DE2825088C2 - Auxiliary system of the type of a Galileo telescope for infrared radiation - Google Patents

Description

IO

The invention relates to an attachment system of the type of a Galilean telescope for infrared radiation. In cameras for visible light designed in the manner of a Galilean telescope attachment systems are used, such. B. from the book "Applied Optics and Optical Engineering", Volume 3 by R. Kingslake, pages 120 to 121, and from US-PS 16 51 493 known. It is known to be advantageous if the attachment for the camera are arranged in this way can change the focal length of the camera lens effectively without affecting its effect or the effect of the camera. The magnification of the attachment is determined by the ratio between the focal lengths of the positive and negative ^{lenses of the attachment. The afocal property is obtained> 5} if the distance between adjacent principal planes of these lenses is made equal to the numerical difference between their focal lengths.

Optical systems for thermal infrared often use a scan of the object space, The incoming radiation first hits a movable mirror and reflects it into an objective and then focused on a detector system. The enlargement of such devices could only can be changed by changing the focal length of the lens, causing a movement of the scanning member and the Detectors with respect to each other. Otherwise space would have to be available for a A variable lens or a lens from a set of lenses with a fixed focal length.

From the book by H. Naumann "Optics for Constructors", Wilhelm Knapp Verlag, 3rd edition, 1970, pages 214 and 215, it is known that infrared lenses, inter alia. made of germanium or silicon can.

The object of the present invention is to provide an attachment system of the type of a Galilean telescope for To create infrared radiation. A first solution to this problem is in the characterizing part of claim 1 and a The second solution to this problem is given in the characterizing part of claim 2.

Such an afocal telescope can be connected upstream of an infrared optical system in order to to achieve a certain magnification or, in reverse, a reduction in size. Here it is from The advantage that the optical system upstream of which the system according to the invention is connected can be kept small can.

Some embodiments of the invention are shown in the drawing and are described in more detail below. It shows
1 shows a first embodiment of an afocal

30th

35

45

i0 infrared telescope with a magnification of 2 ^ 4: 1;

F i g. 2 the telescope according to FIG. 1 in reverse order with a magnification of 1-224;

F i g. 3a, 3b, 3c and 3d calculated correction curves for the telescope according to FIG. 1;

F i g. 4a, 4b, 4c and 4d calculated correction curves for the inverted telescope according to FIG. 2;

5 shows a second embodiment of another afocal infrared telescope with a magnification of normally 2: 1 and in reverse order of 1: 2, and

F i g. 6 calculated correction curves for the telescope according to FIG. 5 at a magnification of 2: 1.

F i g. 1 shows a reversible afocal telescope for Infrared radiation for the wavelength range from 3 to 5 μm with a magnification of 2.24: 1. The telescope is shown in association with a thermal imaging device 8 having a pyramid mirror scan drum 3 is used, which can rotate about an axis 4 which is perpendicular to the plane of the drawing and the the axis 5 of the telescope intersects the axes 4 in here and 5 containing plane, the flat pyramid facets 6 of the drum close an angle of 45 ° with the axis 5 when they are guided through this plane by the rotation of the drum. So will Infrared radiation entering along the axis 5 upwards outside the plane of the drawing into the Infrared lens 7 with an aperture of 28 mm reflects the lens 7 focuses, whereby the parallel bundle of rays falls on a line of infrared detectors (not shown), which line is parallel to the axis 5 is. The effect of the thermal imaging device 8 in decomposing the infrared image and for reconstructing a visible image corresponding to this infrared image is known and will be not described in detail; it should only be noted that scanning with a parallel beam is desirable is, because this means greater manufacturing tolerances in relation to the mutual distances of the 1 telescope, the drum and the lens can be obtained.

The telescope contains a positive meniscus lens 1 made of n-type crystalline silicon with a resistivity of more than 10 Ω · cm and a negative lens 2 made of single crystal / 7-conductive Germanium with a specific resistance of more than 5 Ω · cm. The surfaces of ordinary lenses contain a single layer of anti-reflective coating! for a minimum reflection 4.8 μm. The dimensions of both lenses are given in the table below, none being aspherical Surfaces are used.

