Download The Dobson Space Telescope

The Dobson Space Telescope
A time shared Telescope for NEO and Earth Observation
Tom Segert, Björn Danziger, Matthias Geitner
Technical University of Berlin, Institute of Aerospace
Marchstraße 12, D-10587 Berlin
[email protected], [email protected], [email protected]
1. ABSTRACT
The DOBSON SPACE TELESCOPE (DST) is a current „pre phase A“ study at the Technical
University of Berlin (TUB). Alike Earthguard-1 its primary mission is to find Near Earth
Objects (NEO). Instead of going on a interplanetary journey DST will search for potentially
hazardous objects from LEO and will use the umbra to achieve better observation conditions.
Secondary mission will be commercial remote sensing during sun phase. This dual use
strategy will dramatically decrease the costs of scientific research. The core piece of DST is a
20” f/5 Newton telescope. Its secondary mirror will be placed via four 2,1m Booms when the
spacecraft is already in orbit. In order to fulfill micro satellite requirements it is folded to
minimal space during ascent. This type of telescopes called truss design Dobson was
originally invented by ambitious amateur astronomers on earth. It boosted apertures of
semiprofessional optics to levels where classical Schmidt-Cassegrain (SC) are unable to
follow. Micro Satellites Telescopes using SC optics could be no bigger than 12” Simplesat
without increasing satellite dimension. Therefore further aperture grow demands this new
technology.
2. INTRODUCTION
Shoemaker-Levi 9 in 1994 had a far greater impact on earth than on Jupiter. It created a new
sensibility for the vulnerability of planet earth. For that reason in 1996 Spaceguard (SGF) was
founded. This intergovernmental organization co-ordinates a network of 80 medium sized
telescopes. Being ground based means lesser ability to view into the inner solar system and
dependance to earth weather. In early dawn before sun outshines the minor bodies
astronomers try to get a glimpse of what’s going on. One space based mikrosatellite using the
same technique could far more easily observe potentially hazardous objects.
Sharing the Satellite with a commercial remote sensing mission is an ideal deal. While earth
observation needs sunlight, the search for NEO`s requires shadow. Except the time needed for
maneuver the satellite is permanently in use. This will reduce the observation fees and enable
new quality and quantity of scientific research.
3. SPACESEGMENT
NEO observation* Remote sensing**
DST
Orbit
Polar SSO dusk
700km polar SSO dawn
700km polar SSO
Resolution
1-4m Ground pixel
max. 1m Ground pixel
Field of view
2°
max. 2°
Optical Speed <f/5
>f/8
f/5 – f/12,5
Spectral view
visible; infrared
visible, near infrared
Visible, near infrared
Although there are slight differences in the needs of primary and secondary mission DST
provides good trade-offs for both passengers.
*
Spaceguard requirements
Ikonos data
**
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3.1 Orbit
DST uses a solar synchronous orbit with
an altitude of 700km, giving it a very
good view on earth while staying a
reasonable time in umbra. An angle of
45° between the orbital plane and the
sun will make DST a morning satellite.
As shown in figure 1 all objects with an
aphel greater than 1/3 AU could be
easily identified during the operational
phase of 1 year.
Due to the lack of an orbit control
system the orbital plane will shift during
the time of operation. However long
term studies with TUB-Satellite-A
showed, an inclination precision rate of
Figure 1 :
DST Orbit
0,1° per year and overflighttime changes
within a few minutes every month that seams to be acceptable.
3.2 Attitude Control System
Facing high resolution of DST means putting focus on the ability doing the satellites job with
less vibration as possible. Momentum wheels for 3 axis control are normally great in
disturbing peace of observation but newly invented magnetic bearing reaction wheels similar
to AMSAT-3d-phase would do a great job. Magnetic coils will be used to continuously reduce
the angular momentum. On sensor side onboard GPS-Receiver and star sensors provide
needed data.
3.3 OBDH
Payload generates a maximum of 1.5Gbit of data throughput which has to be handled by
OBDH. DST uses several parallel microcomputers to face with the need of simultaneous
control and transmission cycles.
Although compressed at least x-band transmitters are needed to assure a bandwidth of
700Gbit during interactive video operation. S-Band is onboard only for backup reasons in
case of data relay satellite failure.
