The ATLAST (Advanced Technology for Large Aperture Space Telescopes) effort has presented several design incarnations. Here we will compare the design and performance of the 9.2m segmented, the 8m monolithic on-axis and 8m x 6m off-axis concepts

Aronstein, David

Feinberg, Lee

Gong, Qian

Howard, Joseph

Pasquale, Bert A.

Stahl, Philip

NASA Technical Reports Server

Comparative Concepts for ATLAST Optical Designs
Bert A. Pasquale*', Philip Stahlb , Lee Feinberg', Joseph Howard', Qian Gong', David Aronstein'
'NASA/Goddard Space Flight Center, Greenbelt, MD 20771;
bNASA Marshall Space Flight Center, Huntsville, AL 35812
ABSTRACT
The ATALST (Advanced Technology for Large Aperture Space Telescopes) effort has presented several design
incarnations. Here we will compare the design and performance of the 9.2m se gmented, the 8m monolithic on-axis and
8m x 6m off-axis concepts.	 V
Keywords: Space Telescope, Large Aperture, Segmented Mirror, Monolithic, Off-Axis Design, Optical Design
1. INTRODUCTION
The current ATLAST design effort covers many variants, which have been reported on in response to NRC ASTRO-
2010 Survey', and have been discussed in numerous publications the past few years''. Three such designs, the 9.2m
segmented, the 8m monolithic on-axis and the 8m x 6m monolithic off-axis concepts are the subjects of this review.
Each design shows unique characteristics and design drivers. The performance of each system is also unique and
imposes limitations back on the system. This paper will discuss the design process and trades associated with each
system, and compare the details and performance of each.
1.1 Scope of Study
ATLAST is not a single design or project — it's a general concept for an observatory which NASA and the scientific
community want to build to answer what the science conun unity asks "next." ATLAST stands for "Advanced
Technology for Large Aperture Space Telescopes." This paper presents the work done as part of studies at Goddard
Space Flight Center in 2008 and 2009 to examine and compare possible design options. The three designs we looked at
are: 9.2m Segmented ; the 8m Monolithic on-axis and the 8m x 6m Monolithic off-axis. We will examine each of these.
1.2 TMA Design Characteristics
This paper will discuss the design process as a preamble to the talks about the science and use of the observatory. There
is one unwavering aspect to all ATLAST designs, and that is that they are all based on the Three Mirror Anastigmat, or
TMA optical design. This is simply because TMA telescopes have the highly desirable ability to produce large flat
fields. (Though often a TMA design will alternately be optimized to a desired curved surface, such as a sphere centered
on the exit pupil.) The general TMA design consists of a Primary, Secondary and Tertiary mirror. In Figure 1, a TMA
configuration is shown with a Pupil Mirror, such as is the configuration in JWST. The typical TMA field is biased off-
center to avoid interference of the pupil and,-or image with the optical beam. The TMA layout can be further modified
by the use of fold mirrors, notably a field pick-off mirror located near the intermediate focus. (See Figure lb.)
Figure la/b. Tluree MiiTor Anastigmat (TMA) optical design witli pupil miiTor, unfolded and folded.
https://ntrs.nasa.gov/search.jsp?R=20100032037 2019-08-30T20:56:03+00:00Z
1.3 Optical Design Process
TMA designs developed for small- to mid-class instruments do not directly translate to 8 meter UV observatories, even
when other parameters such as FOV remain constant. As the instrument scales up, so do the wavefront errors.
Tolerance demands also increase, and additional attention is required to meet performance specifications. A set of
optimization constraints and merit functions were developed to achieve the desired results. This process involved
creating a symmetric system and optimizing field performance radially in a field swath covering the science instrument
fields. The weighting on various fields was then balanced iteratively.
The second thing in common with each of the designs is the departure from a "pure" TMA optical design; in that the
conic of the Primary Mirror is modified to allow the intermediate focus to have an improved wavefront over a narrow
field on the optical axis (suitable for specific UV instruments.) This divergence is then corrected by applying conic
terms to the pupil mirror, restoring the TMA field performance. By itself, the conic on the primary mirror may also
improve the TMA base performance.
1.4 Fields of View and the TNIA design
The FOV proportions and location of the science fields directly influences the TMA design. In the example layout
shown in Figure 2; the entre FOV is shown to be 8 arcminutes by 25 arcminutes, but the science fields are in the center
and lower regions.
17 arc min
25 arc rein
Figure 2. Sample ATLAST Field of View alocation.
Knowing that the main science instruments are located in the center and lower wings of the allocated FOV allows the
designer to place the best part of the field along an arc intersecting these higher-requirement instrument fields. The outer
fields may be used for guider pickoffs, which require less stringent wavefront performance and can be allowed to have a
higher design residual. By offsetting the field of regard, the FOV takes on the proportions of a rectangular field. As a
consequence, the best trade off between wide-field coverage and performance is often found in optimizing a field swath
which is farther off the optical axis. The physical design limitation is usually assuring there is no interference between
