Celestron OMNI XLT 102 manual Aligning the Finderscope, Calculating Magnification

Aligning the Finderscope

Accurate alignment of the finder makes it easy to find objects with the telescope, especially celestial objects. To make aligning the finder as easy as possible, this procedure should be done in the daytime when it is easy to find and identify objects. The finderscope has a spring-loaded adjustment screw that puts pressure on the finderscope while the remaining screws are used to adjust the finder horizontally and vertically. To align the finder:

1Choose a target that is in excess of one mile away. This eliminates any possible parallax effect between the telescope and finder.

2Release the R.A. and DEC clamps and point the telescope at your target.

3Center your target in the main optics of the telescope. You may have to move the telescope slightly to center it.

4Adjust the screw on the finder bracket that is on the right (when looking through the finder) until the cross hairs are centered horizontally on the target seen through the telescope.

5Adjust the screw on the top of the finder bracket until the cross hairs are centered vertically on the target seen through the telescope.

Image orientation through the finder is inverted (i.e., upside down and backwards left-to-right). This is normal for most astronomical finderscopes. Because of this, it may take a few minutes to familiarize yourself with the directional change each screw makes on the finder.

Calculating Magnification

You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the formula looks like this:

Focal Length of Telescope (mm)

Magnification = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Focal Length of Eyepiece (mm)

Let’s say, for example, you are using the 25mm eyepiece that came with your telescope. To determine the magnification you simply divide the focal length of your telescope (the Omni XLT 102 for this example has a focal length of 1000mm) by the focal length of the eyepiece, 25mm. Dividing 1000 by 25 yields a magnification of 40 power.

Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The general rule is that 60 power can be used for every inch of aperture. For example, the Omni XLT 102 is 4 inches in diameter. Multiplying 4 by 60 gives a maximum useful magnification of 240 power. Although this is the maximum useful magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 80 to 140 times for the Omni XLT 102 telescope. You can determine the magnification for your telescope the same way.

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