Manual Celestron 76LCM (page 18 of 35) (English)

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TelesCope basICs

A telescope is an instrument that collects and focuses light.

The nature of the optical design determines how the light is

focused. Some telescopes, known as refractors, use lenses.

Other telescopes, known as reﬂectors, use mirrors. The LCM 60,

70, 80 and 90 telescopes are refractor telescopes that use

an objective lens to collect its light. The LCM 76 and 114 are

reﬂecting telescopes with a primary and secondary mirror to

gather and focus light.

Focusing

Once you have found an object in the telescope, turn the

focusing knob until the image is sharp. To focus on an object

that is nearer than your current target, turn the focusing knob

towardtheeyepiece(i.e.,sothatthefocusingtubemovesaway

fromthefrontofthetelescope).Formoredistantobjects,turn

the focusing knob in the opposite direction. To achieve a truly

sharp focus, never look through glass windows or across objects

that produce heat waves, such as asphalt parking lots.

Image Orientation

The image orientation of any telescope changes depending

on how the eyepiece is inserted into the telescope. When

observing through the LCM 60, 70, 80 or 90 using the diagonal,

the image will be right side up. When observing straight

through, with the eyepiece inserted directly into the telescope,

the image will be inverted.

When observing through the LCM 76 or 114, reﬂecting

telescopes, the image will be rotated when looking through

the eyepiece.

For astronomical viewing, out of focus star images are very

diffuse, making them difﬁcult to see. If you turn the focus knob

too quickly, you can go right through focus without seeing the

image. To avoid this problem, your ﬁrst astronomical target

shouldbeabrightobject(liketheMoonoraplanet)sothatthe

image is visible even when out of focus.

Calculating Magniﬁcation

You can change the power of your telescope just by changing

theeyepiece(ocular).Todeterminethemagnication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:

Let’s say, for example, you are using the 25mm eyepiece.

To determine the magniﬁcation you simply divide the focal

lengthofyourtelescope(forexample,theLCM114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LCM114is4.5”(114mm)indiameter.Multiplying

4.5 by 60 gives a maximum useful magniﬁcation of 270 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 90 to 158 times for the LCM 114 telescope.

Determining Field of View

Determining the ﬁeld of view is important if you want to get

an idea of the angular size of the object you are observing. To

calculate the actual ﬁeld of view, divide the apparent ﬁeld of

theeyepiece(suppliedbytheeyepiecemanufacturer)bythe

magniﬁcation. In equation format, the formula looks like this:

As you can see, before determining the ﬁeld of view, you must

calculate the magniﬁcation. Using the example in the previous

section, we can determine the ﬁeld of view using the same

25mm eyepiece. The 25mm eyepiece has an apparent ﬁeld of

viewof50°.Dividethe50°bythemagnication,whichis40

power.Thisyieldsanactualeldofviewof1.25°.

To convert degrees to feet at 1,000 yards, which is more useful

for terrestrial observing, simply multiply by 52.5. Continuing

withourexample,multiplytheangulareld1.4°by52.5.This

produces a linear ﬁeld width of 66 feet at a distance of one

thousand yards. The apparent ﬁeld of each eyepiece that

Celestron manufactures is found in the Celestron Accessory

Catalog(#93685).

General Observing Hints

When working with any optical instrument, there are a

few things to remember to ensure you get the best

possible image:

• Never look through window glass. Glass found in

household windows is optically imperfect, and as a result,

may vary in thickness from one part of a window to the

next. This inconsistency can and will affect the ability to

focus your telescope. In most cases you will not be able to

achieve a truly sharp image, while in some cases, you may

actually see a double image.

• Never look across or over objects that are producing heat

waves. This includes asphalt parking lots on hot summer

days or building rooftops.

• Hazy skies, fog, and mist can also make it difcult to focus

when viewing terrestrially. The amount of detail seen

Magnification =

Focal Length of Telescope (mm)

Focal Length of Eyepiece (mm)

True Field =

Apparent Field of Eyepiece

Magnification

Magnification =

Focal Length of Telescope (mm)

Focal Length of Eyepiece (mm)

True Field =

Apparent Field of Eyepiece

Magnification

LcM 60/70/80/90 - inverTed iMAge when viewing

wiTh The eyePiece sTrAighT Through (no diAgonAL)

LcM 76/114 – uPside down iMAge, As viewed

wiTh The eyePiece direcTLy in TeLescoPe

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Celestron 76LCM Manual

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