Guides / Materials

Invar for Optical Benches and Telescope Structures: Why CTE Stability Matters

September 20, 2026

An optical bench or telescope mount has one job that sounds simple and is not: hold every lens, mirror and sensor in the same relative position as the temperature around it changes. Aluminum and steel structures move enough with temperature to shift alignment measurably, which is why Invar and Super Invar show up so often in optical and aerospace mounting hardware.

The problem Invar solves

Every material expands and contracts with temperature, described by its coefficient of thermal expansion, or CTE. Aluminum 6061 grows around 23.6 ppm per degree C, a typical handbook value, while Invar 36 grows around 1.2 to 1.6 ppm per degree C, roughly a fifteenth as much. Over a one meter structure and a ten degree temperature swing, that difference is the gap between a few tenths of a millimeter of movement and a few hundredths.

For most mechanical parts that gap does not matter. For an optical system where a lens or mirror has to stay aligned to a fraction of a wavelength of light, or a telescope structure that has to hold focus across a temperature range from a cold night to a warm afternoon, it is the difference between a system that works and one that needs constant realignment.

Where Invar and Super Invar show up

• Lens cells and mirror mounts, where the mounting geometry has to hold the optic’s position relative to the rest of the system as the assembly heats and cools.
• Optical bench structures, the frame that multiple optical elements bolt to, where differential expansion between the bench and the elements it holds would shift alignment.
• Laser cavity spacers, where cavity length directly sets the laser’s output characteristics and has to stay stable.
• Telescope trusses and instrument benches, particularly in space applications where the structure sees large swings without the benefit of a controlled indoor environment.
• Composite tooling, layup molds and mandrels for carbon fiber parts, where Invar’s low expansion roughly matches the laminate’s own expansion through the autoclave cycle, covered in the general alloy comparison in Invar 36 vs Kovar vs Super Invar.

Invar 36 versus Super Invar for optical work

Both alloys serve the same purpose, and the choice between them usually comes down to how wide a temperature range the application sees and how tight the precision requirement is.

	Invar 36	Super Invar
Typical CTE	1.2 to 1.6 ppm/°C	0.3 to 0.6 ppm/°C
Where it holds that CTE	Broad room-temperature range	Narrower band, best near its designed midpoint
Typical use	Most optical mounts and benches	Tightest space and laser applications
Relative cost	Baseline	Higher, less common stock

These are typical, handbook style values, not a guarantee for any specific heat lot. If your program needs a verified CTE for the actual material you receive, ask for measured data explicitly, since it is not something a standard mill cert includes by default.

What this means for how the part is machined

Specifying Invar for CTE stability only pays off if the machining process does not introduce its own instability. A part with residual stress from roughing will move after it leaves the shop regardless of how low the alloy’s CTE is, which is why optical grade Invar parts almost always go through the sequence covered in stress relief for Invar machined parts: rough, stress relieve, finish, and for the most demanding parts, a stabilization cycle before final inspection.

Surface finish also matters more on optical mounting faces than on a general mechanical part, since the mounting surface directly sets the optic’s position. A shop machining an optical Invar part typically finishes critical faces with light, sharp cuts specifically to avoid the work hardening and surface disruption covered in why Invar work hardens, rather than treating the finishing pass as just a final sizing step.

Specifying an optical Invar part

A few things worth stating explicitly on the drawing or RFQ for optical and telescope hardware:

• The operating temperature range the part will actually see, not just room temperature, since it affects whether Invar 36 or Super Invar is the better fit
• Whether the application needs measured CTE data for the specific heat lot
• Surface finish requirements on mounting or reference faces
• Whether inspection needs to happen after a soak at a controlled temperature, covered in Invar machining tolerances

Getting a quote for optical hardware

Optical and telescope parts often carry tighter documentation and inspection requirements than a general mechanical Invar part, and not every shop that cuts Invar routinely runs that level of process control. Being specific about your application on the RFQ helps us match you with shops that have the right combination of alloy experience and inspection capability.

Send your drawing through our RFQ form with your operating temperature range and tolerance called out, and we match you with up to three shops suited to the job.

Frequently asked questions

Keep reading

• Machining Invar Layup Molds and Mandrels for Composite Parts
• Machining Invar for Cryogenic and Vacuum Applications