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Formula
Example
F/Ratio = Telescope Focal Length / Objective Diameter
1000mm (FL) / 200mm (OD) = F/5
Magnification = Telescope Focal Length / Eyepiece Focal Length
1000mm (FL) / 10mm (EPFL) = 100X Magnification
Exit Pupil = Eyepiece Focal Length / Telescope F/Ratio
10mm (EPFL) / F5 = 2mm Exit Pupil
Telescope Resolution = 116 / Objective Diameter (mm)
116 / 200mm (OD) = 0.58 Arc-Seconds
Actual Field of View = Apparent Field of View / Magnification
80deg / 40X = 2 Degrees
Aperture Gain = (Objective Diameter / Eye Pupil Diameter)
2
200mm / 7mm
2
= 816.33
x
Mirror / Lens Surface Area = (pi r
2
)
100mm x 100mm x 3.14 = 31,400mm
Minimum Useful Magnification = Objective
Diameter / Eye Pupil Diameter
200mm / 7mm = 58 x
Magnification
Maximum Useful Magnification = Objective
Diameter (mm) x 2
200mm x 2 = 400 x Magnification
Limiting Magnitude = 5 x Log10
(Objective Diameter (cm)) + 7.5
Log(20) = 1.3 so (5 x 1.3) + 7.5 = 14 Magnitude
Lunar Feature Resolution = (2 x Dawes
Limit of Scope * 3476) / 1800
2 x 0.58 x 3476 / 1800 = 2.24 km (For a 200mm
Scope)
Magnification to Split a Binary Star = 480 / Binary Angular
Separation
480 / 34 = 14.12 x Magnification
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Definitions
F/Ratio; is the measure
of a telescopes speed.
Magnification; is the
factor an object appears nearer (larger) than it actually is.
Exit Pupil; is the
diameter of the object that is seen at the eyepiece.
Telescope Resolution; is
the absolute minimum resolvable distance between objects (Dawes
Limit).
Actual Field of View; is
the true field of view seen in the sky.
Apparent Field of View;
refers to how wide a field of view an eyepiece will provide.
Aperture Gain; the gain
in light capture compared to the average human pupil (7mm).
Surface Area; the total
volume of the mirror or lens (in mm).
Minimum Useful Magnification;
the minimum magnification before light is wasted that could enter your
eye.
Maximum Useful Magnification;
the maximum magnification before further magnification degrades the
image.
Limiting Magnitude; is
the faintest object detectable using the telescope.
Lunar Feature Resolution;
is the smallest resolvable lunar feature in km (3476 diameter of the
moon, 1800 angular size of moon).
Magnification to Split a Binary
Star; magnification of telescope required to split a binary star
(eg shown is using Albireo 34deg separation).
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