Wide-field compact catadioptric telescope spanning
0.7–14 μm wavelengths
Daniel L. Marks,1 Nathan Hagen,1 Mark Durham,2 and David J. Brady1,*
1

Department of Electrical and Computer Engineering and Fitzpatrick Institute for Photonics, P.O. Box 90291,
Duke University, Durham, North Carolina 27708, USA
2

DRS Technologies RSTA, 100 North Babcock Street, Melbourne, Florida 32935, USA*Corresponding author: dbrady@ee.duke.edu
Received 12 April 2013; accepted 4 May 2013;
posted 10 May 2013 (Doc. ID 188737); published 18 June 2013

We present a wide-field compact f -1.2, f -1.6 effective illumination catadioptric telescope that spans the
wavelengths 0.7–14.0 μm. Such a telescope replaces several telescopes designed for different infrared
bands, while having a track length shorter than most single-band telescopes. Incorporated with a suitable multiband focal plane array, many wavelength bands may be imaged simultaneously in the same
instrument. We have constructed and tested prototypes of the telescopes and found the performance isnear the predicted values. © 2013 Optical Society of America
OCIS codes: (040.2480) FLIR, forward-looking infrared; (110.3080) Infrared imaging; (220.1250)
Aspherics.
http://dx.doi.org/10.1364/AO.52.004334

1. Introduction

As infrared imaging becomes more widely adopted,
there is a desire to consolidate the optics of multiple
infrared wavelength bands to minimize bulk andweight. Each band of infrared wavelengths has its
particular uses; for example, near infrared light(0.7–1.0 μm) is efficiently emitted by light emitting
diode sources, short-wave infrared (SWIR) (1.0–
1.7 μm) includes common target designation wavelengths as well as night glow, and long-wave infrared(LWIR) (8.0–14.0 μm) is useful for detecting thethermal radiation emitted by objects at terrestrial
ambient temperatures. An optical system that imaged all of these bands simultaneously could be used
in a single extremely versatile and wavelength-agile
camera. We have designed and implemented such asystem using a catadioptric design to keep the effective illumination as large as possible.

1559-128X/13/184334-09$15.00/0© 2013 Optical Society of America
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APPLIED OPTICS / Vol. 52, No. 18 / 20 June 2013

Because reflective surfaces produce no chromaticaberrations, optical instruments that are required toimage a wide wavelength range often consist of onlyreflective surfaces. Axial reflective telescopes, suchas the Cassegrain, Ritchey–Chretien, or Paul–Baker
often have large obscurations reducing the effective
illumination. The obscuration increases as the fieldof view increases. Some unusual off-axis reflective
telescope designs, such as the Schiefspiegler, introduce significant off-axis aberrations and therefore
are limited to a higher f -number. In order to avoid
introducing additional reflective surfaces that add
additional obscurations, refractive surfaces may be
added. Unfortunately, few optical materials are
sufficiently transparent over the 0.7–14.0 μm wavelength bands, and furthermore, these materials
can exhibit significant chromatic aberrations overthis range. In addition, other practical restrictions
include relatively low toxicity, which excludes cadmium and lead compounds, environmental stability,
which excludes hygroscopic materials [1], and theability to easily polish or cut the material, which excludes diamond. The most likely candidate materials

include zinc sulfide, zinc selenide, and chalcogenide
glasses. Only one material, zinc selenide, spans thetransparency range of 0.7–14.0 μm. It would seem a
difficult task to produce a wideband telescope with
only one material. However, new rugged infraredmaterials [2] are becoming available that may add
to these choices.
Other designs that span a wide bandwidth of theinfrared include a fish-eye imager [3] in the midwave infrared (MWIR) and LWIR. A refractive [4]solution studied the ability of reimaging for chromatic correction, and a Mangin-mirror catadioptric[5] solution, similar to a visible catadioptric telephotolens [6], are able to image throughout the MWIR and
LWIR using a combination of materials in a reimaging eyepiece. An interesting SWIR and LWIR design
has been devised [7] that is refractive; however, it
uses cadmium materials to obtain the diversity ofdispersion needed over the wide wavelength band.
An achromatic lens [1] achieves transparency over along wavelength range but is hygroscopic and haslarge f -number. A combination of SWIR and MWIR
designs have been explored [8] including reimaging
and refractive solutions. Another recently publisheddesign is a Mangin-mirror wideband catadioptric