the beam and primary mirror.
2. 8 METER MONOLITHIC MIRROR DESIGN
2.1 Design & Layout
The 8-meter design was conceived as a payload for the inaugural flight of a heavy launch vehicle such as the AresV,
capable of accommodating a 30-foot diameter payload shroud. The 8m monolithic primary mirror design utilizes a f/1.5
primary mirror; a solid glass substrate supported by a mounting truss behind the mirror.
The TMA in this design (and in each of the three we are presenting for consistency in comparison) has an f,-'18 flat focal
plane. Because of the symmetric space behind the primary mirror provided by a heavy launch vehicle, dual TMA
channels can be implemented on opposite sides of the optical axis, as shown in Figure 3a. The 8m TMA optical layout
was designed with compact back ends that can be folded up behind the primary mirror and simultaneously have an
accessible Cassegrain Focus. The tertiary mirrors are then embedded in, and supported by; the primary mirror support
structure. As long as the optical beams do not interfere with main support rings and trusses, this is quite feasible.
Additional fold mirrors allow the final TMA focal plane to be relayed to the center instrument space, or the instrument
can also be integrated behind the primary mirror as shown in Figure 3b.
Figure 3a/b. The 8m monolith optical layout with dual TMA fields shown, and a closeup of a Visible/IR Wide-Field
concept.
The Primary/Secondary combination produces an intermediate "Cassegrain focus" with an f/13.63 convergence. This is
where the UV instruments may access an image that has had only two reflections and has maintained high UV
throughput. The precise qualities of the Cassegrain image can be tailored to the instruments finally slated for
implementation.
2.2 Fields of View
The location of the TMA fields is constrained only by the physical limits of the optical system. Figure 4 shows this
particular TMA dual fields' inner edge at either 4 or 5 arcminutes from the Optical Axis. As before, the particulars of
the optical design can be made to accommodate the necessary fields for the particular science instrument compliment.
TMA FOCUS
8'x 22'
WFC FOV
Ina^:o,o	 (8' x 8')
12
iu
e ^
6 5	 Optical
4 min } ^ Axis
Cass Focus
t30 aresx
TI'AA FOCUS
18'x 22'
WFC FOV
(8' x 8')
Figure 4. The center UV Cassegrain field and dual TMA fields shown together.
2.3 TNIA Performance
During the design process, we first optimize the TMA fields so that the sweet spot of the wavefront performance will
cover the wide field camera FOV, allocated in this design within the swath 4 arcminutes to 13 arcminutes off axis. The
Primary Mirror conic is then allowed to deviate from the standard TMA design parameters to improve the UV focus
region, and finally aspheric terms are added to the pupil mirror to restore the TMA Field performance. Figure 5 shows
the resulting wavefront, shown in nm RMS, across the radial field.
EG	
3s.°
C	 _ 25.0-30.0	 30.0
n 20.0-25.a
3 I15.o-2a.a
10.0-15.a
n 5.0-10.0
n 0.0-5.0
S
.0	 9	 8	 >	 b	 5	
-'	
- 2	 ,	 a	 j 	 9	 -IC
 FOV (%) (amin)
ATLAST Sm (v10b) WFE vs. Radial FOV
30
25
20
L
"' 15
W
3
10
s
0
4	 5	 6	 T	 8	 9	 10	 11	 12	 13	 14
Radial FOV (amin)
Figure 5. The 8111 TNIA wavefont RNIS (mn) as a function of off-axis angle.
From 4 to 12.5 arcminutes off axis, the wavefront is less than 10 nm RMS, and from 5 to 12 arcminutes it's less than 6
nm RMS. When you take that curve and sweep it radially across the entre TMA FOV, you can see how much field is
ideal for science instruments. The entre 8 x 8 arcminute Wide Field Camera FOV has performance 4-10 nm RMS, with
75% of the field is < 5 nm RMS.
Figure 6a shows this curve swept across the full 8 x 20 arcminute field, and Figure 6b a 3-D projection of the wavefront
map across the same field. Guiders or low spatial resolution instruments could be placed in the far wings where RMS
requirements can be relaxed.
ATLAST 8m (v10b) WFE Map	 ATLAST Sm (v1 ) WFE M.p
Figure 6a/b. The 8m wavefront quality over the TMA field.
2.4 UV Cassegrain Performance
If we peal away the back end of the TMA (Figure 7a) and just focus on the Cassegrain section we can see where the UV
science instruments can be placed. This is a very narrow field, but sufficient for many spectrometers or coronagraph
type of instrumentation. As you can see in Figure 7b ; even with the modified design, the wavefront increases almost
linearly as you move away from the optical axis. There is a 10 aresecond field where the RMS is less than 10 nm, and a
20 aresecond field where the RMS is less than 20 nm. The primary and secondary mirrors would have been treated with
high-reflectance UV coatings, but at the expense of decreased throughput in the visible and IR. Further correction could
be done within the UV instrument to allow increased field use ; but you would still want to minimize the number of
reflections.
ATLAST Sm (v10b) Cass Focus WFE
70
60
50
40
303
20
10
0
0	 5	 10	 15	 20	 25	 30
FOV (Axial Radial Offset) (attest)howl
Figure 7A- The "Cassegrain" telescope section; and the wavefront error as a function of field.
2.5 Error Budget Constraints
To understand why the emphasis on wavefront performance is so tight, we must look at the overall budget for the
observatory. This is shown in Table 1:
ATLAST Sm Monolith
Sensitivity Analysis
Rev. 4!8!2009 - 5AP
Page 1 - Pop Level Summary
Cbservatory(TMA)
Requirement
40.0 q rn
Science_ntrumets	 Optical Telescope	 'aFSC Precision	 S. c Pointinghtar in
',^ 	 ^ 	 S
15.0 nm	 33.5 nm	 5.0 nm	 151) nm	 1.7 nm
1 asec )itter — 15()m
Fa b rica tia n
15.0 nm
c, rompersacec(tocLS only)
12.0 rl7
C'cmpensa tec
	