LENS RADIUS RADIUS DICKEODER MATERIAL FREIER VALUE DISTANCE DIAMETER 1 Λ1 132.3 15.5 silicon 105.0 convex Rl 243.8 concave 49.7 air 2 Λ3 641.3 4.5 Germanium 38.5 convex Λ4 118.5 concave

5
continuation RAl) IUS-
Wi-: RT 28 25 088 β LINSi: RADIUS A pertur-
blenclc THICKNESS OR MATERIAL
DISTANCE FREIER
DIAMETER 30.3 28.0

In operation the magnification obtained is 2.24: 1 given by the ratio of the focal lengths of the two Lenses for the afocal telescope is given. The beam of rays incident from infinity points has a diameter of 62.8mm, while the i> The aperture beam behind the objective has a diameter of 28 mm, which passes through the Diameter of the infrared lens 7 is determined. The maximum resolvable spatial frequency of the lines of detectors is ΙΟ / mm and is determined by the distances of 0.050 mm between adjacent detectors certainly.

In Fig. 3a shows the modulation transfer function (Modulation Transfer Function = MTF for short) for the telescope as a function of the spatial frequency in periods / mm for three image angles in the sagittal direction (s), indicated by dotted lines in all diagrams, as well as in the tangential direction (T) , indicated by full lines in the diagrams. The MTF shown is an addition of five monochromatic modulation transfer functions weighed in the following J0 manner:

wavelength

weight

3.60 µm
4.10 µm
4.50 µm

7.74 µm
4.95 at

0.15
0.40
0.70
1.00
0.70

This creates a black body of 100K simulated, whereby it is assumed that the detectors give no more signal at 4.8 μm ("cut-off"). These Modulation transfer functions assume that the lens with which the detectors are imaged does not lead to a significant decrease in the correction.

The maximum usable angle of view in the object space is ± 5.0 °. 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 residual spherical aberration decreases as the distance between the lenses is increased. At the distance chosen in this example, the remaining aberrations are so small that the system exhibits limited diffraction with 0.95 MTF at 10 periods / mm. 3rd order coma and astigmatism are corrected by appropriately choosing the deflection of the lenses, but higher orders of coma prevent complete correction, so that at a field of view of 5 ° the MTFs are 0.90 (S) and 0.72 ( T) is. Some improvement can be obtained by focusing wipers and accepting a slight decrease in power on the axis, as shown in Figure 3 (d). 3 (b), 3 (c) and 3 (d) show the MTF change at two frequencies, namely 4 periods / mm and 10 periods / mm, as a function of the angle of view.

In Fig. 2 shows the telescope in an inverted position with respect to the thermal imaging device 8. The magnification is now 1: 2.24; the diameter of the single 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 value. The enlarged image field and the change in the position of the aperture diaphragm result in a substantial 3rd order astigmatism, with an angle of 25 ° outside the axis with optimal focus the MTF at 10 periods / mm 0.90 (s) and 0.75 (T. ) is. F i g. 4a shows the same information as FIG. 3a, but for a magnification of 1: 2.24. The angles of view of Fig. 4 (a) are 17.5 ° and 25 °. Figs. 4 (b), 4 (c) and 4 (d) show the same information as the corresponding Figs. 3 (b), 3 (c) and (3d), the two frequencies being 5 periods / mm and 10 periods / mm. In this inverted position, the telescope introduces a pincushion distortion of 12%.

Fig. 5 shows a second reversible afocal infrared telescope, also for use with a thermal imaging device, the scanning mirror 9 of which is shown only schematically. The materials that Coatings, the wavelength range and the MT 7th calculation conditions correspond to those for the telescope according to FIGS. 1 and 2. The dimensions of the two lenses on this telescope are shown below Table given:

LENS RADIUS RADIUS THICKNESS OR MATERIAL FREE

VALUE DISTANCE DIAMETER

Rl

130.546
convex

255.882
concave

10.50 ± 0.10 silicon

101.6
98.4

7th RADIUS
VALUE 28 25 088 MATERIAL 8th continuation air LENS RADIUS 501,540
convex THICK OR
DISTANCE FREIER
DIAMETER 118.713
concave 44.19
± 0.50 Germanium 2 * 3 Aperture
cover 5.0
± 0.10 air 43.6 RA 41.4 30.31

Further details regarding dimensions and performance are given in the list below:

the focal length of the germanium lens and
the refractive index of germanium = 4.0

Angle enlargement
Entrance pupil diameter
Total field of view
Wavelength range
Vignetting
Field curvature
Pincushion distortion
Surface 1 aperture diaphragm
Surface 4 aperture diaphragm
Lens 1 diameter
Lens 2 diameter

.v 2 and x- 64.0 mm
10 °

represent.

