Author: csadmin

The purpose of optical alignment is making the optical axis of an optical element, or complete system, coaxial with some other axis that is defined by other optical or mechanical components. This means we must start the discussion of optical alignment by making sure we all mean the same thing when we say the optical axis of a lens.

For a singlet optical element, the definition is simple. The optical axis is the line joining the centers of curvature of the two surfaces. The green arrows in Fig. 1 start at the physical centers of curvature. The red solid arrows start on the optical axis at the optical centers of curvature, that is, the position along the optical axis where the center of curvature appears located due to refraction at the intervening surface when viewed with an autostigmatic microscope (ASM) or an alignment telescope (AT). We will look at the process of using either of these instruments in a subsequent chapter. For now, we are just dealing with the definition.

Fig. 1 Physical and optical centers of curvature that define the optical axis of a single lens

We know one way of defining a line is with two points and those two points are uniquely defined here by the two centers of curvature as in Fig. 1. Independent of whether the surfaces are concave or convex, the optical axis is always normal at its intersection with the surfaces because the axis passes through the centers of curvature. This means here is no refraction or deviation of a ray propagating along the optical axis in either position or angle. This fact is implicit in the definition but seldom stated.

The definition shows why the optical axis is so important to alignment. When a lens is aligned to a reference axis and there is no deviation of a ray propagating along the reference axis the lens is perfectly aligned to the reference axis in tilt and decenter.

(Sidebar – There is a trap in this definition if you don’t think it all the way through. Say I have a lens and I want to center it in a collimated beam relative to some fiducial or datum perpendicular to the beam. I align my ASM or AT to the datum and insert the lens.  By tilting and decentering the lens I get the back focus well centered in my instrument. The image is well centered but by eye the lens looks tilted.

The trap is that I have tried to center the lens using the back focus only. That is a single point, so I only know 3 degrees of freedom (DOF) and I am trying to determine an axis, or a line. I need 4 DOF to do that. I do not have enough information to know the lens is centered, that is, whether the optical axis of the lens is parallel to the axis of the collimated beam. I need a second point such as the center of curvature of one of the surfaces to know the lens is completely centered.)

Optical axis of multiple elements

In looking for a definition of the optical axis of a real assembly of optical elements rather than the design of an assembly I stumbled upon this note by A. E. Conrady [1] from 1919 that states the situation I am discussing perfectly.

So, what is the “optical axis” of set of centers of curvature “scattered around…according to chance”? For the purposes of our discussion, I propose it is the analogue of the optical axis for a single element in a functional sense. What does the “scatter” do to the deviation of a ray propagating through the assembly? My definition is when the optical axis of an assembly of lenses is aligned to a reference axis an optical ray co-axial with the reference axis is not deviated in position or angle while passing through the lens. This begs the question of how we create a single optical ray, but we will get into that in another couple of Chapters.

The same trap occurs here as for the single element. When the system is aligned to the reference axis we must probe the transmitted ray at two distances from the lens to assure that neither the angle of the ray nor its position has changed. This aspect of the problem is getting ahead of myself, but I think you will agree that the definition makes sense assuming we can measure the transmitted ray.

Example of the optical axis definition for a “system” of elements

Consider a cemented doublet whose prescription is shown in Table 1. If the doublet is perfectly centered, the centers of curvature of all three surfaces lie on a straight line that is both the mechanical and optical axis as in Fig. 2a. If there is an error in cementing this is no longer the case. Assume there is a 30 minute of the meniscus relative to the positive element. This is about 10 times larger than the typical centering tolerance for an off the shelf doublet but the large decenter makes it possible to see the errors in Fig. 2

Table 1 Prescription of the cemented doublet used in the example

When the meniscus is rotated about the center of curvature of the 2^nd surface of the positive element, the center of curvature of the 2^nd surface of the meniscus moves 1.81 mm above the optical axis of the positive element. The mechanical vertex of the meniscus is about half a mm below the axis as seen in Fig. 2 (middle) 

To find the optical axis of this “system” the doublet is allowed to rotate and decenter about the 1^st surface of the positive element relative to the initial optical axis of the perfectly centered doublet until the transmitted ray traveling along the initial axis in neither changed in position (ray height) or angle. A ray trace optimization program calculates the doublet must be rotated 0.389° CCW and decentered downward 0.553 mm for no deviation of the ray.

