The first of these is telescope magnification, and by this I mean angular magnification. There are several important factors to consider with telescope magnification: magnification, true field, apparent field, exit pupil, and resolution. The first is the shape of the primary which is: Next is to derive the shape for the secondary. Designing a Cassegrain Telescope Start by picking the apertu re and final focal length. Needless to say, you do need to use optical glass for doing this test. As for the magnification, its sign according to our convention is positive if the image made by the secondary has the same orientation as the object for the secondary. If only the very center of the field is fully illuminated (say the central 0.05"), then we should see the reflection of the edge of the primary mirror right at the edge of the secondary mirror. 1 - Basic Formulas for a Cassegrain Telescope. After you choose a magnification, ... You should be able to now calculate the necessary dimensions for your Cassegrain telescope from the above example. For example, in a 12.5" F/12.5 Classical Cassegrain, Kp = -1 for the primary and Ks = -4.00 for the secondary. The sensitivity to changes in these quantities increases as the magnification rises. This is the case for the secondary mirror in a Cassegrain telescope as shown in Fig 2.7(a). This means that for Foucault testing, we simply multiply the ideal Foucault knife edge positions for a parabola by 1.04167, and that gives us the ideal knife edge positions for the hyperbolic primary mirror (or concave test plate for the secondary mirror with the appropriate multiplier to the parabola values). For example, if you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 40x (1000mm ק 25 = 40). -If the secondary mirror is moved AWAY from the primary mirror, the correction of the system will INCREASE. This system is less sensitive to movement of the secondary than a classical or R-C. endstream endobj 179 0 obj <> endobj 180 0 obj <> endobj 181 0 obj <>stream %%EOF Well, when you decrease the spacing between the secondary and primary, the focal length of the system actually gets longer, and this adds dramatically to the sensitivity to spacing changes. Magnification or power of a telescope is its ability to enlarge small objects from far distances. The primary is a parabola, so we know how to test that. Thus, the distance A ends up being: Next is to figure out what the ROC of the secondary is and that is the formula: This completes the major dimensions of the Cassegrain telescope. When you get that done, you can then start on the actual secondary itself.. Do the ROC on the secondary as needed and don't forget to polish up the back surface (no particular shape is needed, just a clear surface) so you can do interference fringe testing with the test plate which you have already completed.. You want to make the two surfaces (the test plate to the secondary) match so that you get nice straight interference lines between the two surfaces, just like you'd be getting with the comparison of two flats together. Those two variables are dependent on a, B an A, which vary with the location of the secondary mirror. Magnification of a telescope is actually a relationship between two independent optical systems: the telescope itself and the eyepiece you are using. In order to design a Cassegrain, we need to calculate it according to the type of Cassegrain we wish to make. If the fully illuminated field is larger, say 0.5" in diameter, then we should be able to move our eye off the optical axis by 0.25" from the center of the field and still see the entire primary mirror in the secondary mirror, though it will no longer be centered in the secondary. Magnification of a telescope is actually a relationship between two independent optical systems: the telescope itself and the eyepiece you are using. While magnification is really not as important as field of view of aperture, to determine the power of a telescope, simply divide the eyepiece diameter to the telescope focal length: While magnification is really not as important as field of view of aperture, to determine the power of a telescope, simply divide the eyepiece diameter to the telescope focal length: Now, you might wonder why the focal plane moves so much more than we expect. This is done with the two formulas below. h��V�n�8�>6(���F�ǮS�MjDjS���b����&ߙ�%�i�n�(9w�P2L0K���k�)��ϔ�9`��`�T"!bҗ>�I�[�@~B���ٻw|֔M����ER"�n@֔�Ʉ�����-Z]���;͢�-lf���5�|J���9�e�lRcW|9_��. The power of the telescope is the ability to magnify an object. A quick numerical calculation using the mirror spacings for my 12.5" classical Cassegrain indicate that if it were a Dall-Kirkham, then moving the secondary toward the primary would require a slightly less corrected primary. The correction of a Cassegrain optical system with an aspheric secondary mirror (classical, R-C) is dependent on the primary-secondary distance. -If you are figuring a Cassegrain secondary using star testing, increasing the correction of the secondary will decrease the correction of the system. 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