Centering station used for lens inspection for “drop-in” lens assembly
When lenses are assembled using the drop-in method, the lens elements are constrained by the cell bore into which they are “dropped”. Fig. 1 gives an example of a drop-in assembly where the first lens in is supported by a seat and the cell wall while subsequent lens elements are separated by spacers.
For all the lenses in this assembly their diameter is critical. If they are too large, they won’t fit in the cell and if they are too small, they will be randomly decentered. Thought of another way, if the diameter is too small the optical axis of the lens may be parallel to but will not be concentric with the axis of the cell.
If the lenses have wedge, this is also a problem because the optical axis of the lens will not be parallel to the axis of the cell even if the lens is centered. This means that every lens must be inspected for diameter and wedge before kitting will a cell for assembly.
We do drop-in assembly because it is cost effective for commodity type lenses. This means the method of inspection must also be cost effective. The singlet in Fig. 1 is used to illustrate a method of inspection using our Bessel beam centering station in Fig. 2.
To the left in Fig. 2, the centering detector, a PSM, is centered on the projected Bessel beam before the lens in inserted. Since the lens is the correct diameter and free of wedge the Bessel beam propagates through the lens without any deviation. A Bessel beam is used because it propagates like a single ray in a ray trace. The PSM centroids on the Bessel beam with a sensitivity of < 1 μm.
In the middle view, the lens diameter is too large, and this shifts the optical axis off the projected Bessel beam in the plane of the Figure causing the beam to bend so that it passes through the focal point that is on the optical axis. The PSM which was centered on the Bessel beam will show the deviation in proportion to how far it is beyond the focal plane of the lens. Typically, the PSM is placed 10 times the lens focal length so a 2 μm larger diameter will shift the beam by 10 μm at the PSM (10 times the radius difference of 1 μm).
Because the lens has no wedge in this case, its optical axis is parallel to the incident Bessel beam. This means that if the lens is rotated 180 ° against the edge constraint the optical axis does not change position and the decenter of the Bessel beam at the PSM remains constant.
On the right, the lens has wedge, so the optical axis makes an angle with the incident Bessel beam which in turn means the Bessel beam is deviated going through the tilted lens. Now, however, if the lens is rotated 180 ° the optical axis tilt is in the other direction and the Bessel beam will also be deviated in the opposite direction. This is how you tell wedge from a difference in nominal diameter. Fig. 3 is what you see on the PSM monitor if you insert a lens for inspection.
The origin in Fig. 3 is the Bessel beam centroid location before inserting the lens on the window in the centering station and locating it against the edge constraints. For a lens that has a diameter that is too large and some wedge you might get a centroid at the position noted as Beam at 0 deg. Once the lens is rotated 180 degrees you get the second centroid. The distance from the origin to the average location of the two centroids is proportional to the error in diameter from nominal. The distance from the average to either centroid is the error due to wedge.
This simple inspection can be done almost as fast as you can set the lens on the window and push a button to log the centroid location.
u/OpticalBobParks — 6 days ago
▲ 4 r/Optics
Optical axis of an assembly of lenses
Everyone seems to agree that the line between the centers of curvature of a single lens element is the optical axis. Because it is the line between two spherical surfaces, it is normal to the surfaces so a light ray along the optical axis is not deviated but exits the lens coaxial with how it enters. Does anyone see a reason not to adopt the same definition for an assembly of lenses such as a camera lens or Cooke triplet? To the best of my knowledge I do not know of anyone who has discussed this question. I know several people who claim there is no optical axis to an assembly of lenses, but I don't believe they were thinking in terms of my definition.
u/OpticalBobParks — 7 days ago
Optical axis of an assembly of lenses
Everyone seems to agree that the line between the centers of curvature of a single lens element is the optical axis. Because it is the line between two spherical surfaces, it is normal to the surfaces so a light ray along the optical axis is not deviated but exits the lens coaxial with how it enters. Does anyone see a reason not to adopt the same definition for an assembly of lenses such as a camera lens or Cooke triplet? To the best of my knowledge I do not know of anyone who has discussed this question. I know several people who claim there is no optical axis to an assembly of lenses, but I don't believe they were thinking in terms of my definition.
u/OpticalBobParks — 8 days ago
▲ 17 r/OpticalAlignment
Classical optical alignment instruments
The most basic optical instrument used for alignment and testing of optics is a collimator, an instrument designed to produce a parallel beam of rays, or a plane wavefront, from a point source of light. Turning words into hardware we have something like Fig. 1 where a single mode optical fiber serves as the point source and is placed at back focus of an infinity corrected doublet lens.
