The BepiColombo Laser Altimeter (BELA) is one of 11 instruments aboard ESA's Mercury Planetary Orbiter (MPO)
scheduled for launch in 2014. BELA will record the surface profile of the planet while orbiting around it at a distance of
400km to 1500km<sup>1</sup>. The altimetry data constitute an important prerequisite for a number of remote sensing and
observation techniques residing on the same orbiter. The BELA instrument comprises a laser transmitter and a receiver
part, the design of the former is being presented and discussed in this paper. The laser transmitter encompasses a pair of
diode-pumped, actively Q-switched Nd:YAG rod oscillators which have been miniaturized, light-weighted and
dimensioned for high electrical to optical efficiency. The key performance parameters of the laser will be presented.
Laser design trades which are relevant for a space mission to Mercury and the BELA instrument in particular are
discussed. An overview is given to the laser qualification programme which includes performance and environmental
tests. Test results are presented which have been recorded during the qualification test campaign currently in progress at
Carl Zeiss Optronics.
In order to perform spectrometric measurements, the Near Infrared Spectrometer (NIRSpec) aboard the James Webb
Space Telescope (JWST) needs the ability to select various spectral band widths and split these up into its comprised
wavelengths. These functions are achieved by the Filter Wheel Assembly (FWA) and the Grating Wheel Assembly
(GWA). The filters of the FWA select a different bandwidth of the spectrum each while the gratings on the GWA yield
specific diffractive characteristic for spectral segmentation. A high spectral sensitivity as well as the ability to detect the
spectra of various objects at the same time result in high requirements regarding the positioning accuracy of the optics of
both mechanisms in order to link the detected spectra to the 2-dimensional images of the observed objects.
The NIRSpec mechanism including FWA and GWA will operate at temperature levels below 42K which are established
during testing inside of a cryostat. However the alignment and testing of these mechanisms requires a lot of thought since
there is very limited access to the item under test within such a device. Alignment needs to be preloaded based on
simulations and testing is reduced to optical methods and evaluation of electrical signals.
This paper describes the methods used for the various alignment steps, the corresponding tests and their precision of
measurement as well as the achieved accuracies in the mechanism performance.
System designers and end users of diode pumped solid state lasers often require knowledge of the operability limits of
QCW laser diode pump sources and their predicted reliability performance as a function of operating conditions.
Accelerated ageing at elevated temperatures, duty cycles and/or currents allows extended lifetime testing of diode stacks
to be executed on compressed timescales with high confidence.
We present a novel, time-efficient technique for the determination of accelerated lifetime test conditions using
degradation rate data, rather than the traditionally used failures against time data.
To assess the effect of thermally accelerated ageing, 4 groups of 4 stacks each were operated for 60 million pulses at
different temperature stress levels by varying the pulse repetition rate from 100Hz to 250Hz. The measured power
degradation rates fitted to an Arrhenius type model, result in activation energy of 0.47- 0.74eV, apparently indicating
two thermally activated degradation modes with different activation energies.
Similarly, for current accelerated ageing, another 4 groups of 4 stacks were tested at operation currents from 120A to
150A. The optical power degradation rates due to current stress follow a power law behavior with a current acceleration
factor of 1.7.
The obtained acceleration parameters allowed considerable reduction of the lifetime test duration, which would have
otherwise taken an unacceptably long time under nominal operating conditions.
The successful results of the accelerated lifetime have been a major milestone enabling qualification of SCD stacks as
pump sources for the laser altimeter in ESA Bepi-Colombo space mission.
The presented reliability analysis allows life test qualification programs to be accelerated for generic QCW stacks and
their lifetime to be predicted in various operating environments.
The Grating and Filter Wheel Mechanisms of the JWST NIRSpec instrument allow for reconfiguration of the
spectrograph in space in a number of NIR sub-bands and spectral resolutions. Challenging requirements need to be met
simultaneously including high launch loads, the large temperature shift to cryo-space, high position repeatability and
minimum deformation of the mounted optics. The design concept of the NIRSpec wheel mechanisms is based on the
ISOPHOT Filter Wheels but with significant enhancements to support much larger optics. A well-balanced set of design
parameters was to be found and a considerable effort was spent to adjust the hardware within narrow tolerances.
