Sloan Digital Sky Survey Telescope Technical Note
19980215
Sloan Digital Sky Survey 2.5-m telescope: light
baffles
Walter A. Siegmund and Siriluk Limmongkol
University of Washington, ASTRONOMY BOX 351580, Seattle, WA 98195
USA
Charles L. Hull
The Observatories of Carnegie Institute of Washington, 813 Santa
Barbara Street, Pasadena, CA 91101
Daniel Milsom, III
Breault Research Organization, 6400 East Grant Road, Suite 350,
Tucson, AZ 85715
CONTENTS
ABSTRACT
The Sloan Digital Sky Survey 2.5-m telescope is unique with a
3° field of view at f/5. The two-mirror optical design
includes two transmitting correcting elements. To avoid excessive
central obstruction of the entrance pupil, a conical baffle is
necessary in addition to the usual primary and secondary baffles.
This conical baffle is suspended approximately midway between the
primary and secondary mirrors. In addition, an exterior close-fitting
wind and light baffle, not found on most modern telescopes, blocks
rays at large angles from the field of view.
The baffle design was analyzed using stray light analysis
software. Scattered light as a function of source angle was
calculated and the dominant scattering surfaces were identified. For
sources near the field of view, the uniformity of scattered light
over the focal surface was determined.
Keywords: light baffling, stray light, Sloan Digital Sky
Survey
1. INTRODUCTION
The light baffles of the Sloan Digital Sky Survey (SDSS) 2.5-m
telescope must provide effective baffling of the crescent moon. This
translates into a specification for the point source normalized
irradiance transmission (PSNIT) of less than 2x10-6 for sources more
than 30° off-axis. (The PSNIT is the ratio of the stray light
irradiance at the focal surface to the incident irradiance from a
point source.) For sources less than 30° from the field center,
the PSNIT can be higher, but it is desirable that the focal surface
illumination be uniform.
In the analysis of stray light, critical objects are those visible
from the focal surface either directly or via the optical elements.
Illuminated objects are those illuminated by the source under
consideration either directly or via the optical elements. Stray
light on the focal surface comes only from the critical objects.
Critical objects that are also illuminated objects are the main
sources. The elimination of critical objects is the crucial first
step of light baffling.
Practical surface treatments for the baffle surfaces of
ground-based telescopes consist primarily of paints. These aerospace
paints are robust, can be cleaned and are not greatly affected by
mild terrestrial contamination. Suitable black paints have a
hemispherical reflectance of less than 5% for normal incidence.
However, this may rise to 15% or higher if the incidence angle is
near grazing.1 Since coatings have poor performance for
grazing scattering, stray light paths of this sort are of particular
concern. If such a surface cannot be removed from the critical or
illuminated object list by changing its geometry, it may be effective
to add vanes to the surface that interrupt grazing scattering paths.
In this case, edge scatter from the vane tips is a concern and must
be examined.
The roll-off enclosure for the SDSS 2.5-m telescope is compact and
has a low cross-section for wind loading, both of which reduce the
mass and cost of the enclosure.2 However, it leaves the
telescope exposed to the wind and stray light sources. These problems
are addressed by the wind baffle that closely surrounds the telescope
but has a separate low-precision drive system. It transfers wind
loads to the stationary portion of the telescope building. The wind
baffle has a square cross-section that fits closely around the square
secondary frame of the telescope.
Figure 1: Baffles consist of
the wind baffle (outside the beam to the primary mirror), and the
secondary, conical and primary baffles (inside). The conical
baffle is suspended about halfway between the primary and
secondary. Light enters the wind baffle through the annular
opening at the top. Light rays from the edge of the 1.5°
field of view are shown. b. The rays coming from the upper left
illuminate the interior of the primary baffle above the focal
surface. These surfaces are critical objects since they are
visible from the focal surface via the secondary mirror. Rays such
as the one coming from the upper right are blocked showing that no
direct rays reach the focal surface.
