Application of parallel light tube in system star point inspection

Device and condition of star test method

The star test method can be carried out on the optical base, or by the parallel optical tube, microscope and other components of the device.

Objective star point inspection

(1) Diameter of star hole

Figure 1-1 shows the inspection optical path diagram, which is suitable for imaging optical systems with infinite object distance (such as telescopic objectives, photographic objectives, etc.). The star hole should be illuminated by a light source of sufficient brightness (such as a car bulb, ultra-high pressure mercury lamp, etc.). A star hole is an Angle

Figure (1-1) Principle diagram of optical path for objective star point inspection

A light source with a small diameter is close to an ideal point light source, so the size of the star hole should be limited. Theoretical estimates and experiments based on the width of the diffraction ring indicated that the maximum angular diameter αmax of the star hole should be equal to or less than one-half of the angular radius θ₁ supplied by the first dark ring of the Airy spot of the system, as shown in FIG. 1-2, i.e

Figure (1-2)

FIG. (1-3) Relations between maximum angular diameter of starhole αmax and angular diameter θ₁ of Airy spot

Where, D is the entry pupil diameter of the examined objective; λ is the wavelength of the illumination source.

When the star hole plate is placed on the focal plane of parallel optical tube objective lens with focal length of mono '₀, the maximum allowable diameter of the star hole is

Figure（1-4）

(2) Parallel optical tube objective

The parallel optical tube objective lens should be an apochromatic objective lens, the wave difference on the axis should be less than λ/8, the light aperture should be greater than 20% of the human pupil diameter of the objective lens to be measured, and the focal length should be greater than 2 times the focal length of the measured system. If the focal length of the measured system is so large that a suitable collimating objective cannot be configured, the star hole can be placed at a distance of 30 times the focal length value of the measured objective.

(3) Optical axis consistency

It is necessary to pay attention to the relative Angle between the optical axis of the inspected system and the optical axis of the parallel optical tube, especially when the star point image on the inspection axis is ensured that the two axes are accurate and consistent. In order to determine whether it is consistent, the system under test can be rotated around the optical axis in the holder. If the fault orientation of the star point image is also rotated, it indicates that the fault is indeed inherent in the system under test, otherwise it should be adjusted.

(4) Observing the numerical aperture and magnification of the microscope When observing with a microscope, in addition to paying attention to the image quality of the microscope, the numerical aperture and magnification of the microscopic objective should be reasonably selected. In order to enable all the outgoing beams of the examined objective lens to enter the microscope, the maximum aperture Angle Umax of the microscope should be ensured to be greater than or equal to the image aperture Angle U 'max of the examined objective lens. For this purpose, the numerical aperture of the microscope objective can be selected according to the relative aperture of the examined objective in Table 2-1.

Table (2-1) Selection of numerical aperture (NA) based on relative aperture D/ F

D/ Monophora of the examined objective	＜1/5	1/5~1/2.5	1/2.5~1/1.4	1/1.4~1/0.8
NA of the microscopic objective	0.1	0.25	0.40	0.65

The magnification of the microscope is selected in accordance with the first and second diffraction rings that can distinguish the star image when the human eye is observed, and the total magnification of the microscope should meet the following formula when the Angle spacing of the human eye is selected to be 2 '

Figure（1-5）

When the numerical aperture of the microscope is selected, the vertical magnification β is determined Γ=βΓₑ. Therefore, if the magnification of the eyepiece Γₑ is selected, the requirement of the formula (FIG. 1-5) can be satisfied.

Microscopic objective star point examination

The microscopic objective is a limited telephoto imaging system, and the star point and observation microscope (or high power eyepiece) are required as described in the previous section. Artificial star points are made by vacuum aluminization or silver plating on microscopic bed slides. Do not clean the cover glass carefully before steaming, and some pinholes will be formed after steaming, of which several pinholes will be suitable for artificial stars. The inspection device uses a bright light and a condenser under a high-efficiency microscope to perform a star-point inspection of the microscopic objective. The star test is often used for the rapid final adjustment of the air interval in the assembly process of the micro-objective.

Telescopic system star point test

When conducting star point inspection on telescopic systems, glass plates, prisms, etc., it is necessary to use a front mirror (telescopic system) instead of a microscope for magnifying observation, as shown in Figure (1-6) and (1-7). In addition to the requirement of image quality, the diameter of the pupil of the front mirror is also required to be greater than or equal to the diameter of the exit pupil of the telescope being examined, and the magnification should meet the requirement that the human eye can distinguish the details of the star image. Select the front lens magnification as

Figure (1-7) Optical path diagram of telescopic system star test

Figure（1-6）

Where, D 'is the exit pupil diameter of the measured telescopic system (mm); δ is the angular distance (') that human eyes can distinguish.

Star point image interpretation of optical system

The image quality of the optical system can be qualitatively judged according to the deviation of the light intensity distribution from the actual star point image to the ideal star point image, so as to determine the main aberration properties and types of defects in the optical system and the causes of these defects. The star test method can test spherical aberration, coma and astigmatism of optical systems, as well as defects in materials and assembly processes.