Advanced Technology Solar Telescope (ATST) Stray and Scattered Light Analysis

Transcription

1 Advanced Technology Solar Telescope (ATST) Stray and Scattered Light Analysis Geometric Analyses (Tasks 1-6) Prepared for Association of Universities for Research In Astronomy (AURA) Prepared by Scott Ellis Richard N. Pfisterer Photon Engineering, LLC 1 May 2003 Draft Report

2 1.0 Introduction The purpose of this study is to perform an end-to-end scatter and stray light performance analysis of the baseline optical and mechanical design for the Advanced Technology Solar Telescope (ATST). 2.0 System Model 2.1 Geometry The system model for use with the ASAP optical analysis program was constructed from three sources: 1. ZEMAX prescription that described the optical surfaces that constitute the imaging path to Gregorian and coude focus. This file is listed in Appendix A. 2. Series of IGES files furnished by Mark Warner that describe the mechanical structures of the various subassemblies 3. s and discussions with both Rob Hubbard and Mark Warner The complete ASAP model is listed in Appendix B. We also constructed a parallel FRED models to debug some of the results of the ASAP model. Figures 1 and 2 show the complete ASAP model of the telescope. For clarity the dome has been turned off in two of the views. Figure 1 ASAP Model of ATST 1

3 Figure 2 ASAP Model of ATST Dome Subassembly Figure 3 Dome Subassembly 2

4 Figure 3 shows the dome subassembly that includes the inner and outer walls, wind vents, shutter components, and a cylindrical extension surrounding the incoming optical path called the snorkel. The snorkel is centered over the working aperture of the primary mirror. The inner walls of the snorkel and dome are 50% reflective Lambertian scatter surfaces to simulate gray paint on the interior of the dome Mount Base Subassembly Figure 4 shows the mount base subassembly that includes the cylindrical telescope base, two supports on which the telescope articulates, and a mount base center post that supports a fold mirror assembly. The dome floor is 30 m in diameter. It is located 4.75 m below the elevation pivot axis and 15 m above the ground. The dome floor is a gray Lambertian scatter surface. All other surfaces have white Lambertian scatter properties. Light to the Coude focus is directed through a cylindrical tube located in the center of the floor. MOUNT BASE MOUNT BASE CENTER POST (SMALL MIRROR SUPPORTS) 4.75 m 15 m 30 m Figure 4 Mount Base Subassembly OSS Subassembly The Optical Support Structure (OSS) subassembly is a frame structure that supports the primary and secondary mirrors, as shown in Figure 5. The model allows for the rotation of the OSS about the global X-axis to simulate tracking the sun from the horizon to zenith. The OSS is canted degrees from vertical to position it properly relative to the telescope line of sight for zenith pointing. With the exception of the heat shield assembly, all components of the OSS are white Lambertian scattering surfaces. 3

5 OSS SECONDARY SUPPORT OSS.PRIME FOCUS ASSY (HEAT SHIELD) OSS TOP OSS.PRIMARY MIRROR CELL +Y OSS BOTTOM OSS.PRIMARY MIRROR COVER +Z Figure 5 OSS Subassembly Heat Stop Subassembly The heat stop subassembly is a conical/cylindrical structure located at prime focus whose function is to isolate a nominal 5 arcminute field-of-view and reject (via reflection) all other solar radiation. This structure is shown in Figure 6. The actual field stop aperture is a ring whose inner and outer diameters are 13 and 17 mm in diameter, respectively. The ring and cone are mirrored surfaces that reject out of field sunlight into the open air or to the interior surfaces of the dome. The specular reflectance is 90%. The surfaces have been assigned +/-3 degree RMS local random slope errors to simulate the effect of surface roughness resulting from a precision machined (but not optical) surface finish. 4

6 Figure 7 shows the comparison of nominal and actual transmission through the heat stop. In the plane of the optical path, coma from the fast off-axis primary mirror distorts the converging beam such that the full 5 arcminutes FOV is not fully passed. Figure 6 Heat Stop Subassembly Figure 7 Transmission of the FOV Through Heat Stop (Blue lines indicates intended ±2.5 arcminute FOV; red curve indicates actual transmission) 5

7 2.1.3 Optical Path The optical path of ATST is configured for two selectable operational modes: 1. f/13 (52 m effective focal length) Gregorian; plate scale = mm/5 arcminute FOV 2. f/69 Coude (276 m effective focal length); plate scale = mm/5 arcminute FOV Figure 8 shows the optical path of the ATST. Light from the sun entering the telescope passes through a circular entrance aperture and is reflected by the off-axis primary mirror towards the off-axis secondary mirror. The intervening heat stop (not shown in Figure 7) isolates a 5 arcminute FOV. After reflecting off of the secondary mirror, the light passes through a Lyot stop (conjugate to the entrance aperture) and comes to focus at Gregorian focus. If Gregorian focus is not selected, light is deflected by fold mirror M3 towards mirror M4 which is located on one side of the mount base (This defines the rotation axis of the OSS.) Mirror M4 produces a pupil image at deformable mirror M5, which is mounted to the fixed center post on the mount base. Light is then directed downwards (i.e., towards the ground) by fold mirror M6 to Coude focus, which is located on the azimuthal rotation axis of the telescope. Mirrors M1 through M4 all rotate both in elevation with the OSS and azimuthally with Coude. Mirrors M5 and M6 only rotate azimuthally with Coude. SECONDARY MIRROR LYOT STOP M5 DEFORMABLE MIRROR M6 FOLD MIRROR GREGORIAN FOCUS ENTRANCE APERTURE +X +Y M3 FOLD MIRROR TO COUDE FOCUS PRIMARY MIRROR +Z +Z M4 TRANSFER MIRROR (POWERED) COUDE FOCUS Figure 8 Optical Path 6