Alignment
	 IrternaI StabIII	 De si gn Reslcual^	 tl'	 S
	
30.0 nm	 15.0 nm	 10.0 nr'I
- ReaCtlons @ Drift
U{compensarac	 t1`compensatec
	
YOCLS 0111/
	
Focus orl;?
	
com per satec
	
ii ii Compensatec
Table 1. Error Budget for the ATLAST concept ; allocating 1011111 RMS to the TMA Design.
The entire RMS wavefront budget for imaging in the obser vatory is only 40 nm! In this chart you see Telescope
`Design Residual' is only allocated l Onln. This is RSSed with all the fabrication, alignment and stability error sources.
So any design error over l Onm will burden the budget, any less provides margin to the other parameters.
3. 9.2 METER SEGMENTED MIRROR DESIGN
3.1 Design & Layout
The second design looked at was the 9.2 segmented concept, which was referred to as ST-2020 during the study. This
study is the "non-ultra heavy lift vehicle" option; designed to be accommodated on a heavy-up Delta rocket. The
segments are identical in profile to JWST — 1.315m across, flat to flat. By adding the third ring of 18 segments, the total
entrance aperture diameter becomes 9.2m, and literally twice the collecting area as JWST. Like JWST the outer
segments would fold into the narrow shroud. (See Figure 8a/b. 9.2m design configuration, a) stowed for launch and b)
deployed. ;
 below.)
Figure 8a/b. 9.2m design configuration, a) stowed for launch and b) deployed.
In this design, the optical parameters are very similar to the 8m design, though the primary mirror's power had to be
increased to f/1.25 (vs. f1.5 for the 8m monolith.) The internal focus shifted correspondingly to f12.5, but the TMA was
maintained at 1%18. It is again a modified TMA w/ improved Cassegrain focus for UV instruments, and the back end is
designed for a compact folding in the aft space.
2000.00 M
Figure 9. The 9.2m Segmented telescope optical layout.
This is the basic layout of the 9.21m TMA, with the UV focus accessible and the TMA Focal plane biased toward the
bottom of the shroud where the instrument bay is located. The actual focal plane position can be placed as desired as the
instrument compliment is defined.
3.2 Fields and TMA Performance
The TMA field was configured to have its inner edge 4 arcnunutes from the optical axis. The initial design used an
8"x20" field, but the central hole in the primary mirror could accommodate a 23 arcminute wide field. The FOV layout,
and the wavefront performance across the swath of field covering the science area are shown in Figure 10a,b.
8X20
14arcmin I	 \g X 8^
12	 i
10
7 g	 Optical
4rcmin	 Axis
ST2020 (IDL v.]/K) WFE vs. Radial FOV
30
25
20
	
