Blocking Filter In a Collimated Beam?

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marktownley
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Blocking Filter In a Collimated Beam?

Post by marktownley » Sat Aug 17, 2019 11:57 am

Anyone ever tried placing a Coronado / Lunt blocking filter in a collimated beam to see if it trims down continuum leakage in the wings?
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Re: Blocking Filter In a Collimated Beam?

Post by bart1805 » Sun Aug 18, 2019 8:18 pm

Interesting question Mark! Hope somebody has tested it. In addition to that question: what is the effect in theory of tilting the Blocking filter? Will try it out, but curious what to expect.

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Re: Blocking Filter In a Collimated Beam?

Post by Merlin66 » Sun Aug 18, 2019 11:59 pm

I was thinking of another approach...
What about a blocking filter double stack to reduce the FWHM and trim the etalon wings.....
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Re: Blocking Filter In a Collimated Beam?

Post by Merlin66 » Mon Aug 19, 2019 12:00 am

Bart,
Tilting the blocking filter will move the CWL towards the blue, but won't change the FWHM result.
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Re: Blocking Filter In a Collimated Beam?

Post by Bob Yoesle » Mon Aug 19, 2019 9:57 pm

"Anyone ever tried placing a Coronado / Lunt blocking filter in a collimated beam to see if it trims down continuum leakage in the wings?"

Hi Mark, not sure this would get much improvement. If the BF has a Lorentzian profile - just as with FP etalons - the main effect would be to improve field angle rays through the filter so they are not as blue-shifted out of band (e.g. the filter's Jacquinot spot would be enlarged), but I don't think it would do much to change the filter transmission profile.

As Ken notes, double stacking a blocking filter would probably have a greater effect to reduce the etalon's wings. We can see for the one measurement I know of the Coronado blocking filter does appear to have a Lorentzian profile with a peak transmission of about 65%, and a FWHM of 0.6 nm.

Cor-BF15Ha-zoom.gif
Cor-BF15Ha-zoom.gif (19.78 KiB) Viewed 863 times

For a single stack Lorentzian the 1% transmission value is 10 x FWHM = 6 nm, and for double stacking the 1% transmission value is reduced to 3 x FWHM = 1.8 nm. Not sure this would be too significant. However, double stacking the blcoking filter would reduce transmission to about 42%, which would be significant.

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http://www.astrosurf.com/viladrich/astr ... ntrast.htm
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Re: Blocking Filter In a Collimated Beam?

Post by marktownley » Tue Aug 20, 2019 6:22 am

Interesting thanks guys!
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Re: Blocking Filter In a Collimated Beam?

Post by christian viladrich » Tue Aug 20, 2019 8:37 am

Hi Mark,
The question of tilt / collimated or telescentric beam starts to become an issue with filters narrower than about 2 to 3 A.
BF are usually about 5 to 10 A. So, it should not be a big issue.

Regarding stacking an etalon and a BF, I've done many tests. For example here with a top quality 1.5 A BF and a variety of etalons :
http://www.astrosurf.com/viladrich/astr ... -tests.htm

A 1.5A BF would require a TEC body. If ever you are interested in it, you can contact me through PM.
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Re: Blocking Filter In a Collimated Beam?

Post by Merlin66 » Tue Aug 20, 2019 8:43 am

Christian,
Have you looked at double stacked blocking filters to trim the etalon wings??
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Re: Blocking Filter In a Collimated Beam?

Post by christian viladrich » Tue Aug 20, 2019 8:58 am

No, I have not tested nor calculated it.

If I take the example of a SolarSpectrum or DayStar etalons, there is already a 5 to 10 A BF in the body of the filter
Adding a 4 A BF gives a little bit more contrast (see my test pages).

The real benefit is with the addition of a 1.5A filter. I also tested a 2 A filter which was good. Still the 1.5 A is better.

A 1 A would be still better. I have calculated it but no tested it. The price of the filter is rocketing ...

So I would say the best balance between performance / cost for stacking SolarSpectrum or DayStar etalons is with a 1.5A in a TEC.

If we go back to the Lunt / Coronado BF. Adding another 5 to 8 A BF would not change much the selectivity. Having or not a collimated / telecentric beam would not make a big change.
Christian Viladrich
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Re: Blocking Filter In a Collimated Beam?

Post by DSobserver » Tue Aug 20, 2019 8:44 pm

I recently stack a coronado bf15 with a Lunt bf400:
Picture was very very dark

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Re: Blocking Filter In a Collimated Beam?

Post by george9 » Wed Aug 21, 2019 10:43 pm

I put my Lunt 2A prominence filter (from an old Prom 15T blocking filter) in series with my Quark and with my Lunt LS80 SS. It gets darker for sure. It seems a little higher contrast, but hard to tell because darker can fool you and look like more contrast. In fact, I A-B tested the 2A versus an ND filter, and the effect was similar. So I have the 2A mounted in a standard 1.25" cell, but I never use it.

On the other hand, putting my Lunt DSII unit in series with either my Lunt LS80 SS or my Quark produces a great view, not as dim as the 2A and clearly higher contrast.

George

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