The magnification / ;; of the telescope is given by:

zero
zero
1.8%
90.0 mm
30.3 mm
105.0 mm
46.0 mm

(2)

Figure 6 shows the M.T.F. as a function of spatial frequency for the same angle of view and conditions as Fig. 3 (a). The great similarity between these two families of performance curves gives an indication of the range of magnifications over which the telescope can be used and still useful Supplies.

The curvature of field for remote objects places a limit on the range of magnifications over which the array can be used. The curvature of field, which is determined by the Petzval sum P , can be expressed as follows:

P =

/ 2 "2

(D

whereby

/ ι the focal length of the silicon lens,
/ Ji is the refractive index of silicon = 3.4,

When equation (2) is substituted in equation (1), it follows:

- (- -) = - fo, 294 - Hj-

(3)

So for (0.294 - ^) = O is the curvature of field

equal to zero, with the curvatures of the field of view of the two lenses being balanced out. The magnification that would then be obtained is 1.175, which is hardly a useful value. The table below gives values of PaI's function of m calculated from equation (3), where (\ is assumed to be normalized 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, not only is the field curvature of the image plane large, but the negative germanium lens is also strongly curved and is 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 appropriate to use a non-reversible telescope of z. B. to use m = x3 , which is removed when a large viewing angle is required.

In the arrangement according to the invention, the chromatic aberrations of the two lenses can largely be compensated due to the fact that the ratio of the Abbe numbers of silicon and Germanium equals 2.4. This ratio is within the range of magnifications considered considered appropriate. The refractive powers of the lenses are essentially the opposite of their scatter proportional.

In addition, in the arrangement according to the invention, the larger lens is made of silicon. The cost

9 10

price of this material today is only 40% of the extends. The intermediate magnification 1: I then becomes

Cost price of germanium and is therefore obtained by keeping the telescope around this axis

the expensive material only for the smaller of the two over 90 ° in bezut on the one in Fig.! depicted situation

Lenses are used, which rotates the cost price of the. Appropriate openings are then made in the

Telescope is lowered. -, lens mount as, brought to ensure that the

According to Fig. 1, the reversal movement can thereby be obtained by direct observation,

caused, <iaC the telescope around an axis that ensures that incoming infrared rays

rotates which is perpendicular to the axis 5 and this axis development can pass between the lenses.
intersects and which is located between the two lenses

For this purpose 4 sheets of drawings

Claims (3)

Patent claims: 1. Attachment system of the type of a Galileo telescope for infrared radiation, characterized in that that the positive lens element consists of a single lens made of silicon and the negative Lens member from a single lens made of germanium is formed that the focal length ratio is between 2 and 3, the magnification is about 2.24: 1 and about 1: 2 ^ 4, respectively, and that the correction state with that of an attachment system with the following data with respect to the Third-order image errors within the tolerances due to the manufacturing accuracy matches: LENS RADIUS RADIUS
VALUE THICK OR
DISTANCE MATERIAL FREIER
DIAMETER 1 R \
Rl 132.3
convex
243.8
concave 15.5 silicon 105.0 gap 49.7 air Z Λ3
A4 641.3
convex
118.5
concave 4.5 Germanium 38.5 Aperture
cover 30.3 28.0 2. Attachment system of the type of a Galileo telescope for infrared radiation, characterized in that uas positive lens element from a single lens τ from S ^.: - '^; .zium and the negative lens element is formed from a single lens made of germanium, that the focal length ratio is between 2 and 3, that the magnification is about 2 and about 0.5, respectively, and that the corrective cyst level is that of an ancillary system with respect to the third-order aberrations with the following data within the range of the manufacturing accuracy conditional tolerances: LENS RADIUS RADIUS
VALUE THICK OR
DISTANCE MATERIAL FREIER
DIAMETER 1 Al
Rl 130.546
convex
255.882
concave 10.50
± 0.10 silicon 101.6
98.4 44.19
± 0.50 air 2 A3 501.540
convex
118.713
concave 5.0
± 0.10 Germanium 43.6
41.4 Aperture
cover 30.31 air 3. Attachment system according to one of the preceding claims, characterized by such Training that it is reversible upstream of an infrared optical observation system.