Question: Is the LS35THa etalon suitable for use as an internal etalon?

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Question: Is the LS35THa etalon suitable for use as an internal etalon?

Post by michael.h.f.wilkinson » Fri May 17, 2013 7:30 am

As some of you know, I have been planning a 80mm H-alpha scope build using components from my LS35THa (the plan is to use them in such a way that nothing has to be damaged, and I could simply reassemble the LS35. Some people have claimed that the LS35 front mounted etalon is not suitable for use as an internal etalon due to collimation issues (in fact, the statement was that they are rejected LS60 internal etalons). This claim is a bit odd, in that parallel beams are parallel beams, and if you nail the collimation properly (and I bet some colleagues in the Astronomy Institute and Space Exploration departments at our university will help me on that) the beam is essentially the same. The only thing that changes is the field angle, which increases linearly with the size of the objective divided by the beam width at the etalon (34mm in my design, shown below).

Can it be that these etalons have problems with larger field angles? This too seems odd, because when I tested my second-hand B1200 blocking filter on the LS35, I did not see any image degradation as I moved the sun over the 1.7 deg field of view provided by the 12mm blocking filter at 400mm focal length using my XW 10mm EP at 40x. In my design, the maximum field angle is 0.59 degrees, well below the 0.85 degrees I have effectively tested it at.

In short, I can see no physical reason why this etalon could not be used in the way I intend, but I am a bit apprehensive about spending more money before I have sorted this question out.

I am seeking confirmation from Lunt (only yesterday, so it is no surprise I have not had an answer yet), but I would also like to hear from the many experts on here.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.

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