The Near-Infrared Spectrograph (NIRSPEC) on board the James Webb Space Telescope can be reconfigured in space for
astronomical observation in a range of NIR sub-bands as well as spectral resolutions. Reconfiguration of the NIRSpec
instrument will be achieved using a Filter Wheel Mechanism (FWA) which carries 7 transmission filters and one reflective
mirror and a Grating Wheel Mechanism (GWA) which carries six gratings and one prism. The dispersive components
on the grating wheel (GWA) cooperate with the edge transmission filters mounted on the filter wheel (FWA) which block
the higher dispersion orders of the gratings. The paper gives an overview on the design of all optical elements, their key
requirements and the employed manufacturing approach. Test results from breadboard and component level qualification
phase are also given.
Following a warm launch in 2013 the MIRI instrument aboard JWST will be operated for a lifetime of 5-10 years in the L2-orbit at a temperature of ~6 K. The main requirements for its three wheel mechanisms include: (1) reliability, (2) optical precision, (3) low power dissipation, (4) high vibration capability, (5) functionality at 4 < T < 300 K. The filter wheel carries broad and narrow band spectral filters, coronographic masks and a prism on its 18 positions. Each of the two spectrometer wheels is equipped with two disks on both sides of a central torque motor, one of them carries 6 gratings, the other a dichroic/mirror arrangement. The optical positions are defined by a ratchet mechanism. No closed loop control is required; therefore the long time average heat dissipation is negligible. A new ratchet mechanism had to be developed to satisfy a 120° increment of only three positions for the spectrometer wheels.
Extensive cold and warm tests were performed on the development models of the filter and spectrometer wheels at
MPIA. These results stimulated numerous improvements in the mechanical and thermal design which are now to be
implemented in the qualification and flight models developed jointly with Carl Zeiss. Synergies are expected from a
similar development of the NIRSPEC wheels, in which MPIA and Carl Zeiss are involved.
The Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope can be reconfigured in space for astronomical observation in a range of filter bands as well as spectral resolutions. This will be achieved using a Filter wheel (FWA) which carries 7 transmission filters and a Grating wheel (GWA) which carries six gratings and one prism. The large temperature shift between warm launch and cryogenic operation (30K) and high launch vibration loads on the one hand side and accurate positioning capability and minimum deformation of optical components on the other hand side must be consolidated into a single mechanical design which will be achieved using space-proven concepts derived from the successful ISO filter wheel mechanisms which were manufactured and tested by Carl Zeiss. Carl Zeiss Optronics has been selected by Astrium GmbH for the implementation of both NIRSpec wheel mechanisms. Austrian Aerospace and Max-Planck-Institut fur Astronomie Heidelberg (MPIA) will contribute major work shares to the project. The project was started in October 2005 and the preliminary designs have been finalized recently. Critical performance parameters are properly allocated to respective hardware components, procurements of long-lead items have been initiated and breadboard tests have started. This paper presents an overview of the mechanism designs, discusses its properties and the approach for component level tests.
In 2011 NASA and ESA plan to launch the James Webb Space Telescope (JWST) as dignified successor of the Hubble Space Telescope. Three scientific instruments will cover the wavelength regions in the near-infrared (0.6-5μm, NIRCam and NIRSpec) and in the mid-infrared (5-28μm, MIRI), respectively. The ESA-led multi-object spectrograph NIRSpec as major European contribution is presently entering the detailed design phase in a collaboration between European space industries, scientific institutes, ESA and NASA. To allow for various operational modes in the instrument’s optical train several cryo-mechanisms are required, i.e. wheels for exchanging optical elements like filters and gratings as well as linear actuators on refocusing mirrors. We will give an overview on the detailed design, the prototyping and the testing of those mechanisms comprising highest reliability in the cryo-vacuum (~ 35K) combined with minimal power dissipation (~ 5mW on average), ultimate position accuracy (~ 0.5 - 1arcsec) combined with high launch vibration capability (ARIANE 5, ~ 60g) and a very long lifetime (~ 15 years) for ground tests and space operation under various environmental conditions. To reach this goal in a low cost and risk approach we rely on the heritage from ESA's earlier infrared missions, i.e. ISO and HERSCHEL.