AT A GLANCE

Customer:  University of Arizona & the NASA/CNES FIREBall-2 collaboration, led by Dr. Simran Agarwal and Dr. Aafaque Khan

Challenge:  Realign a damaged two-mirror corrector to ±25 µm and 1 arcminute, without touching the UV coatings, with a four-month deadline

Solution:  Two custom CGH nulls on a single substrate: one to verify the damaged mirror’s surface, one to align both mirrors

Outcome:  Alignment within tolerance, image quality better than the original build, and a successful flight in September 2023

01 — THE MISSION

A Telescope Suspended from a Balloon

FIREBall-2 is a NASA/CNES astronomy experiment with an unusual observing platform. Its 1-meter telescope and ultraviolet, multi-object spectrograph are suspended beneath a stratospheric balloon, floating high enough to see UV light that never reaches the ground.

The instruments’ science target is the circumgalactic medium, the faint halo of gas that surrounds galaxies and feeds their star formation. That gas glows near 205 nm, a wavelength that the atmosphere absorbs almost completely. It is, however, readily detectable at balloon altitudes.

The FIREBall-2 observatory was designed to trace this diffuse gas, helping astronomers to understand its role in galactic formation. The unique payload was built and flown by a collaboration led by the University of Arizona and Caltech.

Fig. 1 — The FIREBall-2 payload: gondola (left), optical path (center), and the instrument stack (right). The two focal corrector mirrors, FC1 and FC2, sit at the top of the spectrograph assembly. Image: Agarwal et al., JATIS (2025).

02 — THE PROBLEM

A Hard Landing

On its first flight in September 2018, FIREBall-2 launched cleanly from Fort Sumner, New Mexico. Then, a puncture in the balloon forced an early termination of the flight. The gondola landed harder than planned. When the recovery team reached it, the structure and telescope mirrors showed clear signs of impact. One optic in particular: FC2, the larger of the two mirrors in the telescope’s focal corrector, had fractured at one of its bond pads (Figure 2). A piece of the substrate, pad and all, had broken off.

Fig. 2 — Left: the recovery team reaches the gondola the night after the 2018 flight. Center: the focal corrector assembly before flight. Right: the fracture at one of FC2 bond pads (circled). Image: Agarwal et al., JATIS (2025).

The focal corrector is a critical, precision optical assembly. It consists of a small convex hyperboloid (FC1) facing a large concave ellipsoid (FC2). Together, the two mirrors re-image the telescope’s focal plane onto the spectrograph’s slit mask, correcting coma across a wide field of view while holding the magnification almost exactly at one. Achieving this correction requires the two mirrors to be aligned within 25 microns laterally, with spacing controlled to 40 microns and relative tilt under one arcminute. If these tolerances are not met, the galaxy targets will not align with the slit mask, and the spectrograph cannot perform its analysis.

03 — THE CONSTRAINTS

Why the Original Alignment Method Wouldn’t Work

The focal corrector had been aligned once before, for the first flight. At that  time, the team was able to use a traditional alignment approach: a 5-micron pinhole source, a microscope camera, a crosshair fixture seated in FC2’s central hole, and a CMM (coordinate measuring machine) arm to iterate positions. The team inferred alignment from the quality of an on-axis image spot, then cross-checked it using contact metrology. Careful characterization was needed at every stage to keep tolerances from stacking up. The alignment was successful, but it was slow and indirect.

Following the damage to the corrector, a repair attempt was made during the Covid pandemic to prepare the observatory for a planned 2022 flight. The attempt exposed the difficulty of using this method under pressure. When that flight was canceled, the team decided to fully debond the optics and realign the corrector properly for a Fall 2023 campaign. The deadline was still daunting: all test design, fabrication, alignment, bonding, and verification had to be completed in roughly four months.

