Transcript ATLAST 9.2: A Deployable Large Aperture UVOIR Space

ATLAST-9.2: A Deployable Large Aperture UVOIR Space
Telescope
W. Oegerle1, L. Feinberg1, L. Purves1, T. Hyde1, H. Thronson1, J. Townsend1, M. Postman2, M. Bolcar1, J.
Budinoff1, B. Dean1, M. Clampin1, D. Ebbets3, Q. Gong1, T. Gull1, J. Howard1, A. Jones1, R. Lyon1, B. Pasquale1, C.
Perrygo4, S. Smith1, P. Thompson1, B. Woodgate1 and the ATLAST Concept Study Team
(1NASA/Goddard Space Flight Center, 2Space Telescope Science Institute, 3Ball Aerospace, 4SGT/GSFC)
Abstract
Sunshield and Stray Light
ATLAST-9.2
(folded) inside
Delta IV Heavy
fairing
We present the results of a study of a deployable
version of the Advanced Technology Large
Aperture Space Telescope (ATLAST) that could
be launched on an Evolved Expendable Launch
Vehicle (EELV). The observatory retains
significant heritage from JWST, thereby taking
advantage of technologies and engineering
already developed for that mission. At the same
time, we have identified several design changes
to the JWST architecture, some of which are
required due to the demanding wavefront quality
required for diffraction limited imaging at 500 nm.
A planar sunshield plus a central primary mirror
baffle is sufficient to limit stray light to less than
10% of the zodiacal sky brightness while also
eliminating “rogue path” light that would otherwise
skirt around the secondary mirror. The deployed
sunshield consists of four layers of opaque Kapton
film with a simple flat square shape, 28 m on a
side. To ease manufacture, integration and test
(I&T), and stowage procedures, each of the four
layers is subdivided into four quadrants that are
connected during I&T so that each deployed layer
is continuous. Four 18-meter booms extend in a
cruciform configuration to deploy the membranes.
Four layer
sunshield (corner)
Launch Vehicle, Orbit
Drive with 4
cables
Delta-IV Heavy with 6.5m
diameter x 29.5 m payload fairing
ATLAST fits into a modest upgrade to the Delta IV
Heavy, which will have a 6.5 m (outer diameter) fairing,
and the ability to put 15,800-18,000 kg into SEL2 orbit
(ref: www.ulalaunch.com). The launch trajectory
provides direct injection to a halo or Lissajou orbit with
no eclipses by the Earth over ten years.
Deployed OTA
Folded OTA rear
view, showing
instruments
(colored boxes)
The Optical Telescope
Assembly (OTA)
The OTA is a 9.2m segmented mirror consisting of
36 hexagonal Ultra-Low Expansion (ULE) glass
mirrors. The mirrors are 1.315m in size (flat-to-flat)
with areal density of ~20 kg/m2. The architecture
retains much of its heritage from JWST. The OTA is
operated at room temperature and is thermally
controlled using heaters on its backplane.
18 m boom
Gimbaled mounting of OTA
Deployment Sequence
The OTA is mounted on a multi-gimbal arm that
provides OTA pitch motion, roll about the OTA line
of sight, and center of mass trim for solar torque
control while allowing the spacecraft and sunshield
attitude relative to the Sun to remain fixed. This
enables a very large field of regard with allowed
pointing from 45 to 180° from the sun, permitting
the use of an external occulter (starshade) for high
contrast imaging of exoplanets.
Optical Design
The optical design provides a Three Mirror Anastigmat
(TMA) channel for wide FOV instruments and a
Cassegrain channel that minimizes reflections in the
UV contains coronagraphic instruments for observing
exoplanets. The primary mirror is fast (f/1.25) to
minimize length of the OTA. Primary and secondary
mirrors are coated with Al+MgF2 for UV response. All
optics in the TMA channel and in the exoplanet
instruments with  > 500 nm are coated with protected
silver. The design is diffraction-limited at  = 500 nm
over an 8 x 20 arcmin FOV. The wavefront error map
for the TMA channel is shown below
An active isolation system between the arm and
OTA isolates spacecraft disturbances and, using
the FGS sensor, provides a total image motion (ie.
jitter) of ~1 milli-arcsec.
After fairing
separation
Deploy solar
array and comm
antenna
Sunshield deployed (looking
from sun side; S/C bus
Visible)
Gimbal arm unstowed
and OTA deployed (dark
side of sunshield)
Wavefront Sensing & Control (WFS&C)
The FOV in the Cass channel is ~ 1 arcmin
Cass focus (f/12.55)
TMA focus (f/18)
The core technologies for WFS&C are similar to those employed on JWST, but will run in real-time without
ground intervention. A schematic of one of the combined Fine guidance sensor (FGS) and WFS sensors is
shown in Figure 1. Light from the OTA is fed to the FGS/WFS via a pick-off mirror. A bi-directional fine steering
mirror (FSM) is used to access a 4’x4’ FOV and steer an isolated star onto the FGS/WFS detectors. The beam
is then split 3 ways, with 20% of the light going to the FGS for guidance, and 40% going to each of 2 WFS
detectors. Each WFS beam path contains a dual filter wheel and imagining optics to form an image of
the star on the WFS detectors. The filter wheels are
actuated to place a narrow band (Δλ/λ ≈ 0.01-0.05)
filter in each beam path, as well as a weak lens. One
path will use a positive weak lens while the other uses
a negative weak lens to produce two out-of-focus
images of the star on either side of focus. The images
are sent to an onboard processor where a phase
retrieval algorithm is run and segment-motion
commands are generated every 5-20 minutes to adjust
the shape of each primary mirror segment. Data from 3
FGS/WFS sensors is used to make ~daily adjustments
to the secondary mirror.
Spacecraft & Servicing
The spacecraft bus provides coarse attitude
control, propulsion, power, communication (Ka
band) and data handling modeled after JWST, but
employs a modular architecture (shown below) that
allows servicing and reduces risk during I&T. Each
bay in the octagonal S/C bus is easily accessible
and removable. Refueling of propellant tanks could
also be done robotically during servicing to extend
observatory life. All instruments are modular and
replaceable on orbit using an HST-derived rail and
kinematic latching systems.
Total wavefront error budget = 40 nm
Allocation to wavefront sensing < 10 nm
Modular spacecraft bus