8 2.2 Specular Coatings Optical Mirror Coating All mirror surfaces were assigned a specular reflectivity of 1.0 (100%) Heat Stop Reflecting Surface Per Rob Hubbard s dated 18 April 2003, the specular reflectivity of the outer surfaces of the heat stop surface is 0.9 (90%). 2.3 Scatter Models Since this is a study, many of the actual surface treatments have not yet been established. According to the terms of the Statement of Work (SOW), we were directed to assume realistic surface properties. Consequently the scatter models used in this study are estimates based upon good engineering practice Mirror Surface Scatter Rob Hubbard had proposed (Ref. 1) that fabrication of a primary mirror with an rms roughness of 20 angstroms was not unreasonable and so we used this value for all mirror surfaces. For smooth optical surfaces whose rms roughness is much less than the wavelength of the incident light, the three-term Harvey scatter model is appropriate. The scatter function is given by BSDF ( θ θ ) s 2 sin θ sin θ = b0 1+ L, (1) where θ, θ 0 = the scatter and specular angles (measured from the local surface normal), b 0 = a constant, s = the slope and L = the rollover angle. Lacking specific measured data, we constructed a reasonable model based upon the observations that 1. the slope s of the BSDF function is typically on the order of 1 to 2 and so -1.5 is a reasonable average 2. the rollover angle, while too close to specular to be measured on smooth optical surfaces, is believed to be on the order of or smaller. Since the extreme proximity to specular is not this issue here and since it has little effect on the TIS, we adopted a value of for L. 7

9 Knowing the slope s and the rollover angle L, we can obtain a consistent value for b 0 in a twostep process. First we use compute the TIS from the desired rms roughness using 2 2π n σ TIS = (2) λ where n is the index difference in reflection off of a mirror (=2), and σ = rms roughness. Then we related b 0 to the TIS using the equations s 100 TIS 2π b (3) s + 2 b = b( 100 L) s (4) 0 Following this method, we derive a Harvey model for a 20 angstrom rms roughness surface at a wavelength of 1 micron whose coefficients b 0, s, L are equal to 1.58, -1.5, and 0.001, respectively White Paint White paint is modeled as a simple Lambertian scatterer with a TIS equal to 0.9 (90%) Gray Paint Per Rob Hubbard s dated 21 April 2003, this paint is modeled as a Lambertian scatterer with a TIS equal to 0.5 (50%) Black Paint Per the SOW, the black paint model was to be selected on the basis of reasonableness. Martin Black was proposed. Martin Black is not a paint, per se; it is a surface treatment on aluminum. Therefore it cannot be applied to any arbitrary substrate material. Since we do not know what materials will be used in the construction of the telescope structures, we cannot comment on whether or not Martin Black could actually be applied. Pristine Martin Black on aluminum can have a TIS of approximately 1%. A more generally applicable diffuse black paint, Aeroglaze Z306, has a TIS of approximately 3%. Relative to the other unknowns in the analyses, we felt that the differences were not significant and decided to use Martin Black as the realistic paint. We constructed the Martin Black BSDF model from measured BRDF data shipped with each installation of ASAP. (The data is contained in the file apartlib.dat and it can be found in the ASAP installation\projects\examples subdirectory.) The data was extracted and fit to a generic BSDF function. Figure 9 shows the resulting BSDF model. 8

10 Figure 9 Martin Black BSDF Model (after APART data) Martin Black is applied to only three structures in the model: the edges of entrance aperture and the Lyot stop, and the heat stop inner cone Level 400 Particulate Scatter The SOW directed us to consider particulate scatter based upon the Mie theory which describes the intensity distribution produced by a volume of spherical particles with some arbitrary distribution of diameters and complex refractive indices. Since the particle diameters, distribution, and refractive indices are arbitrary, the SOW did not indicate a specific scatter model. The usual approach in this situation is to adopt a standardized distribution such as MIL-STD MIL-STD-1246A describes a particulate distribution expected in a clean room. The distribution is given by log 2 [ ] 2 ( n) ( log( CL) ) ( log( s) ) = (5) where n = the number of particles per square foot whose diameter is greater than s microns and CL is the cleanliness level. (Note that these are common or base10 logarithms.) According to the definition, the smallest and largest particles have diameters of 1 micron and CL microns, respectively. There are numerous problems with this commonly used particulate model: 1. No clean room is existence actually demonstrates particulates with this distribution. 9

11 2. There is considerable experimental evidence that the slope is far too large and that slopes of are more realistic. 3. It ignores the presence of particles smaller than 1 micron in diameter. 4. Large particles tend to be ellipsoidal or cylindrical rather than spherical, and consequently the Mie-based calculation overestimates the scatter. (However we have not seen a Mie formalism based upon elliptical or cylindrical scattering bodies!) Problems notwithstanding, we followed Rob Hubbard s lead and constructed a particulate model based upon this distribution. Previously we had written a custom DLL for ASAP that efficiently calculates a BSDF based upon this definition. However in the interests of sharing this ASAP model with other reviewers, we decided instead to model the particulate scatter as the sum of two Harvey scatter functions of the form described in Section The values of b 0, s, and L were scaled from those published by Rob Hubbard (Ref. 1). Since Spyak and Wolfe (Ref. 2) have shown that the scatter from particulates is linear shift-invariant, simulating the Mie scatter with Harvey models is appropriate. Figure 10 shows an excellent agreement between a theoretical BSDF function for level 400 cleanliness and the sum of two Harvey functions. Figure 10 Level 400 Model Comparison Note that Rob Hubbard had used a coverage fraction of 0.01% in his calculations; this is equivalent to level 240 which is a reasonable level for a surface immediately after cleaning. However the UKIRT data Rob includes suggests that the accumulated particulates could reach 0.063% coverage at the end of a day; this is equivalent to level 360. For a conservative calculation, we rounded level 360 to level 400 for these analyses Heat Stop Reflecting Surface 10

