On the thin film optics of a Cassegrain telescope

• The filter array was fabricated by photolithography process in a single chip.
• The average transmittance of filters is above 90% in the spectral range of 350–1100 nm.
• The equivalent admittance of the three protection layer has been derived.
• The average reflectance of the reflector is boosted above 99% in visible spectrum.

Optical interference thin films designed and fabricated for use in a Cassegrain telescope were deposited by ion-beam-assisted deposition. The optical parameters of optical interference filters array and the protection layers of silver reflector were simulated and optimized by using admittance loci analysis. The patterned multispectral assembly containing blue, green, red, near infrared, and panchromatic multilayer high/low alternated dielectric band-pass filter arrays in a single chip was fabricated by photolithography process. The corresponding properties of the films were measured by in situ optical monitor and spectrometry. It was found that the average transmittances were above 90% for the multispectral assembly, with a rejection transmittance of less than 1% in the spectral range of 350–1100 nm. The average reflectance of the large area silver mirror (with three layers protective interference coating) is boosted above 99% in visible spectrum. In addition, the equivalent admittance of the three protection layer has been derived.