rus
Issue #2/2016
A.Medvedev, A.Grinkevich, S.Knyazeva
Objective Athermalization Of Sighting And Observation Systems As An Instrument To Ensure Functioning Of Armor And Tank Weapons
Objective Athermalization Of Sighting And Observation Systems As An Instrument To Ensure Functioning Of Armor And Tank Weapons
Operating conditions of military equipment over a wide temperature range (from -50 to 50 °C) dictate strict requirements to the design of optical imaging devices. The paper presents complex of design techniques, which enables athermality of optical parts of the sighting and observation instruments and systems of armored vehicles, allowing to ensure the effective functioning of armor and tank weapons under conditions of a significant temperature differential.
Теги: armor and tank weapons athermality of objective sighting and observation instruments атермальность объектива боевое и танковое вооружение и техника прицельно-наблюдательные приборы
Retention of image high quality within wide range of operating temperatures is the most important requirement to the optical part of any device, which allows its trouble-free functioning under the conditions of heat and frost.
As a rule, all optical calculations are performed for normal climatic conditions, which are characterized by the temperature of 20 °C and pressure of 1 atm. However, the stringent requirements of standards with regard to military hardware normalize the operating temperature range within the limits from –50 °C to 50 °C as minimum. It follows from here that the difference of above-zero temperatures relating to the specified temperature is only 30 °C, whereas the difference of below-zero temperatures is much higher – all 70 °C. In this regard, the problem of operability of the constructions of optical and optical-mechanical units of devices and sights under the conditions of below-zero temperatures arises. In greater degree this refers to the device objectives as the systems, which image the objects on photo-sensitive platforms of different photodetectors or operate in purely visual systems of surveillance and sighting.
It should be noted that under the actual operating conditions of sighting and observation devices the temperature range can be significantly greater. Maximum and minimum temperatures, which have been practically registered, are 63 °C [1] and –91.2 °C [2] but it is commonly assumed that the range of 100 °C (± 50 °C) and 90 °C (from – 40 °C to 50 °C) in some countries is the operating temperature interval in relation to the design of the facilities of AAFV.
In case of temperature variation, the geometry of optical elements varies in accordance with the coefficients of thermal expansion of the material, which they are made of. Also, the refractive index of the majority of used optical materials varies significantly [3].
As a result, the image of infinitely distant object is shifted at some value, and when this shift exceeds the permissible depth of focus the necessity of its compensation will occur. Such situation is mostly typical for high-aperture systems having the minimum depth of focus and primarily topical for thermal imaging systems with uncooled photodetectors, in which the aperture ratio of optics determining the vision range to a large degree is selected as the highest parameter.
It is typical that the shift compensation for high-aperture objectives of sighting and observation devices operating within visible and infrared spectral regions can be provided using the thermal compensator as individual constructive element due to the insignificance of image plane shift upon the variation of ambient temperature. Such method is used in well-known sighting complexes TO1-KO1 and TKN-4S.
One more method for the supply of thermal stability of objectives of optoelectronic devices consists in the availability of the mechanisms of manual or automatic focusing intended for the shift compensation of plane of the best arrangement (PBA) upon the temperature difference and maintenance of image estimated quality.
But any mentioned method results in the construction complication due to the use of additional elements (micro-electromotors, mechanical assemblies with running blocks), operability of which within wide temperature interval is also problematical.
In the pursuit of simplicity and reliability of military hardware optics during the design of optoelectronic devices, the development of so-called athermal optical units gained widespread; in these units the main goal is to obtain the constructive solution of optical scheme, in which the combination of optical elements, their materials and configuration will allow compensating the temperature withdrawals of PBA and retain its concurrency with the plane of photodetector sensitive elements, using the methods of optical calculation.
Let us consider one of such merely optical solutions – optical scheme of 50 millimeter objective with the linear field of view of 14 mm, in which PBA withdrawal is minimized at the expense of optical components only.
Such scheme did not come easily but it turned out to be quite simple and containing only three optical parts and one aspheric surface (Fig. 2). But particularly with such optical construction the photodetector plane remains in the plane of the best arrangement, and the quality of optical image practically varies to insignificant extent.
