That's an interesting comment about AR coated windows...
I use the ASI 178 with the MiniSHG and have imaged in the CaK with no problems.
I should remove the window and see if the difference is measurable with the spectrograph.
The ZWO data shows a 15% QE at 300nm which doesn't seem to match an issue with the coated window.
https://astronomy-imaging-camera.com.../asi178mm-mono
ZWO also push to use a UV-IR filter on colour camera with AR coated windows??!!
https://astronomy-imaging-camera.com...-ir-cut-filter
This infers to me that the AR coating is not equivalent to a UV reject coating.