The correction places the centers of curvature of all 3 surfaces below the optical axis as in the lower part of Fig. 2. The centroid of the image formed at the back focus of the lens for a collimated beam parallel to the original axis would shift about 0.01 mm from the optical axis. Because a tilt about 10 times a typical tilt was used in the example, we get an idea of what to expect from a typical lens by scaling back a factor of 10. A 3 minute tilt is barely perceptible in most instances.

Throughout this discussion we have talked about using a single ray to establish the optical axis. Creating this single reference ray is discussed in a future Chapter.

Other definitions relating to alignment

Before talking about how to use optical instruments to align optics it is helpful to define a few more concepts. We will finish out this segment with these thoughts.

First, we have pointed out that a line, or axis, is defined by 2 points, or a point and 2 angles, 4 DOF altogether. Do not forget that a circle of any radius can be drawn through 2 points. To show 2 points lie on a straight line you need to measure a 3^rd point and prove it lies on the same line.

Next, a plane mirror is a plane defined by 3 points, or 3 DOF. This is why you need 2 plane mirrors to turn an axis from one position and angle to another position and angle. There are not enough DOF to do it with one plane mirror. You need 4 DOF to define an axis. A good example are orthogonal galvo scan mirrors. You need 2 mirrors to scan over all angles in a hemisphere where the beam starts from a particular location and angle.

A spherical mirror or surface is defined by 3 DOF to determine the location of its center of curvature. You need a 4^th point to know its radius of curvature. This is why when using a coordinate measuring machine (CMM), you need to touch the master ball in at least 4 places for the machine to know where the center of the ball is. This is the perfect example of a difference between optical and mechanical measurements, and why we can say that a spherical ball, or any part of a spherical surface, is an analog of a point. If a point source of light is at the center of a spherical surface the light will be perfectly reflected back on itself to the point independent of the radius of the spherical surface. 

For a mechanical measurement we need to also know the radius of the surface to know when the center of curvature is located. Not only do we need 4 points, but in the case of a concave mirror, 4 points around the edge won’t do. We need one point near the middle of the mirror to avoid an ill-conditioned situation that makes the calculation of the center of curvature a poor estimate. Further, most people do not like you to touch the middle of an optical surface. The safest method of finding the center of curvature of an optical surface is to do it optically. This is also the quickest and most precise method of finding the center of curvature.

This is why when using a coordinate measuring machine (CMM), you need to touch the master ball in at least 4 places for the machine to know where the center of the ball is. 

Cylinder – The next most complex surface is a cylinder that is defined by its axis, or 4 DOF. As an analog to a spherical surface, you can find the axis of a cylinder of any radius with 4 DOF, but you need a 5^th point to determine the radius of curvature of the cylinder, just as you needed 3 DOF to find the center of curvature optically but a 4^th point to find the radius. When a point source of light is focused at one of the axes of a cylinder the reflected light comes back to form a line image. Using an autostigmatic microscope you can find the axis in 3 DOF, 2 DOF in translation and 1 in angle.

Symmetric asphere – A symmetric asphere also requires (for alignment purposes) 4 DOF to define because as opposed to a spherical surface, a symmetrical asphere has an axis. A 5^th point is required to define its vertex radius of curvature. We will get into more details about aspheres later but for now these are the basic alignment details.

Off-axis asphere and a toroid – An off-axis asphere requires 5 DOF with an addition point to define a radius in one direction. Toroids are included here because if you mask or stop down an off-axis asphere you effectively are left with a surface with 2 cylindrical surfaces of different radii at right angles to each other. There are cases where a toroid will be a satisfactory substitute for an off axis asphere just as a sphere can substitute for a symmetric asphere if the f/# of the surface is slow enough.

This is enough on definitions. In the next Chapter we will get into the first steps of classical alignment.

[1] Conrady, A. E. (1919). Lens-systems, decentered. Monthly Notices of the Royal Astronomical Society, Vol. 79, p. 384-390, 79, 384-390.

