Celestron 4 - page 23

Figure5-3

Theemblemontheendof

thefo cus knob shows the

correctrot ational direction

forfoc using the NexStar.

TheNexStar's focusingmechanism controls the primary mirror which is mounted on a ring

that slidesback and forth on the primary baffle tube. Thefocusing knob, which moves the

primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece.

Turn the focusing knob until the image is sharp. Ifthe knob will not turn, it has reach ed the

end of its travel on the focusingmechanism. Turn the knob in the opposite direction until

the image is sharp. Oncean image is in focus, turn the knob clockwise to focus o n a closer

object and counterclockwisefor a more distant object. A single turn of the focusing knob

movesthe primary mirror only slightly. Therefore,it will take many turns (about 25) to go

fromclose focus (approximately 20 feet) to infinity.

Forastronomical viewing, out of focus star images are very diffuse,making them difficult to

see. I f youturn the focus knob too quickly, you can go right through focus without seeing

the image. Toavoid this problem, your first astronomical target should be a bright object

(like the Moon or a planet) so that the imageis visible even when out of focus. Critical

focusing is best accomplished when the focusing knob is turned in such a manner that the

mirror movesagainst the pull of gravity. In doing so, any mirror shiftis minimized. For

astronomicalobserving, both visually and photographically, this is done by turning the focus

knobcounterclockwise.

Youcan change the power of your telescopejust by changing the eyepiece(ocular). Todetermine the magnification of your

telescope,simply divide the focal length of the telescope by the focal lengthof the eyepiece used. Inequation format, the

formulalooks like this:

FocalLength of Telescope (mm)

Magnification= 

Focal Length of Eyepiece (mm)

Let’ss ay, for example, youare using the 25mm eyepiece. Todetermine the magnification you simply divide the focal length o f

your telescope(the NexStar has a focal length of 1300mm) by the focal length of the eyepiece, 25mm. Dividing1300 by25

yieldsa magnificationof 52 power.

Althoughthe power is variable, each instrument under average skies has a limit to thehighest useful magnification. Thegeneral

rule is that 60 power can be used for every inch of aperture. For example, the NexStar is 4" in diameter. Multiplying 4 by 60

gives a maximum useful magnificationof 240 power. Although this is the maximum useful magnification, most observingis

donein the range of 20 to 35 power for every inch of aperture which is 80 to 140 times for the NexStartelescope.

Determining the field of view is important if you want to get an idea of the angular size o f 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:

ApparentField of Eyepiece

True Field = 

Magnification

Asyou can see, before determining the field of view, you must calculatethe magnification. Usingthe example in the previous

section, we can determine the field of view using the same 25mm eyepiece. The25mm eyepiece has an apparent field of view of

52°. Divide the 52° by the magnification, whichis 52 power. Thisyields an actual fieldof view of 1.0°.

To convert degreesto feet at 1,000 yards, which is more useful for terrestrial observing, simply multiply by 52.5. Continuing

with our example, multiplythe angular field 1.0° by 52.5. This produces a linear field width of 52.5 feetat a distance of one

thousand yards. The apparent field of each eyepiece that Celestron manufactures is found in the Celestron AccessoryCatalog

(#93685).