I managed to get my first CaK images of the sun today as can be seen above, whilst the image itself is fairly unremarkable - there's some shadowing from a tree that was in the foreground, and the disk is fairly blank apart from the giant spots associated with AR11654, it is how the image was filtered that is significant.
Traditionally there is only a few ways to observe our star in narrowband CaK wavelengths; Lunt CaK module, Coronado CaK PST and Daystar filters. In my mind there are issues with all 3 - the CaK PST is no longer made, the Lunt is expensive and the Daystar is really expensive. For some time I have been using a CaK PST mod - basically using the filters from this in larger refractors. Today represents a mile stone in taking this a step further; the picture to the left shows the filter that I assembled and used to take the image above.
For some time I have been trying to crack the CaK filter code, and now I think I am 90% there. The stack in the image is really simple; a Baader K-Line filter and a longpass filter. There is then a 0.5x reducer on a short nosepeice to get a full disk with the DMK31. The theory behind it is quite simple - the K-line has an 8nm wide pass band, this shows on its own a very watered down and washed out view of CaK. However by inserting a longpass filter into the stack this has the effect of really narrowing down and tightening up the banpass, to the point it is very comparable with the 2.2A pass of the CaK PST and the 2.4A pass of the Lunt. The key is the longpass filter, and after much researching i'm fairly certain I know the specification of this filter, in the coming months I will be purchasing these longpass filters from a range of manufacturers and trialling them to see which performs the best. If the theory is correct, and so far it all is, i'm hoping to be able to put together a filter recipe for imaging at CaK wavelengths at 20% the cost of the cheapest commercial alternatives. Watch this space!