Spectroscopy measure of Omega/PST CaK filter

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Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Between cloud....
I'm in the process of measuring the bandwidth and central wavelength of the DIY CaK filter I use.
Based on the images I've uploaded previously, any bets on the outcome????
111932c_full.jpg
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Re: Spectroscopy measure of Omega/PST CaK filter

Post by DeepSolar64 »

This looks really nice! Is making a CaK filter cheaper than buying one? Who do you buy the filter elements from? Baader?


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Three Omega Bob filters (yes, I know some users say they are rubbish quality!), a Baader Blue CCD filter as an upfront erf and fitted with the #1 "yellow" filter from a PST Cak scope.
Certainly not cheap!!!
I think the Lunt CaK filter is probably more cost effective, but.....I think the DIY has a tighter bandwidth....wait till the results are in.


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Limited test...
The only available spectrograph was a Spectra-L200 with a 1200 l/mm grating, this gave a dispersion of 0.23A
Using the normal set up with the ED80 stopped down to f10.

The bandwidth of the DIY filter is very close to 1.0 A
Bad news: due to the filter assembly "sagging" due to the added weight of the spectrograph the filter CWL was shifted towards the blue wing of the CaK line. Gaffa tape was used to "support" the spectrograph which was not successful, but at least demonstrated the effect of tilt on the CWL....it moved towards the centre of the CaK line. I can only assume at the moment that if it was properly supported and aligned to the optical axis, that the filter would be "on band" at CaK.

I need to rig up a rigid support for the spectrograph to obtain quality results. In the meantime I'm comforted by the fact the bandwidth is pretty good!
Onwards and Upwards.
Cak filter test 130920.png
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Re: Spectroscopy measure of Omega/PST CaK filter

Post by marktownley »

Nice analysis Ken!


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Just checked the Lunt CaK filter the 18mm is over $1000 and rated for 2.4 A (or less???)


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Montana »

Wow!! lovely work Ken, I wish you could come and test all my filters :)

This is very interesting indeed.

Alexandra


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by pedro »

Interesting stuff Ken, thanks for sharing


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by LTHB »

Really interesting, Ken!

Can you explain the marvelous FWHM of 1A? I think the Coronado filter should have 2.2A and the OmegaBob filters are advertised as 2A, but my copy is way above that (according to the transmission curve that came with the filter). Did you tilt the filters or is the low FWHM the effect of "simple" stacking?

Frank


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Frank,
The combination of the cut-off wavelengths and "double stacking" contributes to the reduced bandwidth.
The tilt (sag of the filter assembly) moved the central wavelength (CWL) towards the blue.


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by eroel »

Ken:
Very interesting thread.
Thanks for sharing it.
Best regards,
Eric.


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Thanks everyone for the comments/ feedback.
Using the spectrograph to measure filters/ etalons etc. removes the subjectiveness of assessing the capabilities of your filter(s).

My usual spectrograph (R=10000) is locked up on the C11 for ProAm work at the moment.
Bit difficult at the moment, but if I can assist in the future with any spectrographic measurements I'd love to help.
(There may be a spectroscopist in your area who could help - local astronomy/ science group??)


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by AndiesHandyHandies »

Hi Ken,

Are the two CAK filters in the little box with the corresponding coloured peaks two different CAK set ups?

Are you using three filters? Rather than 3 Omega filters and the other two the wording implies?

Andrew.


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by Merlin66 »

Andrew,
The two profiles show the bandwidth shift when the filter assembly was slightly moved.
It’s the same filter, just different tilts.


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Re: Spectroscopy measure of Omega/PST CaK filter

Post by marktownley »

You seem to have got lucky with that Omega filter Ken, sadly something I never managed...


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