Question about Optics
Posted by Anonymous on
The power of the scope will be the focal length of the main objective (yours is 800mm) divided by the focal length of the eyepiece, so a 9mm eyepiece will give a higher magnification (and be dimmer and harder to focus and find objects) than a 20mm eyepiece. It is usual to have two or three different focal length eyepieces for viewing different objects.
Starting out, you want to use the lowest power, so the highest number, eyepiece. Do NOT use the Barlow lens if one came with the scope. Try it out during the day (but never point a telescope anywhere near the Sun). This will make it easier to find the focus point. There is a very wide range of movement in the focus mechanism, because different eyepieces focus at different points, but the actual focus range for any eyepiece will be a small part of the overall range afforded by the focusing mount.
It is unlikely that the finder scope will be much use in pointing the telescope until you adjust it to precisely line up with the main scope. Most manuals recommend that you do this in daylight, by pointing the scope at an object on the horizon and adjusting the finder to match. Once you have a tree or mountain peak in the center of the main scopes image, you can then adjust the screws around the finder scope to get the crosshairs centered on the same object. It is very difficult to do this job in the dark, especially as objects in the sky are constantly on the move.
Remember that astronomical telescopes usually show an upside down image. There is a good reason for this- erecting the image needs more bits of glass in the light path, which reduces the amount of light and increases aberrations. Even if this is only slight, astronomers prefer to avoid it, and they don't really care which way up the Moon or Jupiter appear. It is possible to fit an erecting prism or eyepiece to most astronomical telescopes, and some of them come with one.
Posted on Dec 31, 2010
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Posted on Jan 02, 2017
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