Compact, Three-Mirror Anastigmat, with "Reflective Lens" Paul K. Manhart K. Scott Ellis James E. Michaiski Raytheon Missile Systems Company P.O.Box 11337B1dg. 840 M/54 Tucson, AZ 85734-11337
pkmanhartccgate.hac.com ksellisccgate.hac.com jemichalskiccgate.hac.com Richard N. Pfisterer Photon Engineering, LLC P0 Box 31316 Tucson, AZ 85751
[email protected]
Abstract A uniquely packaged three-minor anastigmat (TMA) is discussed. The telescope has 5 major parts, including the optics. This compact TMA exhibits diffraction-limited performance in the MWIR and LWIR wavelength regions and operates at fast F-numbers and relatively wide fields of view. Optical surfaces and critical mechanical interfaces are single-point diamond-turned, allowing for drop-together assembly and alignment. This paper discusses the evolution of the optical/mechanical design, developed under the auspices ofthe Standard Missile Company and NAVSEA PMS422. 1 . Introduction For infrared optics it is important to use a re-imaging design with an intermediate field stop to suppress unwanted out-of-field radiation and a buried cold (Lyot) stop to limit the warm radiation seen by the detector. Many times this is done with a re-imaging catadioptric design comprised of a Cassegrain type front end and a refractive group to relay the intermediate image to the detector. Relaying an intermediate Cassegrain image with a refractive assembly compromises spectral coverage, introduces thermal defocus issues (dn/dT), has the wrong field curvature, is less radiation hard, and lowers transmission relative to a pure catoptric (reflective) design. In addition, catadioptric systems typically have more surfaces than do pure catoptric designs. Reducing the total number of warm surfaces in an JR telescope is important because each surface contributes unwanted flux on the detector, reducing signal-to-noise and integration time. An all-reflective three-minor anastigmat (TMA) offers the following optical advantages over the catadioptric relay configuration. TMAs allow the correction of three basic aberrations (spherical, coma, and astigmatism); hence, a larger field of view. The sum of the mirror powers may be zero, yielding a flat field and hence a larger field ofview on a flat focal plane array (FPA). A TMA having both a buried cold stop and an intermediate field stop is necessary for good stray light rejection. Various anastigmatic design forms with two, three, and four mirrors have surfaced over the years spanning focal, afocal, relayed, and direct-imaging telescopes12. Two common TMA design forms are shown in Figures la and lb.
The disadvantages observed in most TMAs manifest in awkward packaging andlor difficult alignment tolerances and procedures. For tube optics, such as those found in missile seekers, packaging constraints are important. This is especially true for gimbaled optics. A TMA that is off-axis in aperture, such as that SPIE Vol. 3482 • 0277-786X/98/$1O.OO 282
og
Figure la. Typical unobscured TMA (F/3).
Figure lb. Typical obscured TMA (F/4).
seen in Figure la, would yield a pupil-to-tube-diameter ratio of about 0.5. The TMA in Figure lb is more symmetrical about a common optical axis, allowing for a more equal ratio of pupil-to-tube-diameter, but
it is relatively long, and the dewar and focal plane assembly face in the wrong direction for many packaging constraints. Typically what is wanted for gimbaled tube optics is an optical telescope with real, accessible pupils, relatively symmetrical about a common axis in aperture and field, and with the focal
plane facing object space to make room for the dewar mechanism. The TMA proposed in Section 2 satisfies these conditions.
In addition to the optical advantages of an anastigmatic form, the proposed compact configuration lends
itself to a unique fabrication and alignment technique. All the optical and mechanical parts may be investment cast to near-net shape from the same material (for passive athermalization). Of the three telescopes currently being fabricated, two will be fabricated in aluminum and the third from a cast beryllium aluminum alloy. Where necessary to form critical opto-mechanical interfaces and optical figures, the parts are single-point diamond-turned. Its axial symmetry allows the mechanical datums and
optical surfaces to be precision diamond-turned in the same setup. Proper tooling design reduces accumulated errors by allowing each part to be finished on a single machine setup. A small parts count also reduces tolerance buildup. 2. Discussion: Optical System Figure 2 shows the optical layout of the proposed TMA. The front end consists of a set of concave and convex mirrors forming an intermediate image at the field stop near the vertex of the primary mirror, just inside the central hole. The fourth mirror, a flat, reverses the direction of the beam path over that shown in Figure ib, reducing the system's overall length and placing the dewar in an accessible position. The primary, secondary, and tertiary mirrors (those with optical power) are ellipsoids. The combined powers sum very nearly to zero, thereby satisfying the Petzval condition for flat field imaging.