12 According to Rob Hubbard s notes, the surface of the heat stop will be reflecting but not optically smooth such that the Harvey scatter model described previously could be employed. Mark Warner suggested that the surface could be machined to a roughness of 32 microinches (0.813 microns) rms. Since this is comparable to the wavelength of light used in these calculations, our feeling was that this would hardly describe a surface with significant specular properties. In discussion with Rob Hubbard, we decided to simulate the rms roughness of the machined surface with a Gaussian slope error similar to what is used to model vacuum metalized reflectors for automobiles and spotlights. In this model, the surface retains its specular properties (90% reflective) but the local surface normal is randomly perturbed according to a Gaussian distribution function with a 3 degree (0.052 radians) slope error. This has the effect of smearing out an otherwise perfectly specular reflection. (Another way of thinking about this: a collimated beam incident on the surface would reflect into a cone of light 6 degrees wide.) Stray Light Calculation Methodology The calculation has been set up so that the mechanical structures are permitted only a single level scatter event, while the optical surfaces are allowed up to second level. The rationale for this approach is to allow a scatter event from a structural object to an optical surface, where a second scatter event may direct light into the imaging field of view. Each optical surface has an importance edge that goes around the periphery of the surface. At a minimum, scatter from any structural element is directed towards every optical surface that has a direct view of the illuminated object. The importance edges for the optical surfaces are generally restricted to images of the focal planes. For example, the primary mirror scatters towards the prime focus, the secondary scatter towards Gregorian focus, the fold mirror M3 scatters towards the virtual image of the Coude plane formed by mirror M4, and so on. There are two other importance edges of note: the top of the tube through the OSS just above Gregorian focus, and the hole in the dome floor leading to Coude focus. These importance edges are added to any surface that are in direct view. This same technique is used for this and all subsequent scatter calculations. 3.0 Source Models 3.1 Sun Model The incident solar radiation was modeled as a planar Lambertian emitter located above the observatory radiating into a solid angle 32 arcminutes wide. The emitter was sized to fully illuminate either the entire dome floor (in the no dome case) or the entrance aperture of the snorkel (in the dome case). (It is important to note that we were unable to preserve the ray density when illuminating the entire dome floor; there were simply too many rays to trace! Consequently we reduced the sampling whenever the dome was turned off.) According to the SOW, the sun was presumed to have a uniform surface irradiance and so we did not apply any angular apodization to simulate limb darkening effects. 11

13 Since there are no dispersive elements in the optical path, it was sufficient to perform all raytracing at a single wavelength of 1 micron. The direct solar irradiance at the surface of the earth was assumed to be 850 W/m 2. (The SOW did not give any specific value for the solar constant or the spectral bandwidth to be used in this analysis.) 3.2 Sky Model Astronauts who have walked on the moon tell us that the sky is pitch black except in the direction of the sun. Since there are no scattering mechanisms on the moon (because there is no atmosphere), there is a clear and sharp delineation between sun and sky. The molecular species comprising the earth s atmosphere are responsible for Rayleigh scatter which, in turn, is responsible for the blue sky. Consequently our sky is actually a contributor to irradiance at the surface of the earth. The American Society for Testing and Materials (ASTM) has published standardized models for solar and sky spectral distributions (Ref. 3). Of course these distributions are functions of spectral bandwidth, position of the sun in the sky, and so on. Based upon references we ve used successfully in the past for solar concentrator design work, we assumed that the hemispheric sky irradiance on the surface of the earth was150 W/m 2. (The SOW did not give any specific value for the spectral bandwidth to be used in this analysis.) Since there are no dispersive elements in the optical path, it was sufficient to perform all raytracing at a single wavelength of 1 micron. The sky was modeled as a Lambertian hemisphere whose center is located on the ground at the center of the dome mount. A point source located at the center illuminates the hemisphere which, in turn, scatters to the ATST. The TIS of the hemisphere and the seed power contained in the point source were scaled such that the irradiance on the ground was 150 W/m Snow Model One of the analysis tasks requires the stray light contributions from snow surrounding the mount base. No specific modeling instructions were given in the SOW and so we made the following assumptions: 1. The snow scatters incident light according to a Lambertian scatter distribution with a TIS equal to 0.9 (90%). 2. There is no specular component to the light reflected by the snow. 3. The snow covers an area 400 meters in diameter surrounding the observatory. The snow was modeled as a Lambertian emitter at ground level with a set of importance edges arranged around the telescope. Since it is not important to the generation of scattered rays how the snow is illuminated, we simply setup an emitter radiating 1000 W/m 2 (sum of sun and sky 12

