Lunt CaK filter transmission curve

[Merlin66]

Does anyone have a transmission curve for the Lunt CaK filter?
Interested in the possible UV and NIR leakage.

[Bob Yoesle]

Hi Ken,

I believe the Lunt CaK modules use a CaK bandpass filter sourced from Barr Associates. Note "some" of these are specified to have OD 4 blocking from "UV to far IR." Christian has published a filter curve for his Barr CaK filter here. There is another curve he recently posted here.

Here's the Lunt CaK filter curves form their product borchure:

Lunt CaK Telescope.jpg (493.2 KiB) Viewed 1316 times

Don't know about any specifics of the other filters employed in the Lunt CaK modules, but from the above and due to the lineage from early Coronado CaK filter systems to Lunt, it seems they may incorporate a a number of additional "conditioning" filters and a sub-diameter 20 nm + wide transmission filter as an ERF. Since Lunt does indicate they are safe for visual use, I would imagine they are otherwise blocked to OD5, but that's only an assumption on my part.

Here's the filter curves of one of my CaK PST filters I obtained from Mark Wagner when he checked it out at his facility a few months back:

Coronado PST CaK MWagner Plot.jpg (498.75 KiB) Viewed 1316 times

Bob

[Merlin66]

Bob,
Yes, I've seen those curves before.
But I haven't seen any 380 to 2500 nm transmission curves to confirm all the marketing hype.
I would have thought by now someone would have put a spectrograph on the Lunt CaK filter.......

[christian viladrich]

Hello Ken,
I think someone on SolarChat shared the transmission curves of the Lunt Ca K etalon. It was a long time ago (3 or 4 years ?).
I don't know whether there is an easy way to make a search on the Forum ?

[Bob Yoesle]

I couldn't find it using Google search, but I did find it on my hard drive:

Lunt CaK blocking filter.PNG (42.79 KiB) Viewed 1277 times

I don't believe this is an etalon, rather it's a typical of a yellow dichroic filter - perhaps hard coated. UV blocking does look good, but anything over 440 nm needs additional blocking.

[Merlin66]

Bob,
Thanks for that.
I can understand why they added all the other blocking filters!!

[christian viladrich]

Hi Bob,
I have this same one and others too on my HD. But for some reasons, I can't find the curve of the etalon.

[Merlin66]

Christian,
I don't believe there is an etalon involved in the Lunt CaK filter design.
Just a series of filter elements, as per the Lunt diagram and Bob's earlier comment.

[christian viladrich]

You are completely right Ken, this is not a Fabry-Perot but a dielectric filter.

Finally, I've found the original SolarChat post :
viewtopic.php?t=12203

Unfortunatly, there is no high resolution plot for the narrow band filter.

[Bob Yoesle]

Thanks Christian - a good re-read of that thread :-)