In addition to the tight timeline, a second constraint made re-alignment even more challenging. The mirrors’ UV-optimized coatings are highly sensitive to contamination, and any contact with the mirrors risked depositing material on the optical surfaces. Contact-based metrology would not be an option; only non-contact methods would be possible

The team needed a way to measure mirror positions directly, in real units, in real time, without touching either mirror surface. It was at this point when the project turned to Arizona Optical Metrology for this mission-critical solution.

04 — A SHORT PRIMER

How a CGH Works

Precision optics are typically tested using non-contact laser interferometry. In the interferometer, a laser source is split into two beams, with one beam directed to the test optic/assembly and the other to a “perfect” reference with similar shape to the test wavefront. When the beams recombine, a pattern of interference fringes is formed, from which the surface or wavefront error can be calculated.

Planar (flat) or spherical references are relatively easy to manufacture, so measuring flat or spherical optics is generally straightforward using an interferometer. To use the interferometer for more complex optical surfaces, however, the test wavefront must be “augmented” Traditionally, testing an asphere like this required a custom null lens assembly, and months of design and fabrication for a one-off stack of precision glass.

When the surface is accurate and perfectly positioned relative to the CGH, light retraces its path and the interferometer sees a “null” — a nearly blank fringe pattern. Any surface error or misalignment will appear as non-null fringes, which the interferometer reads out to nanometer-level precision.

A computer-generated hologram (CGH) is a micron-scale grating pattern that is written onto a substrate using photolithography techniques. When laser light from an interferometer passes through the CGH, it diffracts into a precisely prescribed wavefront that can be designed to exactly match an aspheric mirror surface. A CGH encodes the same correction as a custom null lens, and it can be designed and fabricated in a few weeks. Moreover, a single substrate can carry several independent patterns with nearly perfect co-registration.

05 — THE SOLUTION

Two Powerful Holograms

Working with the University of Arizona team, AOM designed two CGHs around the as-built prescriptions of the flight mirrors.

CGH #1: Was the Damaged Mirror Still Functional?

Before realigning the corrector, the team had to answer a basic question: did the fracture also warp FC2’s optical surface?

The first CGH was built to find out. It consisted of several patterns that function simultaneously. An annular null reshapes the interferometer’s spherical beam to match FC2’s ellipsoidal optical surface for a full-aperture figure error measurement. A retro pattern provides alignment feedback of the CGH itself to the interferometer. Corner patterns project four fiducial points onto the mirror face for coarse positioning, and a central pattern projects a reference point at the mirror’s vertex. (Figure 3)

The verdict from the measurement: FC2’s surface figure was essentially unchanged from its 2016 fabrication report (roughly 87 nm RMS measured, versus 97 nm as built). The CGH approach saved the program the cost and time of refabricating a large aspheric optic and assessed the mirror quality without a single probe touching the coating.

Fig. 3 — Checking the damaged mirror: FC2 in its five-axis mount, measured through the surface CGH with a Zygo Verifire interferometer. Image: Agarwal et al., JATIS (2025).

CGH #2: Aligning the Two Mirrors

The second CGH showcases this method’s power and speed (Figure 4). A single 6-inch substrate, positioned in the converging space between FC1 and FC2, carries three concentric pattern zones. A central circle and four spokes retro-reflect the interferometer beam to position the CGH to the input beam. An inner annulus shapes light to retro-reflect from FC1’s convex hyperboloid. An outer annulus does the same for FC2’s concave ellipsoid.

Fig. 4 — The two CGH designs. Left: the surface-measurement CGH, with corner fiducial projectors, a central vertex reference, and an annular null for FC2. Right: the alignment CGH, with concentric zones serving the CGH alignment reference (center and spokes), FC1 (inner annulus), and FC2 (outer annulus). Image: Agarwal et al., JATIS (2025).

The result is one test setup that measures the position and orientation of both flight optics simultaneously, each against a common, self-registered reference. When the system is adjusted such that every hologram returns a null, the system is aligned!

Fig. 5 — How the light travels. Left: the surface test reshapes the f/1.5 beam to match FC2. Right: the alignment test — separate patterns on one substrate send retro-reflecting beams to FC1 (blue), FC2 (green), and back to the interferometer for CGH alignment (red), all at once. Image: Agarwal et al., JATIS (2025).

