In many optics, it is expected there is focusing point or intermediate image, or filter, in between the components, such as an Keplerian telescope, but how to design and optimize it, here we share an example.
It may be the case that the optical system you want to model needs to be optimized at some intermediate surface, rather than at the image plane. A classic example is a scope system that takes an object at infinite distance, focuses the image, and then collimates that image for coupling with the eye.
Let’s take a telescope beam expander as example to show how to design and optimize it.
Design input
- Wavelength: 532nm
- Laser power: 50mW;
- Laser line width: 1-2nm;
- Beam size: 1.2mm
Output requirement:
- There should be pinhole filter between the lens group.
- Output beam size: 30mm
- Total track: <150mm
Apparently, this is an inverse Keplerian telescope. The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo’s design. It uses a convex lens as the eyepiece instead of Galileo’s concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted.
In most cases, a beam expander is realized as an optical telescope consisting of two lenses (in rare cases, of two curved mirrors). Two different configurations are common:
- A Keplerian telescope consists of two focusing lenses, where the distance between the two lenses is the sum of their focal lengths. There is then a beam waist between the lenses. The beam radius after that telescope is modified if the tool focal length values are different. For example, a doubled beam radius is achieved if the second lens has twice the focal length of the first one.
- A Galilean telescope consists of a focusing and eight the focusing lens. Again, the distance between the lenses equals the sum of the focal lengths – where however one focal length is negative (that of the defocusing lens). The advantage of this type of telescope is that it can be more compact.
About Zemax IMSF Operand
This operand dynamically changes which surface is interpreted as the “image surface” for all subsequent operands. The new image surface is defined by Surface. The primary usage for this
Note this operand only temporarily changes a copy of the lens data use for evaluating the merit function and has no impact on the original lens data. Care should be taken when the selected surface precedes the original stop surface.
The design performance
The intermediate focusing point:
The focusing image performance;
The collimating performance, the beam diverging angle is 16mradian