design [9,10] based on an achromatization with asingle material.
A recent innovation, “origami optics,” [11,12]
achieves an imager that is simultaneously compact
and wideband. In origami optics, the beam path isfolded multiple times by reflecting-off of mirrors
fabricated into the same substrate so that the track
length is reduced. The origami optics example prototype lenses are fabricated in calcium fluoride, as
this material is both of high optical quality and compatible with single point diamond turning (SPDT).
Placing multiple optical surfaces on the same substrate has two advantages: it allows these surfaces
to be fabricated at the same time during SPDT, andbecause the SPDT can register surfaces on the same
substrate to micrometer-scale accuracy, it reduces asignificant potential source of error. Furthermore,
the all-reflective design is naturally broad band anddoes not require difficult-to-fabricate antireflection
coatings over a broad bandwidth. The primary disadvantage of the origami optics approach is the low
effective illumination because of the obscurationsof the mirrors. Because our application required thehighest possible illumination for uncooled thermal
detectors, we looked for an approach that could replace reflective surfaces with refractive surfaces but
remain broad band.
2. Broadband Infrared Telescope

Fig. 1. Layout of the broadband infrared telescope showing thefront and rear optics and ray traces of several image points.
The seven optical surfaces on the two optics are labeled by theletters A to G.

Table 1.

Surface
NumberRadius R(mm)

Thickness
(mm)

1
2
3
4
5
6
7
8
9
10
11
12
13

∞
∞
−161.7723−51.91256−77.95452−51.91256−81.62547
113.1553−651.897
∞
∞
∞
∞

∞
9
18.488258−8−17.98825
16.98825
8
9.116
1
1
2.5

Our wideband optical system is designed to be matedto a custom dual-band focal plane that contains
pixels sensitive to the SWIR (0.7–1.6 μm) wavelengths and the LWIR (8.0–14.0 μm) wavelengths. As
the LWIR sensitivity is expected to be provided by amicrobolometer array, high effective illumination is
crucial to overcome thermal noise. This 1024 × 1024
array of pixels has a minimum 10 μm pitch, so that
the Nyquist rate is a maximum of 50 cycles∕mm. The
telescope full field of view is 10°. The design achieves
70% of the diffraction limit at 25 cycles∕mm.
Furthermore, the track length of the telescope is lessthan half the focal length, allowing for a morecompact system than an equivalent refractive design. Finally, the telescope can also be refocused fromPrescription for the Wide-Field Wideband Telescope Objective

Glass
ZnSe
ZnSe
MirrorMirror
ZnSe
ZnSe
ZnSe

Inner Diameter(mm)

Outer Diameter
(mm)

25
28
0
24.5
0
0
0
0
0
0
0
0

61.05654
60.51518
54.80673
54.80673
54.80673
28
21.5
21.5
19.33909
19.33909
19.11982
17.66941

Comment
ObjectSurface G
Surface E, aspheric, annular stop
Surface A, asphericSurface C, asphericSurface A, aspheric (surface 4)
Surface F, asphericSurface B, asphericSurface D, asphericSensor window front
Sensor window rearImage

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4335

2.2335632e − 024
2.1639841e − 023
4.993244e − 024
2.1639841e − 023−3.6152856e − 020
6.165054e − 018
1.1419969e − 017−8.525799e − 021−6.1349283e − 020−1.6047943e − 020−6.1349283e − 020
4.0551586e − 017−2.9968145e − 015−6.2001605e − 015
Numbers are specified in compact scientific notation, e.g., 5.3e − 010  5.3 × 10−10 .
a6.0720412e − 012
5.8013545e − 011
1.7375931e − 011
5.8013545e − 011−6.0907837e − 010
3.5876138e − 009
1.0140208e − 008−1.875883e − 009−2.233077e − 008−7.5471012e − 009−2.233077e − 008
5.1291145e − 008−2.0043468e − 007−4.1329074e − 007
3
4
5
6
7
8
9

