Quote: It's Schott filter glass, type GG-420. The thickness I use is 1mm, which may be a bit too thin to keep its flatness for imaging purposes. I'd glue it between anti-reflection coated windows, the input side for the Ca-K line and the putput side for broadband green. Broadband visible AR coatings might work on both sides as well, though.
Thanks Bob - great news
As an aside I was mystified why my Conon Rebel with standard zoom in afocal mode recorded nothing of the CaK sun whilst my Minolta D7 would - until I realised the regular glass filter I use on both cameras to protect the front lens element from abrasion blocks light on the Canon - I was using a UV [sky] filter for lens protection and when removed all was fine. Although working blind [eg no LCD preview] the Canon will record a full disk in afocal but I need to clean the dirt-sucking sensor Here a quick+dirty sample for May 2 morning. Bright UV plage[?] near spot #879 not bright in H-alpha.