Visible light and infrared dual-purpose axial vertical infinite target device and calibration method

Abstract

The invention discloses a visible light and infrared dual-purpose axial vertical infinity target device and a calibration method, and the device comprises a concave pedestal; the supporting platform is mounted in a concave part in the middle of the base; the matched reference platform is mounted on the supporting platform; the rack comprises a longitudinal arm and a transverse arm which are perpendicular to each other, the longitudinal arm is vertically fixed on the base, and the transverse arm is arranged above the assembly reference platform; a black body light source, a fluorescent lamp and a Cassegrain collimator are sequentially arranged on the cross arm in the vertical direction, and the optical axis of the Cassegrain collimator is adjusted to be axially vertical. The device disclosed by the invention has relatively high stability.

Description

Technical Field

[0001] The invention relates to a vertical infinite distance target device used for high-precision adjustment of multi-band Kuder optical path imaging optoelectronic equipment, and in particular to a visible light and infrared dual-purpose axial vertical infinite distance target device and a calibration method. Background Art

[0002] Multi-band Kude optical path imaging optoelectronic equipment is usually composed of an infrared system, a visible light system, a laser, an azimuth rotating axis, an azimuth scanning mirror, a beam splitter, etc. The characteristics of this type of optoelectronic equipment are generally to achieve multi-light integration through a beam splitter, and then to achieve circular scanning imaging through a reflector fixed on the azimuth axis. For the installation and adjustment of this type of optoelectronic equipment, one of the core requirements is to align the optical axis of the multi-light integration with the azimuth axis to be parallel, so as to achieve the stability of the center of the optical axis during the azimuth circular scanning process.

[0003] For the adjustment of the parallelism of the multi-photon-in-one optical axis and the azimuth axis, it is generally necessary to borrow the "common plumb" common reference. For the adjustment of the azimuth axis plumb, the "horizontal method" can be used, that is, a level is placed at the end of the azimuth axis, and the level indication changes minimally within the 360° rotation range of the azimuth axis by adjusting the inclination angle of the azimuth axis assembly. For the adjustment of the multi-photon-in-one optical axis plumb, a common method is to place a set of two-dimensionally adjustable 45° reflectors at the front end of the Cassegrain collimator, and the reflectors need to be strictly adjusted to 45°, and the incident surface formed by the Cassegrain collimator optical axis and the 45° reflector normal needs to be adjusted to a plumb plane. This process is relatively complicated, and the error of the 45° reflector will cause the overall plumb error to double. In response to this problem, the present invention has developed a high-precision axial plumb infinitely far target device for both visible light and infrared, which simplifies the adjustment difficulty of the multi-photon-in-one optical axis plumb and improves the adjustment process. Summary of the invention

[0004] The purpose of the present invention is to provide a high-precision, high-stability axial plumb infinite target device for both visible light and infrared, which is used for high-precision adjustment of multi-band Kuder optical path imaging optoelectronic equipment, realizes the parallelism of the multi-light-combined optical axis with the azimuth axis, and ensures the stability of the optical axis center during azimuth scanning.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] Provided is a visible light and infrared dual-purpose axial plumb infinite distance target device, comprising:

[0007] The base is concave;

[0008] A supporting platform is mounted in the middle recess of the base;

[0009] An assembly reference platform is mounted on the support platform;

[0010] The base frame includes a longitudinal arm and a transverse arm which are perpendicular to each other. The longitudinal arm is vertically fixed on the base, and the transverse arm is placed above the assembly reference platform. A black body light source, a fluorescent lamp and a Cassegrain collimator are arranged in sequence along the vertical direction on the transverse arm, and the optical axis of the Cassegrain collimator is adjusted to be axially plumb.

[0011] In accordance with the above technical solution, a light source cover is provided between the black body light source and the Cassegrain collimator, a pair of fluorescent lamps are fixed on both sides of the light source cover, and the fluorescent lamps are placed on the top of the Cassegrain collimator.

[0012] According to the above technical solution, the aperture of the Cassegrain collimator is φ200 mm.

