Methods of Reflective Telescope – Part 1: Getting Started
This is the first of several planned articles methods of designing reflective imaging systems, an area in which Keysight Optical Design Engineering has decades of experience. The goal is to illustrate practical aspects of architecture selection and design methods with CODE V.
Motivation
For many applications, the preferred type of optical imaging system will use only mirrors as the powered elements, with the only refractive elements being filters and windows. Benefits of reflective configurations include:
- Transmit broad spectral bands without chromatic aberration.
- Potentially less sensitive to thermal effects (no need to perform complex athermalization in the optimization process).
- No refractive index uncertainty or inhomogeneity; no ghost reflections.
- More feasible to fabricate large mirrors than large, powered lenses.
- Can be more compact than the equivalent refractive configuration.
- Resistant to radiation.
System requirements and architecture selection
The selection of a reflective configuration is influenced by multiple factors, including entrance pupil diameter, angular field of view, F number, envelope, stray light control, and others. Depending on the application, any one of these factors could be the main driver in selecting a configuration. The best configuration will be chosen on a case-by-case basis.
As a specific example and illustration of the selection process, consider a design with the following requirements:
- 500 mm entrance pupil diameter, with accessible entrance pupil in front of the imager.
- Field of view 4°x4°
- F/4.0 at the FPA
- Broad band infrared spectral band, e.g. 1500-5000 nm
- Accessible field stop, for out-of-field stray light control
- Accessible exit pupil (cold stop) at a high-quality image of the entrance pupil
- Unobscured pupil, for maximum collection area and MTF performance
Typically, the requirements are provided by the customer as a result of a system analysis and are regarded as fixed requirements which the optical designer must accommodate.
In this example, the thought process of the optical designer might be as follows:
- A 500 mm entrance pupil diameter implies that the front elements (at least) will not be refractive due to size and mass.
- A broad IR band points to a mostly or entirely all-reflective design for the powered elements, due to the difficulty of a refractive configuration to control chromatic aberration over a broad band. The maximum to minimum wavelength ratio can be well over 2:1, making antireflection coatings on refractive surfaces less efficient.
- An unobscured pupil rules out classic rotationally symmetric configurations such as the Schmidt.
- An accessible entrance pupil in front of the imager, and accessible exit pupil after the imager, and internal focus for field stop implies a reimaging system. This rules out a 2-mirror configuration such as the Schwarzschild, which is not reimaging.
- The field of view 4°x4°, and F/4 final focus, along with the preceding requirements, imply a design with 3 or more mirrors.
At this point, then, the optical designer will explore multi-mirror unobscured reimaging designs and will work with the customer to discuss the tradeoffs between complexity, performance, and envelope. Some of the possible starting points, configurations with 3, 4, and 5 mirrors, are shown in the accompanying Figures 1-3.
Subsequent articles will describe specific configurations and methods in this trade process.
Figure 1: 3-mirror reimaging configuration
