DE2825088A1 - THERMAL INFRARED AFOCAL GALILIAN TELESCOPE - Google Patents

Description

PUB 32582

Va / VR / Oobb May 31, 1978

.V. Piii:; p- jiv .-.: ...:, '-., ^ ,. «: .. !!, Girihoven 2825088

Thermal infrared afocal Galilelian telescope.

The invention relates to an infrared afocal Galilean telescope. Such a telescope can be used as an attachment in a thermal infrared optical system.
Such afocal attachments are known for use in conventional cameras. They are described, for example, in the book "Applied Optics and Optical Engineering", Volume 2, by R. Kingslake (publisher) on pages 120-121. As is known, it is advantageous that the attachment for the camera can be arranged in such a way that the focal length of the camera lens is effectively changed without the

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Such an afocal telescope can one

Infrared optical system can be added to make a magnification to evoke a certain relationship, or in the opposite position to bring about a reduction in the same ratio.

Some embodiments of the invention are shown in the drawing and will be described in more detail below described. Show it:

Fig. 1 shows a first embodiment of an afocal infrared telescope with a magnification of x2.24;

Fig. 2 shows the telescope. 1 in an inverted position with an enlargement of xi / 2.24;

Figures 3a, 3b, 3c and 3d calculated performance curves for the telescope according to Fig. 1,

· Figures 4a, 4b, 4c and 4d calculated performance curves

for the inverted telescope according to Fig. 2,

Fig. 5 shows a second embodiment of another afocal infrared telescope with a magnification of normally x2 and in the opposite position of xi / 2, and . FIG. 6 calculated power curves for the telescope according to FIG. 5 in the x2 position.

Fig. 1 ■ a reversible afocal infrared telescope for use in the ¥ indicates avelength range of 3 to 5 / ^to a position corresponding to a magnification of x2,24. The telescope is shown in association with a thermal imaging device 8, in which a pyramid mirror scanning tronmiel 3 is used, which can rotate about an axis 4,

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which is perpendicular to the plane of the drawing and which intersects the axis of the telescope. In the plane containing the axes h and 5, the flat pyramid facets 6 of the drum enclose an angle of 45 with the axis 5 when they are guided through this plane by the rotation of the drum. Thus, infrared radiation entering along the axis .5 is reflected upwards outside the plane of the drawing into the infrared objective 7 with an aperture of 28 mm. The objective thereby focuses the parallel beam of radiation on a line of infrared detectors (not shown), which line is parallel to the axis 5. The function of the thermal imaging device 8 for decomposing the infrared image and for reconstructing a visible image corresponding to this infrared image is known and will not be described in detail; it should only be noted that scanning with a parallel beam is desirable because this results in greater manufacturing tolerances with respect to the mutual distances of the telescope, the drum and the lens can be obtained.

· The telescope contains a positive meniscus lens made of monocrystalline η-conductive silicon with a specific ¥ resistance of more than 105i..cm and a negative lens 2 made of monocrystalline n-conductive germanium with a specific resistance of more than 5-ß. «cm. The surfaces Both lenses contain a single layer of anti-reflective coating material for minimum reflection at 4.8 / µm. The dimensions of both lenses are in

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Effect of this lens or the camera is impaired. The enlargement of the attachment wii-d by the ratio between the focal length of the positive and negative lens of the attachment. The afocal property is obtained when the distance between adjacent nodes of these lenses is numerical Difference between their focal lengths is made equal.

Thermal infrared optical systems often use a scan of the object space, the entering Radiation first hits a movable mirror and is reflected into an objective system and then onto a detector system is focused. The enlargement of such devices could only be achieved by changing the focal length of the Lens system can be changed, resulting in a movement of the

- [^ The scanning element and the detectors in relation to each other with sih bx ". Otherwise space would have to be available to accommodate a rubber lens (zoom lens) or any one of a set of fixed focal length lenses. It would be advantageous to design the infrared optical system compact and attach an external afocal attachment.

The invention aims to provide an infrared afocal Galilean telescope, which is characterized is that the positive lens is made of silicon, that the negative lens is made of germanium and that the ratio between the focal length of the silicon lens and that of the germanium lens is in the range from 2 to 3.