The sky-facing end of the wind baffle contains an annular opening
formed by a central disk and a panel with a circular opening (both
supported by the wind baffle frame). This opening provides clearance
for light from the 3° diameter field of view to reach the
telescope entrance pupil (Figure 1a). The effect of the wind baffle
is to block light rays that would otherwise have to be intercepted by
the other baffles and to prevent direct illumination of the primary
mirror by sources more than 28° off-axis.
The inner baffles consist of the secondary baffle (in front of the
secondary), the primary baffle (extending through the primary center
hole), and the conical baffle (suspended between the primary and
secondary). The conical baffle is not present in most two mirror
telescope designs. It is necessary here to avoid an unacceptably
large central obstruction of the entrance pupil that would otherwise
be the consequence of this fast wide-field optical design.
The primary and secondary baffles each consist largely of a stack
of annuli. Short struts connect each annulus to the next one in the
stack and control spacing and centering. The outer surface of each
strut contains a vane to block near-grazing scattered light paths.
The annular baffle design facilitates air circulation near the optics
and is economical to fabricate.
Near the tip of the primary and secondary baffles, flat annuli
would become impracticably numerous. Consequently, each baffle is
terminated by a truncated cone. The cone surfaces are vaned to
eliminate near-grazing scattered light paths. To avoid excessive
light blockage, the cross-section of the conical baffle must be
minimized. This leads to a vaned cone-segment design.
Figure 2: a. Rays reflected by the primary
illuminate the conical baffle. b. Portions of the wind
baffle within 0.9 m of the primary that are directly illuminated
are visible from the focal surface (left rays). A skew ray (out of
the plane containing the optical axis) about 28° off-axis,
entering from the left and passing in front of the secondary and
conical baffles, reflects from the primary mirror and illuminates
the inner surface of the primary support structure. Scattering
from this surface can reach the focal surface.
2. BAFFLE DESIGN
The telescope has two optical configurations. Switching from one
to the other is accomplished by changing the final corrector and the
focal surface package. Since the requirements are less stringent and
the field of view smaller for the spectrographic configuration, the
telescope is baffled for the imaging configuration.
Originally, it was planned that the inner edge of the entrance
pupil be defined by the secondary baffle tip. However, the primary
mirror was damaged during optical generation and its finished inside
diameter was larger than planned. Consequently, the 1.20 m outer
diameter of the primary baffle annulus nearest the primary mirror was
chosen to fully utilize the remaining area of the mirror. For the
inner 0.7° field radius, the inner edge of the entrance pupil is
defined by the secondary baffle tip. Beyond that radius, it is
defined by a combination of the secondary baffle tip and the primary
baffle annulus nearest the primary mirror surface.
The 2.52 m diameter of the outer aperture stop was chosen to mask
the portion of the mirror surface near its outer edge that was rolled
off during polishing. It is located just above the primary mirror and
is integrated with the supports for the primary mirror perimeter air
seal and the seismic mirror cushions.
Figure 3: a. Rays from sources up to 8.8°
off-axis illuminate a portion of the lower surface of each primary
annulus near its outer diameter. The annuli are spaced so that
light scattered from this illuminated portion is not visible from
focal surface instruments. b. Rays from about 6.6°
off-axis, after reflecting from the primary and secondary
illuminate the interior of the conical baffle.
The secondary baffle must be large enough to mask the edge of the
secondary. Also, it is desirable that the baffle and secondary
supports not be critical objects. The 1.28 m diameter of the
secondary mirror baffle tip was minimized consistent with these
criteria. As the diameter of the baffle tip increases, the conical
baffle becomes shorter and moves toward the primary mirror. This
decreases the differential vignetting a bit, but the central
obstruction dominates such that the total blockage increases, even at
the field edge.
Rays that just clear the edges defining the entrance and exit
pupils were traced by a standard optics design program (Zemax, Focus
Software, Inc., Tucson, AZ). These rays define the location of the
baffle edges. The following criteria were used for the design of the
baffles and apply for the worst case combination of component and
installation tolerances and flexure for elevations between 30°
and 90°.