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.
Currently there are electronic ACs on the market with sensitivities of 0.1 µradian or better. This precision can only be obtained if the plane mirror is close to the AC because of environmental influences. A particularly useful AC and mount are made by Nikon, see Fig. 3. The mount is well designed to orient the AC in any possible direction and a single axis filar eyepiece reads the angle in both directions. The instrument is especially helpful for measuring odd prism angles because of the agility of the mount. It can also be focused slightly to keep the crosshair crisp in case the plane surface has a slight curvature.
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.vermontphotonics.com/electronic-autocollimators
https://trioptics.com/us/products/optitest-visual-measurement-instruments
If I have overlooked anyone in this list, I apologize.
u/OpticalBobParks — 9 days ago
Calibration of an alignment telescope using a Bessel beam
Recently, I was asked if you could calibrate an alignment telescope using a Bessel beam. I was sure the answer was yes, but I don't like to claim things without trying an experiment first. Today, I got a chance using a Davidson D-271. It follows a Bessel beam as you focus from about 400 mm to infinity for the Bessel beam I made using a spherical wavefront. I don't know what would happen with plane wavefront illumination, I am not set up for that. A Bessel beam made using a plane wave has a finite extent so this would be an interesting experiment to try.
The Davidson D-271 does not have distance markings on the focus adjust so you would have to make a table of focus distance per turn of the focus adjustment to know any lack of straightness at a particular distance, but I could not see any deviation of the Bessel beam core except for a slight linear shift due to me not being perfectly aligned to the Bessel beam.
Maybe this will inspire someone with a better lab environment and an alignment telescope with a distance marking on the focus to repeat my experiment to see if this idea is really useful. What I can say is that to 1st order it works, and probably works very well given a better environment than my crude lab set up.
u/OpticalBobParks — 23 days ago
Busy, Busy
I must say right now I am very busy but I can see that there is an interest on the subject of Optical Alignment here.
If you would like to I've got several other Chapters and such over on Medium if you would like to go and take a look
https://medium.com/@reparks_11319/chapter-3-classical-optical-alignment-instruments-89ccc2d2c58c
Let me know what you think
u/OpticalBobParks — 1 month ago
▲ 14 r/Optics
👋 Welcome to r/OpticalAlignment - Introduce Yourself and Read First!
Precision optical alignment is where optical design becomes optical performance
Hey everyone! I'm u/OpticalBobParks, a founding moderator of r/OpticalAlignment.
Welcome to a community dedicated to one of the least visible, but most essential, parts of modern optical engineering. The finest optical design, manufactured from nearly perfect optical components, cannot achieve its intended performance unless it is assembled and aligned to the design specifications. Alignment is the final step in realizing the full potential of an optical system.
This community brings together optical engineers, optical designers, metrologists, technicians, machinists, physicists, and hands-on astronomers who design, build, align, test, and troubleshoot optical systems.
What to Post
The Tools of the Trade
Everything from classical autocollimators and alignment telescopes to modern coordinate measuring machines (CMMs) used as large XYZ stages for optical assembly. We welcome discussions on traditional techniques, new instrumentation, and creative shop-floor solutions.
Reference Axes and Alignment Methods
The use of rotary tables, lasers, and Bessel beams as reference axes; sensors and centroiding methods; techniques for establishing mechanical and optical axes; and methods for measuring lens centration and system alignment, whether components are mounted in cells or assembled on an optical bench.
Instrument-Specific Case Studies
Every optical instrument presents unique alignment challenges. Whether the subject is telescopes, microscopes, spectrometers, imaging systems, or other optical instruments, we are interested in practical techniques, lessons learned, and honest discussions of what works—and what doesn't.
Optomechanical Problem Solving
The intersection of optics and mechanics is where many alignment problems are solved. Topics include kinematic design, degrees of freedom, alignment strategies, tolerancing, and the compromises required when a system cannot provide enough adjustment to achieve perfect alignment.
Community Vibe
Whether you are assembling a multi-million-dollar space telescope, using a milling machine as an improvised long-travel heavy-load XYZ stage, or simply trying to measure the focal length of a single lens, you'll find people here who understand the challenges.
We encourage you to share your lab setups, ask questions, and discuss both successes and failures.
Optical designers, engineers, and supervisors have many opportunities to exchange ideas through journals, conferences, and technical societies. The technicians and alignment specialists who assemble and align the hardware often have far fewer opportunities to share their knowledge. We hope this community becomes a place where those working behind the scenes like those spending their days in bunny suits can exchange ideas, solve problems, and ask the practical and mundane questions that need to be asked but aren’t worthy of a paper.
