A large multispectral objective

O. A. Lebedev, V. E. Sabinin, and S. V. Solka)

Scientific Research Institute for Comprehensive Testing of Optoelectronic Devices and Systems, Sosnovy Bor,Leningrad Oblast

(Submitted March 4, 2011)Opticheskiı Zhurnal 78, 24–27 (November 2011)

This paper presents the optical layout and calculated characteristics of a mirror–lens objectivewith a 600-mm aperture and a 1 : 3 relative aperture, intended to operate in the 0.4–10-µmspectral region. c© 2011 Optical Society of America.

Along with radar, space-monitoring systems ordinarilyuse optical systems that are intended to operate in compar-atively narrow sections of the spectrum and that cover thewavelength range of 0.8–13 µm as a whole.1 The requirementson such optical systems are outlined in Refs. 1,2. There arevarious approaches to the design of such systems. The maindifference consists of the following: either a separate opticalsystem is created for each spectral region, or one systemoperates in several ranges. Reference 2 presents two opticallayouts of large (aperture 500 mm), fast (relative aperture1 : 1.2) objectives for operation in the spectral regions 1.8–2.6and 3.2–4.2 µm. Such an approach makes it possible to achievehigh technical characteristics and to obtain good image quality,but it is very expensive in this case to produce the objectives.

We have developed an objective with aperture 600 mm,relative aperture 1 : 3, and field of view 1◦, operating inthe spectral region 0.4–10 µm. The optical layout of theobjective is shown in Fig. 1. The objective is a modifiedRitchey–Chretien layout with a lens corrector. The screeningfactor is 0.3. A modified layout is used in order that thedeviations of the mirror parameters from the theoretical valueswill introduce coma that compensates the coma introduced bythe lens corrector.3 The corrector is a one-lens device and,unlike known correctors,3,4 is not afocal.

In shape, the corrector is made in the form of a meniscuswith its convex surface turned toward the secondary mirror.The meniscus is located a small distance (0.03–0.06 of thefocal length) from the image plane and possesses a small (from−2.8 to−3.4) negative focal power for correcting astigmatismand curvature of the image surface. The longitudinal andtransverse chromatic aberrations introduced by the single-lenscorrector in the working spectral regions (the window oftransparency of the atmosphere) are insignificant. Thus, fora lens corrector made from BaF2, the residual longitudinalchromatism introduced by the corrector in the spectral range0.4–0.8 µm is 8 µm. The residual transverse chromatismfor a field-of-view angle of 1◦ is 16 µm. The residualdistortion of the entire optical system does not exceed 1%. Theeffective correction of the aberrations enumerated above madeit possible to increase the effective diameter with a minimumlength of the optical system. This appreciably increased thespeed and relative aperture of the objective with a substantiallybroadened field-of-view angle—a result that was not attainableearlier.

The use of a single-lens corrector makes it possible to usematerials to fabricate it that are transparent in a wide spectral

600

179

390.4

471.2420

52

1

F

2

3

FIG. 1. Optical system of the telescope objective. 1 and 2—Mirrors,3—single-lens corrector (dimensions given in mm).

region without depositing additional antireflection coatings.In our case, BaF2 was used. Such an approach allows us touse one objective to operate in several spectral regions bychanging the radiation detectors, depending on the problemsand observation conditions, or by using spectral splitters.

Table 1 shows the diameters of the circles of confusionand transmittances for the spectral ranges 0.4–0.8, 1.8–2.7,3.2–4.2, 4.5–5.3, and 8–10 µm at the center and over the field.Figures 2 and 3 show graphs of the geometrical aberrations forthe spectral ranges 0.4–0.8 and 8–10 µm.

The displacements of the image plane when the tem-perature varies within the limits ±40◦ and the items ofthe objective housing are fabricated from Invar, titanium, oraluminum are 0.4, 2.4, or 6.7 mm, respectively. This canbe compensated by displacing the secondary mirror. Thedisplacement will be greatest when the objective housingis fabricated from aluminum and will equal 0.6 mm. Thediameter of the circle of confusion in this case varies by nomore than 5%.