[0013] According to the above technical solution, the bottom of the assembly reference platform is fixed to the support platform via a plurality of groups of adjustment rods.

[0014] According to the above technical solution, an electronic level and a strip level are placed on the base.

[0015] According to the above technical solution, the base and the bottom of the support platform are both provided with multiple sets of adjusting screws.

[0016] In accordance with the above technical solution, a black body light source display control screen is provided on the side of the longitudinal arm.

[0017] According to the above technical solution, the longitudinal arm is provided with a plurality of reinforcing ribs, and is provided with wiring holes and wiring grooves.

[0018] According to the above technical solution, a mounting hole is provided on the cross arm, and the Cassegrain collimator is installed in the mounting hole.

[0019] According to the above technical solution, the aperture of the mounting hole is larger than the aperture of the Cassegrain collimator.

[0020] According to the above technical solution, a mounting hole is provided on the cross arm, an annular mounting flange surface is provided around the mounting hole, and the Cassegrain collimator is installed in the mounting hole.

[0021] Continuing with the above technical solution, the front side of the concave base is provided with three groups of finely machined bosses, which are respectively set as the mounting interface of the stand and two groups of horizontal reference transition plate mounting surfaces; the bottom surface of the base is a rib-beam structure, including multiple bosses, each of which is provided with a threaded hole for installing the adjusting screw.

[0022] The present invention also provides a calibration method for a visible light and infrared dual-purpose axial plumb infinite distance target device based on the above technical solution, characterized in that it comprises the following steps:

[0023] Step 1: After the theodolite is leveled, adjust the elevation angle of the theodolite to the zenith angle, that is, the optical axis of the theodolite is in a vertical upward state, and the elevation angle of the theodolite is 0° at this time; with the theodolite as a reference, adjust the adjustment screw pair of the "concave" base to align the theodolite with the Cassegrain collimator, and set the azimuth angle of the theodolite to zero;

[0024] Step 2: Rotate the theodolite 180°, observe the alignment between the theodolite and the Cassegrain collimator again, adjust the elevation angle of the theodolite so that the horizontal center line of the theodolite scale coincides with the optical axis of the Cassegrain collimator, and record the elevation angle indication δ of the theodolite;

[0025] Step 3: Lock the elevation angle of the theodolite at an angle or Repeat steps 1 and 2 until the vertical error of the Cassegrain collimator optical axis is or Adjust to minimum (can reach within 2 arc seconds);

[0026] illustrate:

[0027] ① This step can also take into account the initial adjustment of the plumb error in the above-mentioned orthogonal direction. The adjustment amount is half of the alignment error between the vertical line of the theodolite scale center and the optical axis of the Cassegrain collimator after the azimuth is rotated 180° in step 2. The specific fine adjustment will be carried out in the subsequent step 4.

[0028] ② For special cases where the zenith angle pitch is other indications (such as 90°), the theodolite elevation is locked at the angle The other installation and adjustment methods are the same.

[0029] Step 4: The theodolite is rotated 90° based on the zero position of the azimuth in step 1, and the vertical error of the Cassegrain collimator optical axis in the orthogonal direction of the azimuth is calibrated according to steps 1 to 3, and the error is also adjusted to the minimum (within 2 arc seconds);

[0030] Step 5: Rotate the theodolite to any position within the 360° range and check the plumb error of the Cassegrain collimator optical axis according to steps 1 to 3. If the error exceeds 2 arc seconds, repeat steps 1 to 4.

[0031] The beneficial effects of the present invention are as follows: the present invention adopts a Cassegrain collimator as an infinite target device, wherein the light source part is of two types, namely visible light and infrared, to meet the requirements of dual use in visible light and infrared bands; a high-stability special base stand is designed in combination with force conditions, center of gravity distribution, environmental adaptability, ergonomics, material properties, design simulation, etc.; the Cassegrain collimator is fixed to the base stand in a "vertical" posture, and a plumb calibration is performed by designing an adjustment mechanism, so that the Cassegrain collimator is calibrated to be axial plumb, which has high stability;

