There is a captivating quality to concepts that propose stripped-down, simplified solutions. For example, becoming increasingly popular among aerospace engineers is the idea that if you can build an entire telescope from a single material, the system will expand uniformly with temperature changes. The goal is for the telescope to stay perfectly in focus in all sorts of conditions like scorching tarmacs or frigid high-altitude flights. From an optical perspective, this matters profoundly. If the physical distances between optical elements and sensors change unpredictably with temperature, your carefully designed imaging system loses focus and invites aberrations to creep in. This leads to degraded image quality—potentially rendering a multi-million-dollar surveillance system useless mid-flight.
The idea of a single-material telescope sounds straight-forward. But in reality, this solution is too good to be true. According to Kevin Sweeney’s research, it does not provide the performance it promises.
As a Principal Optical Systems Engineer at Optikos Corporation, Sweeney has spent a decade developing imaging systems for demanding applications. Through modeling, prototyping and testing compact monometallic telescope systems, he has uncovered how these designs can be sensitive to minute temperature variations. Such scenarios can doom the applications to failure in practice, even if everything looks fine during initial testing. This is the core of Sweeney’s “Sensitivity to temperature variation in monometallic Cassegrains” paper, a presentation that will take place on January 20th during Photonics West 2026.
This presentation is a must-attend for engineering experts. These systems are deployed in defense and surveillance applications where failure isn’t just expensive but compromising for critical missions. Sweeney’s premise promises to show engineers exactly where the boundaries lie between practical and impractical designs, potentially saving companies from discovering their telescope’s flaw only after it’s airborne.
We sat down with Sweeney to understand why this research matters and what attendees can expect to learn from his presentation.
Q: Let’s start with the basics. What makes monometallic telescopes so appealing in the first place?
Kevin Sweeney (KS): The concept is really elegant. When you have glass mirrors mounted in metal housings, all those different materials have varying rates of thermal expansion. So, when the temperature changes, unexpected things can happen. But if you make the entire telescope, from the mirrors, the housing, the sensor mount, all from the same material, like aluminum, then everything should expand and contract uniformly. It’s almost like you just hit the scale button on your computer and everything scales together. As the telescope expands thermally and the focal point moves, the sensor moves with it, so the image stays in focus.
Q: That sounds ideal. So what’s the problem?
KS: While the uniformity premise is exciting, you start noticing a different story in the testing lab. Depending on the prescription that you come up with for that telescope, it may not work in the real world as you intend it to. The issue is that this uniform expansion assumption requires the whole telescope to eventually reach one single, uniform temperature, and that can be a hard condition to meet in practice. Nothing’s ever perfectly uniform. So the question arises: at what threshold do you start to call what is happening uniform? And what our modeling at Optikos shows is that for certain prescriptions of telescope, the amount of uniformity that you would need to achieve is next to impossible.
Q: You mentioned different prescriptions. What makes some designs more problematic than others?
KS: We’re seeing that as designers try to make these systems more compact—as they decrease what we call the telephoto ratio, which is the relationship between total track length and effective focal length—the telescope becomes considerably more sensitive to tiny temperature gradients. We’re talking about minute variations. And it’s not just temperature uniformity. Even if you could achieve a perfectly uniform temperature, you have the issue that the aluminum itself is not necessarily uniform. It can have density fluctuations or variations in chemical makeup that change how it expands. So even with a uniform temperature exposure, it might still grow non-uniformly.
Q: This sounds like what you’d call a “high order problem” in your research. Can you explain what that means for engineers?
KS: Exactly, it is. This is the really dangerous part. If you don’t perform what we call a STOP analysis, which stands for “structural thermal optical performance” analysis, you don’t see in the design phase that this is a problem. Then if you don’t test for temperature changes in the prototype phase, you still don’t see that you have a temperature problem. Everything looks good. Then you put the system on the plane and you find out when it’s in the air that it’s falling apart. That’s a massive problem. You just wasted all that money on that flight, and potentially compromised a critical mission.
What is STOP Analysis? STOP Analysis stands for Structural, Thermal, and Optical Performance analysis. It is a multiphysics modeling process used to predict how environmental factors—like temperature gradients and mechanical stress—affect an optical system’s performance. In aerospace applications, STOP analysis is critical for validating that monometallic telescopes maintain focus during extreme flight conditions.
Q: Who needs to hear this message? What industries are most affected?
KS: We’re seeing these monometallic systems a lot in airborne applications, so, for example, people who are trying to squeeze a large focal length imaging system into a very small package on a plane. This primarily happens for defense and aerospace applications, particularly in airborne surveillance. These are applications where everyone really cares about weight, and where telescopes are looking at something very far away but need a lot of magnification to see it. Reflective systems are advantageous because the optical path gets folded up, making everything more compact, and aluminum can be made really light compared to glass.
Q: So you’re not saying these systems don’t work—you’re saying there are limits to how far you can push the design?
KS: Exactly. An all-aluminum telescope does work in certain scenarios, but you have to be careful how you prescribe the telescope to make sure that you don’t push it into these places where it’s super sensitive to tiny temperature fluctuations. You don’t have complete freedom to squash it down to as small of a package size as possible. There’s a point where you cross a threshold where it becomes too sensitive to non-uniformity. And those points get revealed with proper STOP analyses, something that we provide at Optikos as a service package.
Q: You mentioned that Optikos offers both the analysis and testing capabilities. How do these fit together?
KS: Our engineering services group can perform the STOP analysis during the design phase. Then, after you’ve done your modeling and built your prototype, you can put it on one of our test benches—we have thermal modules for our test benches—and measure performance over temperature. You want to make sure you see performance that matches the model. This is about catching these problems before it’s way too late, before you’ve deployed the system in the field.
Q: What will attendees learn from your talk that they can take back and apply immediately?
KS: The talk will present detailed modeling that shows exactly how sensitivity to temperature variations increases with decreasing telephoto ratio. We systematically vary the system design while maintaining consistent parameters like effective focal length, aperture ratio, and field of view. This gives engineers a framework for understanding where those boundaries are, basically looking at what’s practical and what’s not. The key takeaway is that you need to do this in-depth STOP analysis during the design phase. You can’t skip it and hope for the best.
Q: Any final thoughts on why this research matters?
KS: I think the photonics community needs to understand that elegant theories don’t always translate directly to practical systems. The idea of a monometallic telescope is beautiful in its simplicity, but reality is messier than what was initially promised. There are always imperfections, always non-uniformities to look out for. Our research shows where those imperfections become deal-breakers. For engineers working on high-stakes applications—and most aerospace and defense applications are high-stakes—this talk will be a reminder that STOP analyses are essential to a high-performing outcome.
Kevin Sweeney’s talk “Sensitivity to temperature variation in monometallic Cassegrains” will be presented by David Biss, PhD, Senior Manager Optical Engineering at Optikos Corporation on Monday, January 20, 2026, at 3:50 PM PST in Room 3014 (Moscone West, Level 3) at Photonics West 2026.