Why Astronomik delivers their "unmounted" filters in a protective ring
From Astronomik:
Again and again we receive inquiries as to why we would no longer offer unmounted filters. Many manufacturers of CCD cameras have filter wheels for filters with 36mm or 31mm diameter in their range, an often "unmounted" filters are specified for them.
Unfortunately, it is very difficult to professionally handle and mount a naked, unframed optical component. As an example a picture of a customer showing an unframed filter of a competitor from the USA after two years:
The Astronomik filters in the sizes 31mm, 36mm and 50mm are delivered in a thin protective ring made of aluminum. This protective ring allows normal handling of your filter, as you are used to with filters in threaded cells for 1.25" or 2" eyepieces and filter wheels. The protective rings reliably avoid stray light and reflections.
The protective rings have a thickness of 3.5mm and are compatible with all known filter wheels.
Here is a rendering of what the protective rings look like:
Of course, the protective ring does not change the optical thickness of the filters, and the focal plane is not shifted! (If you are using older, unmounted filters, they can easily be used together with filters in the protective ring.
The filters with 27mm diameter are still delivered as unmounted, naked glass, because the filter wheels from Astrel do not allow any other solution.
We are working on a mount for the square 50x50mm² filters.