[0032] Furthermore, the assembly reference platform and the base frame are separated to prevent the weight of the equipment to be assembled from affecting the axial plumb reference of the Cassegrain collimator. The device has been well applied in the high-precision assembly and adjustment of multi-band Kuder optical path imaging optoelectronic equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0034] Figure 1A This is a front view of a visible light and infrared dual-purpose axial plumb infinite distance target device according to an embodiment of the present invention;

[0035] Figure 1B It is a left view of the axial plumb infinite distance target device for both visible light and infrared according to an embodiment of the present invention;

[0036] Figure 2 It is a top view of a visible light and infrared dual-purpose axial plumb infinite distance target device according to an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of the structure of a light source group according to an embodiment of the present invention;

[0038] Figure 4 Schematic diagram of a "concave" type base according to an embodiment of the present invention, wherein (a) is the front side and (b) is the back side;

[0039] Figure 5 This is a schematic diagram of a “┑” type stand according to an embodiment of the present invention;

[0040] Figure 6 This is a force analysis diagram in the x direction of an embodiment of the present invention;

[0041] Figure 7 This is a force analysis diagram in the y direction of an embodiment of the present invention;

[0042] Figure 8 This is a force analysis diagram in the z direction of an embodiment of the present invention;

[0043] Fig. 9 The overall design dimensions of the embodiment of the present invention are based on ergonomics;

[0044] Fig.10 This is a schematic diagram of the longitudinal distribution of the center of gravity position of an embodiment of the present invention;

[0045] Fig.11 Schematic diagram of horizontal distribution of the center of gravity position of an embodiment of the present invention

[0046] Fig.12 This is a schematic diagram of a design in which the reference platform and the target light pipe are separated according to an embodiment of the present invention;

[0047] Fig.13It is a schematic diagram of adjusting the axial optical axis to be vertical by using the method of straightening and inverting mirrors to eliminate errors of the theodolite according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0049] The overall scheme of the visible light and infrared dual-purpose axial plumb infinite target device of the present invention is as follows: a Cassegrain collimator is used as an infinite target device, a high-stability special base stand is designed, and the Cassegrain collimator is fixed to the base stand in a "vertical" posture. An adjustment mechanism is designed on the base stand, and the optical axis of the Cassegrain collimator is adjusted to an axial plumb position in combination with the axial plumb adjustment process, and the plumb reference is calibrated. The assembly reference platform is fixed on a high-precision and stable support platform, and a leveling mechanism is designed, and a separate design form is adopted with the base stand to achieve a high-precision, high-stability axial plumb infinite target reference.

[0050] like Figure 1A-Figure 4 As shown, the visible light and infrared dual-purpose axial plumb infinite distance target device according to the embodiment of the present invention comprises a base 10 , a support platform 20 , an assembly reference platform 30 and a stand 40 .

[0051] The base is concave; the support platform is installed in the concave part in the middle of the base; the assembly reference platform is installed on the support platform; the stand includes a longitudinal arm and a transverse arm which are perpendicular to each other, the longitudinal arm is vertically fixed on the base, and the transverse arm is placed above the assembly reference platform; a black body light source 41, a fluorescent lamp 42 and a Cassegrain collimator 43 are arranged in sequence on the transverse arm along the vertical direction, and the optical axis of the Cassegrain collimator is adjusted to be axially plumb.

[0052] A light source cover 44 is provided between the black body light source and the Cassegrain collimator. A pair of fluorescent lamps are fixed on both sides of the light source cover. The fluorescent lamps are placed on the top of the Cassegrain collimator.

[0053] The bottom of the assembly reference platform is fixed to the support platform via a plurality of groups of adjustment rods.

[0054] An electronic level 11 and a strip level 12 are placed on the base. Figure 2 As shown, there are 2 sets of electronic levels and 2 sets of strip levels.

[0055] The base and the bottom of the support platform are both provided with multiple groups of adjusting screws.

[0056] A black body light source display control screen 42 is provided on the side of the longitudinal arm.

[0057] The longitudinal arm is provided with a plurality of reinforcing ribs, and is provided with a wiring hole 45 and a wiring groove 46 .

