Not necessarily, as I have first hand experience with my internal SM90 etalon having had compression force applied directly via such foam in order to fine-tune it, which shows just how sensitive the gap-spacing is to very small changes in mechanical pressure in order to effect a change in the CWL (I have since modified the etalon to be pressure tuned).
The original Coronado Patent states:
This invention is based on the concept of applying a compressive force to a conventional air-spaced etalon in order to adjust the thickness of the spacers used to form the cavity and produce a cavity length of precise and uniform optical length. According to one aspect of the invention, such a conventional etalon is coupled to a housing device capable of adjusting the thickness of each spacer, thereby tuning the etalon to the desired filter wavelength. Thus, the resulting assembly is permanently set to the desired parameters for direct use in optical applications.
Whether or not this is used in the PST I do not know, but I have it on good authority that such mechanical pressure tuning was and is used. I believe in some of the Coronado Richview system such mechanical pressure is not only applied at the periphery of the filter in order to compress the circumferential spacers, but is also applied via mechanical pressure applied to the central spacer in order to maintain a parallel gap spacing across the filter.
Coronado is not forthcoming with any information on this scope and is not revealing any internal specifications as of this review. I do not know what the difference may be with the “Rich View” tuning system but it appears to be the same type of compression tuning that appeared on the Coronado PST controlled by a focuser tensioning screw mounted onto the side of a standard compression ring tuner instead of a molded rubber band.
See: http://www.stephenra...Max II 60mm.pdf
This mechanical pressure tuning originated with the later "hi-res" internal modules supplied by Coronado Tucson also included such a mechanical tuning ability. As Bill Dean of the Coronado Tucson days states:
Actually Kevin you can tune your internal etalon as pictured via the set screw... there were some internal etalons with a provision for tuning. You would need to inspect your sample to confirm but if there's a set screw installed you may be able to address your stacked uniformity issue and achieve narrow band nirvana as intended and hopefully originally delivered.
The central actuator or set screw seems to be affixed to either the internal collimation lens, or in the case of the later Coronado Richview front etalons, possibly the ERF.
The Coronado RichView tuning system incorporates a central pressure pad into the ERF assembly which, when rotated, applies pressure to the etalon and reduces the effective gap, giving the wavelength tuning.
Imaging Sunlight Using a Digital Spectroheliograph, page 42, Ken M. Harrison.
Edited by BYoesle, 25 May 2017 - 10:33 AM.