There is no better way to describe an autostigmatic microscope (ASM) than to call it an autocollimator (AC) with a microscope objective attached to the front. This converts the AC from an instrument that measures 2 angular degrees of freedom (DOF) into an instrument that measures the location of the center of curvature of a spherical surface or wavefront in 3 DOF. 

To illustrate this definition there is no better than the original description in the English literature, an article by a Mr. C. V. Drysdale in the Trans. Oct. Soc. London, 1900 called On a simple direct method of determining the curvatures of small lenses. In his introduction, Drysdale says he was surprised there was no instrument capable of measuring the radii of lens surface so “I immediately set to work to devise such a method, … capable of measuring the curvature of any spherical or cylindrical surfaces, from quite shallow curves to those of less than a millimeter radius.”

Drysdale’s Fig. 2 showed the method as reproduced here with my notes to the Figure.

Although Drysdale’s original purpose was to measure radii of lens surfaces by first focusing at the center of curvature and then on the surface and measuring how far the microscope moved between the two measurements, he did not stop there. He went on to show his autostigmatic microscope could view aberrations in lenses by what we would now call the “Star test”, as well as measure object/image distances, focal lengths and principal plane locations. Suggested it could be used to measure index of refraction of glass and be used as a focimeter to aid optometrists.

Modern ASMs have essentially the same optical layout as Drysdale’s and can do all he described and much more thanks to modern light sources and digital detectors. I will continue this discussion using the Point Source Microscope (PSM)* as the example of an ASM because I am most familiar with it and use it on an almost daily basis in my lab. The optical path in the PSM is shown in Fig. 1 when used as an ASM.

Fig. 1 The optical path in the Point Source Microscope when used as an ASM

While the optical paths are the same as in Drysdale’s paper, the light source is the free space end of a single mode optical fiber pigtailed to a laser diode at 640 nm to produce the smallest practical, but very bright, point source producing a near perfect spherical wavefront at the objective focus. In place of the eyepiece and user eye there is a digital camera with a 3.45 µm pixel, megapixel image in the ASM focal plane displayed on a monitor for convenient viewing and saving in a 16 bit format for post processing. At times we forget how much the laser and modern computer have changed optics. Most of the reasons the PSM can do more than Drysdale’s ASM is the vast range of controlled intensity of the source and the sensitivity range of the digital camera.

The PSM is also an autocollimator when the microscope objective is removed because of the collimated beam path inside as shown in the Fig. 2. Because the PSM has a shorter collimator, or tube lens, than most commercially available ACs, it has an angular sensitivity 3-4 times less but a larger angular field of view. This less angular sensitivity is a trade against the instrument size and mass.

Although Drysdale does not mention it, his ASM had a feature that the PSM also has, it can be used as an ordinary reflection inspection microscope as in Fig. 3. There is a second light source in the PSM to give full field, nearly uniform illumination over a ~1 mm object space field of view (FOV) with a 10x microscope objective. This gives the option of locating a particular feature on a sample within the 1 mm FOV and illuminating a very small patch with an intense spot of light via the laser diode, or an external source fiber coupled into the PSM. Figure 4 gives an example of piece of paper with printed lines and the small bright image of the Cat’s eye reflection.

Fig. 2 PSM as an autocollimator

Fig. 3 PSM as an inspection microscope using diffuse illumination, and using the point source illumination

Fig. 4 Image of a 1 mm square area of a sample with a bright Cat’s eye reflection

Now that I have described an ASM and how it works I will discuss a few of the mechanical hardware tools used with the ASM and with autocollimators.

As mentioned earlier, an ASM locates a point in 3 degrees of translational freedom, but a point is an abstract idea. A solid, spherical ball is a physical realization of a point whose location is determined mechanically by touching its surface at 4 or more points to calculate its center. Because an ASM also finds the center of the ball to the same or better precision than it is found mechanically, the ball serves as an artifact that transfers an optical datum to a mechanical one, or vice versa.

