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Old 23-04-2014, 01:39 PM
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ZeroID (Brent)
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Etalons .. ?

Just a query but can anyone tell me the aproximate distance between the two etalon elements (mirrors from my understanding) for solar (Ha?) imaging ?
Been doing some reading and trying to understand the principles but I need a reality check on actual numbers.
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Old 23-04-2014, 02:45 PM
pw (Peter)
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In my head I had the idea that it was 1/4 of a wavelength at the desired bandpass frequency.
One reference that I found suggested an air-spaced etalon would have an air gap of less that 5 thousands of an inch. I found another reference showing a 1mm gap. A third reference suggested a range of centimetres to millimetres.
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Old 23-04-2014, 02:48 PM
pw (Peter)
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Here you go, this really specifies the dimensions without complex formulas, the air gap does indeed vary widely depending on the result required.
http://file.yizimg.com/344592/2010070216185471.pdf
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Old 23-04-2014, 03:14 PM
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Merlin66 (Ken)
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Interesting,
They don't quote the FWHM ( you need around 0.5-0.7A for Ha and at least <2A for CaK.)
The sizes are about the same as the PST and Lunt internal etalons, but I bet the price is x10!
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Old 23-04-2014, 04:13 PM
pw (Peter)
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I think the Colorado and Lunt scopes use solid etalons, the surfaces are mirrored which works the same way as the separated internal surfaces of an air spaced etalon. This was one of the big things that allowed Colorado and Lunt to bring the price down. Only two surfaces have to be flat and polished accurately instead of four.
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Old 23-04-2014, 04:16 PM
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Merlin66 (Ken)
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Peter,
No.
Both Coronado and Lunt still use air-spaced etalons.
The air gap is around 0.7mm for Ha which gives a bandwidth of 0.7A and a FSR around 10A.
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Old 23-04-2014, 04:36 PM
pw (Peter)
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You are right of course Ken, I somewhat misinterpreted something I read earlier and remembered incorrectly what I'd read (I have a lot to learn).
http://www.designerinlight.com/solar...ArticlePt1.pdf
Somehow I conflated the bit about solid etalons and what David and Andy Lunt did to reduce the cost of making etalons.
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Old 23-04-2014, 04:50 PM
pw (Peter)
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I found that article and a part 2 of it on Cloudy Nights website, both are fascinating reading if you are interested in narrowband stuff, solar in particular.
Part 1
http://www.cloudynights.com/item.php?item_id=1791

Part 2
http://www.cloudynights.com/item.php?item_id=2015
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Old 23-04-2014, 05:32 PM
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ZeroID (Brent)
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Excellent information. Thanks all. I shall peruse and ponder.
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