06 — IN THE LAB

Alignment by the Numbers Rather than “Feel”

With the CGH registered to the Zygo interferometer, aligning the mirrors became a deterministic loop rather than an art. Each optic’s interferogram was measured with phase-shifting interferometry and decomposed into Zernike coefficients — the standard vocabulary of optical aberrations. Because AOM’s design process models the exact amount of wavefront error that each micron of decenter or arcsecond of tilt produces, those coefficients convert directly into physical corrections, allowing the team to easily adjust the system, re-measure, and converge on the aligned positions.

Fig. 6 — The alignment station: Zygo interferometer and transmission sphere at left, the 6-inch CGH on a five-degree-of-freedom mount at center, and both flight mirrors on precision stages. Image: Agarwal et al., JATIS (2025).

The same test setup for alignment also allowed the team to verify performance. After the first bonding attempt, releasing the assembly from its ground-support fixtures revealed that FC1 had shifted beyond tolerance. The stressed flexures had relaxed and pulled the mirror with them. With a conventional imaging test, diagnosing that could have consumed weeks. Here, the CGH measurement flagged the issue immediately and quantified exactly how far each mirror had moved. The team debonded the optic, swapped in spare flexures, the installation, and realigned. The second attempt held position and was successful.

Fig. 7 — What the measurement looks like: interferograms before bonding (left) and after release from the fixtures (right). Each concentric zone is a separate, simultaneous measurement — the two mirrors’ returns in the annular regions. Agarwal et al., JATIS (2025).

The measurement also settled a subtler question. A small residual decenter appeared after the optic was released (Figure 7). However, when the assembly was rotated about its optical axis, the offset stayed fixed in the same orientation. That signature identified it as gravity sag in a flexure rather than a bonding error, and since the corrector flies with gravity along its optical axis, it was harmless. The ability to measure the optic positions directly enabled the team to make this kind of diagnosis quickly. That’s the kind of diagnosis you can only make when your test measures positions directly.

07 — RESULTS

Better Aligned than the Day it was Built

The finished assembly met every requirement, with residual lateral errors within the 25-micron tolerance and relative tilts around a tenth of an arcminute — ten times better than the requirement. Alignment was monitored continuously through the seven-day epoxy cure and re-verified in multiple orientations.

As an independent cross-check, the team repeated the classic pinhole imaging test and compared the result against the same measurement from 2017 (Figure 8). The CGH-aligned system produced an image spot roughly half the width of the original and visibly more symmetric, with the coma and astigmatism that had the original alignment when the system was manufactured.

Fig. 8 — Before and after: the on-axis image of a 5-micron pinhole through the focal corrector, as aligned by the legacy process for the 2017 campaign (top) and by the CGH process for 2023 (bottom). The 2023 spot is roughly half the width and far more symmetric. Image: Agarwal et al., JATIS (2025).

08 — TAKEAWAYS

Why Align with CGHs

AOM’s CGH alignment technology has been an essential part of dozens of programs. While most of the alignment systems that AOM builds are proprietary to the programs they serve, FIREBall-2 offers a rare chance to showcase the details of a customer test. The advantages demonstrated by this project apply in most cases:

Direct measurement  —  CGH-based testing reports the actual position of each optic, instead of inferring alignment from downstream image quality. The system reports misalignments that can be corrected in real time, rather than waiting for them to appear as symptoms in the final image quality.

One substrate, many functions  —  Nulls for multiple optics, CGH alignment retros, and projected fiducials can share a single hologram, with near perfect co-registration to each other and at minimal added fabrication cost.

Speed  —  Design and fabrication in weeks, versus months for a conventional null lens. The deterministic alignment loop compresses lab time just as dramatically. The whole FIREBall-2 realignment process was completed within four months.

Non-contact  —  Nothing in the testing process touches the sensitive optical surfaces and coatings.

