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True Field of View =

Apparent Field of View

Magnification

C

alculating the field of view

The  size  of  the  view  that  you  see  through  your  telescope  is  called  the  true  (or  actual)  field  of  view  and  it  is 

determined by the design of the eyepiece. Every eyepiece has a value, called the apparent field of view, which is 

supplied  by  the  manufacturer.  Field  of  view  is  usually  measured  in  degrees  and/or  arc-minutes  (there  are  60 

arc-minutes in a degree). The true field of view produced by your telescope is calculated by dividing the eyepiece's 

apparent field of view by the magnification that you previously calculated for the combination. Using the figures in 

the previous magnification example, if your 10mm eyepiece has an apparent field of view of 52 degrees, then the 

true field of view is 0.65 degrees or 39 arc-minutes.

To put this in perspective, the moon is about 0.5° or 30 arc-minutes in diameter, so this combination would be fine 

for viewing the whole moon with a little room to spare. Remember, too much magnification and too small a field of 

view can make it very hard to find things. It is usually best to start at a lower magnification with its wider field and 

then increase the magnification when you have found what you are looking for. First find the moon then look at 

the shadows in the craters!

C

alculating the exit pupil

Exit Pupil = 

Diameter of Primary mirror in mm

Magnification

The Exit Pupil is the diameter (in mm) of the narrowest point of the cone of light leaving your telescope.  Knowing 

this value for a telescope-eyepiece combination tells you whether your eye is receiving all of the light that your 

primary lens or mirror is providing. The average person has a fully dilated pupil diameter of about 7mm. This value 

varies a bit from person to person, is less until your eyes become fully dark adapted and decreases as you get older. 

To determine an exit pupil, you divide the diameter of the primary of your telescope (in mm) by the magnification.

For example, a 200mm f/5 telescope with a 40mm eyepiece produces a magnification of 25x and an exit pupil of 

8mm. This  combination  can  probably  be  used  by  a  young  person  but  would  not  be  of  much  value  to  a  senior 

citizen. The same telescope used with a 32mm eyepiece gives a magnification of about 31x and an exit pupil of 

6.4mm  which  should  be  fine  for  most  dark  adapted  eyes.  In  contrast,  a  200mm  f/10  telescope  with  the  40mm 

eyepiece gives a magnification of 50x and an exit pupil of 4mm, which is fine for everyone.

C

alculating the magnification (Power)

magnification =  

=   80X 

Focal length of the telescope

Focal length of the eyepiece

800mm

10mm

The magnification produced by a telescope is determined by the focal length of the eyepiece that is used with it. To 

determine a magnification for your telescope, divide its focal length by the focal length of the eyepieces you are 

going to use. For example, a 10mm focal length eyepiece will give 80X magnification with an 800mm focal length 

telescope.

When you are looking at astronomical objects, you are looking through a column of air that reaches to the edge of 

space and that column seldom stays still. Similarly, when viewing over land you are often looking through heat 

waves radiating from the ground, house, buildings, etc. Your telescope may be able to give very high magnification 

but  what  you  end  up  magnifying  is  all  the  turbulence  between  the  telescope  and  the  subject.  A  good  rule  of 

thumb is that the usable magnification of a telescope is about 2X per mm of aperture under good conditions.    

=

0.65°

52°

80X

=

Содержание Reflector AZ1

Страница 1: ...INSTRUCTION MANUAL REFRACTORS REFLECTORS WITH ALT AZIMUTH MOUNT 090103V2...

Страница 2: ...ripod Leg Height Adjustment Clamp AZ1 AZ2 MOUNTS a b c Accessory Tray Tripod Leg Height Adjustment Clamp A B C D E F G H I J K L Dust Cap Mask Remove before Viewing Dew Cap Sun Shade Objective Lens Te...

Страница 3: ...Finderscope Finderscope Bracket Alignment Screws Focus Tube Eyepiece Diagonal Focus Knob 1 2 3 4 Azimuth Flexible Control Cable Altitude Flexible Control Cable Azimuth Adjustment Knob Tube Rings a b...

Страница 4: ...5 9 5 5 6 6 9 9 10 10 10 10 11 11 12 12 12 13 13 13 13 13 14 15 7 7 8 8 13 14 TABLE OF CONTENTS Before you begin Caution This instruction manual is applicable to all the models listed on the cover Ta...

Страница 5: ...ngth when the equatorial mount is level ATTACHING THE ACCESSORY TRAY Fig 2 1 Place the accessory tray on top of the bracket and secure with the locking knob from underneath Fig 5 Fig 6 Fig 5 Fig 6 ATT...

Страница 6: ...p 2 Insert diagonal and re tighten thumbscrews to hold diagonal in place 3 Loosen the thumbscrews on the diagonal 4 Insert the desired eyepiece into diagonal and secure by re tightening thumbscrews AT...

Страница 7: ...be rings from telescope by releasing their thumb nuts and opening their hinges 2 Fasten the tube rings to the mount using the wench provided ATTACHING THE TELESCOPE MAIN TUBE TO TUBE RINGS Fig 4 1 Fin...

Страница 8: ...nder Small finderscope ATTACHING THE FINDERSCOPE BRACKET RED DOT FINDER Fig 6 1 For finderscope only Locate the finderscope bracket and carefully remove the rubber o ring from it Position the o ring i...

Страница 9: ...tool that uses a coated glass window to superimpose the image of a small red dot onto the night sky The Red Dot Finder is equipped with a variable brightness control azimuth adjustment control and alt...

Страница 10: ...e diagonal can become 3X when placed in front of the diagonal In addition to increasing magnification the benefits of using a Barlow lens include improved eye relief and reduced spherical aberration i...

Страница 11: ...T adapter ring for your camera optional onto the T threads on the other end of the camera adapter tube Fig l This makes the telescope into a lens which you then attach to your camera as you would any...

Страница 12: ...average person has a fully dilated pupil diameter of about 7mm This value varies a bit from person to person is less until your eyes become fully dark adapted and decreases as you get older To determi...

Страница 13: ...ich also affects images Astronomy is an outdoor activity The best conditions will have still air and obviously a clear view of the sky It is not necessary that the sky be cloud free Often broken cloud...

Страница 14: ...and works like this Pull off the lens cap which covers the front of the telescope and look down the optical tube At the bottom you will see the primary mirror held in place by three clips 120 apart a...

Страница 15: ...by stopping at the point where the reflected image of the secondary mirror is closest to the primary mirrors edge Fig s When you get to that point stop and keep your hand there while looking at the b...

Страница 16: ...OF THE TELESCOPE FOR VIEWING THE SUN WHEN OBSERVING THE SUN PLACE A DUST CAP OVER YOUR FINDERSCOPE OR REMOVE IT TO PROTECT YOU FROM ACCIDENTAL EXPOSURE NEVER USE AN EYEPIECE TYPE SOLAR FILTER AND NEVE...

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