Constructive data of aspheric athermal objective at the temperature of +20 °C is given in Table 1. Alteration of constructive data of aspheric athermal objective at the temperature of minus 40 °C is given in Table 2.
The circles of diffusion and graphs of contrast characteristics at the operating temperatures of +20 and minus 40 °C are shown in Fig. 3 and Fig. 4 respectively. It is seen from the graphs that the objective has image with high quality along the whole field of view, which practically concurs with diffraction image. Insignificant decrease of sagittal component of the edge of the linear field of view of 14 mm at the temperature of minus 40 °C does not play great role.
Objective athermality and image high quality are achieved at the expense of the use of special import infrared glasses IG4 and IG6 and one aspheric surface of high order.
Further activities were performed in the area of simplification of the objective construction with regard to the elimination of asphere and operability at below-zero temperatures to minus 50 °C without the loss of image quality. For this purpose, the method of temperature compensation by mechanical components of optical units was used with the application of so-called "cup" method.
It allowed not only expanding the temperature range but also simplifying the optical system because the combination of body material and "cup" grants additional capabilities for athermalization even with the simple optical system.
The variant of optical scheme of such thermal imaging objective without aspheric surfaces has the following form (Fig. 5). Constructive data of objective is given in Table 3. The objective length from the first surface to the image plane is equal to ~ 110 mm at the linear field of view of 13.6 mm.
Objective parameters at the estimated temperature of 20 °C are very high – circles of diffusion given in Fig. 6 do not exceed the dimensions of Airy disk.
Temperature variation practically does not influence on the objective quality; the value of back focal distance – S′f ′ (or air gap from the last objective lens to the protective glass of photodetector) varies only. The plane of the best arrangement simply "floats away".
Besides, it "floats’ within the significant limits. Alteration of the back focal distance within the usual range of operating temperature of one hundred degrees varying from normal temperature of 20 °C to ± 50 °C is specified in Table 4.
Thus, in case of the temperature growth PBA is shifted towards the side of decrease of back focal distance by 0.23 mm, whereas in case of the temperature fall the air gap from the last objective surface to the protective glass of photodetector increases by 0.54 mm. Photodetector must "move aside" from the last objective lens at very significant distance.
The variant of mechanical temperature compensation of PBA "withdrawal" is shown in Fig. 7. In this variant of the solution, the fastening end of body concurs with the peak of the last surface of the last objective lens. The "cup" in which photodetector is fastened is connected to the body. With such fastening of photodetector the change of linear dimensions of the "cup" will take place in the direction, which is opposite to the change of body linear dimensions at the temperature variation. Thus, the "cup" plays the role of thermal compensator. It is seen from the figure that:
d6 = A – B.
Evaluation of the potential of the selection of material combination, the temperature coefficients of linear expansion of which would allow the concurrence of PBA with photodetector plane at the expense of coefficient difference, gave the following results.
In accordance with Figure 7 and values of temperature coefficients of linear expansion of the most common construction materials, in order to achieve the expected result it is necessary to choose the material with minimum coefficient of linear expansion for the size A and materials with maximum coefficient of linear expansion – for the size B.
Such combination can be made of titanium, which is commonly used in instrument engineering, in relation to body part and polyamide or fluoroplastic-4 in relation to thermal compensator. Their temperature coefficients of linear expansion kt are equal to 8.6Ч10–6 °C-1 (titanium) and 110Ч10–6 °C-1 (polyamide or fluoroplastic-4).
Linear extension Δl is calculated on the basis of the formula:
Δl = L · kt · ΔT,
where
L is the part length by bearing surfaces;
kt is the temperature coefficient of linear expansion of part material;
ΔT is the range of temperature variation.
Working with the temperature interval from +20 to minus 50 °C and having assumed the size A of body part at +20 °C as arbitrary size, for example, equal to 100 mm, we will obtain the decrease of its length by Δl = 0.0602 mm.
In this case, at the temperature of +20 °C the thermal compensator must have size В = 87.1 mm and its Δl will be 0.67067 mm.