Built-in verification  —  The alignment test doubles as a monitoring and diagnostic tool through epoxy cures, after fixture release, in any orientation. Any shift is immediately apparent, and by quantifiable amounts.

Works with your hardware  —  The CGH is designed around the interferometer, ubiquitous in the optics industry, and often with the reference accessories you already own

Have a System that Requires Precision Alignment?

Arizona Optical Metrology designs and fabricates custom computer-generated holograms for the measurement and alignment of aspheres, freeforms, and multi-element optical systems — from single-surface figure verification to full alignment stations like the one described here. If your program has tight tolerances and a tighter schedule, we’d like to hear about it. Visit aom.us or contact our team.

This article is based on the peer-reviewed paper: Agarwal and Khan et al., “Alignment and performance verification of two-mirror focal corrector optics using computer-generated holograms for balloon-borne ultraviolet Telescope FIREBall-2,” J. Astron. Telesc. Instrum. Syst. 11(2), 025003 (2025), DOI: 10.1117/1.JATIS.11.2.025003. All figures are reproduced from that publication, courtesy of the FIREBall-2 team. FIREBall-2 is funded by NASA’s APRA program and CNES.

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Katie joins AOM with over 20 years’ experience in instrumentation and measurement systems, including seven years as a Physical Scientist at the US Naval Research Laboratory. She holds a  B.S. in Optics from the University of Rochester, and her expertise spans measurement and modeling, image and signal processing, and field trial support. She will be based in our Rochester, NY office.

We’re excited to have her expertise to help our clients find the right optical metrology solutions. Welcome to the team, Katie!

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AZoOptics sat down with Arizona Optical Metrology’s Logan Graves to learn how our Morpheus™ software provides fast, automated CGH data correction for greater accuracy and efficiency.

Read the full interview at AZoOptics

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Thanks for everything, Sam! We’ll miss having you in the office.

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This image captures a small section of NSF–DOE Vera C. Rubin Observatory’s view of the Virgo Cluster, offering a vivid glimpse of the variety in the cosmos. Visible are two prominent spiral galaxies, three merging galaxies, galaxy groups both near and distant, stars within our own Milky Way, and much more.
NSF–DOE Vera C. Rubin Observatory

The Vera C. Rubin Observatory recently released its breathtaking first images, opening a new era for deep-sky exploration. Originally called the Large Synoptic Survey Telescope (LSST), Rubin will build an unprecedented dataset of the universe, a collection of wide-field images of the night sky taken every few nights for ten years!

What many may not realize is that tucked behind these astonishing images is the quiet work of precision metrology, and AOM is proud to have played a key role in all three mirrors and the corrector lenses.

A Mirror Like No Other

The telescope itself uses three large mirrors, but the primary (first reflection) and tertiary (third reflection) mirrors are manufactured onto a single lightweight piece of glass—an extraordinary engineering feat.

AOM co-founder Dr. Jim Burge was deeply involved in the mirror’s design, fabrication, and metrology while at the University of Arizona’s Richard F. Caris Mirror Lab.

AOM co-founder Dr. Chunyu Zhao supported the metrology by providing three different computer-generated holograms (CGHs): one to certify the PM null lens, and two more for directly testing and certifying the tertiary mirror.

Scanning the Sky’s Convex Eye

The convex 3.4 m diameter M2 mirror mated to its active cell and mounted in the metrology tower ready for testing.

The giant convex secondary mirror presented its own unique challenge. It was tested at L3Harris (formerly Exelis) using a custom-built scanning and stitching interferometer developed by Cormic Merle, now Director of Metrology Systems at AOM in Rochester.

William J. Gressler, Douglas R. Neill, John Bagnasco, Paul J. Lotz, Dave Mills, Mike Warner, Oliver Wiecha, Gregory N. Bogan, James M. Ellis, Dale C. Zajac, Steven A. Szklany, Ronald T. Broccolo, Malcolm N. O’Sullivan, Frank A. Carbone, Cormic K. Merle, “LSST secondary mirror assembly,” Proc. SPIE 10700, Ground-based and Airborne Telescopes VII, 1070019 (6 July 2018); https://doi.org/10.1117/12.2311316

The system collected 480 measurements using a 27-inch CGH, a 40-inch transmission flat, and a massive collimator—all rotating around the suspended mirror to build a full-aperture map, resolving details as small as 3 mm (>600 cycles/aperture).

