Sloan Digital Sky Survey Telescope Technical Note 19960616
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 goal for the point source normalized irradiance transmission (PSNIT, the ratio of flux reaching the focal surface to the flux incident on the telescope) of less than 2x10^-6 for sources more than 30° off axis. 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.
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. However, it leaves the telescope completely exposed to the wind and to unwanted light sources. These problems are addressed by the wind baffle that closely surrounds the telescope but has a separate low-precision drive system and 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.
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° field of view to reach the telescope entrance pupil. 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° from the boresight.
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 unacceptably large central obscuration that would otherwise be the consequence of this wide field optical design. The design shown has a central obscuration of 26.0% (Figure 1). The conical baffle adds an additional 2.6%.
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 rib to block near-grazing scattering. The annular baffle design facilitates air circulation near the optics and is economical to fabricate.
Near the tip of each baffle, flat annuli would become impracticably numerous. Consequently, each baffles is terminated by a truncated cone. The cone surfaces are ribbed as necessary to avoid near-grazing scattering. To avoid excessive light blockage, the cross-section of the conical baffle must be minimized. This leads to the ribbed cone-segment design shown.
Practical surface treatments for the baffle surfaces of ground-based telescope consist primarily of paints. The high performance surfaces developed for optical systems in space are not applicable because they do not retain their properties in the presence of terrestrial contamination. Suitable black paints have a hemispherical reflectance of less than 5% for normal incidence light. However, this may rise to 15% or higher if the incidence angle is near grazing.
To achieve adequate baffling, single-scattering light paths must be minimized and grazing scattering must be eliminated. Double scattering light paths are rarely troublesome because the diffuse reflectance of two black surfaces multiplied together is small. Moreover, in most cases, only a small portion of the scattered light from one surface is intercepted by a second surface that can scatter it to the focal surface.
Figure 1: Baffles consist of the wind baffle (magenta) and the secondary, conical and primary baffles (green, top to bottom). Light rays (cyan) from the edge of the 1.5° field of view are shown (left figure). The conical baffle blocks direct rays from the sky from reaching the focal plan. The rays coming from the upper left (right figure) illuminate the inside of the primary baffle above the focal surface. A small portion of light scattered by the primary baffle surfaces is reflected by the secondary mirror to the focal surface. The ray coming from the upper right is blocked showing no direct rays pass between the conical and primary baffles.
Baffles consist of the wind baffle (magenta) and the secondary, conical and primary baffles (green, top to bottom). Light rays (cyan) from the edge of the 1.5° field of view are shown (left figure). The conical baffle blocks direct rays from the sky from reaching the focal plan. The rays coming from the upper left (right figure) illuminate the inside of the primary baffle above the focal surface. A small portion of light scattered by the primary baffle surfaces is reflected by the secondary mirror to the focal surface. The ray coming from the upper right is blocked showing no direct rays pass between the conical and primary baffles.
Since the requirements are much less stringent for the performance of the spectrographic configuration and the field of view (FOV) is smaller than the FOV of the imaging 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 generation and its finished inside diameter was larger than planned. Consequently, the outer diameter of the primary baffle annulus nearest the primary mirror surface was chosen to be as small as possible and still large enough to mask the portion of the mirror surface near its inner edge that was rolled off during polishing. 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 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 support for the primary mirror perimeter air seal.
The diameter of the secondary mirror baffle tip was chosen by trial and error to minimize the central obscuration and differential vignetting of the optical system. As its diameter is increased, the conical baffle can be made shorter. Consequently, the differential vignetting (dominated by the conical baffle) decreases while the central obscuration (dominated by the secondary baffle) increases. Also, if the secondary baffle tip diameter is too small, more than one conical baffle is required and this is obviously undesirable.
Rays were traced used a standard optics design program (Zemax). Rays that just clear the edges defining the entrance and exit pupils were traced (see the following list). These rays were used to define the location of the baffle edges. However, the aperture at the end of the wind baffle is not completely consistent with these rays since it was moved axially without adjusting its radii. The outer edge encroaches on the design ray about 2 mm and the inner edge clears the ray by about 10 mm. The encroachment is inconsequential since only the extreme astrometric CCDs are affected and the effect is negligible.
Table 1: Parameters for the various focal surfaces. The prime focus radius can be used to construct approximate rays between the primary and secondary. This is adequate for most baffle analysis purposes. Rays were traced used a standard optics design program (Zemax). Rays that just clear the edges defining the entrance and exit pupils were traced. These rays were used to define the location of the baffle edges.
Table 1:
Parameters for the various focal surfaces. The prime focus radius can be used to construct approximate rays between the primary and secondary. This is adequate for most baffle analysis purposes. Rays were traced used a standard optics design program (Zemax). Rays that just clear the edges defining the entrance and exit pupils were traced. These rays were used to define the location of the baffle edges.