How to Get Started
- Introduce yourself in the comments below.
- Post something today! Even a simple question can spark a great conversation.
- If you know someone who would love this community, invite them to join.
- Interested in helping out? We're always looking for new moderators, so feel free to reach out to me to apply.
Thanks for being part of the very first wave. Together, let's make r/OpticalAlignment amazing.
u/OpticalBobParks — 1 month ago
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 halfhearted 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 jargons. 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.
u/OpticalBobParks — 1 month ago
👋 Welcome to r/OpticalAlignment - Introduce Yourself and Read First!
Precision optical alignment is where optical design becomes optical performance
Hey everyone! I'm u/OpticalBobParks, a founding moderator of r/OpticalAlignment.
Welcome to a community dedicated to one of the least visible, but most essential, parts of modern optical engineering. The finest optical design, manufactured from nearly perfect optical components, cannot achieve its intended performance unless it is assembled and aligned to the design specifications. Alignment is the final step in realizing the full potential of an optical system.
This community brings together optical engineers, optical designers, metrologists, technicians, machinists, physicists, and hands-on astronomers who design, build, align, test, and troubleshoot optical systems.
What to Post
The Tools of the Trade
Everything from classical autocollimators and alignment telescopes to modern coordinate measuring machines (CMMs) used as large XYZ stages for optical assembly. We welcome discussions on traditional techniques, new instrumentation, and creative shop-floor solutions.
Reference Axes and Alignment Methods
The use of rotary tables, lasers, and Bessel beams as reference axes; sensors and centroiding methods; techniques for establishing mechanical and optical axes; and methods for measuring lens centration and system alignment, whether components are mounted in cells or assembled on an optical bench.
Instrument-Specific Case Studies
Every optical instrument presents unique alignment challenges. Whether the subject is telescopes, microscopes, spectrometers, imaging systems, or other optical instruments, we are interested in practical techniques, lessons learned, and honest discussions of what works—and what doesn't.
Optomechanical Problem Solving
The intersection of optics and mechanics is where many alignment problems are solved. Topics include kinematic design, degrees of freedom, alignment strategies, tolerancing, and the compromises required when a system cannot provide enough adjustment to achieve perfect alignment.
Community Vibe
Whether you are assembling a multi-million-dollar space telescope, using a milling machine as an improvised long-travel heavy-load XYZ stage, or simply trying to measure the focal length of a single lens, you'll find people here who understand the challenges.
We encourage you to share your lab setups, ask questions, and discuss both successes and failures.
Optical designers, engineers, and supervisors have many opportunities to exchange ideas through journals, conferences, and technical societies. The technicians and alignment specialists who assemble and align the hardware often have far fewer opportunities to share their knowledge. We hope this community becomes a place where those working behind the scenes like those spending their days in bunny suits can exchange ideas, solve problems, and ask the practical and mundane questions that need to be asked but aren’t worthy of a paper.
How to Get Started
- Introduce yourself in the comments below.
- Post something today! Even a simple question can spark a great conversation.
- If you know someone who would love this community, invite them to join.
- Interested in helping out? We're always looking for new moderators, so feel free to reach out to me to apply.
Thanks for being part of the very first wave. Together, let's make r/OpticalAlignment amazing.
u/OpticalBobParks — 1 month ago
▲ 0 r/Optics
AI4Wave, a Means of Wavefront Measurement Without an Interferometer?
A common way to measure an optical wavefront is with an interferometer. They're incredibly powerful, but they're also expensive, sensitive to vibration, and not always practical for production or integration into existing test setups.
I've been working with collaborators at Innovations Foresight on an alternative approach using their AI4Wave software that reconstructs the wavefront from a defocused digital image of the point spread function (PSF) magnified by a microscope objective. My interest is in optical alignment, and the software uses the increased information in the defocused image of an autocollimator or autostigmatic microscope to increase the sensitivity to tilt and focus, the low order wavefront terms affecting alignment.
For measuring wavefronts, you are interested in the higher order terms, but you can use the same instrument, a Point Source Microscope, or PSM, in both situations.
There are two ways to use the combination of software and hardware:
Single-pass mode with the PSM acting as an imaging instrument, capturing the defocused PSF directly, or double pass in reflection mode where the PSM provides a near-perfect reference wavefront, and the reflected, defocused wavefront from the test optic is captured and analyzed with the AI software.