3.8408883e − 007
5.5157167e − 006
2.0558711e − 006
5.5157167e − 006
4.4020194e − 007−2.5385046e − 005−2.0017925e − 005−1.1843827e − 014−8.9170571e − 014−2.514486e − 014−8.9170571e − 014
4.4707929e − 012−6.2445709e − 011−1.5742625e − 010

1.3635186e − 017
9.1151044e − 017
2.5061639e − 017
9.1151044e − 017−1.8542139e − 014
5.9028499e − 013
1.3672304e − 012

α8 r16 coeff.
(mm−15 )α7 r14 coeff.
(mm−13 )α6 r12 coeff.
(mm−11 )α5 r10 coeff.
(mm−9 )
α4 r8 coeff.
(mm−7 )
α3 r6 coeff.
(mm−5 )
α2 r4 coeff.
(mm−3 )
Surface
Numberp
Aspheric Coefficientsa for the Prescription of the Wide-Field Wideband Telescope Objective zr   r 2 ∕R 1  1 − r ∕R2  α2 r 4  α3 r 6  α4 r 8  α5 r 10  α6 r 12  α7 r 14  α8 r 16
Table 2.

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10 m to infinity by adjusting the spacing between thelens and sensor. The wide-field broadband infrared
telescope layout is shown in Fig. 1, and the prescription for this telescope is specified in Tables 1 and 2,
including aspheric surface coefficients. The design
is geometrically f -1.2, but with f -1.6 effective illumination due to the secondary mirror obscuration, toachieve a 70% transmission of rays. There are seven
optical surfaces on two optical elements, both madeof zinc selenide. The two elements are the front
element, which is closer to the object, and the rearelement, which is closer to the image. In addition,
each element has two sides, the front side is closer
to the object, and the back side is closer to the image.
Finally, except for the planar front surface of thefront element, each of the surfaces contains two optical surfaces, a center circular surface, and an outerannular surface. Therefore, there are three opticalsurfaces on the front element, and four on the rearelement, which are labeled by the letters A to G in
the figure.
The path of the rays from the object to the image
are given by the following progression:

APPLIED OPTICS / Vol. 52, No. 18 / 20 June 2013

1. propagated from the object and refracted
through surface G (planar front surface of the front
optic),
2. refracted by surface E (the rear outer annulusof the front optic),
3. refracted through surface A (the front annulusof the rear optic),
4. reflected from primary mirror surface C (therear annulus of the rear optic),
5. refracted again through surface A (the front
annulus of the rear optic),
6. reflected from secondary mirror surface F (therear center of the front optic),
7. refracted by the field-flattening window
formed by surfaces B and D (the front and rear centers of the rear optic),
8. propagated through the sensor window, and
9. incident onto the sensor.
In order to maximize the utility that two elements
can provide, several different telescope designs wereincorporated. The two-reflector Cassegrain (surfaces
C and F) was used because additional reflections further obscured the clear aperture. A Mangin mirror(surfaces A and C) was incorporated to allow for better aberration control to achieve a field much wider
than a typical Cassegrain telescope. However, thediverging refractive element of the Mangin mirror
introduced chromatic aberrations. In order to compensate for this, an additional lens (surface E) was
placed on the substrate of the secondary mirror (surface F). Because this lens was the stop, it served bothto help correct chromatic and spherical aberrations
in a manner similar to a Schmidt telescope. Finally,rather than having a hole in the primary mirror, awindow for flattening the field was incorporated(surfaces B and D) because this improved the performance and cutting through the element might have