- All direct rays to the focal surface must be blocked. Even a
small amount of direct illumination will cause photometric
errors.
- Variable vignetting, e.g., due to gravity or wind loads, of
the 2.72° diameter photometric field of view must be much
less than 1%.
- Variable vignetting of the 3.02° diameter astrometric
field of view must be less 2%.
Critical objects
Critical objects are those visible from the focal surface either
directly or via the optical elements. Consequently, optical surfaces
are almost always critical objects as is the case here. Directly
viewed critical objects include the following:
- Secondary baffle. Inner portion of lower surfaces of baffle
annuli and vanes.
- Conical baffle. Interior surface (Figure 3b).
- Primary baffle. Inner portion of lower surfaces of baffle
annuli and vanes (Figure 3a).
Critical objects imaged by the secondary mirror include the
following:
- Secondary baffle. Inner portion of upper surfaces.
- Conical baffle. Interior and exterior surfaces (Figure
2a).
- Primary mirror support structure. Interior surfaces within
about 0.9 m of the primary mirror (Figure 2b).
- Primary mirror aperture stop.
- Primary baffle. Outer portion of upper surfaces.
- Primary baffle. Inner portion of upper surfaces (Figure 4a).
Critical objects imaged by both the secondary and primary
mirrors include the following:
- Conical baffle. Interior surface.
- Secondary baffle. Inner portions of lower surfaces.
Major surfaces that are not critical objects include the
following:
- Secondary baffle exterior. The view of the vane tips and outer
edges of the baffle annuli are hidden by the lower edge of the
secondary baffle tip (Figure 1a).
- Wind baffle interior. The portion that is more than 0.9 m from
primary mirror.
Illuminated objects
Directly illuminated objects include the following:
Figure 4: a. Rays from sources about 3.7°
off-axis, after reflecting from the primary and secondary,
illuminate a portion of the upper surface of each primary annulus
near its inner diameter. Part of these illuminated surfaces are
visible from the focal surface as reflected by the secondary
mirror. b. The primary and secondary baffles consist of
stacks of annuli supported by small struts (not shown). This
section through the right side of the secondary baffle shows the
baffle tip with the vanes that are necessary to interrupt grazing
scattered light paths. The baffle tip, machined out of aluminum
alloy, is 1285 mm outside diameter and 206 mm high. Above the tip
are a series of smaller diameter aluminum annuli each 4.8 mm
thick.
3. BAFFLE DETAILS
The conical baffle is the most challenging to fabricate. It is
supported by steel wires from the telescope truss so it is important
that it be lightweight. To resist tension in the wires, it must have
good bending stiffness. To minimize light blockage on-axis, its
central thickness must be minimized. To minimize vignetting at the
edge of the field of view, the thickness near its lower and upper
edges must be minimized as well. Finally, vanes must be added to the
baffle surfaces to interrupt the grazing scattered light paths of the
Figure 2a and Figure 3b. These constraints lead to a design with a
relatively deep central vane that provides the baffle with bending
stiffness and anchor points for the support wires (Figure 5a).
Extending above and below the central vane are 1.6 mm thick conical
segments with 10.45° and 8.83° half angles
respectively.
The conical baffle was fabricated by fabricated by
Quality Composites, Inc. (QCI, Sandy, UT). It was the largest part
of this sort ever attempted by QCI. Previously, the largest parts
fabricated were straightforward tubes that were longer but smaller
in diameter than the baffle. Three aspects of the baffle required
development.
- fabrication of the inside vanes so that the tips were
smooth and uniform and so that good adhesion to the cone was
achieved.
- fabrication of the central stiffening vane and installation
of the stainless steel inserts that support wires attach
to.
- design of the outside vanes. These are 750 micron diameter
bare acrylic optical fibers tacked to the surface at 80 mm
intervals with cyanoacrylate adhesive (superglue).
