Star simulators are ground testing equipment for star sensors. Based on the optical system requirements of the star sensor, the technical parameters of the optical system of the star simulator are determined. Through analysis, a telephoto objective optical structure is selected to further reduce the secondary spectrum. The positive beam group is separated from the original positive and negative pairs, and a bonding surface is added. The negative beam group is composed of positive and negative pairs of separated lenses, and after image quality balance, the maximum parallel angle error of the entire spectrum is ≤ 10 “.
Modern scientific exploration satellites require high-precision attitude control. Star sensors are the preferred sensors for high-precision satellite attitude control, and we are accelerating our development. Star simulator is a ground simulation testing equipment for star sensors. We have designed the optical system of the star simulator for a certain research institute. This article describes the technical problems encountered in designing the optical system and their solutions, and finally provides the design results that meet the technical requirements.
Star simulators need to simulate stars at infinity, so a collimating objective system should be used to project artificial stars to infinity. In fact, the angle of the star to the ground is very small, so it is required that the collimating objective must reach the imaging quality of the diffraction limit, so that the angle of the artificial star relative to the star sensor meets the requirements.
Due to the need to simulate a star map in the range of 5.06 ° X6.74 °, a large field of view collimating objective is required. To ensure the accurate position and equal energy of all star images within the field, the collimating objective is required to have imaging quality with small distortion flat field and apochromatic aberration. This makes the design very difficult.
To ensure that the transmission of light energy information is not lost, it is required that the star simulator and star sensor be calibrated along the same optical axis, and that the exit pupil of the collimating objective lens coincides with the entrance pupil of the star sensor, as shown in Figure 1.
(l) Optical performance indicators of star sensors
Focal length=75mm
Relative aperture=1: 1.44
Field of view=5.06 ° X6.74 ° (diagonal corresponds to 8.5 °) The size of the CCD photosensitive surface is 6.24mm X 8.832mm
Spectral range=0.48~0.85 μ M
Central design wavelength=O.7 μ M
System transmittance>80%
Dispersion element size=50 after defocusing μ 90% of m concentration can freeze
Working distance>10mm
CCD unit size=23 μ M x 23 μ M
Working temperature=-40 ℃~+40 ℃
(2) Determination of parameters for collimating optical systems
Due to the fact that the aperture of the collimating objective is determined by its exit pupil diameter, and the human pupil diameter of the star sensor is equal to 52mm, the exit pupil diameter of the collimating objective should be greater than or equal to 52mm. The field of view of the star sensor should also match it, which should be greater than or equal to 5.06 ° X6.74 °, and the field of view of the star simulator should also match it, which should be greater than or equal to 5.06 ° X6.74 °. In this way, the main optical parameter that needs to be determined is the focal length of the collimating objective.
The size of the applied LCD screen is shown in Figure 2.
Due to the fact that the long side 193mm corresponds to a 6.74 ° field of view and the short side 145mm corresponds to a 5.0 ° field of view, it is appropriate to take f ‘=1638.77 for the focal length determined by them.
(1) System performance indicators
Focal length f ‘=1638.77mm
Field of view angle 2 ω ＝ 5.07 ° X6.74 °
The effective aperture diameter is 52mm, located on the side of the emitted parallel light, at a distance of 30mm from the vertex of the spherical surface.
Band range 0.48~0.85 μ m. Center wavelength input.= zero point seven μ M.
(2) Image quality requirements
This system is used to collimate the spherical waves emitted by any star point displayed on the fluorescent screen into emitted parallel light. The parallelism error Am ‘
(3) Selection
The system belongs to long focal length (J ‘=1638.77), small relative aperture (d/f,=1:32.77), and large field of view (5.06 ° X 6 74 ° collimation system, with prominent secondary spectrum and wide spectral band (0.48~0.855) μ m) , which further increases the difficulty of design. Referring to a double glued lens with the same focal length, such as the combination of ZK4-ZF4, the secondary spectrum is approximately 1.9492mm.
The theoretical field of view angle corresponding to the diagonal of the display screen is 2 ω′ = 8.4211 °. If a close contact lens group is used, the display screen is about 1.6m away from the lens, and the coaxiality and verticality of the system are difficult to ensure. At the same time, the temperature stability of the system is poor, and it is also difficult to install and adjust, and the parallelism error of 1O “cannot be guaranteed. Therefore, a telephoto objective form with positive and negative binary separation is selected. The positive group is in front, the negative group is behind, and the distance between the display screen and the last side is 715.5mm. The length from the first side of the system to the last side is 474.22mm, so the distance from the first side of the system to the screen is L=715.5+474.22=ll89.72mm, and L/f ‘=0.725.
In order to further reduce the secondary spectrum and ensure that the parallelism error of all wavelengths is within 10 “, the positive light group is separated from the original positive and negative pairs, and a bonding surface is added. The latter group is composed of a positive and negative pair of separation lenses to form a negative lens group. Finally, image quality balance proves that this is the best structure to ensure image quality requirements and suitable cylinder length. As shown in Figure 3.
(4) Aberration structure
This design calculates aberrations for eight wavelengths, with a secondary spectral value of approximately 1.4mm, resulting in a parallelism error of A ω’= 9.652 “.
Table 1 lists the corresponding six wavelengths, with a maximum field of view of 2 ω ＝ The angle value of parallel light emitted at 8.4211 °.
The maximum angle error of the full spectrum is 9.6526 “.
The one with the largest parallelism error on the axis is F light at full aperture=4.12 “, U ‘=3.063” at 0.85 aperture, U’=2.25 “at 0.707 aperture, and in=0.85 μ M light, U ‘=2.16 “at full aperture. The maximum parallelism error of the other spectra does not exceed l”. In the case of off-axis light, A=0.85 μ Maximum field of view angle at m ω’= 4.2056675 °, which is the highest angle among all spectral lines; When λ＝ zero point five eight μ At m, ω’= 4.202986 ° is the smallest angle among all spectral lines; The difference between them Δω’= 9.6526 “. The rest of the light is in between. The above requirements for parallelism error are less than 10 “.
Due to the fact that the full spectral range of light is within the full field of view angle, the parallelism error of the emitted light Δω'
(l) The lens radius error should result in N~l~3 circles and surface shape error Δ N ≤ 0.3.
(2) Lens thickness processing error Δ D1=0.05~0.1mm
(3) Gap error between lenses Δ When d2=0.52mm, Δ D2=± 0.02mm; When d2=427mm Δ D2=± O.1mm; When d2=1mm, Δ D2=± 0.05mm. Display the distance to the first side, l1=715.5mm, error Δ L=± 0.01mm.
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