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Lunt 50 or Quark with ED 80?

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Rear mounted Etalons are designed for a certain f/stop and need to be placed at the correct distance from the objective lens.  It's all about the angle of the light passing through.  I don't have the physics to hand but a PST is f/10, so really you want to keep the final scope at or near f/10 and the same relative distance from the objective as the PST. 

In front mounted Etalons the light [from the sun] is near enough parallel, in rear mounted Etalons, the light is converging, but not so much in a f/10 system.  Restricting the aperture of a objective lens, results in light which converges less (at the edge of the aperture) and increases the f/stop, so might actually help the Etalon work better.

If you use a different f/number then you will need to experiment to find the best place to put the Etalon.  Remember that the Etalon has a fixed aperture so it is likely that it will need to move backwards in the tube towards the eyepiece. 

Using a Etalon on a lower f/stop scope will probably lead to a smaller field of view over which the Etalon will work (smaller sweetspot) which would be counter-productive.

Peter [Drew] will know more about this than the rest of us put together, interesting that he uses a 6" f/10 lens.

It is the same with a Quark, Daystar recommend a F/4 to F/8 scope although the Quark has a 4.2 x barlow installed, which sets the f/stop to f/16 to f/20 which is the range they decided on.

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The PST is configured for the converging beam of its 40mm F10 objective. The 20mm diameter etalon has a positive and negative lens fitted to it to provide the required convergence angle and is placed 200mm inside the focal point of the objective. PST mods have to use the same  etalon placed 200mm inside the focal point. If the objective is shorter than F10, the etalon will vignet it to F10. There used to be a kit available as a PST mod based on a 80mm F5 objective, the PST etalon would have vignetted the aperture to 40mm, the same aperture that the PST started out with.   😥   

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