Steel balls are a perfect type of ball for this purpose. They are commodity items available in standard sizes and qualities. Half inch, 7/8” and 1.5” balls are particularly useful because these are the sizes of spherically mounted retroreflectors (SMR) and their mounts, or nests, used with laser trackers. Fig. 5 shows a 0.5” ball and nest while Fig. 6 shows a 0.5” SMR with a corner reflector mounted so its vertex is at the ball center. Because SMRs are mounted in spheres, an ASM is useful to locate them within a few µm of their required location.

 Fig. 5 ½” steel ball and nest designed to place the ball center ½” above the base

 Fig. 6 ½” Spherically Mounter Retroreflector. The cube corner apex is at the ball center

Pin or plug gauges are similar commodity items for use with an ASM, see Fig. 7. When the ASM is focused on the axis of the cylindrical gauge the reflection is a line image located in 2 translational and one angular DOFs. If the gauge is measured at 2 points along its axis, an ASM establishes the axis against which the plug gauge rests. Later is this series when we talk about alignment of asphere we show how plug gauges are useful datums for locating sagittal and tangential radii of curvature.

Obviously plane mirrors are useful with autocollimators. Aids in tilting plane mirrors are precision rotary tables and goniometers, sine plates and angular gauge blocks. See Figs. 8 and 9.  Sine plates and angle gauge blocks are relatively inexpensive methods of controlling angles to about 1 second of arc. Because you can use an angle gauge block in 2 directions, it only takes 9 gauge blocks to duplicate any angle between 0 and 90 degrees to 1 second of arc. Compound sine plates create 2 orthogonal angles by inserting standard gauge blocks between the platform and roll. 

Fig. 7 Plug gauge

Fig. 8 Set of angle gauge blocks

Fig. 9 Compound sine plate

To make an angle of 5°, for example, with a 10” sine plate you use a 0.872” gauge block because sin(5) = 0.08715.

Now that we have covered most of the hardware used to perform optical alignment we will move on to the definition of an optical axis and how to locate it for single elements as well as lens assemblies in the next Chapter.

*Full disclosure, my company, Optical Perspectives Group, LLC makes and sells the PSM.

Fig. 1 A simple collimator with a point source of illumination. An illuminated target in the same plane could serve as the source.

Collimators are used as a light source for testing camera lenses on a nodal slide optical bench. The collimator simulates a point source, or in astronomical terms, a star, at infinity. For lens testing, the focal length of the collimator is typically 5 times or more the focal length of the lens under test so that the star appears “perfect” to the lens under test. Collimators are also used in MTF measuring instruments to project targets with a structured pattern into the lens under test to measure the lens quality.

An autocollimator (AC) is a collimator with a beamsplitter and an eyepiece so you can see where the reflected “star” falls in the eyepiece of the instrument, as in Fig. 2. A common method of packaging an AC is with a precision ground barrel designed to mate with a mount so that the axis of the barrel can be adjusted in 4 degrees of freedom (DOF). The reticle crosshair in the AC is centered on the axis of the barrel. When the AC barrel axis is normal to a plane mirror in front of the AC, the reflected image from the mirror will be centered on the crosshair and will not move when the AC is rotated about its axis.

Fig. 2 Simple autocollimator shown for visual use, or for a point source and digital camera.

For autocollimators that do not have a mechanical reference axis such as a precision barrel, a cube corner reflector is used to center the reference crosshair on the axis of the instrument. The cube corner reflects light back upon itself so an image of the source as seen in the eyepiece is centered on the source. The crosshair in the eyepiece is set to zero on the image of the source. 

Notice that an AC is only sensitive to two DOF, the two angles the plane mirror is tipped from being normal to the axis of the AC. Typical barrel type ACs have a full field of view of about +/- 1° but the reticle is labelled to give the angle between the plane mirror and the axis of the AC, or +/- 30 are minutes.

Fig. 3 Nikon autocollimator fitted with a digital camera (from Nikon online catalog)

Notice that ACs cannot be used without some auxiliary hardware because all they measure is departure from normal. As an example, assume the AC, mounted vertically looking downward, is zeroed out against a plane surface such as the base of the Nikon instrument. Then the AC measures parallelism when sample plane mirrors or windows are set on the base plane surface.  They are also useful for measuring errors in prism angles where the faces of the prisms are parallel to each other looking through the prism.

