Quite often we receive requests for a single filter in an off-standard size. In all cases we are sorry that we must answer as follows:
Sorry (we know it would be so very much cheaper in production - and we would be so much more flexible to fill special requests) - but we have decided long ago to not cut or saw our filters from large plates because this would leave the coating stack open and mutilated (with microscopic cracks) all around, prone to aging and peeling.
Many times we had the chance to inspect our competitors filters after several years of use (due to our 30+years of servicing SBIG-CCD-cameras/and filter wheels) and we realized already 15 years ago how moisture and heat stress can deteriorate even most modern hard coatings, slowly peeling off from the carrying substrate over time, unless the coating stack is sealed all around the filter stack.
As a consequence - in order to create our filters to remain impermeable - we only offer all filter substrates already cut to final shape and run each substrate on an double deck auto-polisher to achieve perfect optical flatness and freedom from cone errors.
Then we do individually coat these substrates in 500 pc per run as minimum to fill a complete coating chamber, in a way that the coating stack (many times 50+ layers) applied onto each filter won't reach to the very edge of the round or square substrate, so that the coating stack remains completely sealed from all sides. In this way we can ensure that our filters will not age at all.
The sad effect is that we cannot offer other sizes unless the inquired production quantity were in the range of 250 to 500 pc (depending on size) and the tooling rings or square holders will be paid for, which serve to precisely center each individual filter substrate within the rotating calotte inside the coating chamber. Sorry - as explained above - we just will not coat onto large plates and cut any shape from them, also because such large plates cannot be polished optically flat in the same way we do it.
For your most urgent need and for single piece solutions we can only recommend to order the next larger size of our respective filter and have that cut to shape by an ophtalmologist locally. We can supply the round filters without the metal cell in such cases; square filters come without cell in any case.
