-R. The Cover-R is designed to be installed without the cumbersome requirement of reinstalling the 111 original screws. Rather, by flipping the two Chassis Latches, the Cover-R will be locked into place. The Cover-R also includes Contingency Latches. EMI Gaskets provide electrical shielding for the entire enclosure, with redundant coverage designed to provide maximum protection for the electronics cavity.


I

LVPS-2

.a Fig. 4: Inside the MEB, the astronauts will be able to view the STIS electronics. The LVPS-2 card, indicated here, contains the failed power converter. This card will be pulled and replaced.

Fig. 5: The failed LVPS2 card will be extracted from STIS, and the new LVPS2-R card will be inserted, using one common tool, the Card Extraction and Insertion Tool, or CEIT, shown here.

STIS-R represents a significant step forward in our ability to perform on-orbit spacecraft servicing. STIS-R is the first ever board-level operation that was planned for on-orbit execution by EVA astronauts. In addition, the tools and techniques developed as part of STIS-R have been of high value in preparing for repair of the Advanced Camera for Surveys (ACS). In fact, many of the elements included within ACS Repair are derived from STIS-R.


MEB Chassis

Card guide (fine)

Card Guide Sracket

Card guide coarse)

LVPS-2R

Connector Shroud

Connector

Card slot

Motherboard

Fig. 4. The mechanical elements of the LVPS-2R card are designed to ensure successful seating of the card into the STIS MEB. The card guide, mechanically fastened to the card, provides both coarse and fine alignment features to properly register the card within the card slot. The connector shroud also provides alignment features to properly seat the connector pins to the MEB motherboard. All alignment features are designed to allow the astronauts to insert the card without visibility to the actual electrical connections.

EMI Gasket (enclosure)

ME

Latches

Cross Section of MEB Cover-R

Main Cover-R Latches Reed Pad TIM

Bottom View of Cover-R

Fig. 7. The new MEB Cover-R is designed to replace the original MEB cover, without requiring the reinsertion of the 111 cover screws. Instead, it makes use of two chassis latches; by simply throwing these latches, the astronauts will lock the cover into place.

3. TESTING Both the LVPS-2R board and the MEB Cover-R will be fully tested prior to launch. EMI and vibration testing of the STIS-R boards was completed in July 2007. Thermal vacuum testing was successfully completed in December 2007, providing two flight-qualified LVPS-2R boards and two Cover-Rs. All test requirements and objectives were completed. System level tests were completed in January 2008. Further testing is ongoing, with over 1000 hours of run time on the flight electronics, and over 600 hours of run time on the flight spare


After the LVPS-2R board and cover have been installed, an aliveness test and functional test will be run during SM4, soon after the repairs have been made. “ATs” and “FTs” are standard tests run on replacement hardware during Hubble servicing missions. The verification process for STIS-R will be similar to the one used for the initial STIS installation in 1997. The aliveness and functional tests will verify instrument power, and will test all mechanisms and lamps. These tests will also provide some preliminary data to demonstrate the functionality of the detectors. Brief exposures using internal lamps through selected apertures will be used to test the performance of the on-chip amplifiers. The detector logic circuitry for the MAMA detectors will also be verified during this phase; low voltage threshold settings will be adjusted to produce false positive counts from noise. The MAMA detectors themselves will not be checked out during on-orbit verification, as they require high voltage power supply. This power will not be applied until several weeks after SM4, as this provides sufficient time to ensure that the gas pressure in the aft shroud is safely below electrical breakdown pressures. Once the HST has been released from the shuttle, full orbital verification of STIS can occur. The mechanisms will be fully tested, followed by an annealing cycle of the CCD detector. The detector will then be cooled down for complete testing. The four on-chip amplifiers will be fully tested, dark current will be characterized, and the bias settings will be optimized. Internal lamps will provide a means of checking the instrument focus and stability for CCD spectroscopic and imaging modes. Once the pressure in the aft shroud reaches safe limits, testing of the MAMA detectors will begin. These detectors have been powered down for over four years; therefore, the high voltage to the detectors will be gradually increased while the logic circuitry is monitored for high dark count rates. We have examined the history of similar MAMA detectors left in the laboratory for a number of years and find little risk of bright spots or arcing, but conservatism suggests careful testing as the detectors are returned to nominal operation. Once the MAMAs are determined to operate safely at optimal voltages, a detailed sequence of testing will begin with internal lamp exposures, followed by observations of external sources. Initial calibration exposures of standard stars will provide information on any changes in instrument performance. Once the instrument is found to be nominal in performance, the instrument will return to science operations, first using CCD modes and later with the MAMA UV modes. A robust Cycle-17 calibration is planned for supporting the general investigator science programs.


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Fig. 5. All elements of STIS-R have been tested in the laboratory and validated in the Neutral Buoyancy Laboratory (NBL) by the crew of SM4. Here, J. Grunsfeld, in the NBL, is installing the LVPS-2R into STIS using the Card Extraction/Insertion Tool.

ACKNOWLEDGEMENTS STIS-R is only possible through the efforts of a number of dedicated individuals and organizations, including Ball Aerospace, ATK, Jackson & Tull, SGT, Lockheed Martin, Goddard Space Flight Center, and others. Ball and ATK worked design; J&T led manufacture, integration and testing; SGT and LM provided system engineering; Goddard provided management and HST systems support.

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