Calculations
Calculating magnification:
Although magnification
is not the main factor in choosing and using a telescope, it is useful to
know how to calculate the magnification for a given eyepiece. To compute the
magnification of an eyepiece divide the telescope focal length by the eyepiece
focal length.
Magnification
= Scope Focal Length(mm) / Eyepiece Focal Length(mm)
Examples:
SCT with an aperture
of 203mm and focal length of 2030mm Power with a 20mm eyepiece = 2030mm /
20mm = 101 times (101 Power) 8" F/6 Newtonian = 8" * 25.5mm / inch * 6 = 1220
focal length Power with a 20mm eyepiece = 1220mm / 20mm = 61 times (61 Power)
NOTE: The F ratio is calculated by dividing the aperture into the focal length
of the telescope E.g. 1220mm / 203mm (8") = F6
Field Of View:
Apparent (afov)
versus True Field of View (fov) The apparent field of view is determined by
the manufacturer and is the area of the sky that the eye sees. The true field
is the amount of the sky actually seen through the eyepiece when it's attached
to a telescope.
See Diagram Below:
Commonly
used equation:
Field of view
in degrees = afov / magnification E.g. for an SCT of 203mm aperture = 65 (afov)
degrees / 101 magnification = 0.64 degrees And for an 8" Newtonian: 65 (afov)
degrees / 61 Magnification = 1.06 degrees. More accurate equation requires
the field stop specification for a given eyepiece, which is generally supplied
by the manufacturer. FOV = Field Stop (mm) x 57.3(deg / radian) / Scope Focal
Length (mm) Example [19mm Panoptic in 9.25" SCT]:- Field of View = 21.3mm
(field stop) x 57.3 / 2350 (focal length) = 0.52 degrees.
Calculating Exit
Pupil:
The exit pupil
is the size of the light cone exiting the eyepiece.
The light exiting
the eyepiece should fit inside your dark adapted pupil as it does with the
3mm lines in the diagram above. An 8mm exit pupil may be too large for your
pupil and so wastes light.
NOTE:
In young people
the pupil in the eye can be as large as 7 - 8mm diameter, whereas in older
people the pupil is usually limited to about 5mm diameter, so getting an eyepiece
with an exit pupil of greater than this is probably of no benefit.
Exit Pupil = Eyepice
focal length (mm) / Scope Focal Ratio (F#) or Exit Pupil = Scope Aperture
(mm) / Magnification.
Example:
55mm Plossl F/10
SCT = 55mm / 10 = 5.5mm Exit Pupil. F/5 Newtonian = 55 / 5 = 11mm Exit Pupil.
Or
35 Panoptic or
35mm Plossl: F/6 = 35 / 6 = 5.8mm F/4.5 = 35 / 4.6 = 7.8mm
