How to Focus a Telescope

How to Focus a Telescope. A telescope must be focused to show a proper image. An out-of-focus telescope will show blurs or nothing at all. The first time you use a new telescope, it will most likely be out of focus. Get it into focus and begin enjoying the new vistas your telescope provides.

Understanding the Focusing Mechanism

Step

Determine what type of focusing mechanism the telescope has. Small telescopes use one of two basic types - you either move the eyepiece, or you move the primary mirror.

Step

Check for an eyepiece-movement focusing mechanism. It will be either a geared system or a simple sliding eyepiece.

Step

Turn a wheel or pair of wheels in a geared system, called rack and pinion. As you turn the wheel, the eyepiece moves closer to or farther away from the primary mirror or lens. A rack-and-pinion focus is better than a sliding or twisting focus. Most refractors and Newtonian reflectors use a rack-and-pinion focus.

Step

Slide or turn the eyepiece in a slide-focusing system. This action moves the eyepiece closer to or farther from the main lens of the telescope. Sometimes, a locking screw will keep the telescope from slipping out of focus. Small refractors and lower-cost reflecting telescopes use slide-focusing mechanisms.

Step

Look for a screw knob on the back of a telescope whose focus mechanism moves the mirror. Turn the knob to shift the mirror forward or backward. This focus method is very common in standard Schmidt Cassegrain and Maksutov-style telescopes.

Focusing

Step

Set up the telescope where you will be viewing the stars. Avoid direct bright lights, which will keep you from seeing faint objects.

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Select a bright star high in the sky or the moon to focus on.

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Use the finder telescope to center the star or the moon in the telescope. With most new telescopes, you will have to align the finder scope (see the instructions for your telescope) with the main telescope.

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Look through the eyepiece. You should see some light. If the telescope is way out of focus, the light may be very dim. If the telescope is just slightly out of focus, stars will appear as large balls, and the moon will be just a blur.

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Shift the telescope slightly if you are not sure what you are seeing. Look for a change in the light coming from the eyepiece as the telescope is moved. The light may be very dim, so pay close attention.

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Look for some light from a bright star (it will appear as a disk), then find the edge of the disk. Shift the focus mechanism so that the disk becomes smaller. It should focus into a bright point.

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Move the telescope when viewing the moon so that the light is brightest. Shift the focus until the moon comes into focus. You should be able to see craters and other lunar details, even with a small telescope.

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Focus a planet to a disk, not a point of light. Venus and Mercury may have a crescent or half-moon shape when properly focused. Saturn will focus to a disk with a ring around it. With smaller telescopes, Saturn will appear as an oval.

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Shift the focus slowly in one direction and check to see if it improves. If it does not, shift it in the opposite direction slowly and see if it gets better. Once you discover which direction sharpens the image, continue in that direction until you achieve the best view.