Another instrument in this class is the alignment telescope (AT), an AC with more parts to give it more functionality. We use an AT to determine where an axis is or use it to set up an axis because the AT focuses in any plane between the instrument and infinity.  This gives information to determine a line, 4 DOF, in space rather than just two angles.

I have used Fig. VIII from the 1957 Kueffel AT US Patent 2,784,641 in Fig. 4 to show the optically important parts. Starting at the left there is a Galilean 5:1 reverse beam expander and a meniscus element (714) which if decentered, decenters the field of view in the eyepiece, without changing the focus. This is followed by a Newtonian telescope (716) focused on a crosshair reticle that is projected from the AT. Following the reticle is a beamsplitter to bring in a light source, and an erecting eyepiece so the view is right side up when viewing thorough the telescope.

The objective (716) on the Newtonian telescope is used to focus the AT from very close to the front of the instrument all the way to infinity in an almost perfect straight line. Because the aperture and corresponding focal length are about 5 times smaller than the main objective (702), the distance the Newtonian objective must move to achieve this large focus range is very much reduced from moving the main objective.

As opposed to an AC, an AT determines 4 DOF, and thus an axis. It does so by first focusing on a far target, and then on a close target. When both targets are centered on the crosshair in the AT, the axis of the AT is coaxial with the line between the two targets. Making this adjustment is a little trickier than it first sounds. It is an iterative process, and the adjustments must be made in the right order, or you get farther and farther from alignment.

When focused on the far target, change the angle of the AT tube to bring the far target on the crosshair. When focused on the near target, translate the AT to bring the near target on the crosshair. Even following this order of adjustments, you are usually not rotating about the optimum center, so you end up either over or under-shooting the angle adjustment. With patience good alignment is achieved to the precision of the instrument and your ability to set the target on the crosshair.

Since this is a rather long blog, I will put off a discussion of the last instrument, the autostigmatic microscope until next week. I will also discuss a few mechanical gauges and tools that complement optical alignment tools.

For reference see:

https://www.brunson.us/products/optical-tooling-products/optical-instruments-amp-accessories/alignment-telescopes.html

https://industry.nikon.com/en-us/products/optical-manual-measuring/others/autocollimators-6b-led-6d-led/

https://www.vermontphotonics.com/electronic-autocollimators

https://trioptics.com/us/products/optitest-visual-measurement-instruments/

https://www.taylor-hobson.com/products/alignment-level/autocollimators/ultra-dual-axis-digital-autocollimator

If I have overlooked anyone in this list, I apologize.

In the Introduction to this series of articles on optical alignment, I said there were three basic methods of alignment. This article presents my thoughts on these methods. My approach may be a bit unconventional, but I hope this way of beginning makes the whole idea of alignment easier to understand. To illustrate the three approaches, I will consider the case of a concave mirror as an initial example. 

(After reviewing this note it is clear I have not discussed the tools used for alignment such as autocollimators and alignment telescopes. I will do that in the next article, so you know the details of each instrument and have some idea of its sensitivity and range of measurement before I discuss each of the 3 methods of alignment in detail.)

The three methods are 1) exact alignment, 2) alignment by aberration and 3) alignment with a Bessel beam. The exact alignment method should be familiar to everyone and is most easily illustrated by means of a concave, spherical mirror. The center of curvature of a spherical mirror, or surface, is found by focusing an alignment telescope (AT), an interferometer (INT) or an autostigmatic microscope (ASM) at the center of curvature as in Fig. 1. When the reflected light is centered on the cross hair of the AT or ASM, or there are no tilt fringes in the (INT), the focus of the test instrument is at the center of curvature, a point, in 3 translational degrees of freedom (DOF).

Fig. 1 Alignment instrument at center of curvature of a sphere, a point defined by 3 DOF

The second method, alignment using aberrations, is best illustrated by thinking of aligning a concave, parabolic mirror autocollimated with a plane mirror as in Fig. 2. Here the test instrument is at the focus of the parabola looking at the reflected image in a double pass test arrangement. Unless the plane mirror is tilted precisely perpendicular to the optical axis of the parabola, the reflected image will not be centered on the crosshair or tilt free interferometer. The plane mirror is first tilted in 2 degrees of freedom (DOF) to center the reflected light on the test instrument. Unless you are extremely lucky, the centered image will be comatic or aberrated. You use the coma to adjust the tilt of the plane mirror while keeping the image focused on the crosshair while reducing the coma as much as possible to achieve alignment.