15	 ^wce lnm,
LL
3
10
s
0
	
4	 6	 8	 10	 12	 14
Radial FOV (amin)
Figure l0a/b. The 9.2m field allocation and wavefront performance.
The field performance is mostly comparable to the previous design - under 12 n1m from 4 to 12.5 arcminutes. The main
contributor for it being 1-5 nm greater than the 8m monolithic design is the faster Primary Mirror. However,
performance is still excellent in a wide area, as shown in the Figure l Ia/b.
ST2020 (1 ­1K) WFE  Map
Figure l la/b. The 9.2m telescope field performance and 3-D projection.
3.3 UV Cassegrain Performance
Like the 8m design, the 9.2m system has a narrow field of focus at the intermediate focus, useable by various UV
instruments. The faster primary mirror and Cassegrain focus constrain the size of high-quality field achievable. The
field for RMS 10nm is 8 areseconds, and <20iun is 16 areseconds. (See Figure 12a/b.) Again, the UV instruments can
incorporate some level of correction into their design, but reflections must me minimized to maintain UV throughput.
77
ST2020 ILL Base design K	 Bnz 14-J-09
Figure 12a/b. The 9.2m telescope intermediate focus and performance.
4. 6 X 8 METER OFF-AXIS MIRROR DESIGN
4.1 Design & Layout
While the first two designs compared on-axis TMA designs, the final configuration or discussion is the 6m x 8m
monolithic Off-Axis TMA design. This is further related to the 8m monolith design in that it is also envisioned for a
heavy launch vehicle with a 30m payload shroud, as shown in Figure 13. Everything is comparable to the general
aspects of the on-axis designs, except that the Primary Mirror had to be slowed down to fi 1.8 since the system is in
essence using a section of mirror wider than even the 9.2m design — the entire parent mirror would be 14 meters in
diameter. The TMA maintains f'18 focus (flat plane, orthogonal to Optical Axis) but the Cassegrain focus slows slightly
to f,-15. The TMA back-end is folded behind the primary mirror, making both the Cassegrain focus and TMA focal
plane accessible. Can accorninodate extra-large pickoff field (in lieu of using Cass Focus.) Though shown here with a
single pick-off mirror,
Am
2500.00 MM
AlLA6T OH-Axis $^Bln (v8e7 wEE Map
Figure 13. Off-Axis 6x8 meter TMA shown in a 30-foot diameter shroud.
4.2 Fields & Performance
The off-axis design's FOV is also more skewed away from the optical axis than the on-axis designs. (See Figure 14.)
The FOV of interest begins about 7 arcminutes from the axis, and continues out to 16 or 17 arcminutes. So, we needed to
design this with a very different wavefront profile. And here you see that from 7 to 16.5 arcminutes are below 10 rim
RMS, while the entire range from 8 to 1.5 is below 5 nm RMS.
ATLAST Off-Axis TMA FOV Layouts
TPAA FOCUS
8'x 22'
wFC wv
24 arcmin,
l	 ^
(8' x 8,^ \
20,
1s. 16 1412 	
Optical
J8arcmin	 Axis
1	 \	 Cass Focus	 J
1	 ±30 aresec
Figure 14. Off-Axis Field and wavefront performance.
Again a large TMA FOV is able to accommodate various science instruments, as shown in Figures 15:
ATLAST Off-Axis 6x8m (v8c) WIFE Map
IF is S
• 25.0- 30.0
n 20.035.0
—	 10.035.0
•s.oaao
.o a iu	 s	 e	 ..	
rov (x((amm).	
r	 s	 io u
Figures 15a/b. 6n1 x 8111 Wavefront RMS (tin) over the TMA Field of View.
4.3 Expanded Field Options