14 irradiances) immediately over the snow and launched the rays into the snow. The scattered rays were directed towards the telescope. 3.4 Lake Model One of the analysis tasks requires the stray light contributions from a lake surrounding the mount base. No specific modeling instructions were given in the SOW and so we made the following assumptions: 1. The lake acts like a mirror with 2% reflectivity per the Fresnel reflection calculation for air over a dielectric substrate with a refractive index of The lake is deep enough to complete attenuate the transmitted component of the incident light. 3. The surface of the lake is randomly roughened such that an incident ray could be reflected into any arbitrary direction over 2π sr. (We felt that the heights of the waves and ripples were insignificant relative to the propagation distances to the telescope, and so only the randomized direction was important to the model.) 4. There is no scatter component to the light reflected by the lake. 5. The lake covers an area 400 meters in diameter surrounding the observatory. During our initial attempt at modeling the lake, we implemented the same technique as used to model the snow except that the lake was specular and hence there were no importance edges around the telescope. However the statistics were extremely poor; only a small fraction of the rays reflected by the lake reached the telescope. (The distinction here is that you can aim scattered rays via importance edges but you cannot aim reflections!) After some thought we concluded that, statistically, the lake is essentially a scaled version of the snow. A ray intersecting the lake is randomly reflected into 2π sr; this looks very much like a ray intersecting the snow and scattering into 2π sr. Taken as a whole, therefore, the lake can be modeled as a 2% Lambertian scattering surface. Since the snow is modeled as a 90% Lambertian scattering surface, our approach to calculating the stray light contributions from the lake is to scale the snow scatter calculations by 45x. 4.0 Analysis Tasks 4.1 Aperture Stop Task Description Quantify the scattered light contribution from the top and bottom surfaces (The bottom is irradiated by light reflected by M1), and the edge of the entrance aperture stop, assuming realistic surface properties (Martin Black or similar) and edge geometry (details to be provided by ATST) Analysis 13

15 The entrance aperture is modeled as a thin annular disk centered over the off-axis segment of the primary mirror and oriented perpendicular its parent axis. The outer diameter of the disk is 6 m. The size of the inner hole is 4 m in diameter. The front (facing the sky) and back (facing the primary) surfaces are white Lambertian with a reflectivity of 90%. The inside edge is a simple cylinder 12.7 mm long. It is coated with Martin Black. The current location of the entrance aperture places it inside the field of view of the instrument. Illumination of the top of the aperture scatters light directly to the secondary mirror, both through the center and around the periphery of the heat stop assembly. These paths both require only a single scatter event to illuminate the focal plane(s). The aperture obscures about 6%, by area, of the light passing through the aperture and reflected by the primary mirror. The illuminated portion of the back side can scatter light into the field of view only via a second level scatter event from the primary mirror. The edge of the aperture can scatter light both towards both the primary and secondary mirrors. Its small size keeps it from being a significant contributor. As the following figures indicate, scatter from the front side of the aperture generates orders of magnitude more flux at both the Gregorian and Coude foci than either the edge or back surface. The effect of the dome on these contributions is minimal. Variations between the dome and no dome configurations are more likely due to ray sampling statistics than any real difference in the scattered power reaching the analysis planes. 14

16 GREGORIAN FOCUS, NO DOME ********************** SCATTER FROM APERTURE STOP ********************** ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 1.8E-4 W (497 RAYS) LEVEL 2 SCATTER FROM FRONT = 1.5E-7 W (30500 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 4.E-12 W (162 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 1.3E-7 W (18318 RAYS) TOTAL SCATTERED POWER FROM EP APERTURE = E-4 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E Figure 11 Scatter to Gregorian focus from the entrance aperture without the dome. Object 133 is the front surface of the aperture. Object 139 is the secondary mirror reflector. 15

17 COUDE FOCUS, NO DOME ********************** SCATTER FROM APERTURE STOP ********************** ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 1 LEVEL 1 SCATTER FROM FRONT = 1.6E-4 W (466 RAYS) LEVEL 2 SCATTER FROM FRONT = 3.1E-7 W (38986 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 3.E-12 W (111 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 2.3E-7 W (17937 RAYS) TOTAL SCATTERED POWER FROM EP APERTURE = E-4 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E Figure 12 Scatter to Coude focus from the entrance aperture without the dome. Object 133 is the front surface of the aperture. Object 134 is the back surface of the aperture. Object 136 is the primary mirror reflector. Object 139 is the secondary mirror reflector. 16

18 GREGORIAN FOCUS, WITH DOME ********************** SCATTER FROM APERTURE STOP ********************** ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 1.6E-4 W (700 RAYS) LEVEL 2 SCATTER FROM FRONT = 3.5E-8 W (10611 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 4.E-12 W (243 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 1.6E-7 W (17841 RAYS) TOTAL SCATTERED POWER FROM EP APERTURE = E-4 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E Figure 13 Scatter to Gregorian focus from the entrance aperture with the dome. Object 165 is the front surface of the aperture. Object 166 is the back surface of the aperture. Object 168 is the primary mirror reflector. 17

19 COUDE FOCUS, WITH DOME ********************** SCATTER FROM APERTURE STOP ********************** ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 1 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 1.5E-4 W (669 RAYS) LEVEL 2 SCATTER FROM FRONT = 1.7E-7 W (30269 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 4.E-12 W (247 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 5.4E-8 W (17698 RAYS) TOTAL SCATTERED POWER FROM EP APERTURE = E-4 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E Figure 14 Scatter to Coude focus from the entrance aperture with the dome. Object 165 is the front surface of the aperture. Object 171 is the secondary mirror reflector. Object 181 is reflecting surface of mirror 5. 18