The dashed lines in Fig. 2 show that when the focus of the reflected light does not return coincident with the focus of the outgoing light, the rays are not normally incident on the plane mirror. This amounts to retrace error, a term from interferometry, where the reflected rays follow a different path than the incident ones. This is why the return spot of light must be kept centered while reducing aberrations.

Fig. 2 Alignment of a parabola using aberrations. The outgoing rays must be coincident with reflected rays and the normal to the plane mirror parallel to the axis of the parabola to eliminate aberrations

These two methods are simultaneously similar but different. I call the first method exact because depending on the sensitivity of the test instrument, you can locate the center of curvature to less than 1 um, and often better assuming you have a good test environment. This is about as exact as you can get, and the degree of precision is largely independent of the f/# or cone of light the mirror subtends. On the other hand, you have only located the mirror in 3 translational degrees of freedom, because a sphere is a point in space if its radius is reduced to zero, and a point is defined by only 3 translational DOF. The corollary to this is that a sphere, or spherical mirror, has no optical axis. It does have a mechanical axis defined by its periphery, but no optical axis because it is still a sphere no matter how it is rotated about its center of curvature.

The parabola, however, does have an optical axis given to it by the slight departure from a pure spherical shape by its being a parabola. The same goes for any asphere. When the reflected light comes back to the test instrument centered, all this guarantees is that the light collimated by the parabola is incident on the flat mirror normally. It does not mean the flat is perpendicular to the optical axis of the parabola. This takes two more DOF to satisfy this condition and if the condition is not satisfied, you have coma. It then becomes a matter of how sensitive you are to the coma, the aberration, with your test instrument to determine how well you can align the parabola. This sensitivity governs which test instrument you must use to achieve the desired degree of alignment.

(Sidebar – A further consequence of the difference between a sphere and an asphere is that a spherical wavefront remains spherical independent of how far it propagates because all lines from its center of curvature intersect the sphere at normal incidence. This is not the case for an aspheric wavefront. It changes shape as it propagates. To see why to first order, consider an off axis ray from the center of curvature of a parabola, or any asphere. From similar triangles the ratio of y/R is constant as the ray propagates. The difference between a sphere and asphere is proportional to y^4/R^3 and this ratio is not constant.)

You may ask why can’t I use the center of curvature and the focus of the parabola to use the exact method to do the alignment? In theory, you can, as some textbooks show. However, in practice it is difficult to simultaneously view both the center of curvature and focus. This is why aberrations are generally used in this alignment situation. In turns out there are many similar situations where it is easy to draw pictures of optical tests, but it is difficult to impossible in practice to get to the required centers of curvature to do exact alignment. Then you are forced to use aberrations. 

However, alignment with aberrations is a bit tricky because at the beginning of alignment the aberrations will be large and it is not always obvious whether your adjustments are making the aberrations worse or better. This is why you will want to stop down the aperture of the system to reduce the aberrations to the point where you can make sense of them and see that you are going in the right direction to reduce the aberrations. Just as when using a microscope to examine a specimen, you start with the lowest magnification objective to locate the feature of interest and then switch to higher and higher power objectives using the turret.

(Sidebar – The first time you attempt to align some optical component always presents you with the worst possible alignment situation. You have no idea how sensitive any part of the setup is to being in the correct location before the next is added. It is not obvious which adjustment to make first, or even which way to turn the adjustment to make the alignment better. It is very frustrating, particularly if you are working alone and there is no one to ask for advice. There are also times when it is best to work alone; too many cooks… This is why I am excited about the third method of alignment. It is pretty easy to see what you are doing because you have direct feedback.)