Cormic led the development of the system, but he’s quick to note that the project wouldn’t have been possible without the critical contributions of current AOM Senior Project Manager Melissa Ryczek, who operated the test setup and processed much of the data as an Optical Technician at the time. “If it weren’t for her, I’d probably still be there processing the data,” he said. “She was my right hand.”

World’s Largest Contact Lenses

The Rubin camera includes two massive lenses—measuring over 1.6 and 1.2 meters in diameter and just over 82 mm and 30 mm thick—earning them the nickname “the world’s largest contact lenses.”

These optics were fabricated, measured, and assembled at Arizona Optical Systems (AOS), co-founded by Jim Burge. Computer-generated holograms from AOM were used to measure the surfaces of the lenses, the lens alignment in the carbon fiber cell, and the final transmitted wavefront for the assembly.

From Holograms to the Heavens—and Beyond

RubinObs/NOIRLab/SLAC/DOE/NSF/AURA/H. Stockebrand

At AOM, we’re honored to have contributed to the Rubin Observatory—a project that exemplifies what’s possible when precision engineering meets astronomical ambition. The same expertise that supports billion-dollar, one-of-a-kind instruments is available to help our customers tackle demanding optical challenges of their own.

Our team brings deep, hands-on experience in optical metrology—from designing CGHs for massive telescope mirrors to aligning ultra-thin lenses with micron precision. Today, we’re applying that knowledge to commercial and research applications where the technical stakes are just as high, but the budgets and timelines are often much tighter.

Want to learn how computer-generated holograms or other products from AOM can streamline your next large optics project? Explore our custom CGH and optical alignment capabilities or contact us to get started.

The post Behind the Scenes of the Rubin Telescope: How AOM Helped Shape the Sky’s Sharpest Eye appeared first on AOM - Arizona Optical Metrology.

Enter Morpheus™, a full-featured CGH data reduction software designed to take the guesswork out of data processing. With just a few clicks, Morpheus processes your measurement data, corrects for test misalignments, and provides clear, visual feedback—so you can move confidently to the next step in your workflow.

Why Use Morpheus for CGH Metrology?

Traditional CGH data reduction can be cumbersome, relying on manual processing or command-line tools that require expertise to interpret. Morpheus software changes the metrology game by providing:

✅ Intuitive UI – No more command-line ambiguity. Easily process, visualize, and validate results in a user-friendly interface.
✅ Automated Misalignment Correction – Remove residual misalignment errors without relying on approximations.
✅ Uncertainty Analysis – Gain insights into expected measurement accuracy, even before making a measurement.
✅ Seamless Integration – Compatible with common interferometry data file formats.

How It Works

Getting started with Morpheus is easy:

1. Include a .cgh file with your next CGH procurement from AOM, containing all the CGH details and test analyses – the Morpheus software application is included as a free download!
2. Acquire your measurement data using a CGH test setup.
3. Process the data instantly in Morpheus with just a few clicks.
4. Review processing with rich visuals, ensuring confidence in the accuracy of results.
5. Take processed data directly to polishing correction, saving time and improving efficiency.

With Morpheus, you can streamline your measurement workflow and ensure reliable, repeatable results for even the most complex optics.

Want to learn more about the steps behind the data processing? Check out our article on our resources page for more details.

See Morpheus in Action

Want to see how Morpheus simplifies CGH data reduction? Watch our short demo video below, where Dr. Logan Graves measures an off-axis parabola and talks through the software’s key metrology features.

Ready to Experience Effortless CGH Data Reduction?

Morpheus software is free to download and included with your .cgh file purchase. Take the next step toward faster, more reliable metrology today.

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