Ultra-high precision NiP mirror fabrication using ion beam figuring for space applications

Area of Science:

• Optical Engineering
• Materials Science
• Space Instrumentation

Background:

• Nickel phosphorous (NiP) is a key optical material for space-based visible to mid-infrared instruments.
• Electroless NiP deposition followed by single-point diamond turning (SPDT) is a common fabrication process.
• SPDT introduces low- and mid-spatial frequency errors, degrading optical performance.

Purpose of the Study:

• To demonstrate the efficacy of ion beam figuring (IBF) in correcting surface errors on NiP-coated mirrors.
• To quantify the improvement in surface quality achieved by IBF for both flat and spherical NiP mirrors.

Main Methods:

• Application of ion beam figuring (IBF), a noncontact, deterministic surface correction technique.
• IBF was applied as a final fabrication step to NiP-coated flat and spherical mirrors.
• One flat mirror underwent additional chemical and mechanical polishing before IBF.

Main Results:

• IBF reduced root mean square (RMS) height error on a flat NiP mirror from 16.3 nm to 3.4 nm over a 25x15 mm² clear aperture.
• IBF decreased surface irregularity on a spherical NiP mirror from 13.8 nm to 4.4 nm RMS.
• Diamond turning marks remained as a limitation, but a polished and IBF-treated mirror achieved 1.9 nm RMS irregularity.

Conclusions:

• Ion beam figuring is a viable and effective method for correcting low-spatial frequency errors in NiP mirrors.
• IBF significantly enhances the surface quality of NiP optics for demanding space applications.
• Combining polishing with IBF offers a pathway to achieve even higher surface precision.