The ratio of the focal lengths can be almost equal to the inverse ratio of the stretches of silicon and Be germanium.

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given in the table below, with no aspherical Surfaces can be used.

LENS SURFACE RADIUS mm THICK O) ER
DISTANCE mm MATERIAL FREE
APERTURE mm 1 R1
R2 132.3
convex
243.8
concave 15.5 silicon 105.0 gap 49.7 air 2 R3
Rk 641.3
convex
118.5
concave 4.5 Germanium 38.5 Aperture
cover 30.3 28.0

In operation, the magnification obtained is = x2.24, given by the ratio of the focal lengths of the two Lenses for an afocal telescope is given. The parallel input beam has a diameter of 62.8 mm, while the output beam has a diameter of 28mm which is determined by the diameter of the infrared lens 7.

The maximum resolvable spatial frequency of the rows of detectors is 10 Hz and is determined by the distances of 0.050 mm between adjacent detectors.

In Fig. 3a the modulation transfer function (Modulation Transfer Function = M.T.F. for short) for the telescope as a function of the spatial frequency in periods / mm for three Field angle in the sagittal direction (s), by dotted

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Lines indicated in all diagrams as well as in the tangential direction (τ) indicated by solid lines in the diagrams. The MTF shown is a weighted addition of five monochromatic modulation transfer functions in the following ways: Wavelength Weight

3.60 / around 0.15

4.10 / around 0.40

4.50 / around 0.70

7.7 ² I / um 1.00

4.95 / µm ■ 0.70.

This simulates a 300 K black target in a range of 2000 m, assuming that the detectors are cut-off at 4.8 / µm. * These modulation transfer functions assume that the objective lens that is used to generate the image on used on the detectors does not cause a significant decrease in performance /

The maximum field angle in the object space is + ^ 5j0 The corresponding movement of a facet of the pyramid drum is + _ 11.2 and the ratio between these two

Angles is 2.24, i.e. the magnification of the telescope.

The remaining chromatic aberration is a fraction of a wavelength. The rest of the spherical Aberration decreases as the distance between the lens elements, enlarged like 3 ^ d. With the distance chosen in this example, the remaining variations are the same little that the system has limited diffraction at 0.95 M.T.I

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at 10 periods / mm. Primary coma and astigmatism are corrected by appropriately choosing the shape of the lenses, but higher orders of the coma prevent complete correction, so that at an angle of 5 ° off-axis the MTF is 0.90 (s) and 0.7i (T). Some improvement can be obtained by refocusing and accepting some on-axis power reduction, as shown in Figure 3 (d). Figures 3 (t>)> 3 (c) and 3 (d) show the MTF change at two frequencies, namely k periods / mm and 10 periods / mm, as a function of focus.

In FIG. 2, the telescope is shown in an inverted position with respect to the thermal imaging device 8. The enlargement is now. = xi / 2, 2k; the diameter of the input beam is reduced to 12.5 mm and the maximum field angle of the object space is increased to + _ 25. With such a reduced beam diameter, the axial aberrations are negligible and high-order coma is reduced to a low level. The increased field angle and the change in the position of the

Aperture diaphragms lead to a substantial primary astigmatism > , with the MT, F. at 10 periods / mm is 0.90 (s) and 0.75 (τ). Fig. J | a shows the same information as Fig. 3a, but for the xi / 2.24 magnification. The field angles of wing 4 (a) are 17.5 and 25. Figures 4 (b), k (c) and 4 (d) show the same information as the corresponding Figures 3 (t>)> 3 (c) and 3 (d), the two frequencies being 5 periods / mm and 10 periods / mm '. In this reverse or

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xi / 2.24 position, the telescope carries a level of $ 12 Pillow distortion.