Thus, at the temperature of minus 50 °C the size A will be equal to 99.9398 mm and the size B – 86.42933 mm. As a result, the air gap d6 will increase and obtain the value of 13.51046 mm instead of the required value of 13.44 mm (Table 4). In order to remove the obvious "overcompensation" and provide the required value of the air gap d6, it is necessary to decrease the body length and thermal compensator length by selection.
However, such constructions are capable to perform shifts with insignificant value, as a rule. In the cases of more intense defocusing of optical system at the temperature variations when greater values of shift are required for the compensation of PBA withdrawal, the relevant material for the production of compensation element cannot be selected.
When providing the capability of adjustment of PBA position within large limits, it is also necessary to take into account the spread of the values of coefficient of linear expansion in various delivery batches of the material, which the compensation elements are made of.
In order to solve the set task, the construction of three-lens infrared objective shown in Fig. 8 was suggested.
The objective contains lenses 1, 2 and 3. The lens 3 is installed in its own mounting with the capability of shift along the optical axis in relation to the body 5 of optical system. The assembly of temperature compensation includes the compensation element 9 made of the material with the coefficient of linear expansion, which considerably differs from the body material and is installed in parallel with the optical axis of objective with elastic one-way stop into the mounting 7. On the other end of the compensation element rigidly fixed on the mounting 6, the longitudinal slots are formed. The second compensation element consisting of two components 10 and 11 is installed between the body 5 and photodetector 4. The component 10 is made of the material with the coefficient of linear expansion, which significantly differs from the material of body 5, and the second component 11 – of the material with the coefficient of linear expansion, which is equal to the expansion coefficient of body material. Two-component compensation element is located in parallel with optical axis and elastically pressed to photodetector mounting 8 with one end, to objective body 5 through fixed stop – with the other end. The elastic element 13 is installed between the mountings of the third lens 7 and photodetector 7 with the observance of the following ratios:
d4 = (0,5 ч 1,5) · fоб;
Ln ≤ 0,5 · L1k;
10 ≥ n ≥ 2,
where
d4 is the air gap between the second and third lenses;
fob is the focal distance of objective;
Ll is the length of longitudinal slot in the first compensation element;
L1c is the length of the first compensation element;
n is the number of longitudinal slots in the first compensation element.
In the suggested optical scheme of objective the parameter d4 is comparable with the objective focal distance and selected from the ratio:
d4 = (0,5 ч 1,5) · fоб,
where
d4 is the air gap between the second and third lenses;
fob is the focal distance of objective.
The longitudinal slots formed on the compensation element 9 are selected from the following ratios:
Ln ≤ 0,5 · L1k;
10 ≥ n ≥ 2,
where
Ll is the length of longitudinal slot in the first compensation element;
L1c is the length of the first compensation element;
n is the number of longitudinal slots in the first compensation element.
In case of temperature variation, the length of compensation element 9 changes, and as a result of the difference of the coefficients of linear expansion of the materials of body 5 and compensation element 9 the alteration of relative position of lens 3 occurs compensating the major part of mismatch of PBA and photodetector 4.
The longitudinal slots executed on the rigidly fixed end of compensation element 9 provide the bore diameter of this element with spring-type properties at the areas equal to the length of slots and exclude the cracking of rigidly fixed end of compensation element 9 due to the alteration difference of diametric dimensions of conjugated components (body 5, mounting 6 and compensation element 9) in case of temperature variation.
In order to reach complete compensation (compensation of remained minor part of mismatch of PBA and photodetector 4), the second compensation element consisting of two components 10 and 11 is installed between the body 5 and photodetector 4.
When the temperature reaches minus 50 °C in accordance with the optical estimation, the objective PBA shifts from the initial position by maximum value increasing the back focal distance of objective by ~ 1.27 mm.
If titanium (type VT-1) is used in the capacity of the material of body 5, mountings and rings 6, 7, 8, 11, 12, complete shift of PBA which must be compensated by components 9 and 10 will be equal to 1.364 mm.
It can be achieved in case of the selection of fluoroplastic F-4 for the first compensation element 9 and component 10 with the length of component 9 equal to ~ 120.4 mm and length of component 10 equal to 47 mm.
The total value of compensation shift of PBA will be 1.364 mm providing the concurrence of objective PBA with the plane of photodetector 4 in case of temperature drop to minus 50 °C.
