What is a Schmidt Camera?

A Schmidt camera is a particular kind of telescope. The most important thing that makes Schmidt cameras different from other telescopes is that it has a much larger field of view. The Yerkes Schmidt is able to record good images over a field of view of 4 degrees square. For comparison, the 40-inch or the 41-inch has a field of view of 0.5 degree! So, you'd have to take 64 exposures with the 40-inch to cover the same amount of sky that you would get in one exposure with the Schmidt! At, say, 10 minutes per exposure, that would be more than 10 hours longer using the 40-inch! If you were searching for something, you can see that a Schmidt camera would be the better tool. It would also be used for taking pictures of things that cover a large section of sky, like comets. The 40- or 41-inch would be better for studying the details of [apparently] smaller objects when you already know where to look.

As you would expect, then, the Yerkes Schmidt is used for wide-field surveys and studying comets. Several pictures were taken of Comet Halley during its last visit to the inner Solar System.

A Schmidt camera uses both a lens and a mirror. By a clever combination of the lens and mirror, you avoid things like chromatic aberration, mentioned before on the page on why most research telescopes are reflectors. However, the trade-off is that the set-up [as desribed here, without modifications] cannot be used visually because the focus of the instrument is actually inside the instrument! So, you put a photographic plate there instead of your eye. Interestingly, the optics combine to make the focal surface curved, so you have to bend your photographic plate exactly right.

Now, since the film most regular cameras work with is very flexible, you may not be impressed with the fact that you have to bend your film for use in a Schmidt. However, for astronomical work, we don't use the same kind of film as your regular camera uses -- we use photographic plates, where the emulsion is on glass plates. So you have to bend glass to get it into the plate holder on the telescope! Fortunately, the glass plates are specially made quite thin, so they usually bend to the appropriate curvature without cracking.

The Schmidt that Yerkes owns is 17.75 centimeters (7 inches) in diameter and is f/4. The photographic plates for it are nearly 6 centimeters square and only 1 millimeter thick! (2.25 inches square and 0.040 inches thick)! There is a famous Schmidt at Palomar Observatory (it was used for the Palomar Sky Survey) that is 1.2 meters (48 inches) in diameter -- the plates for that instrument are 35.5 centimeters (14 inches) on a side and still only 1 millimeter thick! According to Professor Kyle Cudworth, picking up those Palomar plates feels like picking up a piece of paper. (!!)

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Content originally generated in March 1995.
This file was last modified on 20 April 1999.