Central to this design concept is the reflective lens, a term used to indicate that the combination of surfaces fabricated on opposite sides of the same substrate have a center thickness, wedge, and relative tilt tolerance as does a conventional lens. The all-reflecting configuration arises from a desire to increase the sensitivity of an existing catadioptric seeker by reducing the number of emissive surfaces seen by the detector. After the initial optical design
depicted in Figure 6a was established, several key issues were addressed to optimize system level performance. Among these were the change in obscuration as a function of field, stray light on the
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detector, and thermal energy from the warm housing and optics. Reducing thermal energy on the detector increases signal to noise and integration time. It was therefore necessary to revisit the original design and re-optimize the system in parallel with respect to the analytical results of the above issues.
The unique configuration of this design poses a number of constraints related to obscuration, stray light, and thermal noise as seen by the detector. Along the optical axis are 5 obscurations/holes. Sequentially, these can be seen as a secondary mirror obscuration, the central hole in the reflective lens, an obscuration (hole) in the flat mirror, a hole in the tertiary, and finally a gold dot on the dewar window. Each of these Reflective Lens
Primary (Conic)
I Cold Shield (Stop)
/
Secondary Mirror
(Conic)
Figure 2. Optical layout of folded, centered, TMA (F/l.2).
obscurations/holes is separated in optical space and hence, parallax causes a different apparent motion for
each relative to field angle. As a result, the off-axis imaging bundles are increasingly obscured. The driving obscuration in this design is the hole in the flat mirror as the light propagates from the secondary to the tertiary mirror. The hole size in the flat is determined by the numerical aperture of the imaging bundle and is therefore nearly constant for a given aperture, focal length and dewar position. To minimize its impact, it is necessary to shorten the focal length of the primary and secondary and to maximize the separation between the intermediate field stop and the flat mirror. Both of the above techniques increase the ratio of the flat to hole diameter, reducing the obscuration caused by the hole. However, too much divergence and/or too much separation between the field stop and the flat result in either an overly long system or an overly large tertiary mirror. In addition, a short focal length front end tightens tolerances and
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makes wavefront correction more difficult as surfaces become steeply curved. The tradeoffs between wavefront aberration, tolerances, obscuration, system size, and aperture constrain this design.
The configuration shown in Figure 2 was evaluated parametrically to examine RMS wavefront error as a function of F-number and field of view. Figures 3 and 4 show the results of this trade. 1-lowever, RMS wavefront error does not efficiently show the effects of obscuration as a function of field. A more
appropriate performance metric that demonstrates the impact of obscuration change is ensquared energy on a detector, because obscuration affects the distribution of energy within the point spread function. Figure 5 shows the variation of obscuration and toleranced ensquared energy on a 30-micron square pixel as a function of field for an F/1.2 system. This metric has additional significance because the fractional
obscured area of the pupil grows increasingly large as the F-number is increased, resulting in poor ensquared energy performance. It is left to the reader to examine this parametric trade space to observe and understand the intricate relations and behavior of this particular optical design.
0.2
E 0.15
FI1.0
E
0.1
—-—-—-—-
0
1(X)
2.00
1.50
2.50
3(X)
F/#
0
0.4
0.8
1.2
1.6
Semi-FOV (degrees)
Figure 3. Field averaged RMS wavefront error(lO j.im). Figure 4. RMS wavefront error @ 10 jim as a function of field and F-number. 70.00
o %Obscuration 60.00
j%Ensquared Energy
50.00 40.00 30.00 20.00 10.00
0.00
r 0.00
0.71
1.00
1.41
Semi-FOV(deg.)
2.00
Figure 5. Percent obscuration by area and toleranced ensquared energy on a 30-tm square pixel (X=10.0 jim, F/1.2).
2
3. Stray Light "Stray light" encompasses the range of light propagation mechanisms that cause unwanted light to reach the detector. These mechanisms include surface scatter from optical surfaces and baffles, ghosts formed by refractive/reflective surfaces, and thermal emissions from the components and diffraction effects.
