An optical filter consisting of a multilayer spectral coating on a flexible membrane has been designed to be placed in front of the 200-in. (5.08-m) Hale telescope on Mt. Palomar. The filter is intended to protect the telescope against solar radiant flux and limit solar heating of the interior of the telescope dome while transmitting light at the 1,064-nm wavelength of the Mars Laser Communication Demonstration.
For supporting a multilayer spectral coating in this application, the flexible membrane was chosen as a lightweight, less-expensive alternative to a conventional thick optical-glass substrate. Multilayer spectral coatings have been used for decades, and membranes have more recently come into use as substrates for mirrors. However, until now, there have been few (if any) published instances of multilayer coating of membranes to form lightweight solar-rejection/narrow-band-pass filters.
The main problem to be solved in designing the present filter was to satisfy both (1) the need for a large number (hundreds in a typical first approximation) of coating layers needed to obtain the desired broad-band-solar-rejection/ narrow-band-pass spectral characteristic and (2) the need to limit the number of layers to no more than the maximum (≈60) that the membrane could support. The solution is a design of fewer than 50 layers having the following features:
- The front surface of the membrane is coated with 25 dielectric layers alternating between higher and lower indices of refraction. These layers are designed to efficiently reflect the visible and near-infrared (wavelengths up to 1.0 μm) light in which the Sun predominantly radiates.
- The back surface of the membrane is coated with very thin layers of copper and silicon chosen to reject the remainder of the spectrum.
- Multiple dielectric layers stacks are deposited on what would otherwise be the exposed opposite outer surfaces of the aforementioned layers. The materials and thicknesses of these layers are chosen to induce the desired narrow passband, centered at 1,064 nm, in the broad-band-reject spectrum of the silicon and copper layers.