20 4.2 Heat Stop and Trap Task Description Quantify the scattered light contribution from the heat stop, including the aperture edge, and the light trap surface, assuming realistic surface properties and edge geometry (to be provided by the ATST project) of the stop and trap. Compare the scatter from the inside of the heat trap to the no-trap option of dumping the diverted light into the dome and surrounding structures Analysis Per the instructions of Rob Hubbard, the no-trap option was eliminated from this task. What remained was an assessment of a reflecting conical heat shield that directs light out of the field of view to either the open air or to the internal walls of a dome. As discussed earlier, the conical reflector and leading edge of the heat shield are machined specular and are 90% reflective. Small perturbations to the local slope simulate errors due to the residual roughness of the surfaces. Additionally, the specular surfaces have a MIL-STD-1246A contamination model (CL 400) applied to them as well. These two effects direct light into a larger angle set than that of a purely specular process allows, thereby increasing the number of objects illuminated in reflection and increasing the likelihood of generating scattered rays that can reach either of the detector planes. Figure 15 The yellow dots indicate what the Gregorian focus sees. As the following figures indicate, the Gregorian focus is, by many orders of magnitude, more susceptible to stray light than is the Coude focus. The difference between the two has to do primarily with what the detectors can see, i.e. the critical objects. Gregorian focus is much more exposed. It sees substantial portions of the OSS top structure, which are directly illuminated by specular reflections from the heat shield (Figure 15). Conversely, the Coude focus does not have 19

21 a direct view to the top of the OSS, so multiple level scatter events are required for light reflected from the heat shield to reach the focal plane. GREGORIAN FOCUS, NO DOME ********************** SCATTER FROM HEAT SHIELD ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT EDGE = 1.4E-6 W (8 RAYS) LEVEL 2 SCATTER FROM FRONT EDGE = 2.4E-9 W (7242 RAYS) LEVEL 1 SCATTER FROM OUTER CONE = 1.7E-5 W (562 RAYS) LEVEL 2 SCATTER FROM OUTER CONE = 7.8E-8 W (57750 RAYS) LEVEL 1 SCATTER FROM INNER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INNER CONE = 2.E-10 W (50 RAYS) TOTAL SCATTERED POWER FROM HEAT SHIELD = E-5 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E E E E E E E Figure 16 Scatter to Gregorian focus from the heat shield without the dome. Object 58 is the reflective front edge of the heat shield. Object 59 is the reflective conical surface on the heat shield. Object 87 is the cylindrical tube that passes through the OSS support arms. Object 15 is the dome floor. Object 41 is one of the vertical support arms for the top of the OSS structure. Object 133 is the top side of the entrance aperture. 20

22 COUDE FOCUS, NO DOME ********************** SCATTER FROM HEAT SHIELD ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 1 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT EDGE = 5.E-11 W (1734 RAYS) LEVEL 1 SCATTER FROM OUTER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM OUTER CONE = 6.E-11 W (5989 RAYS) LEVEL 1 SCATTER FROM INNER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INNER CONE = 1.E-10 W (46 RAYS) TOTAL SCATTERED POWER FROM HEAT SHIELD = E-10 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter... NONE Figure 17 Scatter to Coude focus from the heat shield without the dome. Object 58 is the reflective front edge of the heat shield. Object 59 is the reflective conical surface on the heat shield. Objects 136 and 145 are the reflecting surfaces of the primary mirror and mirror M3, respectively. 21

23 GREGORIAN FOCUS, WITH DOME ********************** SCATTER FROM HEAT SHIELD ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT EDGE = 1.3E-6 W (14 RAYS) LEVEL 2 SCATTER FROM FRONT EDGE = 2.5E-9 W (9662 RAYS) LEVEL 1 SCATTER FROM OUTER CONE = 1.6E-5 W (1090 RAYS) LEVEL 2 SCATTER FROM OUTER CONE = 4.5E-8 W ( RAYS) LEVEL 1 SCATTER FROM INNER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INNER CONE = 0 W (0 RAYS) TOTAL SCATTERED POWER FROM HEAT SHIELD = E-5 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E E E E E E E Figure 18 Scatter to Gregorian focus from the heat shield with the dome. Object 90 is the reflective front edge of the heat shield. Object 91 is the reflective conical surface on the heat shield. Object 119 is the cylindrical tube that passes through the OSS support arms. Object 47 is the dome floor. Object 2 is one of the wind vents. Object 165 is the top side of the entrance aperture. 22

24 COUDE FOCUS, WITH DOME ********************** SCATTER FROM HEAT SHIELD ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 1 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT EDGE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT EDGE = 1.E-10 W (3866 RAYS) LEVEL 1 SCATTER FROM OUTER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM OUTER CONE = 5.E-11 W (1053 RAYS) LEVEL 1 SCATTER FROM INNER CONE = 0 W (0 RAYS) LEVEL 2 SCATTER FROM INNER CONE = 0 W (0 RAYS) TOTAL SCATTERED POWER FROM HEAT SHIELD = E-10 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter... NONE Figure 19 Scatter to Coude focus from the heat shield with the dome. Object 90 is the reflective front edge of the heat shield. Object 91 is the reflective conical surface on the heat shield. Objects 168 and 177 are the reflecting surfaces of the primary mirror and mirror M3, respectively. 23

25 4.3 Lyot Stop at P Task Description Quantify the scattered light contribution from the edge and front surface of the Lyot stop located at P1 below the secondary, assuming realistic surface properties and edge geometry (to be provided by the ATST project). Analyze the effectiveness of this stop to mitigate the stray light from the edge of the entrance-aperture stop over a small range of undersized diameters Analysis The Lyot stop is located between the secondary mirror and Gregorian focus. Its location is coincident with the best image of the entrance aperture. Like the entrance aperture, the top and bottom surfaces are white Lambertian and the inner edge is treated with Martin Black. 24