In case if the ambient temperature is +20 °C corresponding to the normal climatic conditions, the coefficient of objective contrast transfer has the following values:
for the point on axis
(diffraction quality) ....................... CCT = 45.3%
for the point on axis
(aberration quality) ........................ CCT = 42.1%
for the field point 6 mm from
the image center ........ CCTМ = 39.6%, CCTС = 39.4%
In case if the ambient temperature is minus 50 °C corresponding to the minimum set temperature for operating conditions:
for the point on axis
(diffraction quality) ....................... CCT = 45.4%
for the point on axis
(aberration quality) ....................... CCT = 42.0%
for the field point 6 mm from
the image center ...... CCT М = 36.9%, CCT С = 37.4%
As it is seen from the calculations, in case of variation of ambient temperature the infrared objective provides the concurrence of the plane PBA with photodetector plane keeping the permissible quality of image, which is close to diffraction image, for the optoelectronic devices using the micro-bolometric arrays in the capacity of photodetectors with the pixel size up to 17 µm.
However, the material, which the compensation elements 9 and 10 are made of, has the spread of the coefficients of linear expansion in the area of temperatures from minus 10 to + 20 °C, which is regulated by GOST 10007–80 and equal to (80ч250) Ч10–6. The calculations described above are based on the average value of the coefficient, which is assumed to be equal to 165Ч10–6, and length of component 10, which is equal to 47 mm. In case of the deliveries of material for the first compensation element 9 and component 10 of the second compensation element, the deviations of the values of coefficient of linear expansion from the average value are possible in different delivery batches, and the compensation of these deviations can be provided by the change of the length of component 10 and corresponding change of the length of component 11 with the retention of their total length:
L2к – 1 + L2к – 2 = const,
where
L2c-1 is the length of component 10;
L2c-2 is the length of component 11.
The increase (or decrease) of the length L2c-1 of component 10 and respectively the decrease (or increase) of the length L2c-2 of component 11 with the retention of their total length allow eliminating the influence of the spread of the coefficients of linear expansion in the materials of compensation elements 9 and 10, which the different delivery batches of these materials have, using the simple method of the selection of lengths of elements 10 and 11.
Determination of the length of component 10 and corresponding length of component 11 is performed once for every material batch using the method of collimating measurement of focusing "undercompensation" or "overcompensation" under the conditions of cold chamber of minus 50 °C.
Thus, described complex of constructive methods allows providing the athermality of optical parts of observation and sighting devices, enhances their operational properties and provides the object survivability under the actual conditions of operation.
As a rule, all optical calculations are performed for normal climatic conditions, which are characterized by the temperature of 20 °C and pressure of 1 atm. However, the stringent requirements of standards with regard to military hardware normalize the operating temperature range within the limits from –50 °C to 50 °C as minimum. It follows from here that the difference of above-zero temperatures relating to the specified temperature is only 30 °C, whereas the difference of below-zero temperatures is much higher – all 70 °C. In this regard, the problem of operability of the constructions of optical and optical-mechanical units of devices and sights under the conditions of below-zero temperatures arises. In greater degree this refers to the device objectives as the systems, which image the objects on photo-sensitive platforms of different photodetectors or operate in purely visual systems of surveillance and sighting.
It should be noted that under the actual operating conditions of sighting and observation devices the temperature range can be significantly greater. Maximum and minimum temperatures, which have been practically registered, are 63 °C [1] and –91.2 °C [2] but it is commonly assumed that the range of 100 °C (± 50 °C) and 90 °C (from – 40 °C to 50 °C) in some countries is the operating temperature interval in relation to the design of the facilities of AAFV.
In case of temperature variation, the geometry of optical elements varies in accordance with the coefficients of thermal expansion of the material, which they are made of. Also, the refractive index of the majority of used optical materials varies significantly [3].
As a result, the image of infinitely distant object is shifted at some value, and when this shift exceeds the permissible depth of focus the necessity of its compensation will occur. Such situation is mostly typical for high-aperture systems having the minimum depth of focus and primarily topical for thermal imaging systems with uncooled photodetectors, in which the aperture ratio of optics determining the vision range to a large degree is selected as the highest parameter.