Figure 6a. Pre-optimization layout of the TMA system. Figure 6b. Post-optimization layout of the TMA system.
The techniques used to improve the scattered light rejection are often in conflict with those used to minimize the thermal load onto the detector. (For example, a low-emissivity surface reduces the amount of thermal energy emitted, but can lead to glints and other artifacts that increase the scattered light.) In the process of simultaneously minimizing the scattered light and the thermal loading onto the detector, the impact of following parameters were considered. 1.
Length of main baffle (sun shade)
2. Shape ofprimary mirror baffle/field stop (baffle eliminated in optimized design) 3. Shape ofthe secondary baffle and unused mirror vertex 4. Reflectivity (emissivity) of front- and rear-facing field stop 5 . Curvature of the field stop 6. Shape of secondary support struts 7. Diameter of front and rear surfaces of the reflective lens 8. Diameter of secondary mirror 9. Diameter of flat mirror 10. Diameter ofprimary mirror central hole 1 1 . Taper of the primary mirror central hole 12. Reflectivity (emissivity) of the primary mirror central hole 13. Shape of the conical dewar obscuration (eliminated in optimized design)
During this optimization phase, the field stop was moved from its traditional place (in front of the primary) to the hole in the reflective lens. Thus, the conical primary baffle/field stop was replaced with a simple rectangular aperture. However, to minimize the thermal load, the side of the field stop facing
image space was made a low-emissivity surface and the side facing object space was made a highemissivity surface. While this is an unconventional approach, detailed computer analyses indicated that it made sense as a trade between stray light, thermal load, packaging, and image quality. Figures 6a and 6b show the original and optimized layouts of the system. (Only one of the three struts is shown in these figures.)
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3 . 1 Scattered Light Analyses A common measure of the scattered light rejection characteristics of a sensor system is its Point Source Transmittance (PST). While there are several definitions in use, the most common is given by
I\
irradiance at detector plane
PST(8)= irradiance at the entrance aperture Essentially, the PST is a transfer function relating the irradiance at the detector plane to that at the entrance aperture due to a distant point source located at some angle to the optical axis or line-of-sight. However, being an irradiance-based definition, its most significant limitation is that it does not convey information about the distribution of energy at the detector.
Computing the PST is a matter oftracing distributions ofrays from various off-axis angles through the sensor. Where necessary, scattered rays were generated at surfaces subject to their scatter. Total power reaching the detector is then calculated. Table 1 lists the component scatter models. Table 1. Component scatter models.
Component Mirrors (50 A JJ5)1 Ge windows (20 A RMS)1 CdTe windows (20 A RMS)1 Structures (baffles, struts, etc.) 300 level particulates3 500 level particulates3
Scatter Model Harvey b = 0.0047, s = -1.5 Harvey b = 0.0017, s = -1.5 Harvey b = 0.00053, s = -1.5 Polynomial based upon "Stinger black" measurements2 Harvey b = 0.0245, s = -1.446 Harvey b = 0.284, s = -1.284
Notes: 1.
Estimates based upon experience with similar systems.
2. "Stinger black' is a proprietary diffuse black paint used by Raytheon in its seeker systems. It produces approximately 1% Lambertian scatter when illuminated at normal incidence; like most diffuse black paints, it becomes progressively more specular as the angle of incidence increases toward grazing. 3. See Reference 3.
Figure 7 shows the PST plotted from 0 to 60 degrees off-axis in the meridional (yz) plane for three different cleanliness levels. The PST curve has 4 distinct areas of interest:
1. The direct signal reaches the detector following the as-intended trajectory out to approximately I .5 degrees, and thus dominates the PST at these angles. 2. Scatter dominates from 1.5 degrees out to approximately 2.8 degrees. 3 . From 2.8 to 4.6 degrees off-axis, light reaches the detector from a variety of stray light paths. Several mechanisms have been identified to block many ofthese paths. 4. Beyond 4.6 degrees and out to 60 degrees, the PST falls offmonotonically. Some ofthe structure is caused by the light scatter off the secondary mirror support struts.
The sun shade blocks light to the primary mirror past 63 degrees and so the PST drops off precipitously beyond this angle.
