Talk:Schmidt camera

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I've removed this text from the article:

A simpler lensless Schmidt can be made by placing an aperture stop at the center of curvature, stopping the aperture to f/8 or longer. This degrades the light gathering ability of the camera but produces a sharp image while preserving the wide field of the shorter focal length mirror.

because this does not describe any variant of a Schmidt camera. While it is true that a slow aperture stop at the COC will result in sharper images, Schmidt's innovation was the introduction of an aspherical corrector plate that allowed a very fast camera. Hence the text at the beginning about Schmidt cameras being "noted for very fast focal ratios."

The optical design described above would probably be better described as a Newtonian camera, since the original Newtonians had spherical optics and were slow. Also, the slow lensless design was known to optical engineering long before Schmidt's innovation, so the lensless design can't be said to be a variation on or development of Schmidt's technology. The lensless design would differ from a Newtonian telescope in the absence of a secondary mirror and the placement of a detector at prime (rather than Newtonian) focus.

I've also removed this text from the article:

This variant is called the Baker-Schmidt camera or more popularly, a Schmidt-Cassegrain.

A Schmidt-Cassegrain is not the same as a Baker-Schmidt. A Baker-Schmidt configuration has the focal plane somewhere in the middle of the telescope tube, in front of the primary mirror. Incoming light passes through a corrector, reflects off the primary, reflects off the secondary, and falls upon a forward-facing (or sky-facing) detector that is placed in front of the primary (and with its back to the primary).

In a Schmidt-Cassegrain, incoming light passes through a corrector, is reflected off the primary, is then reflected off the secondary, passes through a perforation in the primary, and comes to focus behind the primary mirror.

The distinction is important, since to use a Schmidt-Cassegrain you simply stand behind the telescope and look through it (or place your camera there); to use a Baker-Schmidt, you must climb inside the telescope and sit above the primary to look "through" it (this isn't, of course, done - you put a camera there instead).

The confusion is probably a result of a number of telescopes being designed such that they can be converted between the Baker-Schmidt and Schmidt-Cassegrain configurations. These are typically observatory instruments that achieve the conversion by an exchange of secondaries, and such instruments are sometimes called Baker/Schmidt-Cassegrains, apparently just to confuse us.

I recommend checking Rutten and van Venrooij, Telescope Optics, Willmann-Bell 1988, for additional detail, technical informaiton, and elaboration on these topics. There are also additional references cited there.

I've merely brought the article into a state of correctness with the established literature in the field; if the community thinks that more elaboration is needed in the article generally I'd welcome it, but I don't want to add something like the above to the article as I think that would make it read a bit contentious.

User:Jeffmedkeff

The "lensless Schmidt" is well-known amung amateur telescope makers and is known by no other name that I can find. I have reworded the text to make it clear that the design predated Bernhard Schmidt's design. Perhaps it needs an article of it's own but I'll work on that later.

I must have misread the description of the Baker-Schmidt and was merely trying not to redescribe it. Thanks for the correction. Rsduhamel 07:07, 24 Jan 2005 (UTC)

Thanks very much; it looks good to me.

I was unaware that "lensless Schmidt" was a term having currency amongst ATMs. But I don't doubt for a minute it is true, so thanks for bringing it up and keeping the information in the article. If people are likely to look for information on the lensless design and it goes by that name, they are likely to end up here.

I've given this a little more thought as well. I don't know enough about the history of the lensless Schmidt to know whether it deserves its own article, but I recall seeing references to the broad strokes of the design on the Oldscope (Antique Telescope Society) mailing list that dated from the late 1800's. I don't know when the first one was actually built. It is possible that the first one actually made (or the first made in recent times) was in fact inspired by the Schmidt camera and in ignorance of the old trick to control Newtonian coma, in which case what I said above about the lensless design not being a Schmidt camera derivative would be false. This might be a very interesting topic to research, and I would not regret being proven wrong. Jeff Medkeff 10:09, Jan 24, 2005 (UTC)

Came across the article wpo - lensless Schmidt camera. It has the intro "Bernhard Schmidt, announcing his unique astrographic camera in the 1930s, noted that if the camera was of slow f-ratio and scaled down in size, the expensive corrector plate could be substituted by a simple hole". To bad it doesn't reference a source. His original statement may be the source of Schmidt inspiring "Lensless Schmidts" even though he probably did not invent them. 69.72.93.199 12:59, 18 October 2006 (UTC)