[0058] The cross arm is provided with a mounting hole 47, the periphery of the mounting hole is provided with an annular mounting flange surface, and the Cassegrain collimator is installed in the mounting hole.

[0059] Further, based on Figure 1A , Figure 1B Schematic diagram of the device composition. The Cassegrain collimator is used as the target for the visible light and infrared axial vertical infinity, with a diameter of φ200mm, and is fixed in a "vertical" posture at the end of the cross arm of the "┑"-shaped stand. The assembly reference platform is fixed on the support platform and forms an adjustment station with the base stand. The "concave"-shaped base in the base stand is designed with 5 sets of M42×3 adjustment screw pairs 21 with ball heads as support legs, which are used to adjust the optical axis of the Cassegrain collimator vertically. The assembly reference platform is used as the assembly reference table, and is designed with 3 sets of M30×2 adjustment screw pairs 22 with ball heads as support legs, which are used to level the assembly reference table. The support platform is designed with 4 sets of M42×3 adjustment screw pairs 23 with ball heads as support legs, which mainly play the role of fixing and supporting the assembly reference platform.

[0060] In one embodiment of the present invention, the base frame is designed as a combination of "concave" type + "┑" type (bolted connection), wherein the "concave" type is the base part, the "┑" type is the frame part, and it is a box beam structure, with the Cassegrain collimator fixed at the end of the beam. The materials are all gray cast iron, combined with design simulation, and reinforced ribs are used in many places. The overall weight is as high as 2 tons, achieving high stability of the base frame. The "concave" type base is designed with 5 sets of M42×3 adjusting screw pairs as an adjustment mechanism for axial plumb reference adjustment, and the end of the "┑" type frame cross arm is designed with a Cassegrain collimator installation interface. Figure 4 , Figure 5 They are schematic diagrams of the structures of the “concave” type base and the “┑” type frame which constitute the base frame.

[0061] There are three groups of finely machined bosses on the front of the "concave" type base, which are the installation interface of the "┑" type stand and two groups of horizontal reference transition plate installation surfaces; the bottom surface adopts a rib beam design, which not only reduces weight but also maintains overall strength, and is designed with 5 bosses, each of which is designed with an M42×3 threaded hole for installing the adjusting screw leg.

[0062] The vertical reinforcement of the “┑” type gantry adopts a rib beam design, and the horizontal arm adopts a box beam design. The end of the horizontal arm is designed with a φ220mm mounting hole for the Cassegrain parallel light tube, and an annular mounting flange surface is designed around the mounting hole.

[0063] The “concave” type base and “┑” type stand are both made of grey cast iron and have good environmental resistance and stability.

[0064] The assembly reference platform and the base frame are designed to be separated to prevent the dead weight of the equipment to be assembled from affecting the axial plumb infinite target reference when the equipment is fixed to the assembly reference platform for optical adjustment.

[0065] The theodolite is placed on the assembly reference platform, with any orientation of the theodolite as the starting zero position. At the orientations of 0°, 90°, 180°, and 270°, the Cassegrain parallel light tube is aimed at the zenith by using the method of correcting and inverted mirrors to eliminate errors, and the optical axis is adjusted to be plumb.

[0066] The assembly reference platform and the target light pipe part (including the base stand) are designed to be separated to avoid the influence of the axial plumb infinite target reference due to its own weight when the equipment to be assembled is fixed to the assembly reference platform for optical adjustment.

[0067] Figure 3 It is a structural diagram of the light source group, that is, a fluorescent lamp and a black body are designed at the reticle position of the Cassegrain collimator, which serve as the light sources of visible light and infrared respectively, and a light source cover is designed, which plays a role in overall aesthetics on the one hand, and focuses light on the other hand. There is no need to use a guide rail to switch the light source to meet the use of visible light and infrared at the same time.

[0068] Figure 6 to Figure 8 For the simulation analysis of the base rig, the main analysis is the deformation degree of the base rig caused by the Cassegrain collimator and light source group (about 60kg in total) fixed to the end of the "┑"-shaped rig cross arm beam, as well as the influence of room temperature changes and ambient vibrations on the stability of the plumb reference.