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Since the detector "looks" forward toward the secondary mirror directly as well as in reflection off of the tertiary and flat mirrors, it is not surprising that spurious paths to the detector exist at selected angles. (Many of these paths could have been eliminated by putting the primary conical baffle back into the system, at the expense of considerably higher thermal noise.) By interrogating the PST at fine angular increments, more than 20 such direct paths to the detector were identified, but in all cases the out-offocus light did not contribute significantly enough to affect the SNR budget.
Off-Axis Angle (deg)
Figure 7. PST vs. off-axis angle in the meridional (yz) plane.
3.2 Ghost Image Analysis The dewar assembly contains three refractive elements: the dewar window, (nearly co-planar with the flat), a cold filter, and the CdTe window located over the detector. These surfaces have the potential to introduce ghost images around the signal via Fresnel reflection. The surfaces of the cold filter and the detector window are too close to the detector to produce a large ghost image. However the surfaces of the dewar window, being in close proximity to the optical flat, can reflect light back towards the tertiary mirror. Some of this energy can find its way back to the detector through various mechanisms. Ghost paths involving these surfaces were analyzed in detail. An irradiance distribution at the detector shows that the signal is surrounded by a ring produced by ghosts from the two surfaces of the dewar window; the irradiance of this ring is approximately 5 orders of magnitude down from that of the signal. Towards the lower portion of the detector exists a very diffuse ghost produced by the reflection off of the primary mirror central hole and the side of the field stop facing image space. Both ghosts are entirely negligible for this application. 3 .3 Thermal Analysis
The detector, looking back through the system, sees the optical elements and structures as Lambertian thermal radiators at various temperatures and emissivities. These sources illuminate the detector, thereby reducing the overall sensitivity of the system. Table 2 lists the thermal models used in this system. Table
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3 lists the thermal contributions to the detector for the two wavelength bands. A catadioptric model is shown for comparison. Table 2. Component thermal models.
Temperature
Component Mirrors Structure (baffles, struts, etc.) Dewar walls and internal baffles Dewar Ge window Dewar cold filter and CdTe detector windows
(deg K) 300 300 90
Emissivity 0.01 1.0 1.0
270 90
0.01 0.01
Goiddot
270
0.01
Primary mirror central hole and rear-facing field stop
300
0.1
Table 3. Breakdown of (normalized) thermal power on detector.
Component Primarymirror
Thermal Power On Detector (Normalized)' Catadioptric Original Optimized 0.105
0.101 0.428
0.125
I .368
0.008
Primary mounting ring Primary central hole
-
Secondarymirror
0.108
0.151
0.130
Secondarymountingring
0.034
0.076
-
0. 1 53
0.127
0.107
0.153
-
-
-
-
1 .757
-
-
0. 1 0 1
0.620 0.101
0.285 0.101
1.484
4.863
1.0
Flat mirror
Tertiary mirror Lens 1
-
Lens2
0.226 0.225
Lens3
0.241
Lens4
0.241
Field stop (front) Field stop (rear) Field stop baffle2 Stop outer cone Struts Dewar optics and structure (including gold dot)
-
Total
0.036 0.166
-
0.0 16 0.047
Notes: 1. Normalized relative to optimized design concept. 2. In original design, thermal power from the field stop baffle is a composite of the contributions from the walls; front and back surfaces of the field stop.
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As a result of the simultaneous optimizations for scattered light and thermal loading, this system may be considered well baffled; that is, the stray light levels are dictated by the apertures, surface finish, and cleanliness of the optics and the effective use of the field and Lyot stops. This is surprising considering the unconventional design which, at first glance, should be plagued with stray light problems. Extensive computer modeling combined with good design practices have proven otherwise. 4. Opto-Mechanical Design and Fabrication
To successfully build a reflective design of this F-number and field of view care must be taken in the allocation of tolerances between optical or opto-mechanical component parts and the alignment of these components. This presupposes that both a philosophy of alignment and an alignment plan have been formulated. This plan must consider which critical relationships can be built into the parts and which tolerances require an active alignment where additional clearances are built into the parts so that their respective relationships can be modified to peak performance. In addition, one must consider fabrication technology and be judicious in the apportioning of tolerances between optical components and their support structures. This is critical because in most applications the fabrication methods, tooling, and resulting fabrication accuracies differ between the two. The alignment plan is also important because it forces one to focus on the needed test equipment and tooling required to assemble, align, and test the optical system. Opto-mechanical simplicity is a key feature of the design. A goal of the opto-mechanical design was to transfer the burden of alignment from the assembly to the fabrication process, so that only focus needs adjustment in the final assembly. This is achieved by reducing parts count, carefully structuring datums to
take into account fabrication methods and processes, and by utilizing ultra-precision fabrication technology. There are five major assembly parts, plus retaining rings, shims, and hardware. These five parts are: reflective lens, rear housing, flat mirror, secondary mirror, and secondary mirror cage (spider). The key parts and datum surfaces are indicated in Figure 8.
