Generally speaking, a "large" optic is anything with dimensions exceeding 10 cm. Designing a large optical system is not intrinsically harder than designing a smaller system, but the stakes are higher. The classic example of a large optics project gone wrong is the Hubble Space Telescope, in which an error in metrology resulted in a mirror that was made "exactly wrong" to very high precision.
Keep in mind that, when designing a system that includes large optical elements, the effects of the usual cost factors increase and some new factors appear. The following design points will help.
Availability: Because large optics naturally require more space, optical suppliers are less likely to maintain a standing stock. Make sure that you secure a source and establish a supply chain early in the design process. If a part is not available in the quantity or rate that you require, you need to be able to adjust your design accordingly.
Inventory: Large optics require more space for both storage and handling. Moving large optics in and out of storage increases the risk of damage. If there is a drop in product demand, you may be stuck storing and maintaining expensive optics that you do not need. Rather than maintaining your inventory internally, consider working with an optics supplier that can provide just-in-time delivery.
Coatings: Large optics take up more space in a coater, reducing production rate per machine. The reduced throughput usually creates the need to run multiple coating machines in parallel. Large optics also require special fixtures within the coater and may need additional in situ measurements to ensure coating uniformity. If your production volume does not justify that kind of equipment investment, then you need to identify an optical coating house that can meet your production rate and performance specifications. It is ideal to find an optical supplier that can provide this service in-house, which saves you the cost and stress of shipping products between locations.
Quality control: Inspecting large optics for damage and performance is considerably more difficult than for smaller optics. Interferometry is particularly difficult because it requires an inventory of large, expensive reference spheres. It is almost always best to find an optical supplier that can perform quality control inspection and metrology before shipping the parts to you. A supply partner who can guarantee quality large optics saves you the cost of maintaining a metrology lab and the headache of return-shipping rejected parts.
Opto-mechanics: Environmental and mechanical sensitivities scale with size. Issues that have little bearing on small systems can be major problems when the system is scaled up. Factors include thermal expansion, structural integrity, and gravitational sag. It is important to perform physical modeling early in the optical and opto-mechanical design of large optical systems.
These tips underscore the importance of working with an optics supplier that understands your application and the implications of your design choices. To reduce risk and control cost, you need a partner that can assist you from conceptual design through final testing. Honour Optics has the qualifications to be that partner. We can collaborate with you on optical design analysis, establish a reliable supply-chain, provide inventory support, and coat and test optics all within our facilities. Working with us makes your big problem less to worry about.