[0069] Through ANSYS software analysis, the deformation in the x direction is basically symmetrical, with a maximum value of 0.0033mm; in the y direction, the maximum absolute value of the deformation is 0.057mm; in the z direction, the maximum absolute value of the deformation is 0.068mm. The influence of room temperature changes and ambient vibration on the stability of the plumb reference does not exceed 0.5 arc seconds.

[0070] Since the weight of the collimator and light source is constant, and the verticality of the dimmer axis is only installed after the two components are fully tightened, the vertical optical axis reference will remain unchanged. According to actual measurements, the vertical optical axis reference error changes by ≤1 arc second after standing for 24 hours.

[0071] Fig. 9 The following is a schematic diagram of the main dimensions of the device designed according to the principles of ergonomics.

[0072] The overall dimensions are 1500mm (length) × 1200mm (width) × 2795mm (height); the dimensions of the "concave" base are 1500mm (length) × 1200mm (width) × 560mm (height); the dimensions of the assembly reference platform table are 750mm (length) × 750mm (width), and the height of the table from the ground is 790mm; the bottom space of the supporting platform working surface is 460mm (width) × 520mm (width).

[0073] Fig.10 , Fig.11 The longitudinal and horizontal distribution diagrams of the center of gravity of the device are shown in Figure 2. Fig.10 As we know, the center of gravity is 720mm from the ground, which is much lower than the overall height of 2795mm. Fig.11 It is known that, relative to the five sets of supporting legs, the center of gravity is centered in the left-right direction, and in the front-back direction, the center of gravity is 148 mm backward, but still in the middle area of ​​the five sets of supporting legs. Therefore, the design of the device is safe and reliable.

[0074] Fig.12 The schematic diagram of the assembly reference platform and the target optical tube part (including the base stand) is designed to be separated. The two parts are "embedded" with each other, and the separation gap is 10mm. Through this design, it is prevented that the self-weight factor of the equipment to be assembled will affect the axial plumb reference of the infinite target when it is fixed to the assembly reference platform for optical adjustment.

[0075] Fig.13 This is the principle diagram of adjusting the axial plumbness of the Cassegrain collimator optical axis by using a theodolite to eliminate errors by setting a positive and negative mirror.

[0076] The calibration process of the above-mentioned visible light and infrared dual-purpose axial plumb infinite distance target device is carried out according to the following steps:

[0077] Step 1:

[0078] After the theodolite is leveled, adjust the elevation angle of the theodolite to the zenith angle position, that is, the optical axis of the theodolite is in a plumb upward state, and the elevation angle indication of the theodolite is 0° at this time; with the theodolite as a reference, adjust the adjustment screw pair of the "concave" base to align the theodolite with the Cassegrain parallel light tube, and set the azimuth angle of the theodolite to zero.

[0079] Step 2:

[0080] Rotate the theodolite 180°, observe the alignment between the theodolite and the Cassegrain collimator again, adjust the elevation angle of the theodolite so that the center horizontal line of the theodolite scale coincides with the optical axis of the Cassegrain collimator, and record the elevation angle indication δ of the theodolite.

[0081] Step 3:

[0082] Theodolite elevation is locked at angle or Repeat steps 1 and 2 until the vertical error of the Cassegrain collimator optical axis is or Adjust to minimum (can reach within 2 arc seconds);

[0083] illustrate:

[0084] ① This step can also take into account the initial adjustment of the plumb error in the above-mentioned orthogonal direction. The adjustment amount is half of the alignment error between the vertical line of the theodolite scale center and the optical axis of the Cassegrain collimator after the azimuth is rotated 180° in step 2. The specific fine adjustment will be carried out in the subsequent step 4.

[0085] ② For special cases where the zenith angle pitch is other indications (such as 90°), the theodolite elevation is locked at the angle The other installation and adjustment methods are the same.

[0086] Step 4:

[0087] The theodolite is rotated 90° based on the zero position of the azimuth in step 1, and the vertical error of the Cassegrain collimator optical axis in the orthogonal direction of the above azimuth is calibrated according to steps 1 to 3, and the error is also adjusted to the minimum (within 2 arc seconds).

