NRL VAULT Very-high-resolution Advanced ULtraviolet Telescope
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Instrument Description and Status

Figure 1 shows the ideal ultraviolet modulation transfer function (MTF) of a 30 cm telescope at Lyman-alpha wavelength 121.5 nm. The VAULT 0.33 arcsecond resolution is close to the Lyman-alpha Rayleigh diffraction limit (~0.1 arcsecond). The principal components of VAULT are shown in Figure 2. The instrument and in-flight technical performance are described in Korendyke et al. (2001). VAULT uses an excellent optical quality (wavelength/16 rms at 121.5 nm), 30 cm diameter telescope followed by a zero dispersion spectroheliograph (Bartoe and Brueckner 1978) to obtain 0.33 arcsecond spatial resolution UV images. The solar radiation passing through the field stop is collimated and dispersed by G1, reflected by a single folding mirror and recombined by G2. The G1 and G2 grating configuration was selected to obtain a UV solar image with minimal geometric blur (<5 microns, 0.07 arcsecond, predicted by a ray trace), moderate bandpass (15.0 nm at Lyman-alpha) and high efficiency (~13%). The solar image is projected onto a lumogen coated, 2048x3072, 9 micron pixel KAF-6301 Kodak CCD. The CCD UV quantum efficiency (>10%), linearity and dynamic range are substantially improved over the 101 film used in NRL's High Resolution Telescope and Spectrograph (HRTS) flights. The throughput allows short exposure times (1 second) even with a plate scale of 72 microns/arcsecond.

Figure 1. VAULT MTF. Diffraction-limited sinusoidal MTF plotted for a 30 cm telescope with 25% obscuration at wavelengths 546.1 nm and 121.5 nm.


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Figure 2. VAULT Instrument Optical/Mechanical Layout

VAULT uses an f/24.6, 30 cm diameter, 25% obscuration Cassegrain telescope. The optical parameters were chosen to allow reuse of the HRTS Gregorian graphite tube and structure. While prior HRTS Cassegrain and Gregorian telescopes had f/3 primary mirrors with a secondary mirror magnification of 5, VAULT uses an f/4.92 primary with a magnification of 5. The thermal loading of the Zerodur primary and secondary is similar to the HRTS Cassegrain. During the five minutes of solar observations, no image quality degradation was observed.

The VAULT telescope produces a solar image with a plate scale of 72 microns/arcsecond compared to the HRTS value of 22 microns/arcsecond. Coma and astigmatism are <0.01 arcsecond over the instrument field of view. The slower primary mirror reduces the sensitivity of the telescope to secondary decenter, focus, and tilt. The telescope tube and structure have been flown on Spacelab 2 as well as HRTS 8 and 9. The amount of tilt and decenter has been measured to be <1 arcminute and <50 microns respectively, even under relatively severe stress conditions. The amount of blur induced by this secondary tilt and decenter is <0.05 arcsecond. The combined telescope system optical/mechanical performance allows 0.33 arcsecond spatial resolution to be achieved.

The payload makes extensive use of existing HRTS hardware and mechanical design heritage. The overall mechanical design of VAULT is identical to HRTS. The latter has benefited from several experience-based modifications over the years, particularly in the telescope design area, and meets the VAULT performance objectives. The telescope tube and focal plane deck structure are cantilevered from a central bulkhead. The primary mirror is potted in a central hub, to avoid stressing the optic. Curing procedures, which effectively avoid molecular contamination, have been developed. The hub is mounted directly to the central bulkhead. The focal plane deck structure is launch locked to the rocket skin using a retractable pin mechanism. The front and aft rocket skins are bolted to the central bulkhead. A vacuum compatible, commandable aperture door, provided by NASA's Wallops Flight Facility (WFF), is used to expose the telescope to the Sun and to protect it from damage during reentry. A pair of specially designed breather filters on the telescope door slowly bleeds the instrument to ambient pressure while the payload is descending to the desert floor. VAULT reuses the front and aft HRTS skins from HRTS 8 and 9, but a new vacuum bulkhead and electronics skin section were fabricated for VAULT.

The superb, demonstrated performance of the Mark-7 SPARCS system allows VAULT and other high-resolution solar payloads to be launched without image motion compensation systems. The new system incorporates a fiber-optic communication line, which relays the sensor output information to the ACS system. The new electronics includes a microprocessor for digital control of the SPARCS gas jets. HRTS 10 was the first scientific flight of the new Mark-7 digital SPARCS system. It has subsequently flown several times. The performance of the SPARCS system during the HRTS 10 and VAULT 1 flights was superb. The drift and jitter, generally associated with electrical pickup on the signal lines in the long cable between the SPARCS skin section and the analog sensor, were entirely eliminated. A stability of the payload of 0.1 arcsecond was obtained. During VAULT 2, a drift rate of 0.3 arcsecond/second was observed between the digital sensor axis and the telescope axis. The source of this error was traced to the mounting of the spectroheliograph structure to the rocket skin. A new design will be implemented and should reduce the drift by a factor of 3 for VAULT 3.

CCD Camera and Flight Computer Description

A block diagram of the VAULT electronics bulkhead is shown in Figure 3. A PC-104 computer sequences the mechanisms, the CCD camera, and the telemetry interface. A number of auxiliary boards provide power conversion, internal/external power relays, and analog conditioning and mechanism driver functions. The computer is comprised of Commercially available Off The Shelf (COTS) PC-104 format boards; these boards are repackaged at NRL to withstand the sounding rocket environment. The small footprint (90x90 mm), low power dissipation, and surface mount PC-104 format boards are technically well suited to the sounding rocket application; the boards are also readily available and reasonably priced. The computers are used routinely in embedded systems; the DOS and BIOS of these computers are specially modified to prevent fatal operating system errors.

The VAULT camera electronics and camera head are a modified version of the COTS Apogee AP-9 thermoelectrically cooled camera. The VAULT camera incorporates a Kodak 6301e, class-2 CCD detector. The CCD is lumogen-coated to obtain ultraviolet sensitivity. The measured performance of the AP-9 VAULT camera is given in Table 1. Existing software drivers for the commercial camera were used as the building blocks to construct the VAULT flight and laboratory software.


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Figure 3. VAULT electronics block diagram.

Table 1. CCD Camera Performance
CCD Kodak KAF6301e, 2048x3072, lumogen coated, 9 micron pixels
Dark current Minimal with cooling to -10 degrees C
Full well 85,000 e-
Read noise 40 e-, standard deviation
Digitization 14 bits, 0.5 MHz readout, 5 e-/DN

    Very High Resolution Advanced Ultraviolet Telescope
 

2003 U.S. Naval Research Laboratory