26 GREGORIAN FOCUS, NO DOME ********************** SCATTER FROM LYOT STOP ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT = 2.8E-6 W (3606 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 1.4E-7 W (2 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 1.E-10 W (393 RAYS) LEVEL 1 SCATTER FROM BACK = 6.9E-5 W (2 RAYS) LEVEL 2 SCATTER FROM BACK = 1.1E-8 W (153 RAYS) TOTAL SCATTERED POWER FROM LYOT STOP = E-5 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E Figure 20 Scatter to Gregorian focus from the Lyot stop without the dome. Object 155 is the front surface of the Lyot stop. Object 156 is the back surface of the Lyot stop. Object 157 is the inside edge of the Lyot stop aperture. Object 139 is the reflecting surface of the secondary mirror. Object 87 is the tube that passes through the OSS support structure between the secondary mirror and fold mirror M3. 25

27 COUDE FOCUS, NO DOME ********************** SCATTER FROM LYOT STOP ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT = 2.6E-6 W (3298 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 1.2E-6 W (11 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 1.1E-9 W (1035 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 4.E-11 W (119 RAYS) TOTAL SCATTERED POWER FROM LYOT STOP = E-6 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E Figure 21 Scatter to Coude focus from the Lyot stop without the dome. Object 155 is the front surface of the Lyot stop. Object 156 is the back surface of the Lyot stop. Object 157 is the inside edge of the Lyot stop aperture. Objects 136, 139, and 152 are the reflecting surfaces for the primary mirror, secondary mirror, secondary mirror, and fold mirror M6, respectively. 26

28 GREGORIAN FOCUS, WITH DOME ********************** SCATTER FROM LYOT STOP ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT = 3.1E-6 W (7461 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 5.1E-7 W (11 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 1.E-10 W (483 RAYS) LEVEL 1 SCATTER FROM BACK = 1.3E-5 W (2 RAYS) LEVEL 2 SCATTER FROM BACK = 8.0E-9 W (315 RAYS) TOTAL SCATTERED POWER FROM LYOT STOP = E-5 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E Figure 22 Scatter to Gregorian focus from the Lyot stop with the dome. Object 187 is the front surface of the Lyot stop. Object 188 is the back surface of the Lyot stop. Object 189 is the inside edge of the Lyot stop aperture. Object 171 is the reflecting surface of the secondary mirror. Object 119 is the tube that passes through the OSS support structure between the secondary mirror and fold mirror M3. 27

29 COUDE FOCUS, WITH DOME ********************** SCATTER FROM LYOT STOP ************************ ANGLE FROM ZENITH = 0 DEGREES FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm LEVEL 1 SCATTER FROM FRONT = 0 W (0 RAYS) LEVEL 2 SCATTER FROM FRONT = 2.8E-6 W (6710 RAYS) LEVEL 1 SCATTER FROM INSIDE EDGE = 4.4E-7 W (8 RAYS) LEVEL 2 SCATTER FROM INSIDE EDGE = 5.E-10 W (620 RAYS) LEVEL 1 SCATTER FROM BACK = 0 W (0 RAYS) LEVEL 2 SCATTER FROM BACK = 2.E-10 W (252 RAYS) TOTAL SCATTERED POWER FROM LYOT STOP = E-6 W ************************************************************************ LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 1 (SOLAR DIRECT) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E LIST OF MOST SIGNIFICANT SCATTER PATHS FOR SOURCE 2 (INDIRECT SKY) OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E E E E E Figure 23 Scatter to Coude focus from the Lyot stop with the dome. Object 187 is the front surface of the Lyot stop. Object 188 is the back surface of the Lyot stop. Object 189 is the inside edge of the Lyot stop aperture. Objects 168, 171, 177, 184 are the reflecting surfaces for the primary mirror, secondary mirror, fold mirror M3, and fold mirror M6, respectively. 28

30 4.3.3 Stray Light Control w/lyot Stop Since the Lyot stop is conjugate to the entrance aperture, undersizing the Lyot stop clear aperture has the effect of projecting a mask onto the entrance aperture. Properly sized, the Lyot stop can completely prevent the scatter from the entrance aperture from reaching the Gregorian focus. However this level of obstruction usually involves vignetting the imaging rays as well. Figure 20 shows a plot of the amount of level 1 scatter from the entrance aperture that reaches Gregorian focus as the semidiameter of the Lyot stop is varied from 190 to 280 mm. In this calculation, the entrance aperture and primary mirror are illuminated by solar radiation, and the entrance aperture is allowed to scatter to Gregorian focus. At a semidiameter of approximately 212 mm, scatter begins to leak into the FOV. By the time the semidiameter reaches 240 mm, scatter from the entrance aperture reaches a constant level. Figure 24 Calculation of Stray Light Reaching Gregorian Focus as a Function of Lyot Stop Clear Aperture Figure 21 shows the same calculation from the point of view of Gregorian focus. In this calculation, the Gregorian focal plane is treated as an emitter radiating into the Lyot stop aperture and we re looking at the raytrace backwards to the entrance aperture and the primary mirror. When the semidiameter is 190 mm, the footprint of the beam is well within the entrance aperture. At a semidiameter of approximately 212 mm, Gregorian focus can just see the edge of the entrance aperture. At the nominal semidiameter of 260 mm, the entrance aperture vignettes the 29

31 view of the primary mirror. At still greater apertures, Gregorian focus can see almost all the way around the entrance aperture. Lyot Stop Semidiameter = 190 mm: Rays underfill clear Entrance Aperture Lyot Stop Semidiameter = 212 mm: Rays just clear Entrance Aperture Lyot Stop Semidiameter = 260 mm: Rays vignetted by Entrance Aperture Lyot Stop Semidiameter = 280 mm: Rays vignetted by Entrance Aperture Figure 25 Beam Footprints from the Point of View of Gregorian Focus as a Function of Lyot Stop Clear Aperture 30