A' (3x)
Figure 8. Side view showingrnajor components and datums
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The principal datum surface is the front and outer perimeter of the azimuthal ring around the reflective lens. This ring is diamond-machined with both sides flat and parallel. In addition, the azimuthal ring is
cast with an undercut to reduce the potential for distorting the mirror surfaces when loaded. The secondary cage and rear housing supporting the reflective lens have three small raised pads that are machined to a high degree of flatness and coplanarity. The two parts are held together with a spring loaded locking pin. In this fashion, mounting stresses on the primary mirror element are reduced because
the load paths do not pass through the minor surface. The risk to this approach is that it relies on the ability of the machinist and machine to maintain optical tolerances.
For this telescope design a drop-together philosophy was chosen after detailed examinations of tolerance requirements and of the current state of the art in optical/opto-mechanical fabrication. This requires that the tolerances on clearances and interfaces be held in such a manner that there are no interference fits but at the same time relative clearances are small enough to maintain elementcentration tolerances to 0.002" or less. Parallelism or total indicated runout (TIR) of the mounting surfaces on both the mirrors and the which corresponds to tilts in the 125-250 microto support structures ranges from radian range. This philosophy also requires that thickness variations of each of the components along the so that vertex to vertex spacings are controlled to to optical axis be held to tolerances of The one exception to this is the position of the secondary mirror which is to within being used as a focus compensator. The position of the secondary is controlled by a nominal shim which can be modified to control the system focus position. Since the three powered surfaces of the TMA are all conies and the substrate material is Ni-plated A1Be, the fabrication method of choice is precision diamond
turning. The most critical tolerances for this F/l.2 system are angular tilts or wedges. The inherent accuracies of the diamond-turning process can be exploited by machining datums and mounting interfaces in conjunction with the turning of the optical surface. If properly employed, this can result in exceedingly small errors in concentricity and TIR of mounting surfaces with respect to the optical surface. Figures 9a and 9b show the Pro E solid model of the telescope in exploded view and final assembled form.
Figure 9a. Exploded view of major components and housing
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Figure 9b. Assembled solid model 5. Conclusion
A new packaging of an anastigmatic telescope design with a reflective lens has been proposed. It is diffraction limited for MWIR and LWIR applications. Its unique configuration allows for a new approach
to optical fabrication and alignment. The capability to produce surfaces to very high accuracy with today's precision diamond point turning machines is well documented. It is believed that this concept is among the few that has integrated this capability as a key element of the optical design philosophy. That is, the system has been opto-mechanically designed to manufacture without significant impact on optical performance. Detailed design and fabrication efforts are ongoing, the results of which will be reported on at a later date. 6. Acknowledgments The authors would like to thank the Standard Missile Company and NAVSEA PMS422 for their support in this effort and Ken Sklaroff (RMSC) for securing the funds to develop the design and build hardware. In addition, special thanks for reviewing the design configuration for uniqueness goes to Lacy Cook, Dick Buchroeder, Bob Hubert, Aden Meinel, Jose Sasian, Dave Shafer, Mike Rogers, Ken Moore, and
Kevin Thompson. The opto-mechanical design credit goes to Gary Mladjan (RMSC). Vic Wagner (RMSC) and Greg Hanauska (RMSC) did the finite element analysis and the Pro E modeling. References
1. L. G. Cook. Lens Design, (W. J. Smith editor), SPIE Press, Bellingham, 1992. 2. D. Korsch, Reflective Optics, Academic Press, San Diego, 1991.
3. Spyak, P. and Wolf, W., "Scatter from particulate contaminated mirrors. Part IV: Properties of scatter from dust for visible and far infrared wavelengths", Optical Engineering, Volume 31 #8, p. 17751784, 1992
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