[0088] Note: When adjusting the adjustment screw pair of the "concave" base in this step, the alignment reference between the vertical center line of the theodolite scale and the optical axis of the Cassegrain parallel light tube must be kept unchanged (the alignment reference is the plumb reference that has been adjusted in steps one to three).

[0089] Step 5:

[0090] Rotate the theodolite to any position within the 360° range and check the plumb error of the Cassegrain collimator optical axis according to steps 1 to 3. If the error exceeds 2 arc seconds, repeat steps 1 to 4.

[0091] The above method can eliminate the leveling error and axis backlash error of the theodolite, and the calibration accuracy can be controlled within 2 arc seconds through the positive and negative mirror error elimination method.

[0092] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A visible light and infrared dual-purpose axial plumb infinite distance target device, characterized in that: include: The base is concave; A supporting platform is mounted in the middle recess of the base; An assembly reference platform is mounted on the support platform; The stand includes a longitudinal arm and a transverse arm which are perpendicular to each other. The longitudinal arm is vertically fixed on the base, and the transverse arm is placed above the assembly reference platform. A blackbody light source including visible light and infrared, a fluorescent lamp and a Cassegrain collimator are arranged in sequence along the vertical direction on the transverse arm, and the optical axis of the Cassegrain collimator is adjusted to be axially plumb.

2. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: A light source cover is arranged between the black body light source and the Cassegrain collimator, a pair of fluorescent lamps are fixed on both sides of the light source cover, and the fluorescent lamps are placed on the top of the Cassegrain collimator.

3. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: The bottom of the assembly reference platform is fixed to the support platform via a plurality of groups of adjustment rods.

4. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: An electronic level and a strip level are placed on the base.

5. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: The base and the bottom of the support platform are both provided with multiple groups of adjusting screws.

6. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: A black body light source display control screen is provided on the side of the longitudinal arm.

7. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: The longitudinal arm is provided with a plurality of reinforcing ribs, and is provided with wiring holes and wiring grooves.

8. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 1, characterized in that: The cross arm is provided with a mounting hole, a ring-shaped mounting flange surface is provided around the mounting hole, and the Cassegrain collimator is installed in the mounting hole.

9. The visible light and infrared dual-purpose axial plumb infinite distance target device according to claim 3, characterized in that: The front side of the concave base is provided with three groups of finely machined bosses, which are respectively set as the mounting interface of the stand and two groups of horizontal reference transition plate mounting surfaces; the bottom surface of the base is a rib-beam structure, including multiple bosses, each of which is provided with a threaded hole for installing the adjusting screw.

10. A calibration method for a visible light and infrared dual-purpose axial plumb infinite distance target device according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: After the theodolite is leveled, adjust the elevation angle of the theodolite to the zenith angle, that is, the optical axis of the theodolite is in a vertical upward state, and the elevation angle of the theodolite is 0° at this time; with the theodolite as the reference, adjust the "concave" base to align the theodolite with the Cassegrain collimator, and set the azimuth angle of the theodolite to zero; Step 2: Rotate the theodolite 180°, observe the alignment between the theodolite and the Cassegrain collimator again, adjust the elevation angle of the theodolite so that the horizontal center line of the theodolite scale coincides with the optical axis of the Cassegrain collimator, and record the elevation angle indication δ of the theodolite; Step 3: Lock the elevation angle of the theodolite at an angle or Repeat steps 1 and 2 until the vertical error of the Cassegrain collimator optical axis is or Adjust to minimum. Step 4: The theodolite is rotated 90° based on the zero position of step 1, and the vertical error of the Cassegrain collimator optical axis in different orthogonal directions is calibrated according to steps 1 to 3, and the error is also adjusted to the minimum; Step 5: Rotate the theodolite to any position within the 360° range and check the plumb error of the Cassegrain collimator optical axis according to steps 1 to 3. If the error exceeds 2 arc seconds, repeat steps 1 to 4.

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