Removal of lunt erf double stack possible?

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Stardust5858
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Removal of lunt erf double stack possible?

Post by Stardust5858 » Tue Oct 29, 2019 12:01 pm

Hi all, I've a double stack 60mm lunt and in double stack the image isn't bright compared to the single stack view. So my question is can the erf be removed from the single stack elliment? I've no idea of the sckmatics of the lunt 60mm.
Was just pondering and if it could be done would it be safe in double stack mode?

Thanks, BTW I'm new to the group

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Re: Removal of lunt erf double stack possible?

Post by solarchat » Tue Oct 29, 2019 1:52 pm

Welcome to the group! Whenever you add a second etalon to a hydrogen alpha scope, you reduce the brightness by 40% as you narrow the passband of the scope.
While the ERf may be removed, it won’t make any noticeable difference.
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Re: Removal of lunt erf double stack possible?

Post by Stardust5858 » Tue Oct 29, 2019 4:32 pm

Thanks, good to know.

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Bob Yoesle
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Re: Removal of lunt erf double stack possible?

Post by Bob Yoesle » Fri Nov 22, 2019 2:40 pm

Double stacking reduces the overall transmission of the filter system, but also removes the parasitic continuum from the photosphere itself. Both contribute to the overall perception of reduced image brightness, but vastly improved disc contrast. For two Lunt etalons in series with a transmission of 85%, the resulting transmission is theoretically 0.85 x 0.85 = 0.72 > 72%, or about a 25% reduction in overall filter transmission.

Parasitic continuum.jpg
Parasitic continuum.jpg (337.14 KiB) Viewed 947 times

The images above are normalized to show approximately equal brightness for the prominences and highlight the contrast improvement due to removal of parasitic continuum from the photosphere. This continuum begins to intrude at +/- 0.65 Angstrom off the H alpha center line, and its removal also adds to the perception of reduced image brightness - which I would call noise removal (or an improved S/N ratio).

The two ERF's contribute a few percent to this depending on the particular melt and thickness of the ERF(s).

You can generally safely remove/replace the ERF of the Lunt front 60 mm DS element - the internal ERF and safety filtering will remain in place and is all that is needed. Once you remove the front etalon cover, you can access the ERF filter cell retaining ring and remove the RG630 ERF. When doing this with a Coronado SM90 etalon, I found the improvement in image brightness to be significant. https://www.cloudynights.com/topic/3465 ... ble-stack/

However, you will then have exposed the front etalon itself to the environment, and the etalon will not have any pre-filtering - especially the IR filtering which helps with etalon thermal stability. I would add a Baader 75 mm DERF to the front of the filter. It will provide essentially un-attenuated transmission at 656 nm, yet provides UV/IR filtering. If that does not fit the budget use a piece of Baader Turbo film or a suitable small optical window to at least protect the etalon from dirt and dust...

If you do not find this of benefit, you can always reinstall the ERF and return or sell the DERF, or if using a piece of Turbo film you haven't lost much of an investment.

I would not attempt the removal of an internal ERF - it might even include a UV/IR coating which is important for thermal stability of the internal etalon and additional filtering not found elsewhere in the filter system.
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