Radius (°)
f/5 radius (mm)
f/5 mean scale (µm/arcsec)
Prime focus radius (mm)
Extreme photometric FOV
1.363
295.696
60.258
133.812
Extreme astrometric FOV
1.51
327.641
148.244
Spectrograph FOV
1.5
327.372
60.624
147.262
Baffle raytrace FOV
1.523
330.35
60.252
148.66
The following criteria were used for the design of the baffles. These criteria apply for the worst case combination of the tolerances of the individual components and flexure for elevations between 30° and 90°.
In the design of the baffles, it was necessary to make assumptions regarding the precision of fabrication of the component parts and the positioning of these components with respect to the optics (Table 2). The components are conical or annular with no diametrical bracing. For such parts, the most difficult tolerance is the runout, i.e., the variation of the radius. Certain components can be positioned much more precisely than others. The camera baffle and the primary and secondary baffle tips have fairly direct load paths to the optics and the position tolerances are correspondingly tight. The conical baffle and the wind baffle aperture load paths are much less direct and the tolerances are more relaxed.
Table 2: Tolerances for baffle components. Runout is the tolerance on the radius of the part. Position is the tolerance on centration of the part with respect to the telescope instrument rotator axis over the elevation range and with operating wind loads present.
Table 2:
Tolerances for baffle components. Runout is the tolerance on the radius of the part. Position is the tolerance on centration of the part with respect to the telescope instrument rotator axis over the elevation range and with operating wind loads present.
Component
Runout (mm)
Position (mm)
Camera baffle
±1
Primary baffle tip
±3
±2
Conical baffle tips
±6
Wind baffle aperture
±1.5
±10
Secondary baffle tip
The conical baffle is a novel feature of this baffle design. It was constructed as follows:
Light rays from the sky enter the annular opening at the end of the wind baffle and illuminate interior baffle surfaces.
Figure 2: The primary and secondary baffles consist of stacks of annuli supported by small struts (not shown). Direct rays illuminate a portion of the upper surface of each secondary annulus near its outer diameter (right ray in left diagram). However, the annuli are spaced so that light scattered from this illuminated portion is not reflected by the secondary to the focal surface instrument. Rays reflected by the primary illuminate the secondary baffle (left) and conical baffle (right).
Figure 2:
The primary and secondary baffles consist of stacks of annuli supported by small struts (not shown). Direct rays illuminate a portion of the upper surface of each secondary annulus near its outer diameter (right ray in left diagram). However, the annuli are spaced so that light scattered from this illuminated portion is not reflected by the secondary to the focal surface instrument. Rays reflected by the primary illuminate the secondary baffle (left) and conical baffle (right).
Figure 3: Rays from sources up to 8.8° off axis illuminate a portion of the lower surface of each primary annulus near its outer diameter (left diagram). The annuli are spaced so that light scattered from this illuminated portion is not visible from focal surface instrument. Rays from about 6.6° off axis, after reflecting from the primary and secondary illuminate the inside of the conical baffle (right diagram).
Figure 3:
Rays from sources up to 8.8° off axis illuminate a portion of the lower surface of each primary annulus near its outer diameter (left diagram). The annuli are spaced so that light scattered from this illuminated portion is not visible from focal surface instrument. Rays from about 6.6° off axis, after reflecting from the primary and secondary illuminate the inside of the conical baffle (right diagram).
Light rays from the primary mirror from sources outside the field of view illuminate interior baffle surfaces.
Light rays from sources outside the field of view after reflection from the primary and secondary illuminate interior baffles.
Figure 4: Rays from sources about 3.66° off axis, after reflecting from the primary and secondary, illuminate a portion of the upper surface of each primary annulus near its inner diameter (left diagram). Part of these illuminated surfaces are visible from the focal surface as reflected by the secondary mirror. Portions of the wind baffle within 0.9 m of the primary that are directly illuminated are visible from the focal surface (left rays in right diagram). 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 off the primary mirror and may illuminate the inner surface of the primary support structure. Forward scattering from this surface can reach the focal surface.
Figure 4:
Rays from sources about 3.66° off axis, after reflecting from the primary and secondary, illuminate a portion of the upper surface of each primary annulus near its inner diameter (left diagram). Part of these illuminated surfaces are visible from the focal surface as reflected by the secondary mirror. Portions of the wind baffle within 0.9 m of the primary that are directly illuminated are visible from the focal surface (left rays in right diagram). 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 off the primary mirror and may illuminate the inner surface of the primary support structure. Forward scattering from this surface can reach the focal surface.
