On the choice of focal length of parallel light tube

Focal length is an important technical index of parallel optical tube, and it is also the first technical parameter to be considered in the selection of parallel optical tube. On the one hand, according to the image relation expression of geometric optics, the technical parameters of the optical instrument to be measured can be calculated and deduced theoretically as long as the focal length of the parallel optical tube is accurately measured, without considering the size of the focal length. However, in the actual use process, due to the inevitable position error of the target plate of the parallel optical tube, according to Newton's formula, when the defocus of the target plate is certain, the parallelism error of the outgoing light is inversely proportional to the focal length, so it is necessary to weigh the focal length of the parallel optical tube according to the requirements of the parallelism of the outgoing light and the calibration accuracy of the target plate.

On the other hand, when measuring the geometric parameters of optical instruments (focal length, relative aperture, etc.) and imaging quality, in order to reduce the measurement error, the focal length of parallel optical tubes is generally 3 to 5 times that of the instrument to be detected.

There are two main benefits of telephoto

1. According to Newton's formula, the output parallelism error caused by defocusing can be derived as follows: It can be seen that the output parallelism error caused by defocusing of the optical tube is inversely proportional to the square of the focal length:

In the formula, 2θ is the light Angle, D is the aperture of the optical tube, Δ is the defocus amount, and f is the focal length of the system.

For the same defocus error control accuracy of 0.01mm, for 300mm aperture, focal length 3000mm, 6000mm, 8000mm caused by parallelism error are:

2. Corresponding to the same Angle, the size of the target face of the telephoto tube can be enlarged for easy production, or the resolution target with the same line width can be placed on the focal surface of the telephoto tube with higher measurement accuracy.

For 300mm aperture, focal length 3000mm, 6000mm, 8000mm optical tube, Angle 1 arc second corresponding to the target surface dimensions are: