Magnification or power of a telescope is its ability to enlarge small objects from far distances. Then the image resolution at this magnification = 120/30 = 4 arcseconds. The power of the telescope is the ability to magnify an object. 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: m = 1 for flat. For an example, a 14.25" F/21 Cassegrain is: Here is a page that describes the different ways of defining this asphereic shape, eccentricity, Conic Constant and so forth. Nowadays, it is most often used in the Cassegrain configuration FIG. 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. Those two variables are dependent on a, B an A, which vary with the location of the secondary mirror. 201 0 obj
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Calculate K values for the mirrors in your Cassegrain. 1", asphereic shape, eccentricity, Conic Constant and so forth. The f-ratio of the primary can be just about anything but above F6 the total effective f-ratio becomes too high. The things that make an R-C difficult are the typically fast focal ratio of the primary that is often used to gain a relatively fast telescope and the significant correction that must be applied to the secondary mirror as a result. Physics - Formulas - Telescope Magnification: A common question when purchasing a telescope is what "power" it is.
Cassegrain telescopes are laid out in a kind of juggling act to get the telescope to work well, seduce the problems making the optics and give a telescope that is comfortable to use - all at the same time. "Advanced Telescope Making Techniques, Vol. For example, if you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 40x (1000mm ק 25 = 40). What Telescope Magnification Really Means. This is why adjusting the spacing between an aspheric secondary and the primary changes the system correction. There are several important factors to consider with telescope magnification: magnification, true field, apparent field, exit pupil, and resolution. 178 0 obj
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Everything in the night sky is so far away that it’s not the actual distances between things that we see, it’s the differences in the angle from our vantage point. %%EOF
Magnification of a telescope is actually a relationship between two independent optical systems: the telescope itself and the eyepiece you are using. This is formula is needed if you already have a primary that you wish to use in a Cassegrain. This is because the secondary's conic constant, Ks is a function of how far it is from the primary mirror and the distance to the focal plane. Here's a thread on the ATM list that discusses testing a convex hyperbolic secondary through the glass. For example, if we know the location of the focal plane and we put our eye there at its center, we should see the reflection of the primary mirror centered in the secondary mirror. Many are surprised to find out that the primary mirror for an R-C is only 1.04167 times more corrected than a parabola. However, if you see spherical aberration, digest the following: For Dall-Kirkham, the position of the (spherical) secondary makes a difference, too, as evidenced by the presence of the variables s and m in the equation for the value of Kp for the primary mirror. This is the opposite of what happens in the other Cassegrains with hyperboloidal secondaries. This formula is a bit more lengthy and is: You should be able to now calculate the necessary dimensions for your Cassegrain telescope from the above example. The telescope’s focal length (for example, 1200mm) 2. Designing a Cassegrain Telescope Start by picking the apertu re and final 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: With a magnification of 3 and the effective focal length of 90 inches which will make it an F10 system. The second number means that an error of plus (or minus) one millimeter in R2 moves the focal planeda / dR2mm farther from the secondary mirror with a negative number meaning that the distance is closer. Since Ks = -4.00 for the secondary, for example, it is a hyperbola with 4.00 times the correction of a parabola, so if we are to make a concave test plate, we simply multiply all the ideal knife edge displacements by 4.00 to get the proper shape of the secondary.. This system is less sensitive to movement of the secondary than a classical or R-C. Thankfully, the formulas are already worked out in Richard Buchroeder's article mentioned above - he has provided a convenient table, and I have merely substituted the variable names I have been previously used here. 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