ETALONS: EXTERNAL AND INTERNAL
Answer: An etalon refers to an interference type filter typically used in Solar Telescopes because of the desire for an ultra narrow bandpass (less than 1 Angstrom).
An etalon is comprised of 2 extremely flat and parallel optical surfaces that have been optically coated with a ultra-uniform, high reflective dielectric layer with the high reflector layer peaking at the desired bandpass point for best results. These optical surfaces are separated by a very small gap. This gap can be either air or a solid material. The light resonates in the gap by internal reflection off the highly reflective layers on the surfaces. Thru interference at this gap, only light meeting the correct angle of incidence to the surface and is not “interfered with” can pass, all other light is lost.
The main parameters that define an etalon are: bandpass, peak transmission, free spectral range, and optical flatness and parallism.
Answer: The defining difference between an External etalon vs an Internal etalon has typically been price compare to performance; however, that gap is getting very narrow.
External etalons are used on the front of an existing scope, and the user typically asks that the etalon be full aperture. Internal etalons are situated internally to the scope, allowing for a full aperture objective to pass the light thru a smaller etalon due it being placed further down the optical path.
The use of tilt at the front etalon is simply a method of shifting the bandpass (the frequency of light passing through the etalon). Due to the acceptance angle of the etalon, this tilt must remain very small. The more the tilt, the wider the bandpass becomes, and the non-collimated Sun angles begin to play a factor.
Answer: The jury is still out on that one. It is still a function of Price vs Performance.
The new evolution of internal design has certainly gone a long way to improving the performance of internal systems. However, the added cost of the pressure system has now made the external system seem just a little more affordable.
The removal of the tilting from the internal system is providing stunning results. The ability to Doppler Shift instantly to and away from you adds a 3D feel to the image that cannot be matched. This technology has certainly improved the dedicated system.