been difficult. Surface G was left as planar so that theregistration of this surface to the others was lesssensitive and it allowed for an exposed surface that
could be cleaned.
The Mangin mirror of surfaces A and C allows forbetter correction of off-axis aberrations over a simple
aspheric mirror. However, the refractive surface ofthe mirror has negative power resulting in chromaticaberration. In order to balance this, the refraction at
surface E compensates with positive power of thesame material. In doing so, aberrations may be corrected using both surfaces C and E, but the chromaticaberration is only partially corrected over the band.
To estimate the effect of the correction, we note thatthe focal length of the lens consisting of surfaces
G and E is 114.14 mm. The Mangin mirror has afocal length of 70.48 mm, but a focal length of
39.98 mm without the negative power of the refractive surface, so that the power of the refractive surface is −92.38 mm. This negative refractive power
approximately balances the positive refractive powerof 114.14 mm. Therefore, most of the net power in thesystem is due to reflection alone, with the refractive
surfaces used for aberration correction. Second, thechief ray angle incident on the sensor is 2° from normal at the edge of the field, so that any chromaticaberrations result in largely longitudinal focus shift.
The combination of these two effects serves to greatly
reduce the effects of chromatic aberrations.
The nominal short-wave performance and longwave performance is characterized by the modulation transfer function (MTF) plots of Figs. 2 and 3.
The MTF is 70% or greater than the diffraction limit
at 25 cycles∕mm for both the SWIR and LWIR wavelengths, albeit the MTF is somewhat lower than thisat the periphery of the field. The diffraction-limited
MTF is lower for LWIR than SWIR because spatial
frequencies scale inversely with the wavelength sothat the MTF curve abscissa is scaled by a factorof 10 between the two bands. Figure 4 shows the
root-mean-square (RMS) wavefront error at the exit
pupil as a function of wavelength. The performanceis nearly diffraction limited in the LWIR and aberration limited in the SWIR as one might expect from thelarge difference in wavelengths. Satisfactory imaging
performance is maintained throughout the entire
band, so this design may be used for SWIR, MWIR,
LWIR, or any combination of these. The challenges
then become fabricating broadband antireflection
coatings as well as constructing broadband sensors
for achieving compact multiband instruments.
Because the design extensively used aspheric surfaces, special attention was paid to how the surfaces
would be registered, both on the same element andbetween the two elements and the sensor. The surfaces of the same side of an element can be diamondturned together so that a high degree of registration
accuracy is possible. As for registration between the
optical surfaces on opposite sides of an element, flat
fiducial areas and beveled edges are diamond turned
onto the perimeter of the element that allow thecentration and runout of each side to be registered
to these fiducial areas. The two elements are thenactively aligned by either using an image quality
metric or an interferogram before these are potted
into place. Stray light occurring due to scatter atthe boundaries between the surfaces on the same
part was minimized by ensuring that the rays within
the field of view would not cross these boundaries.
A honeycomb baffle on the lens could be used toFig. 2. Nominal MTF of the telescope for the SWIR band.
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4337

Fig. 3. Nominal MTF of the telescope for the LWIR band.

minimize the amount of stray light from outside thefield of view.
Because the diamond turning fabrication and activealignment processes are able to achieve very tight tolerances, the element and surface decenter toleranceswere estimated as 10 μm, the element and surface
tilt tolerance as 0.01°, and the element thickness
and longitudinal placement tolerances as 20 μm.
These tolerances were considered reasonable by the
fabrication engineer. When tolerancing the telescopein the LWIR, the nominal RMS wavefront error is
0.0453 waves at 11 μm wavelength, and it is 0.0514

waves when including the expected manufacturing
errors. In the SWIR, the nominal RMS wavelength
error is 0.268 waves at 1.1 μm wavelength, or 0.428
waves when including the manufacturing errors. A
Monte Carlo analysis was also performed at the two
wavelength bands, SWIR in Fig. 5 and LWIR in
Fig. 6. The Monte Carlo MTF is shown at four field
angles: 0°, 2.8°, 4°, and 6.4°. As expected, the LWIR
is not sensitive to geometric aberrations due to its
comparatively long wavelength. The SWIR may be
expected to experience a 5%–10% reduction in MTF
at 25 cycles∕mm. While not ideal, this reduction

Fig. 4. RMS wavefront error of the telescope over the entire band 0.7–14.0 μm. With appropriate antireflection coatings, this design isusable in a combination of SWIR, MWIR, or LWIR.
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APPLIED OPTICS / Vol. 52, No. 18 / 20 June 2013

Fig. 5. Monte Carlo MTF calculation of the SWIR performance.

