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So, what is it? How does it work? The article is amazingly scarce on even the most basic information. --Srleffler (talk) 04:12, 27 June 2008 (UTC)
A search of Amazon gives several references, none of which I have. I think this is how it works:
In general, in a lens system with relays, you have a choice of where to put the system aperture, since you have multiple pupil planes. To a first order, I think you want to put it at the first pupil plane because that minimizes the number of surfaces the the stopped light hits, and so there is less opportunity for stray light. However, light may still be scattered due to reflection, glass impurities, etc., and so some of those rays will hit the final image sensor coming from somewhere other than the exit pupil. In particular, some rays will reflect and/or refract off the edge of the stop. Those rays can be stopped by putting a Lyot stop at the final pupil plane, making it just smaller than the exit pupil defined by the previous stop making the Lyot stop be the system stop, thereby cutting back on stray light that hits the image sensor. This is particularly true if you have an intermediate field stop because without a relay, bright sources outside the field can scatter from the edge of the aperture stop, but with a relay and a field stop and a Lyot stop, only sources in the field make it to the Lyot stop.
In the context of a coronograph, in which you are trying to image the corona of the sun while blocking out the disk of the sun, my sense is that the primary stray light is due to diffraction from the aperture stop. Naively, you might make a simple lens and put a field stop on the image sensor to block out the sun. The problem with that is that the sun is so bright that the point-spread function due to diffraction lets significant energy spill out to the non-stopped part of the image. My understanding is that the original Lyot corongraph solves this as follows: You have a first lens, then a field stop that blocks out the sun and defines the edge of the image. Then you relay that stopped image, stopping down slightly further (the Lyot stop). If you didn't have the first field stop, a second aperture wouldn't reduce diffraction, but with the field stop, the Lyot stop doesn't see the sun's full energy it just has to deal with the diffracted light which, I think, looks like it is coming primarily from the edge of the first stop, so stopping down removes it. From page 136 of The Solar Corona by Leon Golub and Jay M. Pasachoff, it sounds like a central disk is added to the Lyot stop because a lot of energy from internal reflections winds up there. —Ben FrantzDale (talk) 13:55, 21 September 2011 (UTC)