The third method of alignment uses a Bessel beam. This is a new method under development because we have recently shown a Bessel beam propagates through an optical system as though it was a single paraxial ray [1]. With a paraxial ray, if you know the ray height as it comes from infinity and enters the first surface, first order optics tells you the height and angle the ray will exit the lens. The plane within the lens where the two paraxial rays intersect, the principal plane, tells us where the lens is along the optical axis of the system (1 DOF) and the equation of the optical axis of the lens determines the remaining 4 DOF. Since simple optical systems have rotational symmetry, the Bessel beam is all that is needed to locate the lens in 5 DOF. We will have more to say about this new method of alignment as we proceed.

In the next couple articles, I will discuss each of the three methods of alignment in more detail to give a better idea where one or the other is more useful depending on other considerations, for example, what equipment you have on hand. Speaking of equipment, I will discuss typical instruments used for optical alignment.

REFERENCES:

[1] Parks, R. and Kim, D., “Physical ray tracing with Bessel beams”, Proc. Winter Topical Meeting, Precision Optical Metrology Workshop, pp. 72-6, ASPE (2023) 

Introduction to a Series of Articles on Optical Alignment

For some time, I have been encouraged to write a book about optical alignment. There have been several half-hearted attempts at beginning, but it never seemed there was enough to talk about and I kept finding new ideas about alignment. I didn’t want the book to be out of date before it was ever published. For this reason, I have a fresh approach for starting again. 

The book will be written as sort of a blog with each stand-alone part being a piece of the bigger picture. It will be a little like Charles Dickens who wrote his novels as serials with a chapter published weekly. This will be a little more complicated, as I feel there are three basic methods of alignment and I want to contrast the three as the serial is written. To help with this scheme, I will also use a set of example systems to illustrate the methods, and the systems will get more complex as the serial develops.

Along the way I intend to toss in tips and references about performing various steps in alignment. For example, if when you first look at an image or interferogram and it looks like a bowl of spaghetti because there is so much aberration it is hard to know where to start, simply stop the system down to reduce the aberrations until it becomes apparent which is the most offending aberration. Then you will have an idea for corrective action. As the alignment is improved, you can increase the stop size until eventually you are viewing the full aperture. Alignment is, after all, governed by paraxial optics.

Before getting into any details, I want to say a few words about why alignment is important and why there is any need for a series such as this. With modern computers and the work of some very smart people, the optical design of lenses and mirrors are about as good as can be achieved. Perhaps a new glass will come along that will help with a certain design defect but this is a detail in the bigger picture, lens design will probably not get much better than it already is.

In addition, with modern CNC polishing techniques and interferometric testing you can get about any degree of optical surface quality you want. Once you have 0.1 rms wave surfaces, even if you have a system with many such surfaces, you are not really going to improve your system performance by asking for 0.05 rms wave surfaces, at least in the visible. The only way to improve the performance of an optical system these days is to put it together more precisely, that is, to align your system better. In this area there is a long way to go for several reasons.

The main reason there is room for improvement is that the design of a system and its assembly are far apart in time and space. By the time hardware shows up in the assembly area, the design people are working on a whole new project. In addition, the designers and assembly people have entirely different skill sets and speak different jargon. There are mechanical engineers in between the two groups but they often hinder communication between the two rather than improve it. My hope is that this set of articles will help improve the situation.

This gives you some idea of where this project is headed. Consider the material a draft that may eventually get organized into a real book, but for a long time it will remain fluid and subject to revision. I solicit your help in this regard. If after reading these articles you have a comment,  suggestion or to point out an error in my thinking, please let me know. My background is limited and if you can share your experiences, it will only make this effort better. All additions to the text will be acknowledged unless you wish to remain anonymous. 

One other matter about the organization of the material, I would like to keep the text and ideas as simple as possible so that the articles can be read and appreciated by people with any skill set. There are people who may want more detail, and I will try to keep these more detailed explanations as side bars for the more interested. I will try to make this detailed material obvious, and suggest it be ignored by those who want just the basic ideas. This is in line with my feeling that when you push an engineering problem hard enough it becomes science, interesting science, but stopping to look at the science doesn’t necessarily get the hardware out the door, the thing your boss wants most.

Wish me good fortune in this effort,

Bob Parks,

Optical Perspectives Group, LLC