Subsequently, a fillet of epoxy was applied between the surface
and either side of the fiber.
Figure 5: a. Section through the right side of
the conical baffle. The vanes that are necessary to interrupt
grazing scattered light paths are apparent. The details are
magnified a factor of 5. The deep central vane provides
attachment points and bending stiffness. The vanes are small
near the tips to minimize field-edge vignetting. Made of
graphite fiber reinforced epoxy, it is 1239 mm outside
diameter, 725 mm high and weighs 96 N. b. Section
through the right side of the tip of the primary baffle. The
tip is a machined aluminum alloy truncated cone with vanes. It
is 858 mm in diameter at its upper edge and 222 mm high. Below
the tip are a series of progressively larger aluminum annuli
each 4.8 mm thick.
The baffle was formed on a machined aluminum mandrel out of
graphite fiber reinforced epoxy. Machined into the mandrel are
grooves that result in the vanes on the interior surface. The
mandrel was made in three pieces so that it could be collapsed
inside the finished baffle and removed. To improve handling, its
weight was minimized. This reduced the thermal inertia which
speeded the curing of the epoxy. Openings that allowed oven air to
circulate through the inside of the mandrel helped as well. The
mandrel weight is about 1000 N. The maximum minus minimum diameter
of its outer surface is 1.8 mm or less.
Finished, the conical baffle weighs only 96 N. All
specifications were met or exceeded. In particular, the maximum
minus minimum diameter was 3 mm or less for both the upper and
lower edges of the baffle. The surfaces are smooth and uniform.
The edges of the inside vanes are sharp and free of voids. Joints
between ends of the acrylic fibers of the outer ribs are carefully
made and visible only with careful inspection. Earlier tests of
the bond strength between the acrylic fibers and the surface of a
prototype part gave the excellent result that the fiber often
failed before the adhesive.
Figure 6: Point source normalized irradiance
transmission (PSNIT) v. source off-axis angle. For angles less
than 20°, the PSNIT is calculated with no scattering from
optical surfaces and with scattering from "clean" optics. The
steep drop at 25° corresponds to the source moving so far
off-axis that the aperture stop at the primary mirror is no
longer directly illuminated.
The primary baffle tip is a machined aluminum truncated cone
with vanes (Figure 5b). The balance of the baffle consists of
progressively larger aluminum annuli with sharp inner and outer
edges. The spacing of the annuli increases toward the primary
mirror.
The outer edges of the annuli are illuminated directly from the
sky. The inner edges are illuminated directly from the sky and by
the secondary. The edge bevel faces the secondary in both cases.
Viewed from the focal surface, as reflected by the secondary, it
should be darker than if the edge were beveled on the opposite
side. The finished primary baffle assembly weighs 619 N. The
maximum minus minimum diameter of the upper edge of the primary
baffle tip is 0.8 mm.
The secondary baffle is similar to the primary baffle but is
inverted and shorter. The baffle tip is a machined aluminum
truncated cone with vanes (Figure 4b). Its tip is more conical
than the rest of the cone and blocks the view of the rest of the
baffle exterior from the focal surface. The balance of the baffle
consists of smaller diameter aluminum annuli with sharp outer and
cylindrical inner edges. The spacing of the annuli increases
toward the secondary mirror.
The outer edges of the annuli are illuminated directly from the
sky. The edge bevel faces the sky and is not a critical object.
The inner edges of the lower surfaces are illuminated by the
converging beam from the primary mirror. The cylindrical inner
surface is not illuminated. The finished secondary baffle assembly
weighs 248 N. The maximum minus minimum diameter of the lower edge
of the secondary baffle tip is 0.8 mm. The conical baffle mandrel
and the primary and secondary baffles were fabricated by
Machinists, Inc., Seattle, WA.
The primary mirror aperture stop is located just above the
primary mirror and defines the outer edge of the entrance pupil.
