Mounting Rings protect optics from damage.
Press Release Summary:

Original Press Release:
Mounting Rings Protect Optics From Damage
A new line of mounting rings for finishing and protecting the edges of optics such as lenses, filters, gratings, pellicles and mirrors is being introduced by Cockrill Precision Products of Holliston, Massachusetts.
Cockrill Optics Mounting Rings are precision machined from aluminum to ±0.05 mm and then black anodized to provide a non-reflective surface. Protecting the edges of lenses, filters, gratings, pellicles and mirrors from damage, the rings are available in a wide range of sizes, in single-, double-sided, and two-piece configurations that nest within each other.
Featuring an internal recess to hold the optics, Cockrill Optics Mounting Rings are offered in 9 mm to 50 mm O.D. sizes in 3 mm increments that range from 0.75 mm to 7 mm deep and have 0.2 mm or 0.5 mm thick walls to accommodate different optics. Metric sizes are also offered and specials including threaded O.D.s can be provided upon request.
Cockrill Optics Mounting Rings are priced from $0.95 each, depending upon size, configuration, and quantity. Literature is available upon request (distributor inquiries are invited).