Fig. 6. Monte Carlo MTF calculation of the LWIR performance.

should still be satisfactory and provide useful resolution up to the Nyquist rate of the sensor array.
3. Construction and Testing

Four prototypes of the telescope were constructed.
The two elements of each telescope were fabricatedby SPDT. The surfaces were used without any

further finishing or polishing after diamond turning.
The rear optic is shown in Fig. 7 after diamondturning but before coating. An antireflection coating
for both wavelength bands was used on surfaces A, B,
D, E, and G, while gold was used as the mirror coating on surfaces C and F. Because of the relatively
high refractive index of ZnSe, high quality coatings
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Fig. 7. Photographs of the rear optic after diamond turning but before coating.

Fig. 8. Photographs of the completed telescope.

Fig. 9. LWIR sagittal and tangential MTFs of two telescopes.
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APPLIED OPTICS / Vol. 52, No. 18 / 20 June 2013

Fig. 10.

SWIR sagittal and tangential MTFs of two telescopes.

were required for sufficient light throughput andwere nontrivial to manufacture for dual-band use.
The elements were aligned using an active alignment process that optimized the image quality whenpresented with a collimated beam. A mechanicalhousing was machined that held the lens assembly
precisely relative to the sensor plane, and the focus
was varied by rotating the lens in a threaded mountto vary the sensor to lens assembly distance. A completed prototype is shown in Fig. 8.
The MTF was determined from the image of a rearilluminated narrow slit collimated by a telescope.
Two of the four telescopes were evaluated this way,serial numbers 001 and 003. Figure 9 shows fourgraphs indicating the LWIR sagittal and tangential
MTF of the two tested telescopes. As expected, thelong wavelength greatly relaxes tolerances and enables the diffraction limit to be reached. The SWIR
sagittal and tangential MTF of the two telescopes
are shown in Fig. 10. The performance, especially forthe sagittal MTF at the outer field angles for S/N 003,
is below the performance expected from the Monte
Carlo simulation of Fig. 5. Because of the tight tolerances, it is likely tilt and translation alignment
errors between the two elements that caused some ofthis performance loss, especially because the elements
are aligned primarily to optimize on-axis performanceand minimize on-axis point spread function asymmetry. Optimizing the alignment of the elements forimage quality over a wide field is a difficult task andtherefore remains a challenge for active alignment ofthese telescopes. A further problem that was encountered on the unit S/N 004 not shown here was a small
step-like discontinuity introduced into the sag of the
Mangin mirror during diamond turning. Such yieldproblems can be reduced with further refinement of
the manufacturing process.
4. Conclusion

We have designed, assembled, and demonstrated acompact, wide-field telescope capable of imaging over
the SWIR and LWIR bands. The aperture, at f -1.6
effective illumination, was chosen conservatively fora first test of the design and can be decreased if morecurved surfaces are fabricated and tighter tolerances
are held. Diamond turning several surfaces onto thesame element is a useful way to hold higher tolerances
between surfaces as well as reduce fabrication steps,
eliminating potential sources of alignment error somore aggressive tolerances can be held. In the future,these designs could be molded from chalcogenide
glasses for mass production. As seven optical surfaces
are molded onto two elements, a complex optical system could be manufactured with few components. As
sensors and optics continue to be integrated, the costsof deploying and maintaining separate systems fordifferent infrared bands can be reduced or eliminated.
Furthermore, such integration provides other opportunities for data fusion and sensor networking.
Ultimately, such multiband sensors may help exploit
the information riches available in the electromagnetic band for environmental awareness.
This project was supported by the DARPA Dual
Mode Detector Ensemble program under contract
NBCHC090032. We thank Gary Blough and BarbaraChampion of Photon Gear for the mechanicalassembly and alignment assistance. We thank Arnold
Karpel and others of Reynard Corporation for providing the elements and antireflection coatings.
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