Fig. 5 shows a second reversible afocal

infrared telescope, also for use with a thermal imaging device, whose scanning mirror 9 only is shown schematically. The materials, coatings, wavelength range, and M.T.F. -Rechmmgsbedurigen correspond to those for the telescope according to FIGS. 1 and The dimensions of the two lenses of this telescope are in mentioned in the table below:

LENS Q SURFACE - RADIUS mm THICK or
DISTANCE mm J
MATERIAL free
APERTURE mm • 1 R1
R2 130,546
convex
255.882
concave IO.5O
+ 0. 10 silicon 101.6
98.4 . ■ '. ■ '. 44.19
+, O.5O air 2 R3 501,540
convex 5.00
+ 0. 10 Germanium 43.6 R4 118.713
concave 41.4 Aperture
cover • 30.31 air

Further details regarding dimensions and performance are given in the list below: ¥ angle magnification x2 and x-J

Entry pupil diameter 64.0 mm

Total field of view 10

Wavelength range 3 to 5 / µm

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Vignetting
Field curvature zero

(Pillow) Distortion 1, 85ο

Surface 1 aperture diaphragm 90.0 mm

Surface 4 aportur aperture 30.3 mm

Lens 1 diameter 105> 0 mm

Lens 2 diameter 46.0 mm.

Fig. 6 3 (which shows the MTF al ·? Function of spatial frequency for the same field angle and conditions as Figure a. The great similarity of this ^one two power curves are an indication of the range of Vergrösscrungen that the telescope can be used for · and still provides useful benefits.

The curvature of the image plane for distant objects sets the range of magnifications over which the arrangement may have been used, a limit. This curvature, which is determined by the Petzval sum P, can in this case can be expressed as follows:

^f 1 ⁿ 1 ^f 2 ⁿ 2
where f .. is the focal length of the silicon lens,

n _{1 represents} the refractive index of silicon, = 3 »4, fp the focal length of the germanium lens and n_ the refractive index of germanium, = 4.0. The magnification in the telescope is given by:

m = -—-. (2);

2
Venn substitutes equation (2) in equation (1)

will follow;

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) h -5) (3).

f hl S) h 5 1 \ 1 2/1

So for (0.294 - I ») = O the curvature is equal to zero, where sicli compensate for the field curvatures of the two lenses.

The magnification that would then be obtained is 1.175, which is hardly a useful one. The table below gives values of P as a function of m, which are calculated from equation (3), where f .. is assumed to be standardized to 1. m 1.5 2.0 2.24 2.5 3.0 3.5 P -0.075 -0.2 -0.266 -0.325 -0.45 -0.575 · above a magnification of 3.0 is not only the curvature of the The image plane is large, but the negative germanium lens is also strongly curved and difficult to manufacture. This could be corrected by increasing the distance between the lenses, but this results in a longer and more extensive telescope. With m = x2, it can be much more useful to use a non-reversible telescope of, for example, m = x3, which is removed when a large viewing angle is required.

'. It is a feature of the arrangement according to the invention, that the chromatic aberrations of the two lenses are largely offset by the fact "that the ratio of the scatter of silicon and germanium is 2.4. This ratio is within the Range of enlargements, i.e. within the range of proportions of the focal lengths that are considered appropriate. On the whole, the achievements of the

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Lenses are inversely proportional to their scatter.

Another feature of the invention

The arrangement is that the larger lens is made of silicon. The cost price of this material today is only kofi the cost price of germanium and therefore the expensive material is only used for the smaller of the two lenses, which means that the cost price of the telescope is hotly discounted.

According to Fig. 1, the reverse movement can be caused by the fact that the telescope rotates around an axis, which is perpendicular to and intersects axis 5 and which extends between the two lenses. The intermediate enlargement x1 is then obtained by moving the telescope about this axis over 90 with respect to the one shown in FIG position shown is rotated. Suitable openings will be then attached in the lens mount to achieve that the direct χ 1 observation can be obtained by causing infrared radiation to enter can be passed between the lenses.

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Claims (2)

PHB 32582 May 31, 1978 PATENT CLAIMS ; 1 . Infrared afocal Galilean.es telescope, characterized in that the positive lens consists of Silicon is made up of that the negative lens is made of germanium and that the ratio between the focal lengths the silicon lens and the germanium lens in the range of 2 to 3. 2. Telescope according to claim 1, characterized in that that the ratio of the folcallengths is almost equal to the inverse ratio of the scattering of silicon and Is germanium. 3 · Infrared optical observation system with a telescope according to claim 1 or 2, characterized in, that the telescope is reversibly attached to the observation system. 809851/0855