3.4 Payload
Micro satellites for remote sensing normally use SC optics. In consequence on their concept
the focal ratio of these reliable robust telescopes is greater than 8 which made resolution near
theoretical limits possible to small apertures. Compared to earth and stars NEO are flimsy
objects therefore more aperture is needed. This aim can not be achieved by compact
telescopes. Despite the folded optical path of SC telescopes 12“ optics with a length of 80cm
outnumber the possible size of the satellite as lately shown with Simplesat. More Aperture
leads inevitable to truss design Dobson’s. This type of telescopes was invented by amateur
astronomers who desired huge apertures in combination with the capability of smooth
transport and humble costs. A truss Dobson is an azimuthally mounted Newtonian telescope
which could be easily disassembled for transportation. High aperture and less optical surfaces
combined with a small focal length made them perfect for deep sky observation as well as
NEO research.
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DST comes with a 20“ f/5 Newtonian
telescope which is a good trade-off between
optical speed and aberration. Normal 20“
Mirrors weight about 20-30kg. They have to
be that heavy in order not getting deformed by
gravity. For that reason in space the use of
ultra thin mirrors is possible. A prototype of a
21“ Mirror weighting only 1kg was presented
by the University of Arizona in 2002.
Increasing the resolution of the telescope
for remote-sensing is quite simply made by
using optical multipliers. A „Barlow Lens“
with a factor of 2.5 pushes the focal ratio up to
f/12.5 which assures maximum possible
magnification and a ground pixel resolution of
about 1m. Optical multipliers and filters put
into optical path via a simple wheel
mechanism has proven to be a reliable
solution in astronomy for decades.
Another highlight of this telescope is the
Figure 2 : DTS Launch Configuration
deployable second mirror. 4 booms each 6
times diagonal folded lay over the main mirror. In Order to make DST operate they will erect
to a 2.10m high tube which is covered by a foil against stray light.
In contrast to a SC telescope the camera is next to the secondary mirror. For this reason
special precaution have to be made to protect the CCD`s from radiation and heat. Part of a
possible solution is the shadow produced by the unfolded solar arrays as shown in figure 3
A slice difficulty is in orbit adjustment of the secondary mirror after unfolding the
telescope. Astronomers on earth therefore invented easy to handle collimation lasers which
allow in combination with 3 actuators behind the second mirror the exact alignment of the
optic path.
In order to get pictures for further electronic processing there are two main CCD Sensors
onboard. One large area panchromatic 28Mpixel and one 2Mpixel HDTV for interactive
video observation. By putting a splitting prism into the optic path during earth observation
simultaneous use of both cameras is possible and the CCD is secured from getting glared by
earth albedo.
Figure 3 : DST unfolded
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4. GROUND SEGMENT
Figure 4 : Groundsegment of DTS
With the growing bandwidth of direct internet application a new controlling strategy is made
possible. In order to enable interactive video observing, even when the ground antenna is out
of sight, DST uses geostationary data relay satellites to communicate via internet with the
user. Therefore DST simulates a ground based server by automatically login to a new host
every half orbit. Since the maximum continuous operation cycles are that long there isn’t
much fear of loosing connection while observing. It is now possible to connect to DST from
everywhere on the planet.
The renunciation of a central ground station may be useful to the decentralize concept of
SGF but it’s believed to be perfect for commercial users. Once they bought observation time
from a marketing agency, customers login to DST. Then only guided by onboard failure
control they choose freely where to watch what to observe. The collected data will be
encrypted and send to them via internet. This strictly confidential use of DST will be greatly
attractive especially to intelligence and military customers.
5. CONCLUSION
Although DST study is in a very early stage the great potential of truss Dobson’s for space
application is already visible. It is our aim to point out the strength and difficulties of this
newly space optic design. To open path for a new quality in micro satellite based observation
where once much heavier platforms would have been used. Since 2.4m HUBBLE could only
be beaten by 8m VLT with special seeing suppression it may be that a few years to go Dobson
telescopes mean the same to micro sat application than to amateur astronomers today. That
there is a fleet patrolling the sky; a fleet of micro satellites carrying affordable midrange
telescopes. Following the basic principle of astronomy: aperture can be replaced by more
aperture but dark transparent skies cannot be replaced by anything else. Dark transparent
skies
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