Practical Optical Alignment with the Point Source Microscope
In optical design, the concept of an optical axis is basic, it exists naturally within the description of the system. In the laboratory, however, the optical axis does not exist until it is physically established through measurements and alignment. Optical components must be positioned and oriented relative to mechanical hardware, gravity, and the constraints of real mounts and fixtures. The process of optical alignment is therefore the task of translating an ideal optical system into a physical system whose elements occupy the correct positions and orientations in space. The Point Source Microscope (PSM) is a tool developed to make this translation as easy as possible by allowing key optical reference points, particularly centers of curvature, to be located and placed on a common axis with high precision.
What distinguishes the PSM from many traditional alignment instruments is the variety of optical information it measures. Instruments such as autocollimators, interferometers, or mechanical indicators are extremely useful for specific alignment tasks, but they typically only individually measure either angle, wavefront quality, or mechanical runout, respectively. The PSM, in contrast, can measure all these properties with one small footprint, lightweight instrument. The PSM serves as a practical tool for establishing the geometric framework on which precise optical alignment depends.
Although the Point Source Microscope (PSM) in its present form has existed for more than twenty years, potential users still approach me with the same basic questions: What is it? What does it do? How would I use it? I have also been asked by several customers if I could give training talks on how to use the PSM effectively. These questions, together with the chapters I have been writing about optical alignment, made me realize that my thoughts needed to be organized more systematically to explain the subject clearly.
The PSM is adjusted in three degrees of freedom so that it focuses on the center of a ball, which acts as a convex mirror. In this way, the optical center of curvature of the mirror is tied directly to a mechanical datum to a micrometer of so.
The PSM itself does not stand alone. It is only one tool within the broader discipline of optical alignment. However, using the PSM tends to highlight other aspects of alignment that are often overlooked. Some of these aspects have little to do with optics in a narrowly defined sense, but they are essential to understanding what alignment means in practice, what can realistically be achieved, and what cannot.
For this reason, the discussion in this book expands beyond optics alone and includes the mechanical side of alignment, specifically kinematics and the degrees of freedom that define the position and orientation of an optical element in space. Once alignment problems are expressed in terms of kinematic degrees of freedom, it becomes much easier to understand which aspects of a system can be precisely aligned and which cannot.
Returning to these mechanical fundamentals may sometimes seem obvious. However, in practice I have found that when the underlying kinematics are overlooked, alignment efforts often wander down blind alleys. It is far better to begin with a clear understanding of what adjustments are physically possible, and necessarily needed, taking into consideration the hardware available, than to proceed without thinking the whole matter through.
To establish this framework, the discussion begins by describing the PSM in terms of the kinematic degrees of freedom it can measure, depending on how it is configured. We then examine common optical elements and how their geometries determine which degrees of freedom can be aligned with precision.
In addition to precise alignment, I will also introduce what I will call “soft alignment.” Soft alignment concerns the direction and distribution of a cone of light, whether it is propagating in the intended direction and whether it fills the aperture of the system well centered. While soft alignment is necessary to reasonable precision, it does not usually introduce aberrations that significantly affect optical performance. Precise alignment, in contrast, is concerned with placing key optical features, such as centers of curvature, on a common axis to within small tolerances, usually measured in µm.
It is also important to remember that alignment is fundamentally a problem in first-order optics. We are concerned with the positions and directions of rays rather than with detailed wavefront structure. In most alignment situations we assume that well-polished optical surfaces are locally spherical. If the center of an optic is spherical, we assume the same radius applies across the aperture. For this reason, it is not necessary to fill the full aperture of an optic during alignment as it would be if measuring wavefront quality with an interferometer. While filling the aperture may increase sensitivity to misalignment, it is not necessary for performing precise alignment.
Once we understand what the PSM can measure and which properties of optical elements are accessible to measurement, we can examine how to characterize those elements in terms of their
first-order optical properties. These properties reveal the degrees of freedom available when adjusting optical elements into alignment.
Finally, we will consider alignment under the mechanical constraints imposed by the hardware that supports the optics. To keep the discussion general, optical elements, be they lenses or mirrors, will be treated simply as glass objects with defined mechanical features, while the supporting structures will be treated as metal hardware with their own datum features. This approach allows alignment to be described in terms of kinematic relationships, some determined optically, but all ultimately tied to mechanical features or datums.
Although the final scope of this learning project is not fully determined, my goal is to describe the interaction between the PSM and the elements being aligned in a schematic way similar to how mechanical parts are characterized in ISO Technical Report 5460-1985. This document, although now out of print, can still be found online.
One of the strengths of ISO 5460 is that its examples explicitly show the kinematics of each measurement. They illustrate how the correct number of degrees of freedom constrain both the measuring instrument and the features being measured, the measurand, or unit under test (UUT). When alignment setups are described in this way, it is easier to avoid kinematic errors. While it is always possible to make mistakes in any measurement, careful attention to constraints eliminates many common problems such as having more adjustments than necessary, or not enough to perform the desired alignment.
I always welcome feedback. If you have suggestions for correcting errors, improving examples, or expanding the scope of the material, please let me know.
The chapters that follow are organized to move from fundamental concepts to practical procedures. We will begin with a description of the PSM itself and the types of measurements it can perform. From there the discussion will expand into the kinematic description of optical elements and the degrees of freedom that determine how those elements can be positioned and aligned. With that framework established, later chapters will focus on practical alignment methods and examples drawn from real optical systems. The intention is that the reader can either follow the material sequentially to build a complete understanding of the subject or consult individual sections as a reference when working on a specific alignment problem.