Appendix A: Calculating Eyepiece Power - Meade 4504 Instruction Manual

APPENDIX A: Calculating Eyepiece Power

The power, or magnification of the telescope depends on two optical characteristics:
the focal length of the main telescope and the focal length of the eyepiece used dur-
ing a particular observation. The focal length of the 4504 telescope is fixed at 910mm.
To calculate the power in use with a particular eyepiece, divide the focal length of the
eyepiece into the focal length of the main telescope. E.g., using the H 25mm eyepiece
supplied with the telescope, the power is calculated as follows:
Power =
The maximum practical magnification is determined by the nature of the object being
observed and, most importantly, by the prevailing atmospheric conditions. Generally
powers of perhaps 35x to 175x will be useful most often, consistent with high image
resolution. When unsteady air conditions prevail (as witnessed by rapid "twinkling" of
the stars), extreme high-power results in "empty magnification," where the object detail
observed is actually diminished by the excessive power.
The 3x Barlow lens included with your telescope triples the eyepiece magnification. To
calculate the power of an eyepiece used with a Barlow lens, multiply the eyepiece
power by the Barlow value. E.g., using the H 25mm eyepiece, the power or magnifi-
cation of the 4504 telescope (as calculated in the example above) is 36x and the
Barlow value is 3x. The total power using the Barlow lens is:
Total power = eyepiece power x Barlow value = 36 x 3 = 108x
To install the Barlow lens, first place the Barlow lens into the focuser assembly
(remove the focuser dustcap, if necessary), followed by the eyepiece (Fig. 30).
Tighten the Barlow thumbscrews and the focuser thumbscrews to a firm feel only.
Focuser
Assembly
Fig. 30: Insert Barlow into focuser
assembly.
telescope focal length
eyepiece focal length
Eyepiece
Thumbscrew
Barlow
Thumbscrew
Focus knob
page 38
910mm
= = 36x
25mm