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SCHMIDT-CASSEGRAIN TELESCOPES

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A CLOSER LOOK AT REFLECTORS

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Schmidt-Cassegrain Reflectors

Moving up the price bracket a bit (around double) comes the first of the Cassegrain design telescopes. A Newtonian reflector telescope bounces the collected light from it's main mirror out the side of the tube via the secondary mirror to the awaiting eyepiece. The light travels up the tube once, whereas the Cassegrain, the light bounces back from the secondary mirror towards the main mirror and through a hole in it's center to the eyepiece behind it. By bouncing the light back down the tube a second time it has enabled the
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telescope to be virtually cut in half, but with the same focal length- dimensions.
Dimensions
Balancing
The compact design allows focal ratios to be longer by making the secondary mirror hyperbolic. This is the same mating curve as the main mirror but smaller. Doing this magnifies the image producing the longer focal ratio. This makes them good for planets and the Moon as well as being able to see thousands of other sky objects. Unfortuneatly the telescopes unique
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capabilities offset some of the light gathering power.
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The next main feature of the Schmidt is it's corrector plate. The first thing the light does before being collected by the main mirror is to go through the corrector plate at the front of the telescope. This plate eliminates the spherical abberations. The addition of the corrector plate makes the Schmidt-Cassegrain a sealed unit. Maintenance is much lower than a Newtonian since the plate keeps out the dust from the mirrors and also anything else which may cause the mirror to loose it's optimum reflectivity. The corrector plate does drop the light gathering power down a bit,
especially on small models. Also they are the first thing to dew.
Collimation of Schmidt-Cassegrain reflector is easier physically than the Newtonian reflector, but the design of the telescope means it must be highly accurate. The sturdiness of the main mirror construction has made it unadjustable. If the alignment does drop out, it's usually the secondary mirror. There are three adjusting screws on the corrector plate attached to the secondary mirror. These types of telescope shouldn't readily go out of alignment. If your unsure please call. If it does appear drastically out of alignment by using the star test as for the Newtonian reflector on the previous page and you want to have a go at adjusting it, don't undo the central screw! This screw holds the secondary mirror in place and will drop inside the tube, possibly onto your mirror.
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