It is typical that the shift compensation for high-aperture objectives of sighting and observation devices operating within visible and infrared spectral regions can be provided using the thermal compensator as individual constructive element due to the insignificance of image plane shift upon the variation of ambient temperature. Such method is used in well-known sighting complexes TO1-KO1 and TKN-4S.
One more method for the supply of thermal stability of objectives of optoelectronic devices consists in the availability of the mechanisms of manual or automatic focusing intended for the shift compensation of plane of the best arrangement (PBA) upon the temperature difference and maintenance of image estimated quality.
But any mentioned method results in the construction complication due to the use of additional elements (micro-electromotors, mechanical assemblies with running blocks), operability of which within wide temperature interval is also problematical.
In the pursuit of simplicity and reliability of military hardware optics during the design of optoelectronic devices, the development of so-called athermal optical units gained widespread; in these units the main goal is to obtain the constructive solution of optical scheme, in which the combination of optical elements, their materials and configuration will allow compensating the temperature withdrawals of PBA and retain its concurrency with the plane of photodetector sensitive elements, using the methods of optical calculation.
Let us consider one of such merely optical solutions – optical scheme of 50 millimeter objective with the linear field of view of 14 mm, in which PBA withdrawal is minimized at the expense of optical components only.
Such scheme did not come easily but it turned out to be quite simple and containing only three optical parts and one aspheric surface (Fig. 2). But particularly with such optical construction the photodetector plane remains in the plane of the best arrangement, and the quality of optical image practically varies to insignificant extent.
Constructive data of aspheric athermal objective at the temperature of +20 °C is given in Table 1. Alteration of constructive data of aspheric athermal objective at the temperature of minus 40 °C is given in Table 2.
The circles of diffusion and graphs of contrast characteristics at the operating temperatures of +20 and minus 40 °C are shown in Fig. 3 and Fig. 4 respectively. It is seen from the graphs that the objective has image with high quality along the whole field of view, which practically concurs with diffraction image. Insignificant decrease of sagittal component of the edge of the linear field of view of 14 mm at the temperature of minus 40 °C does not play great role.
Objective athermality and image high quality are achieved at the expense of the use of special import infrared glasses IG4 and IG6 and one aspheric surface of high order.
Further activities were performed in the area of simplification of the objective construction with regard to the elimination of asphere and operability at below-zero temperatures to minus 50 °C without the loss of image quality. For this purpose, the method of temperature compensation by mechanical components of optical units was used with the application of so-called "cup" method.
It allowed not only expanding the temperature range but also simplifying the optical system because the combination of body material and "cup" grants additional capabilities for athermalization even with the simple optical system.
The variant of optical scheme of such thermal imaging objective without aspheric surfaces has the following form (Fig. 5). Constructive data of objective is given in Table 3. The objective length from the first surface to the image plane is equal to ~ 110 mm at the linear field of view of 13.6 mm.
Objective parameters at the estimated temperature of 20 °C are very high – circles of diffusion given in Fig. 6 do not exceed the dimensions of Airy disk.
Temperature variation practically does not influence on the objective quality; the value of back focal distance – S′f ′ (or air gap from the last objective lens to the protective glass of photodetector) varies only. The plane of the best arrangement simply "floats away".
Besides, it "floats’ within the significant limits. Alteration of the back focal distance within the usual range of operating temperature of one hundred degrees varying from normal temperature of 20 °C to ± 50 °C is specified in Table 4.
Thus, in case of the temperature growth PBA is shifted towards the side of decrease of back focal distance by 0.23 mm, whereas in case of the temperature fall the air gap from the last objective surface to the protective glass of photodetector increases by 0.54 mm. Photodetector must "move aside" from the last objective lens at very significant distance.
The variant of mechanical temperature compensation of PBA "withdrawal" is shown in Fig. 7. In this variant of the solution, the fastening end of body concurs with the peak of the last surface of the last objective lens. The "cup" in which photodetector is fastened is connected to the body. With such fastening of photodetector the change of linear dimensions of the "cup" will take place in the direction, which is opposite to the change of body linear dimensions at the temperature variation. Thus, the "cup" plays the role of thermal compensator. It is seen from the figure that:
d6 = A – B.
