Caption: A diagram of a Schmidt-Cassegrain telescope, a classic reflector telescope.

The Celestron C8 telescope is an example of popular Schmidt-Cassegrain telescope. It has an 8-inch primary as its name implies.

Features:

1. At the front end, a Schmidt-Cassegrain telescope has a Schmidt corrector plate (the thin thing) and the secondary (mirror) (the thicker thing).

   The Schmidt corrector plate makes the telescope a SCHMIDT TELESCOPE.

   Note that unlike most telescopes, the primary is NOT the first optical device in the formation of the observed image: Schmidt corrector plate is.

   In a sense, the Schmidt corrector plate is the "real primary" since its area must be about the same as the primary's in order for the primary to have its full light-gathering power.

2. The Schmidt corrector plate is an aspheric lens that corrects for the spherical aberration of the primary (mirror) and secondary both of which may be spherical mirrors though they don't have to be in some designs (see Wikipedia: Schmidt-Cassegrain telescope: Derivative designs).

   Why have a correction for spherical aberration?

   Two reasons:

   1. Spherical mirrors corrected for spherical aberration can be made to give a wider field of view (FOV) with good focusing as compared to parabolic mirrors which need NO correction for spherical aberration.

   2. Highly exact spherical mirrors are vastly easier to construct than any other kind of highly exact curved mirror (e.g., parabolic mirrors).

      This is because inner and outer spherical surfaces rubbed together any which way to smooth them tend to become more and more exactly spherical. NO other curved shape allows for such a simple procedure for exact shaping. Of course, two flat surfaces rubbed together can be smoothed to greater flatness.

3. The primary (AKA objective) is the bigger mirror at the end of the telescope tube.

   The diameter of the primary is the main parameter of any telescope. It is the first parameter anyone should know or ask about.

   The reason for this is that the light-gathering power of a telescope is proportional to the area of the primary, and thus square of the diameter.

   The primary gathers the light from the observed object (e.g., an astronomical object).

   The primary usually starts the process of image formation, but in the case of the Schmidt-Cassegrain telescope where the Schmidt corrector plate does.

4. The light gathered by the primary is reflected to the secondary---which is smaller than the primary.

   In general, when a telescope has a secondary, the secondary is used to create a real image that is viewed by an eyepiece.

   A Schmidt-Cassegrain telescope is a CASSEGRAIN TELESCOPE because the secondary is a convex mirror is on the main optical axis of the telescope and creates the real image behind the primary---which is often a convenient place to have it.

   The secondary in the Schmidt-Cassegrain telescope is embedded in the Schmidt corrector plate at the front of the Schmidt-Cassegrain telescope as shown in the diagram.

   The convex mirror (usually a spherical mirror) gives a Cassegrain telescope an effective focal length longer than the focal length of the primary.

   The longer effective focal length allows greater telescope magnification than otherwise, and so allows for a compact long focal-length telescope. For example, the Celestron C8 telescope (which is a Schmidt-Cassegrain telescope as aforesaid) has a telescope tube length of about 400 mm, but the effective focal length is 2032 mm = 80 inches (see Wikipedia: Celestron: Products).

   The secondary gives the longer effective focal length by decreasing the angles the light rays have with respect to the optical axis. The angles then act as if they came from a longer focal-length primary.

5. The light reflects from the secondary and is directed through a hole in the primary.

6. The real image forms somewhere behind the primary as aforesaid. For visual astronomy, the real image probably forms in the eyepiece cylinder in many cases (see e.g., Starzona: How Schmidt-Cassegrains Work).

   If there were no secondary, the real image would form in somewhere front of the primary where it would be very awkward to view in visual astronomy.

   Actually, large, professional telescopes are almost never used for visual astronomy, and so almost always have NO eyepiece setup. A CCD camera is usually put on the focal plane. The CCD camera records the real image.

7. It may seem odd that the real image can be complete given that there is a hole in the primary and that the secondary occults (i.e., blocks) part of the aperture.

   But the diagram shows (with some close looking and imagination) how light rays from every part of a distant object make it to the real image, and so the image is complete.

   But we can say a bit more:

   1. When the image is in focus, the light-ray group coming from a point light source outside the telescope is focused to a point which is a point real image.

   2. It doesn't matter what shape the light-ray group cross section had along the way since the beam group is focused to a point.

   3. But if the image is out of focus, the beam group is NOT focused to a point, but to some shape that is like the light-ray group cross section.

      Since the telescope aperture is circular, the beam group cross section would be circular image for a point light source if the secondary were transparent to incoming light rays.

   4. Since the secondary is NOT transparent, you get a donut unfocused image for a point light source. The hole in the donut is the shadow of the secondary.

      If you have an extended object, each point on it is a point light source.

      So an out of focus telescope gives a continuum of overlapping donut unfocused images.

      Formally, unfocused image formation is called defocus aberration. One usually just calls an unfocused image an unfocused image or an out-of-focus image or a blurred image.

   5. Yours truly thinks this is the gist, but there is a lot more detail to image formation. See also Collimating a Schmidt-Cassegrain Telescope.

8. For most optical telescopes, the image formed by the primary and the secondary is a real image: i.e., light rays from the object converge to actual points in physical space.

9. A real image can be observed directly in the directions that the light rays are traveling as they diverge from the real image or it can be cast on a screen or recorded on photographic film or recorded digitally by a CCD camera as aforesaid above.

10. The diagram shows a representative set of light rays that converge to form representative points on the real image.

11. In visual astronomy, the real image is viewed by a lens called an eyepiece (not shown in the diagram).

12. Telescope magnification is determined by the formula M = -f_p/f_e , where f_p is the effective focal length of primary and f_e is the focal length of the eyepiece.

    The minus sign just means that the image is point inverted and can be dropped if the point inversion is considered to be understood. Using a star diagonal usually adds a axis reflection to the image.

13. For a given telescope, f_p is fixed and f_e is adjustable by changing eyepieces.

14. There is a trade-off. The greater the magnification provided eyepiece, the smaller the field of view (FOV).

15. The field of view (FOV) is actually the full image of that part of the sky that can send light rays through the telescope.

16. The FOV is circular because the telescope aperature and other controlling optical parts are circular.

17. The telescope limits the FOV: i.e., limit how far off the optical axis (the symmetry axis of the telescope) one can see.

    How the limit on FOV arises:

    1. A point light source too far off the optical axis of the telescope can only shoot light rays that hit non-reflecting surfaces in the telescope, and thus will NOT reach the primary.

       So you just see nothing at angles too far off the optical axis

    2. Light rays that do make it through the eyepiece, but at too large an angle to the optical axis are effectively out of the FOV.

       You don't usually want to look at such large angles when comfortably doing visual astronomy.

       Also Gaussian optics breaks down for sufficiently large angles and you would see some sort of distorted image.

18. Telescopes are usually designed to observe faraway objects.

    Objects sufficiently far away that the light rays from them are effectively parallel are said to be at optical infinity. in optics jargon

19. Optical infinity is often not that far. For small telescopes (like the Celestron C8 telescopes), it could be of order of magnitude 10 meters.

    By focusing, objects at less than optical infinity can be focused, but how close they can be depends on your focusing setup.

    For example for C8 telescopes, focusing for a distance less than the length/width an ordinary classroom is NOT possible.