32 4.4 The Open Dome Telescope and Baffles Task Description Assuming a snorkel-style dome enclosure with wind vents (details have been provided), provide a rough order of magnitude estimate of the stray light contributions from the sky, external surroundings (a lake or snow covered ground), the observatory floor, and the inside of the dome. Consider both Gregorian and coude foci. If the contribution is important, investigate baffling strategies to mitigate these sources. Examples include baffling vanes, tube or box structures extending toward the secondary mirror, and any baffles around the secondary mirror that may be useful. Only general recommendations for shapes and sizes are needed at this time. This task quantifies the level of concern when not completely enclosing the entire path from the heat stop to coude to allow for easy flushing with ambient air, cleaning and servicing Analysis This analysis examines the effectiveness of the dome at reducing the scatter contributions to both Gregorian and Coude foci, from distant objects outside of the telescope as well as those inside the dome enclosure that would otherwise be in full view of the sun and the sky. Only zenith pointing has been considered, which is very likely a best-case scenario since the primary mirror and entrance aperture are facing upwards. As a reference point, it is useful to consider the scatter contributions from only the optical surfaces; these contributions for both Gregorian and Coude optical paths are listed in Figure 26. We would expect that the total direct power in these two cases would be identical, however they are different by less than 2%. The ray statistics involved in these calculations prevent an exact agreement. This will be evident in all the following ASAP calculations. All other things being equal, we expect that the mirrors optically closer to the image plane will show higher scatter contributions because the projected solid angles of the image plane from these mirrors are larger; we observe this in the Coude path. Why the primary and secondary mirrors should contribute nearly equal amounts of scatter is less obvious. Consider the paraxial radiometric transfer equation Pc = Ps BSDF Ω (6) where P c = power scattered to collector, P s = power incident on scattering surface, BSDF = scattering function, and Ω = projected solid angle of collector. Since we ve assigned the same scatter models to all mirrors, the BSDFs are identical. If we assume that the power incident on the primary mirror is unity, then the power incident on the secondary mirror must be (5 arcminutes/32 arcminutes) smaller; this factor is equal to 2.45e-2. The projected solid angle subtended by the Gregorian focus relative to the primary mirror is approximately equal to (area of heat stop aperture)/(distance from primary mirror to heat stop aperture) or 2.07e-6 sr. The 31

33 projected solid angle of Gregorian focus relative to the secondary mirror is approximately equal to (area of Gregorian focus)/(distance from secondary mirror to Gregorian focus) or 6.41e-5 sr. Multiplying everything together, we get (1)(BSDF)(2.07e-6) (2.45e-2)(BSDF)(6.41e-5) 2.07e e-6 (7a) (7b) which is good agreement for such a crude calculation. Therefore the primary and secondary mirrors contribute equal amounts of stray light because the solid angles and incident powers balance each other out. Under these conditions, the signal-to-noise ratio (SNR) is approximately 4460 for the Gregorian optical path and approximately 869 for the Coude optical path. It is important to note that these are best-case results because we ve excluded everything but the optical surfaces. Figures 27 and 28 show the results of the stray light analysis of the entire ATST including the dome, shutter, snorkel, etc for both Gregorian and Coude foci for direct solar and indirect sky illumination. Despite the somewhat different total powers reported for first level scatter (comparing Figure 26 with Figures 27 and 28), the analyses show that the scatter from the optical surfaces far exceeds the contribution from any other structure. (The reason for the apparent discrepancy between first level scatter powers is ray sampling: it is much more time consuming to trace a dense distribution of rays over the area of the entire ATST than it is to trace the same density of rays over the primary mirror aperture. In the interests of time, we were forced to reduce the ray density for the complete calculation and the result is the variation in total direct and scatter power shown in these figures.) Ray statistics not withstanding, we can still draw conclusions from these results. 1. The mirror surfaces are the major cause of stray light. 2. The Coude optical path shows a greater amount of stray light because more directly illuminated mirror surfaces are involved. 3. The mount structure, entrance aperture, Lyot stop are comparatively minor contributors. 4. Scatter from indirect sky illumination is negligible compared to the scatter from direct solar illumination in the Coude optical path. 5. Scatter from indirect sky illumination is an order of magnitude below the scatter from direct solar illumination in the Gregorian optical path because the Gregorian focus is more exposed. 6. Light reflected off of the heat stop that is then scattered by the dome is not significant. 7. Additional baffles are probably not necessary, at least when the telescope pointing upwards towards zenith. In order to make a stronger statement about the need for additional baffles, we would have to analyze the system for several telescope positions over the intended range of motion. 32