Evaluation of the potential of the selection of material combination, the temperature coefficients of linear expansion of which would allow the concurrence of PBA with photodetector plane at the expense of coefficient difference, gave the following results.
In accordance with Figure 7 and values of temperature coefficients of linear expansion of the most common construction materials, in order to achieve the expected result it is necessary to choose the material with minimum coefficient of linear expansion for the size A and materials with maximum coefficient of linear expansion – for the size B.
Such combination can be made of titanium, which is commonly used in instrument engineering, in relation to body part and polyamide or fluoroplastic-4 in relation to thermal compensator. Their temperature coefficients of linear expansion kt are equal to 8.6Ч10–6 °C-1 (titanium) and 110Ч10–6 °C-1 (polyamide or fluoroplastic-4).
Linear extension Δl is calculated on the basis of the formula:
Δl = L · kt · ΔT,
where
L is the part length by bearing surfaces;
kt is the temperature coefficient of linear expansion of part material;
ΔT is the range of temperature variation.
Working with the temperature interval from +20 to minus 50 °C and having assumed the size A of body part at +20 °C as arbitrary size, for example, equal to 100 mm, we will obtain the decrease of its length by Δl = 0.0602 mm.
In this case, at the temperature of +20 °C the thermal compensator must have size В = 87.1 mm and its Δl will be 0.67067 mm.
Thus, at the temperature of minus 50 °C the size A will be equal to 99.9398 mm and the size B – 86.42933 mm. As a result, the air gap d6 will increase and obtain the value of 13.51046 mm instead of the required value of 13.44 mm (Table 4). In order to remove the obvious "overcompensation" and provide the required value of the air gap d6, it is necessary to decrease the body length and thermal compensator length by selection.
However, such constructions are capable to perform shifts with insignificant value, as a rule. In the cases of more intense defocusing of optical system at the temperature variations when greater values of shift are required for the compensation of PBA withdrawal, the relevant material for the production of compensation element cannot be selected.
When providing the capability of adjustment of PBA position within large limits, it is also necessary to take into account the spread of the values of coefficient of linear expansion in various delivery batches of the material, which the compensation elements are made of.
In order to solve the set task, the construction of three-lens infrared objective shown in Fig. 8 was suggested.
The objective contains lenses 1, 2 and 3. The lens 3 is installed in its own mounting with the capability of shift along the optical axis in relation to the body 5 of optical system. The assembly of temperature compensation includes the compensation element 9 made of the material with the coefficient of linear expansion, which considerably differs from the body material and is installed in parallel with the optical axis of objective with elastic one-way stop into the mounting 7. On the other end of the compensation element rigidly fixed on the mounting 6, the longitudinal slots are formed. The second compensation element consisting of two components 10 and 11 is installed between the body 5 and photodetector 4. The component 10 is made of the material with the coefficient of linear expansion, which significantly differs from the material of body 5, and the second component 11 – of the material with the coefficient of linear expansion, which is equal to the expansion coefficient of body material. Two-component compensation element is located in parallel with optical axis and elastically pressed to photodetector mounting 8 with one end, to objective body 5 through fixed stop – with the other end. The elastic element 13 is installed between the mountings of the third lens 7 and photodetector 7 with the observance of the following ratios:
d4 = (0,5 ч 1,5) · fоб;
Ln ≤ 0,5 · L1k;
10 ≥ n ≥ 2,
where
d4 is the air gap between the second and third lenses;
fob is the focal distance of objective;
Ll is the length of longitudinal slot in the first compensation element;
L1c is the length of the first compensation element;
n is the number of longitudinal slots in the first compensation element.
In the suggested optical scheme of objective the parameter d4 is comparable with the objective focal distance and selected from the ratio:
d4 = (0,5 ч 1,5) · fоб,
where
d4 is the air gap between the second and third lenses;
fob is the focal distance of objective.
The longitudinal slots formed on the compensation element 9 are selected from the following ratios:
Ln ≤ 0,5 · L1k;
10 ≥ n ≥ 2,
where
Ll is the length of longitudinal slot in the first compensation element;
L1c is the length of the first compensation element;
n is the number of longitudinal slots in the first compensation element.