The edge bevel faces the primary. Integrated with the primary stop
is the primary mirror perimeter bulb seal and elastomeric
retention cushions that protect the primary mirror during seismic
accelerations, particularly when pointed at the horizon.
4. SCATTERED LIGHT
MODEL
To analyze the scattered light performance, the baffles and
other elements of the telescope design are simplified as
follows.
- Tension elements. These support the secondary assembly, the
central disk at the end of the wind baffle and the conical
baffle and are not modeled. However, they are critical objects
and must be treated to minimize grazing scattered light
paths.
- Secondary backup structure. The view of the secondary
backup structure from the focal surface is blocked by the
secondary and secondary baffle. It is not modeled.
- Secondary truss and frame. These are not critical
objects.
- Primary and secondary baffle supports. Each annuli of the
primary and secondary baffles is located and supported from its
larger neighbor by eight 25 mm diameter struts. The struts have
little area compared to the baffle annuli and have a vane in
the middle. They are not modeled.
- Wind baffle interior. The wind baffle has a rectangular
cross-section. In the model, it is replaced by an inscribed
cylinder.
The APART computer program (Breault Research Organization,
Tucson, AZ) was used to perform stray light analysis of the
simplified baffle model.3 It calculates the power that
propagates from a stray light source to the focal surface for each
segment of a propagation path. Since it is not a ray-based
program, it avoids the sampling density problems of the ray-based
programs. The design that was modeled was a somewhat earlier
version of the baffle design than described above. However, the
changes made subsequent to the analysis are not likely to have a
significant effect on the results. The APART model has seven basic
components.
- Wind baffle
- Aperture stop
- Secondary mirror baffle
- Conical baffle
- Primary baffle
- Focal plane baffle
- Optical system
The baffles are very complex either consisting of series of
sharp-edged annuli or cone segments with vaned surfaces. The APART
vane algorithm uses conical shaped objects. The surface of each
object is defined to be coincident with the annuli or vane edges.
The algorithm modifies the geometric configuration factor (GCF)
and the bidirectional reflectance distribution function (BRDF)
characteristics of the surface according to vane cavity parameters
input into the program to accurately calculate the energy
scattered by the vane system toward other objects in the
model.
The PSNIT was calculated for angles from 1.6° to 50°.
For source angles of less than 20°, the PSNIT was calculated
both with and without the scattering contribution from clean
high-quality optics. Also, the distribution of power on the focal
surface was calculated. For this calculation, the focal surface
was divided into ten rings of equal area that were, in turn,
divided by radial lines into ten regions of equal area.
For source angles of 25° or less, the most important
critical object is the directly illuminated aperture stop located
just above primary mirror. For angles greater than 25°, the
interiors of the primary and conical baffles are the most
important critical objects. They are illuminated by light
scattered from the interior of the wind baffle. The PSNIT
decreases abruptly between 25° and 30° (Figure 6). At
30° and more, critical objects are no longer directly
illuminated.
The stray light irradiance over the focal surface varies
smoothly for the five source angles that were examined. It is
plotted in Figure 7 for four source angles. (Artifacts in the
contour plots are associated the finite size of the focal plane
regions and the contouring algorithm.) The source image would fall
on the +y axis beyond the upper edge of each plot. Power increases
in the -y direction because more of the illuminated baffles are
visible from the opposite side of the focal surface, e.g., the
primary mirror aperture stop viewed via the secondary mirror. The
stray light contribution from optical surfaces is not included in
the data plotted. It dominates the contribution from the baffles
for angles of less than 7° and moves the peak of the stray
light distribution to the +y edge of the field of view.
Figure 7: The relative power distribution of
scattered light reaching the focal surface is plotted v.
location on the 3° diameter focal surface. The source
image would fall on the symmetry axis above each plot. The
sources are at 2.7°, 4.5°, 7.4° and 12.1°
(left to right and top to bottom). The data in each plot have
been scaled so that the maximum value is 100. Scattering from
optical surfaces was not included.
