For some time I have given thought to running a CaK filter in a telecentric beam, the benefits should be apparent, the filter will operate closer to its specified FWHM compared to shorter focal ratios, and, in a telecentric beam the FWHM of the filter will not change across the field of view. With CaK and optics comes the caveat of transmission, at shorter wavelengths glass becomes increasingly opaque and coatings on optics are designed to not let UV wavelengths pass through.
I've been impressively pleased recently with the 2,7x Airylab telecentric as the image amplifier on my Quark when used in conjunction with my 100mm Tal refractor. Normally this telecentric would be used with the HaT, but, plagued with poor seeing the scope gets less use than I would like, so lockdown curiosity meant I tried the telecentric on the f10 refractor, figuring it was designed to be used on the f10 HaT. Results have been impressive and the setup has become my preferred setup for imaging prominences.
So when I rigged up the scope with the CaK filter on the back, straight away I was beaming at the view I was getting on the screen, the contrast was some of the best I have seen. The final stacked quality value in AS3 was up in the high 900s, which, is very very good. Sadly after a couple of weeks activity the sun has gone quiet, but I was still impressed in the details I was pulling out in the above images. Looking very much to being able to image more with this setup when activity picks up again. Camera used was the FLIR IMX174.