In case of temperature variation, the length of compensation element 9 changes, and as a result of the difference of the coefficients of linear expansion of the materials of body 5 and compensation element 9 the alteration of relative position of lens 3 occurs compensating the major part of mismatch of PBA and photodetector 4.
The longitudinal slots executed on the rigidly fixed end of compensation element 9 provide the bore diameter of this element with spring-type properties at the areas equal to the length of slots and exclude the cracking of rigidly fixed end of compensation element 9 due to the alteration difference of diametric dimensions of conjugated components (body 5, mounting 6 and compensation element 9) in case of temperature variation.
In order to reach complete compensation (compensation of remained minor part of mismatch of PBA and photodetector 4), the second compensation element consisting of two components 10 and 11 is installed between the body 5 and photodetector 4.
When the temperature reaches minus 50 °C in accordance with the optical estimation, the objective PBA shifts from the initial position by maximum value increasing the back focal distance of objective by ~ 1.27 mm.
If titanium (type VT-1) is used in the capacity of the material of body 5, mountings and rings 6, 7, 8, 11, 12, complete shift of PBA which must be compensated by components 9 and 10 will be equal to 1.364 mm.
It can be achieved in case of the selection of fluoroplastic F-4 for the first compensation element 9 and component 10 with the length of component 9 equal to ~ 120.4 mm and length of component 10 equal to 47 mm.
The total value of compensation shift of PBA will be 1.364 mm providing the concurrence of objective PBA with the plane of photodetector 4 in case of temperature drop to minus 50 °C.
In case if the ambient temperature is +20 °C corresponding to the normal climatic conditions, the coefficient of objective contrast transfer has the following values:
for the point on axis
(diffraction quality) ....................... CCT = 45.3%
for the point on axis
(aberration quality) ........................ CCT = 42.1%
for the field point 6 mm from
the image center ........ CCTМ = 39.6%, CCTС = 39.4%
In case if the ambient temperature is minus 50 °C corresponding to the minimum set temperature for operating conditions:
for the point on axis
(diffraction quality) ....................... CCT = 45.4%
for the point on axis
(aberration quality) ....................... CCT = 42.0%
for the field point 6 mm from
the image center ...... CCT М = 36.9%, CCT С = 37.4%
As it is seen from the calculations, in case of variation of ambient temperature the infrared objective provides the concurrence of the plane PBA with photodetector plane keeping the permissible quality of image, which is close to diffraction image, for the optoelectronic devices using the micro-bolometric arrays in the capacity of photodetectors with the pixel size up to 17 µm.
However, the material, which the compensation elements 9 and 10 are made of, has the spread of the coefficients of linear expansion in the area of temperatures from minus 10 to + 20 °C, which is regulated by GOST 10007–80 and equal to (80ч250) Ч10–6. The calculations described above are based on the average value of the coefficient, which is assumed to be equal to 165Ч10–6, and length of component 10, which is equal to 47 mm. In case of the deliveries of material for the first compensation element 9 and component 10 of the second compensation element, the deviations of the values of coefficient of linear expansion from the average value are possible in different delivery batches, and the compensation of these deviations can be provided by the change of the length of component 10 and corresponding change of the length of component 11 with the retention of their total length:
L2к – 1 + L2к – 2 = const,
where
L2c-1 is the length of component 10;
L2c-2 is the length of component 11.
The increase (or decrease) of the length L2c-1 of component 10 and respectively the decrease (or increase) of the length L2c-2 of component 11 with the retention of their total length allow eliminating the influence of the spread of the coefficients of linear expansion in the materials of compensation elements 9 and 10, which the different delivery batches of these materials have, using the simple method of the selection of lengths of elements 10 and 11.
Determination of the length of component 10 and corresponding length of component 11 is performed once for every material batch using the method of collimating measurement of focusing "undercompensation" or "overcompensation" under the conditions of cold chamber of minus 50 °C.
Thus, described complex of constructive methods allows providing the athermality of optical parts of observation and sighting devices, enhances their operational properties and provides the object survivability under the actual conditions of operation.
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