Figure 8: Obstruction of the 2.5-m
aperture. The conical baffle blockage, while small, contributes
most of the differential vignetting.
5. PUPIL OBSTRUCTIONS AND
DIFFERENTIAL VIGNETTING
A two mirror on-axis telescope is typically baffled so that the
primary baffle is contained within the shadow of the secondary
baffle, even at the field edge. Consequently, the central obstruction
of the entrance pupil is constant with field angle.
For the SDSS 2.5-m telescope, this would result in a central
obstruction of more the 50%. This is due to its fast final focal
ratio and wide field of view. The design described herein produces a
much lower central obstruction, at the cost of minor blockage by the
conical baffle, some differential vignetting with field angle and a
more complex diffraction pattern.
The obstruction of the telescope aperture is found by projecting
each component onto the primary mirror from the point on the prime
focal surface corresponding to the field angle of interest. This
portion of the mirror is not visible from the final focal surface and
consequently contributes no light to the images. For complicated
objects such as the conical baffle, the tips of the small vanes on
the central vane and the upper and lower edges on both the interior
and exterior surfaces are projected to the primary. Each projection
is assumed to be circular on the mirror surface. The envelope of the
projections is the obscured area. This process is facilitated by the
use of a computer aided design program. The program calculates the
area of bounded 2D regions, e.g., obscured areas.
The total obstruction, not including vanes that support the
secondary, conical baffle and the central disk at the end of the wind
baffle, is 28.6% on-axis and 31.8% at the field edge. The
differential vignetting is 3.2%. Most of the obstruction is due to
the secondary baffle and most of the differential vignetting is due
to the conical baffle. Two breaks occur in the obstruction curve v.
field angle (Figure 8). At 0.5°, the tips of the conical baffle
are no longer obscured by the central stiffening rib. At 0.7°,
the primary baffle emerges from the secondary baffle shadow.
6.
CONCLUSIONS
In the design of the baffles for an optical system, the
identification and minimization of critical objects and the coupling
of critical objects to illuminated objects are more important than
the treatment of baffle surfaces with coatings and vanes. Stray light
analysis software can play a crucial role by identifying dominant
stray light paths and by validating the final design. Despite the
many constraints on the design of the baffles for the SDSS telescope,
its stray light performance is calculated to be almost a factor of 10
better than its specification of 2x10-6 for a 30° source
angle.
The uniformity of stray light over the focal surface is
satisfactory. The stray light irradiance from the baffles varies
gradually and by less than a factor of two over the focal surface for
source angles from 4.5° to 12°. At source angles less than
7°, the average irradiance and nonuniformity of stray light from
optical surfaces dominates that from the baffles.
The central obstruction of the pupil is 26.0% on-axis and 26.9% at
the field edge. The conical baffle adds 2.6% on-axis and 4.9% at the
field edge. The supports for the secondary mirror and the conical
baffle will add a small amount of additional blockage.
7.
ACKNOWLEDGMENTS
It is a pleasure to thank Ed Mannery of the University of
Washington (UW) for the ray tracing analysis that was used to design
the baffles and for suggesting the final shape of the conical baffle.
An earlier analysis by Steve Pompea and subsequent discussions with
Gary Peterson and Robert Breault of Breault Research Organization led
to the final design of the light baffling system. Jim Gunn of
Princeton University, Don York of the University of Chicago and
Patrick Waddell of the UW provided encouragement and advice.
8.
REFERENCES
1. S. M. Pompea and R. P. Breault, "Black Surfaces for Optical
Systems" in Handbook of Optics, ed. M. Bass, McGraw-Hill, New York,
p. 37.22, 1995.
2. C. Hull, S. Limmongkol and W. A. Siegmund, "Sloan Digital Sky
Survey telescope enclosure: design", Proc. of S. P. I. E.,
2199, 1994, p.1178.
3. R. P. Breault, "Control of Stray Light" in Handbook of Optics,
ed. Michael Bass, McGraw-Hill, New York, pp. 38.1-38.35, 1995.