6. With the two setting circles thus accurately reading the RA and DEC ofthe bright object. turn the
telescope tube so that the setting circles now correctly read the RA and DEC of the object you wish find. If
the above calibration was done carefully, and if the polar axis of the telescope is reasonably well-aligned to
the pole, then the object will be in the field of a low power eyepiece.
The above procedure can be followed to locate any object in the sky with known RA and DEC. Note that the
DEC circle, as stated above, need not bere-calibrated once step 4 above has been performed. Because RA
is a function of time, however, the RA circle must always be calibrated to an object of known RA first,
followed by movement of the telescope to the correct RA of the object to be located.
The operating magnification of the telescope is a function of two distinct optical characteristics: the focal
length of the telescope and the focal length of the eyepiece in use. For example,using the 25mm eyepiece
with the 16" Starfinder yields 72X, computed as follows:
1800mm (focal length of the telescope) 72X
• 25mm (focal length of the eyepiecef
The type of eyepiece,whether Modified Achromatic, Orthoscopic,or Plossl, has no effect on magnification,
but does have a bearing on such optical characteristics as field of view,flatness of field, and color correction.
Maximum practical magnification is about 50X per inch ofaperture. Generally;however, lower powers will
produce higher image resolution. When unsteady air conditions prevail (as witnessed by rapid "twinkling"of
the stars),·extremely high powers result in "empty" magnification, where the object detail observedis actually
diminished by the excessive power.
When beginning observations on a particular object, start with a low power eyepiece; get the object
well-eentered in the field of view and sharply focused. Then try H;e next step up in magnification. If the
image starts to become fuzzy as you work up into higher magnifica :ons,then back down to a lower power:
the atmospheric steadiness is notsufficient to support high powers at the time you are observing. Keep in
mind that a bright, clearly resolved, but smaller, image will show far more detail than a dimmer, poorly .
resolved, larger image. "
Because of certain characteristics of the human eye (in particular,eye pupil diameter) and because of
optical considerations inherent in the design of the telescope, there exist minimum practical power levels
also. Generally speaking the lowest usable power is approximately 4Xper inch of telescope aperture.
Even in normal city conditions, with all of the related air and Iignt pollution,there are many interesting
celestial objects to observe. But, to be sure, there is no substitute for the clear, dark, steady skies generally
found only away from urban environments, or on mountaintops:oQjects previously viewed only in the city
take on added detail or are seen in wider extension, or even become visible at all for the first time.
The amateur astronomer is faced typically with two broadly defined problems when viewing astronomical
objects through the Earth's atmosphere: first is the clarity, or transparency, of the air, and secondly the
steadiness of the air. This latter characteristic is often referred
to
as the quality of "seeing." Amateur
astronomers talk almost constantly about the "seeing conditions,"since, perhaps ironically, even the clearest,
darkest skies may be almost worthless for serious observations if tl'e air is not steady.This steadiness of
the atmosphere is most readily gauged by observing the "twinkling" of the stars:rapid twinkling implies air
motion in the Earth's atmosphere, and under these conditions, re~,blutionof fine detail (on the surface of
Jupiter, for instance) will generally be limited.When the airis ste3.dy,stars appear to the naked eye as
untwinkling points of unchanging brightness, and it is in such
a
situation that the full potential of the
telescope may be realized: higher powers may be used to advantage, closer double stars resolved as
distinct points,and fine detail observed on the Moon and planets.