34 This calculation assumes a Cleanliness Level 400 contamination model and 20 Angstrom RMS surface roughness. Only solar direct illumination is considered. ************* LEVEL 1 SCATTER FROM OPTICS TO GREGORIAN ***************** NUMBER OF SOURCE RAYS = TOTAL INCIDENT POWER = W (850 W/m^2) SUN AOI (LINE OF SIGHT OFFSET) = 0 degrees TOTAL DIRECT POWER (NO SCATTER) = W (2192 RAYS) M1 --> PRIMARY MIRROR = 2.6E-2 W ( RAYS) M2 --> SECONDARY MIRROR = 2.6E-2 W (10626 RAYS) M3 --> FOLD MIRROR 1 = 0 W (0 RAYS) M4 --> TRANSFER MIRROR = 0 W (0 RAYS) M5 --> FOLD MIRROR 2 [DM] = 0 W (0 RAYS) M6 --> FOLD MIRROR 3 = 0 W (0 RAYS) TOTAL SCATTERED POWER FROM OPTICS ONLY = E-2 W ************************************************************************ *************** LEVEL 1 SCATTER FROM OPTICS TO COUDE ******************* NUMBER OF SOURCE RAYS = TOTAL INCIDENT POWER = W (850 W/m^2) SUN AOI (LINE OF SIGHT OFFSET) = 0 degrees TOTAL DIRECT POWER (NO SCATTER) = W (2154 RAYS) M1 --> PRIMARY MIRROR = 2.5E-2 W ( RAYS) M2 --> SECONDARY MIRROR = 2.5E-2 W (10118 RAYS) M3 --> FOLD MIRROR 1 = 1.5E-2 W (18275 RAYS) M4 --> TRANSFER MIRROR = 4.9E-3 W (8217 RAYS) M5 --> FOLD MIRROR 2 [DM] = 9.8E-2 W (21348 RAYS) M6 --> FOLD MIRROR 3 = 9.5E-2 W (21350 RAYS) TOTAL SCATTERED POWER FROM OPTICS ONLY = W ************************************************************************ Figure 26 Scatter Contributions from Optical Surfaces (Note that Gregorian focus is located between the secondary mirror and fold mirror 1; this is why there are no contributions from the mirrors in the optical path after the secondary mirror in the upper table.) 33

35 GREGORIAN FOCUS, WITH DOME *************************** SUMMARY DATA ******************************* ANGLE FROM ZENITH = 0 DEGREES DOME SETTING (0 = OFF, 1 = ON) = 1 ANALYSIS TASK (0=DIRECT, 1=LAKE, 2=SNOW) = 0 FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 0 LYOT STOP INNER APERTURE RADIUS = 260 mm ======================================================================== TOTAL DIRECT POWER (NO SCATTER) = W TOTAL SCATTERED POWER (LEVEL 1) = E-2 W SCATTERED POWER FROM SUN = 7.684E-2 SCATTERED POWER FROM SKY = 4.836E-3 TOTAL SCATTERED POWER (LEVEL 2) = E-5 W ************************************************************************ LIST OF MOST SIGNIFICANT LEVEL 1 PATHS FOR SOURCE 1 (SOLAR DIRECT) =================================================== OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E Secondary Mirror E Primary Mirror E Primary Mirror E Entrance Aperture Front E Heat Stop Outer Cone E Entrance Aperture Front E Heat Stop Edge E-02 LIST OF MOST SIGNIFICANT LEVEL 1 PATHS FOR SOURCE 2 (INDIRECT SKY) =================================================== OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E Mount Base E Primary Mirror E OSS Frame E Lyot Stop Back E OSS Frame E Secondary Mirror Frame E Primary Mirror E Entrance Aperture Front E-03 Figure 27 Results of Stray Light Analyses of Complete ATST including Dome, Shutter, Snorkel, etc. Multiple occurrences of a scattering object in the list indicate that the object can scatter light to the image plane via multiple scatter paths. 34

36 COUDE FOCUS, WITH DOME *************************** SUMMARY DATA ******************************* ANGLE FROM ZENITH = 0 DEGREES DOME SETTING (0 = OFF, 1 = ON) = 1 ANALYSIS TASK (0=DIRECT, 1=LAKE, 2=SNOW) = 0 FOCUS POSITION (0=GREGORIAN, 1=COUDE) = 1 LYOT STOP INNER APERTURE RADIUS = 260 mm ======================================================================== TOTAL DIRECT POWER (NO SCATTER) = W TOTAL SCATTERED POWER (LEVEL 1) = W SCATTERED POWER FROM SUN =.323 SCATTERED POWER FROM SKY = 6.064E-5 TOTAL SCATTERED POWER (LEVEL 2) = E-4 W ************************************************************************ LIST OF MOST SIGNIFICANT LEVEL 1 PATHS FOR SOURCE 1 (SOLAR DIRECT) =================================================== OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E Mirror M E Mirror M E Secondary Mirror E Primary Mirror E Primary Mirror E Mirror M E Mirror M E Mirror M E Mirror M E Mirror M E Mirror M E Entrance Aperture Front E-01 LIST OF MOST SIGNIFICANT LEVEL 1 PATHS FOR SOURCE 2 (INDIRECT SKY) =================================================== OBJECTS Path Rays SumTOTAL Percent Hits Curr Prev Split/Scatter E Mount Base E Primary Mirror E Entrance Aperture Front E Primary Mirror E-05 Figure 28 Results of Stray Light Analyses of Complete ATST including Dome, Shutter, Snorkel, etc. Multiple occurrences of a scattering object in the list indicate that the object can scatter light to the image plane via multiple scatter paths. 35

37 4.4.3 Stray Light Contributions from Lake and Snow with Dome With the dome in place, light scattered by the snow can reach the telescope structure only through the six open windows arranged about the base of the dome structure, as shown in Figure 29. Consequently there is considerable shading (obscuration) of the telescope structure. Figure 29 With the Dome in Place, Scattered Light from the Snow can only reach the Telescope through the Dome Windows Figure 30 shows the results of the stray light analysis for snow illumination only; no solar or sky illumination of the telescope is included in this calculation. We did this to make it easier to identify the effects of different illumination sources. The calculation identified a handful of first-level scatter paths to Gregorian and Coude foci, but none of these paths involved any of the mirror surfaces; only telescope structures are involved. Therefore it is not surprising that the total scattered power is less than that of the mirrors, approximately 100x smaller. There are hundreds of second-level scatter paths to Gregorian and Coude foci. Rather than trying to list them all in Figure 30, we included the raw ASAP output in Appendix D. (In order to identify the scattering objects, the ASAP names are given in Appendix C.) 36