All baffle surfaces will be painted with Aeroglaze Z306 paint (Lord Industrial Coatings, Erie, PA). Its hemispherical reflectance at 546 nm v. incidence angle in degrees is given in "Black Surfaces for Optical Systems", Stephen M. Pompea and Robert P. Breault, in the Handbook of Optics, ed. Michael Bass, McGraw-Hill, New York, 1995, p. 37-1.
The conical baffle is the most challenging to fabricate. It is supported by steel wires from the telescope truss. To resist tension in the wires, it must have good bending stiffness and be lightweight. To minimize light blockage, its 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. Finally, ribs must be added to the baffle surfaces to interrupt the grazing scattered light paths of Figure 2 (right diagram) and Figure 3 (right diagram).
These constraints lead to a design with a relatively deep central rib that provides the baffle with bending stiffness and anchor points for the support wires (left diagram of Figure 5). Extending above and below the stiffening rib are 1.6 mm thick cones with 10.45° and 8.83° half angles respectively. The baffle is formed on a machined aluminum mandrel out of graphite fiber reinforced epoxy. Machined into the mandrel are grooves that result in the ribs on the interior surface. The exterior surface of the finished part is wrapped with ø0.8 mm bare acrylic optical fiber to form the outside ribs.
Figure 5: This section through the right side of conical baffle shows the ribs that are necessary to interrupt grazing scattered light (left diagram). The details are magnified a factor of 5. The central rib provides attachment points and bending stiffness. The ribs are small near the ends to minimize field-edge vignetting. The section through the right side of the tip of the primary baffle is a machined aluminum tapered cylinder with ribs (right diagram). Below the tip are a series of progressively larger aluminum annuli each 0.64 mm thick.
Figure 5:
This section through the right side of conical baffle shows the ribs that are necessary to interrupt grazing scattered light (left diagram). The details are magnified a factor of 5. The central rib provides attachment points and bending stiffness. The ribs are small near the ends to minimize field-edge vignetting. The section through the right side of the tip of the primary baffle is a machined aluminum tapered cylinder with ribs (right diagram). Below the tip are a series of progressively larger aluminum annuli each 0.64 mm thick.
The primary baffle tip is a machined aluminum truncated cone with ribs (right diagram of Figure 5). 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 edge bevel faces the sky. Viewed from the focal surface, as reflected by the secondary, it should be blacker than if the edge were beveled the opposite way. The inner edges are illuminated directly from the sky and by the secondary. The edge bevel faces the secondary. Viewed from the focal surface, as reflected by the secondary, it should be blacker than if the edge were beveled the opposite way.
The primary mirror was damaged during generation. As a result, the inner diameter of the useful area of the primary was set by the damage and not be the demands of the baffling. A consequence of this and the large field of view is that the outer edges of the primary baffle annuli are visible from the focal surface.
The secondary baffle is very similar to the primary baffle but is inverted. The baffle tip is a machined aluminum truncated cone with ribs (left diagram of Figure 6). 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 secondary mirror.
The outer edges of the annuli are illuminated directly from the sky. The edge bevel faces the sky and is not visible from the focal surface. The inner edges are not illuminated except by scattered light. The edges are not beveled. They would be visible from the focal surface either directly or reflected by the secondary independent of their bevel.
Figure 6: This section through the right side of secondary baffle shows the baffle tip with the ribs that are necessary to interrupt grazing scattered light (left diagram). Above the tip are a series of progressively larger aluminum annuli each 0.64 mm thick. The scale is the same as the right diagram of Figure 5. The primary stop is 0.064 mm thick with a sharp edge (right diagram). The primary mirror is blue, the field edge rays are cyan and the primary support structure is black. A rib can be added about 0.3 m above the primary mirror, if necessary, to block illumination from the primary, subsequently scattered from the primary support structure, from reaching the secondary mirror. (Drawing by S. Limmongkol.)
Figure 6:
This section through the right side of secondary baffle shows the baffle tip with the ribs that are necessary to interrupt grazing scattered light (left diagram). Above the tip are a series of progressively larger aluminum annuli each 0.64 mm thick. The scale is the same as the right diagram of Figure 5. The primary stop is 0.064 mm thick with a sharp edge (right diagram). The primary mirror is blue, the field edge rays are cyan and the primary support structure is black. A rib can be added about 0.3 m above the primary mirror, if necessary, to block illumination from the primary, subsequently scattered from the primary support structure, from reaching the secondary mirror. (Drawing by S. Limmongkol.)
The primary stop is located just above the primary mirror and defines the outer edge of the entrance pupil (right diagram of Figure 6). The edge bevel faces the primary. Integrated with the primary stop is the primary perimeter bulb seal and the elastameric retention pad that protects the primary from overtravel during seismic accelerations, particularly when pointed at the horizon.