The Off-axis TMA design can actually accommodate an expanded FOV if desired. This provides plenty of prime real
estate for additional science instruments, as shown in Figure 16. Another option is to allow instruments to pick off fields
ATLAST Off-Axis 6x8m (v8e) WFE Map
00
• 25.0-30.0
, 0 3	 IS.O-20 0
• 10A 15.0
• 5.0-10.0
n	 •0.0-5.0
to either side of the optical axis, though additional folds and tertiary mirrors would have to be integrated into their
design.
ATLAST Off-Axis 6x8m (v8e) WFE Map
Is
I	 11
10 m
9 O
I
s 3
5 ..
Lu
2
p	 ^
o
11 13 12 11 10 9 0 2 6 5 a 3. 3V 1(X) (amin)3 	 -2 -0 -e 101. i2-13-u -1
025.0-30.0
•z0.o-25.0
e ls.p-z0.o
olo.o-ls.o
•5.0-1o.0
.o.o-5.0
Figure 16. Options for art 	 FOV for the Off-Axis Monolith Design.
Unlike the on-axis designs. the 6m x 8m off-axis design does not afford a well-corrected intermediate focus using the
method described. This seems to be a limitation of using such an off axis section of the parent pupil. Instruments placed
at the Cassegrain focus would have to have internal correction built in. Ideally, the telescope design would be reworked
in tandem with instrument designs to maximize performance.
5. COMPARISION & SUMMARY
5.1 Component Sizes
Tables 2-4 show the sizes of the optical components for the 8m Monolith, 9.2m Segmented, and 6x8m Off-Axis designs.
Radius (mm) Conic Aperture(mm) Other
Primary 24,000.00 -0.994680 C 8,000 Concave
Secondary 3006.791 -1.487700 C 1, 050 Convex
Fold 1 Flat 0 R 780 x 480
Tertiary 3,414.393 -0.729426
R 1, 150 x 590
350 Off-Axis Concave
Pupil Flat
4lhorder Asphoft:
-0.292698e-10x
-0.205033e-10Y
C 160
('Fiar with
amorphic
aspheric terms)
Fold 2 Flat	 0 R 620 x 220
Table 2. 8m monolithic design optical component sizes.
Radius (mm) Conic Aperture (mm) Other
Primary 23,000.000 -0.996326 C 9,200 Concave
Secondary 2,665.3656 -1.442873 C 1,100 Convex
Fold 1 Flat 0 R 760 x 440
Tertiary 2,942.0139 -0.762194 R 1,160 x 560320 Off-Axis Concave
Pupil Flat B: 0.108072e-14 C 150
(01,&Msp	 a idterms;
Fold 2 Flat 0 R 440 x 260
Table 3. 9.2m segmented design optical component sizes.
Radius Conic Aperture (mm) Other(mm)
.
Primary 28,800.00 -0.991702 E 8,000 x Concave6,000
E 1, 140 x 880Secondary 4,117.562 -1.526240 Convex600 Off-Axis
Fold 1 Flat 0 R 850 x 500
R 1,200 x 620Tertiary 4,717.870 -0.695919 Concave
360 Off-Axis
Pupil Flat 4h-der Asphedc:0.312237e16 C 16 ('FI.V	 Aha,ph eric'P.	 )
Fold 2 Flat 0 R 620 x 220
Table 4. 6m x 8m off-axis design optical component sizes.
5.2 Primary f/# and TMA Performance
One very interesting point that came from this study is the relationship of Primary f/# to best wavefront performance.
Because in each design a wide radial field (-10 arcminutes) was being maintained in optimization, the average
wavefront error was inversely related to the Primary Mirror f/#.
The monolithic designs take advantage of the heavy lift vehicle capacity to avoid phasing segments. Only the off-axis
monolith provides an unobstructed pupil, desirable fore 10 coronagraphy of exoplanets.
REFERENCES
[1] Marc Postman, et.al., "Advanced Technology Large-Aperture Space Telescope (ATLAST): A Technology
Roadmap For The Next Decade," R.F.I. Subnutted to NRC ASTRO-2010 Survey (2009).
[2] http://www.stsci.eduiinstitute/atlast/index html documents gage


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