Sloan Digital Sky Survey Telescope Technical Note 19980215
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
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
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.
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°.
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:
Critical objects imaged by the secondary mirror include the following:
Critical objects imaged by both the secondary and primary mirrors include the following:
Major surfaces that are not critical objects include the following:
Directly illuminated objects include the following:
Objects illuminated via the primary mirror 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 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.
To analyze the scattered light performance, the baffles and other elements of the telescope design are simplified as follows.
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.
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.
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.
