Prototype Aspect System for the Solar Orbiter / STIX Instrument
This
work will develop and test a prototype aspect system for a hard X-ray telescope
to be proposed for flight on the Solar Orbiter mission.
The
project will involve assembling the aspect system optics, checking out the data
acquisition hardware and software, acquiring data with the system and analyzing
the results. The student will do much of the hardware assembly, data
acquisition and analysis but with enough advice/encouragement to ensure success
by the end of the summer. It should
provide a good overall exposure to real-world hands-on experimental physics and
make a real contribution to future proposed spaceflight instrumentation.
Please
contact me (643-9653, ghurford@ssl.berkeley.edu)
if you are interested or have any questions (but note that I will be out of
town until April 11). Details on the
project, taken from a Cal Space proposal are given below....
Solar
Orbiter is a planned ESA mission that will orbit as close as 0.22 AU from the
Sun for in situ studies of the inner heliosphere. NASA involvement is anticipated through the International
Sun-Earth Connection Program. Solar
Orbiter’s strawman payload of in situ and imaging instruments now includes
STIX, a ‘Spectrometer/Telescope for
Imaging X-rays’, suggested by Lin and Hurford in 2001. Its role is to establish the timing, location
and spectra of energetic electrons near the Sun so that these electrons can be
related to in situ observations by solar energetic particle and radio
instruments. STIX uses non-focusing
Fourier transform optics to image solar x-rays from 3 to 150 keV with
2.5-arcsecond angular resolution using the same imaging principles as the
Yohkoh/HXT payload.
The
imager consists of two 10x10cm grid trays, separated by 90 cm. Each tray carries an array of 64 1x1cm grid
elements while each grid element features a large number of parallel slits and
x-ray absorbing slats with judiciously chosen pitch and orientation. X-rays transmitted by the grids are measured
by a 64-element CZT detector array located behind the rear grid. Imaging information is encoded by the
relative transmission of the 64 grid pairs.
A C-C heat shield in front of the grids not only deals with the
significant solar heat load (20x that at 1 AU), but also suppresses the intense
flux of unwanted, low-energy solar flare x-rays.
Such
telescopes (e.g. RHESSI) typically use an aspect system based on a lens mounted
on the top grid tray that focuses a solar image on the rear grid tray where a
linear diode array accurately locates the solar limbs. Because of the co-planarity of aspect and
grid components, this not only enables accurate, absolute placement of x-ray
images on the solar disk but also fully compensates for pointing and telescope
structural variations.
For
STIX, two novel variations on this aspect approach are necessary. The first is to use a metal-etched Fresnel
zone plate (instead of glass) as the focusing element. This is desirable to avoid the potential
risk of the degradation of a glass lens that would be exacerbated by the
intense solar illumination at 0.22 au.
The second is to use only a narrow slice of the Fresnel zone plate
(through its diameter) to minimize the optical heat load transmitted to the
rear grid tray. Diffraction then
spreads the solar image orthogonal to the focused direction, while still
enabling accurate location of the projected solar limb in 1-dimension. An identical, but orthogonal system locates
the limb in the perpendicular direction.
In
this work we plan to use inexpensive, non-space qualified components to build a
1-D prototype optical aspect system as described in order to optimize the
optical parameters and to demonstrate the feasibility and performance of such a
system. Achievement of these goals
will greatly enhance the credibility of an SSL proposal to be submitted in
response to an anticipated AO for Solar Orbiter instrumentation in
2006-2007.
The
prototype components include a 0.5 x 30mm segment of a Fresnel zone plate with
90cm focal length (custom fabricated by MikroSystems Inc.), a narrow-band
optical filter, an off-the-shelf 2048x1 element CCD with USB interface (to an
existing laptop), corresponding data acquisition software, and miscellaneous
components to fabricate a tube and adjustable mount for outdoor testing on the
Sun.
Testing
is based on rapid readout (up to 128 Hz) of the CCD array as the Sun drifts
through the field of view of the stationary telescope. Post-analysis of the limb positions
establish the aspect system linearity, resolution and effective field of
view. The role of seeing can be
evaluated since atmospheric fluctuations tend to be stable on timescales less
than ~40 ms.
Additional
information on the Solar Orbiter mission can be found at:
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=45