One advantage is that everything is integrated into a single software package. The same interface controls the illumination intensity, camera exposure, image acquisition, and AI-based wavefront reconstruction, making the measurement process seamless.
The figure shows a single pass example based on an image viewed through a telescope with a deformable mirror to correct for atmospheric turbulence. The top row is the open-loop system viewing a star giving a low Strehl ratio of 0.17 due to the atmosphere. After correction by activating the deformable optics (bottom row), the reconstructed wavefront produces an improved PSF with a Strehl ratio of 0.75.
I think this is a great example of how an instrument’s sensitivity can be increased using some insight and well-designed and trained software.
I'm curious what this community thinks.
Do you think it is worthwhile to effectively refurbish instruments working at the limits of their resolution, or sensitivity, by applying AI trained software? Is this a cost effective approach, possibly the only approach to more sensitivity?
Where do you see this approach fitting into optical testing or adaptive optics workflows?
What limitations would concern you most?
I'd love to hear your thoughts and experiences.
u/OpticalBobParks — 1 month ago
▲ 17 r/Optics
A short history of the CaliBall™ and the Random Ball Test
Back in the late 1990’s NIST had a number of firms that wanted to send their interferometer transmission spheres there for calibration but NIST was not in this sort of calibration business. While I was at NIST consulting for Chris Evans in the Precision Machining Facility we thought of the idea of a self-calibration test for transmission spheres that was a spherical analog of the plane surface test(1) used to self-calibrate interferometric surface roughness testing microscopes. We called our method the Random Ball Test (RBT). It relied on averaging multiple interferograms of random patches of a precisely polished ball. The test worked as expected and we published the results in a fairly obscure meetings proceedings(2).
While the RBT worked as we expected and provided the desired method of self-calibration, it was not a practical method because it used a ball made of black filter glass that was rather soft and easily damaged. The glass had to be opaque to eliminate a coherent reflection from the far side of the ball. Another ball was made of harder, transparent glass where a small hole was drilled through the center of the ball to block this reflection but this made the ball more expensive and the surface not completely random. The idea was left as an interesting exercise that solved a serious calibration problem but had little practical value.
About 5 years later I became aware of commercially available, precision silicon nitride balls and these made it look like the RBT could be commercialized. The CaliBall™ was first marketed in 2005 and well over 300 have sold since then. The 1” diameter, Grade 5, silicon nitride ball is extremely hard and tough, has a reflectivity of about 11%, a good compromise for use with both uncoated and highly reflective transmission spheres, and resists stains and finger prints much better than steel balls. Further, the SiN balls do not dent as some steel balls do with mishandling.
In the random ball test the ball artifact, sitting on a kinematic support of 3 hard points, is placed so its center is at the focus of the transmission sphere to be calibrated. The ball surface facing the transmission sphere acts as a convex mirror whose center of curvature is at the transmission sphere focus. An interferogram is taken and the resulting contour map is saved. The ball is removed from its kinematic support, arbitrarily rotated and replaced on the support. Another interferogram is taken and averaged with the first. This process is repeated a number of times although about 10 times is enough to get a good idea of the errors in the transmission sphere as can be seen from this paper with typical examples(3).
The question then comes up, how good is the RBT? For one, it should not be used to calibrate slow transmission spheres; diffractions effects start to creep in around f/7 or slower that compromise the results. On the other hand, for faster transmission spheres rather extensive tests were run at CSIRO by Jan Burke. In a paper(4) covering not only the RBT but several other self-calibration methods for transmission spheres, Burke comes to the conclusion that the RBT gives the most precise and consistent results of all methods tried, but that the RBT is somewhat tedious due to having to move and replace the ball between interferograms. This seems a small price to pay for a robust calibration method that takes but a few minutes to perform.
1 Creath, K. and Wyant, J. C., “Absolute measurement of surface roughness”, Appl. Optics, 29, 3823–7 (1990).
2 Parks, R. E., Evans, C. and Shao, L., “Calibration of interferometer transmission spheres”, OSA, Technical Digest Series, Optical Fabrication and Testing, Hawaii (1999).
3 W. Cai, D. W. Kim, P. Zhou, R. E. Parks, and J. H. Burge, “Interferometer Calibration Using the Random Ball Test,” in International Optical Design Conference and Optical Fabrication and Testing , OSA Technical Digest (CD) (Optical Society of America, 2010), paper OMA7.
4 Jan Burke and David S. Wu, “Calibration of spherical reference surfaces for Fizeau interferometry: a comparative study of methods,” Appl. Opt.49, 6014–6023 (2010)
u/OpticalBobParks — 1 month ago