709 J. Opt. Technol. 78 (11), November 2011 1070-9762/2011/110709-03/$15.00 c© 2011 Optical Society of America 709

TABLE 1. Diameters of circle of confusion and transmittances for various spectral ranges.

Diameter of circle of confusion for 83.8% energy concentration, mm

Spectral range, µm Center of field of view, ω = 0◦00′ Edge of field of view, ω = 0◦30′ Transmittance neglecting central screening

0.4–0.8 0.011 0.031 0.681.8–2.7 0.026 0.039 0.743.2–4.2 0.041 0.049 0.854.5–5.3 0.053 0.060 0.878.0–10.0 0.098 0.159 0.82

H

dYdY

dX dX

H HY1

Y2

Y0

Y1.Z s

Z m

dis

0.02

0.020.05 0.005

0.020.005

(a) (b) (c)

(d) (e) (f)

mm

mm

mm

mm

mm

mm

2

FIG. 2. Graphs of geometrical aberrations for the range 0.4–0.8 µm. (a) Spherical aberration for three wavelengths: 0.546 µm—curve Y ′0, 0.4 µm—curve Y ′1,0.8 µm—curve Y ′2; (b and c) field aberrations for field angles in object space ω = 0.35◦ and ω = 0.5◦, respectively (linear field 2Y ′ = 32 mm); (d) distortion;(e) components of astigmatism Z′m (meridional) and Z′s (sagittal); (f) transverse chromatism for the first and second wavelengths—Y ′1,2.

HHH

Y1

Y2

Y0

dYdY dXdX

0.02 0.020.02

Y1.2Z s Z mdis

0.02 0.010.05

mm

mm

mm

mm

mm

mm

(a) (b) (c)

(d) (e) (f)

FIG. 3. Graphs of geometrical aberrations for the range 8–10 µm. (a) Spherical aberration for three wavelengths: 9 µm—curve Y ′0, 8 µm—curve Y ′1, 10 µm—curveY ′2; (b and c) field aberrations for field angles in object space ω = 0.35◦ and ω = 0.5◦, respectively (linear field 2Y ′ = 32 mm); (d) distortion; (e) componentsof astigmatism Z′m (meridional) and Z′s (sagittal); (f) transverse chromatism for the first and second wavelengths—Y ′1,2.

710 J. Opt. Technol. 78 (11), November 2011 Lebedev et al. 710

Experience at our plant in designing and fabricating largeobjectives shows that the telescope’s primary and secondarymirrors can be fabricated from glass–ceramic or Zerodur witha high degree of lightening. Lassel-type unloading of theprimary mirror makes it possible to carry out scanning overthe azimuth (0–360◦) and the tilt angle (0–90◦). A reflectivecoating of the mirrors (aluminum with a shield) makes itpossible to obtain 85% reflection in the visible and 96% inthe IR regions.

ACKNOWLEDGMENT

A request for patent No. 2010152188 from 12/20/2010has been granted for an optical system with the compensatordescribed above.

a)Email: solk@sbor.net

1E. A. Grishin, V. L. Milovidov, and V. D. Shargorodskiı, “The current stateof IR systems for observing space objects from the earth,” Prib. Tekh. Eksp.No. 1, 3 (1999).

2N. I. Potapova, A. D. Starikov, and A. D. Tsvetkov, “Fast mirror-lensobjective for the infrared region,” Opt. Zh. 70, No. 4, 76 (2003). [J. Opt.Technol. 70, 284 (2003)].

3N. N. Mikhel’son, Optical Telescopes. Theory and Design (Nauka, Moscow,1976).

4V. N. Churilovskiı, Theory of Chromatism and Third-Order Aberrations(Mashinostroenie, Leningrad, 1968).

711 J. Opt. Technol. 78 (11), November 2011 Lebedev et al. 711