I got the Baader CaK filter
today, but with a distinct lack of sun I couldn't test it on the primary subject. So the thought occurred, could it be used for UV imaging? The filter has a narrow bandpass around 395nm, which is about entering UV from visible. Looking through the filter outside my window to the overcast view, I could make out a faint purpleish image.
I thought, what could I test it on? One feature of UV photography that struck me was that flowers often look different in UV, with a dark pattern in the middle where for visible light they appear uniform. There was a dandelion on my lawn, so that would have to do. Looks pretty ordinary?
Yup, it worked, and there is a the dark middle bit. Sensitivity was a bit of an issue. Ok, it was an overcast day so not the brightest. But on the modified 450D I had to use ISO800, f/1.8 and 1/30s to get this, although strictly speaking due to my unique way of holding the filter in place, it would likely have restricted the aperture to something a bit less. This could be challenging to use hand held in all but the brightest conditions.
This is the unique mounting method I chose! The filter itself is a standard astronomical 1.25 inch fitting, which isn't that big in photographic terms. My chosen lens here is the Canon EF 50mm f/1.8 II, with a cardboard holder to centre it roughly, and then some white tack to make sure it doesn't fall out.
Not sure I'll actually use this for much outside of solar imaging, but it will get the obligatory test in the park some time. Baader also do a "Venus filter" which is a UV shortpass filter, which appears to be the general UV imaging filter of choice for those with the cameras and lenses to make the most of it. That would help with the sensitivity too, since normal glass blocks reduces UV pretty effectively.
As a further comment, processing the image was... weird. The detection was quite strong in both the red and blue channels, with very little in green. I think this confused the camera and RAW processors. I first put a raw through DxO, which looked pretty bad. I tried ACR instead after that, and it looked even worse! The raw image seemed low contrast and due to the moderate-high ISO used, possibly with the converters trying to boost the green channel, resulted in a highly speckled output. Photoshop's dust filter was very effective at removing that in post thankfully.
Conversion using PixInsight, manually coarsely balanced for white, then finish off in Photoshop including noise reduction. I kept it in colour this time unlike the earlier B&W conversion. Note a bit of a colour shift left to right, with possibly something resembling a hotspot in the middle.