To analyze the scattered light performance of this design, the baffles and other elements of the telescope design are simplified as follows (left diagram of Figure 7).
Figure 7: In the simplified baffle model the wind baffle is represented by a cylinder (magenta in the left diagram).The conical baffle is projected onto the primary mirror from the prime focus field edge (right diagram).
Figure 7:
In the simplified baffle model the wind baffle is represented by a cylinder (magenta in the left diagram).The conical baffle is projected onto the primary mirror from the prime focus field edge (right diagram).
The APART computer program (Breault Research Organization) was used to perform stray light analysis of the simplified baffle model. 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 any significant effect on the results. The APART model of the SDSS telescope system has seven basic components.
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 GCF and BRDF charateristics 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 system.
For angles of 25° or less, the most important object that is visible from the focal surface (a critical object) is the aperture stop located just above primary mirror. For angles greater than 25°, the inside of the primary baffle is the most important critical object. It is illuminated by light scattered from the inside of the wind baffle. The point source normalized irradiance transmission (PSNIT) decreases abruptly between 25° and 30° (Figure 8). This decrease occurs because at 30°, the aperture stop is no longer directly illuminated. (The PSNIT is the ratio of the stray light irradiance at the focal surface to the irradiance from a point source measured in front of the optical system.)
Figure 8: 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.
Figure 8:
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.
For angles less than 20°, the distribution of power on the focal surface was calculated. For this calculation, the focal surface was divided into 10 rings of equal area. Each ring was divided by radial lines into 10 regions of equal area. The distribution varies smoothly over the focal surface for all field angles. The power distribution is plotted in Figure 9. Artifacts associated with rounding and the finite size of the focal plane regions are apparent. The source image would fall on the +y axis. Power increases in the -y direction because more of the illuminated baffles are visible from the opposite side of the focal surface.
Figure 9: 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 1.6°, 2.7°, 4.5°, 7.4° and 12.1° (left to right and top to bottom). The data have been scaled so that the maximum value is 100. Scattering from optical surfaces was ignored.
Figure 9:
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 1.6°, 2.7°, 4.5°, 7.4° and 12.1° (left to right and top to bottom). The data have been scaled so that the maximum value is 100. Scattering from optical surfaces was ignored.
A two mirror on-axis telescope is typically baffled with a secondary baffle that is somewhat larger in diameter than the secondary mirror. The primary baffle is contained within the shadow of the secondary baffle, even at the field edge. Consequently, the central obscuration is constant with field angle.
For the SDSS 2.5-m telescope, such an approach would have resulted in a central obscuration of more the 50%. This is due to its small final f/ratio and its wide field of view. The design described herein produces much less central obscuration, at the cost of addition blockage by the conical baffle and significant differential vignetting from the field center to its edge.
The obscuration of the telescope aperture was found by projecting the obstruction onto the primary mirror from the point on the prime focal surface corresponding to the field angle of interest (right diagram of Figure 7). 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 ribs nearest the middle and the upper and lower edges on both the inside and outside surfaces must be 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 was facilitated by the use of a computer aided design (CAD) program, Vellum (Ashlar, Inc., San Jose, CA) to project the obstructions onto the primary mirror. The program calculates the area of bounded 2-d regions, e.g., obscured areas.
The total obscuration, 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 obscuration is due to the secondary baffle and most of the differential vignetting is due to the conical baffle. Two breaks occur in the obscuration curve v. field angle (Figure 10). At 0.5 degrees, the tips of the conical baffle are no longer obscured by the central stiffening rib. At 0.7 degrees, the primary baffle emerges from the secondary baffle shadow.
Table 3: Fractional obscuration of the 2.5-m aperture.
Table 3:
Fractional obscuration of the 2.5-m aperture.
Field angle (degrees)
Conical baffle blockage
Central obscuration
Total obscuration
0.000
0.026
0.260
0.286
0.500
0.700
0.029
0.289
1.500
0.049
0.269
0.318
Figure 10: Obscuration of the 2.5-m aperture. The conical baffle blockage, while small, contributes most of the differential vignetting.
Figure 10:
Obscuration of the 2.5-m aperture. The conical baffle blockage, while small, contributes most of the differential vignetting.
I wish to thank Ed Mannery for performing the raytrace analysis that was used to design the baffles and for suggesting the final shape of the conical baffle. Discussions with Charles Hull were helpful throughout the design process. Dan Milsom of Breault Research Organization, Tucson, AZ, performed the scattered light analysis.
Date created: 6/16/96 Last modified: 2/17/98 Copyright © 1996, 1997, 1998 Walter A. Siegmund Walter A. Siegmund
