NASA Dawn’s Early Light – April 2004

The fifth issue of the Dawn team newsletter, Dawn’s Early Light,
has been posted on the Dawn website. Follow the links below to
view individual articles, or obtain the pdf version. We look
forward to obtaining your feedback.

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20040422/status.html

NASA Dawn Mission Status April 2004

Dawn proceeds towards critical design review

Christopher T. Russell

Dawn Principal Investigator, UCLA

Dawn has now entered Phase C/D, the implementation phase, and is moving
forward quickly toward launch. Dawn has made excellent progress, as
described below, as has the Discovery program itself.

As previously reported, the Discovery Program Office has moved to JPL
where the office has the technical support it needs to evaluate and
assist the missions in development. At NASA Headquarters, Orlando
Figueroa has appointed a deputy, Andy Dantzler, who will oversee the
Discovery Program. These changes will ensure that Dawn and all the
Discovery missions receive the support and advice that is needed to
achieve success.

Great progress has been made with the Dawn spacecraft, the payload and
the ground system. All spacecraft subcontractors are on board and
hardware is beginning to appear. Subsystem CDRs are taking place weekly.
The GRaND CDR was the first payload CDR to take place on March 30 and 31
with VIR following on April 22 and 23. The Framing Camera CDR takes
place on May 18 and 19.

The technical reserves of the project are appropriate to the mission
phase and green is becoming Dawn’s official color. On April 13, an
independent power and mass margin review was successfully conducted,
confirming the project’s assessment of health. Schedule reserve continues
to be tight; however, an aggressive schedule is key to executing a
successful low-cost mission. Recent events within the Discovery program
have increased the level of oversight and the effort required to support
this oversight, directly impacting schedule. As we begin to build and
integrate the spacecraft and payload, the schedule is and will remain a
top priority.

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20040422/vir.html

Dawn’s Visible and Infrared Mapping Sectrometer (VIR)

Dawn’s Visible and Infrared Mapping Spectrometer (VIR)

Angioletta Coradini
VIR Team Lead, Istituto Nazionale di Astrofisica (INAF), Rome

Hubble Space Telescope observations of Vesta’s surface made ten
years ago revealed a large southern polar crater and geological
diversity, with regions that can be characterized spectroscopically
as basalts. Figure 1 shows the surface composition map of Vesta
produced from separate images in blue (439 nm), orange (673 nm),
red (953 nm), and near-infrared (1024 nm) light.

The map shows that all of Vesta’s surface is
igneous, indicating that either the entire surface was
once melted, or lava flowing from its interior
completely covered its surface. A firm identification
of surface geology on Vesta requires medium-high
resolution spectra. The Dawn Visible-IR Mapping
Spectrometer (VIR) addresses this need with a
capability to acquire high spectral and spatial resolution
data. Spectral coverage is important for both Vesta and Ceres, as
diagnostic minerals show absorption bands in the visible and
near IR regions. For this reason we developed a single
spectrometer able to cover both visible and IR spectral
ranges.

The Visible and Infrared sensors, that are the heart
of the VIR imaging spectrometer, are housed in the
same optical subsystem. The instrument is derived
from the VIRTIS experiment that is presently flying
on the Rosetta mission. Figure 2 shows the result of
in-flight calibration of VIRTIS, compared with the
ground-based measurements. The difference at the
long wavelengths is due to the higher temperature of
the VIRTIS box in-flight.

The optics module OM (Figure 3) contains the
optical system, scan mirror, the entrance and
sunshield, cover, shutter, cryocooler, in-flight
calibration units (lamps), radiators, focal plane
arrays (FPAs), and the proximity electronics
(PEM). The OM architecture has been maintained
as similar as possible to the VIRTIS for Rosetta;
only the spacecraft-instrument interfaces have been
modified. The optical concept is inherited from the
visible channel of the Cassini Visible Infrared
Mapping Spectrometer (VIMS-V) developed at
Galileo Avionica. This concept matches a Shafer
telescope to an Offner grating spectrometer to
disperse a line image across two FPAs. The Shafer
telescope and Offner spectrometer are aligned separately,
then mounted and co-aligned.

The Shafer telescope combines an inverted Burch telescope and
an Offner relay (M4/6 and M5). The Offner relay takes the curved,
anastigmatic VIR virtual image of the inverted telescope and makes
it flat and real without losing the anastigmatic quality. Coma
optical aberration is eliminated by putting the aperture stop on
M5 near the center of curvature of the primary mirror and thus
making the telescope monocentric. The result is a telescope
system that relies only on spherical mirrors yet remains
diffraction limited over an appreciable portion of the spectrum
and all vertical field (slit direction). The Shafer telescope is
matched to the Offner grating spectrometer because both are
telecentric; the entrance pupil is positioned in the front focal
length (FFL) of the optical system at 750 mm in front of the
primary mirror (M1). Because the pupil optics conjugate is on
the grating, the same spectral beam splitting is performed for
each FOV angle. The grating profiles are holographically recorded
into a photoresist and then etched with an ion beam. Higher groove
density in the central 30% of the conjugate pupil area generates
the higher spectral resolution required in the "visible" channel,
extending from the ultra-violet to the near infrared. The smaller
pupil area allows the visible channel to operate partially
coherently and achieve a smaller point spread function.

A laminar grating is used for the visible channel’s pupil zone to
increase the grating efficiency spectrum and compensate for low
solar energy and low CCD quantum efficiency in the ultra-violet
and near infrared regions. This improves the instrument’s dynamic
range by increasing the S/N at the extreme wavelengths and
preventing saturation in the central wavelengths. For the
infrared zones, a blazed groove profile is used that results in a
peak efficiency at 5 µm to compensate for the low signal levels
expected at this wavelength. These features, combined with custom
designed FPAs, result in an instrument able to collect spectra of
very large dynamic range with high spatial resolution
that satisfies Dawn’s requirements.

Dawn’s Early Light is published on an occasional basis and
distributed electronically. To contribute material or query the
team, email us at dawnnews@igpp.ucla.edu.

Editor: Carol A. Raymond, Jet Propulsion Laboratory