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Once your telescope is polar aligned, you must set the hour circle in order
to use the measurements listed on the mount to nd celestial objects. Once
the hour circle is properly set, you will be able to use the coordinates listed
on star charts to nd objects for viewing in the night sky. Setting the hour
circle will require that you recognize and be able to nd a star other than the
ones used for alignment of the telescope.
1. Loosen the declination clamp and rotate the telescope to the nearest de-
gree of declination to the star you will be viewing (58° for Dubhe, -63° for
Acrux). Retighten the clamp to lock the declination in place.
2. Loosen the right ascension clamp and rotate the telescope on the R.A.
axis until the star you are using to set the hour circle is near the center of the
nderscope. Retighten the clamp to lock in the R.A. axis.
3. Center the star in the eyepiece using the R.A. and Dec. adjustment cables.
Once it is centered, turn the hour circle until the arrow points at the ap-
propriate measurement for the star you are looking at (11h23m for Dubhe,
12h33m for Acrux). This sets the hour circle to the appropriate setting for
your viewing location and time.
ASTRONOMY FOMULAE
Magnification
To determine the magnification of a telescope and eyepiece combina-
tion, divide the telescope focal length be the eyepiece focal length.
Magnification (x) = Telescope Focal Length (mm)/Eyepiece Focal Length (mm)
Ex: 6.3mm Eyepiece with a 114x1000mm telescope.
Magnification = 1000mm/20mm
Magnification = 50x
Focal Ratio
To determine the focal ratio of a telescope, divide the focal length of
the telescope by the aperture.
Focal Ratio (F/x)= Telescope Focal Length (mm)/Aperture (mm)
Ex: Focal Ratio of a 114x1000mm telescope.
Focal Ratio (F/x)= 1000mm/114mm
Focal Ratio (F/x)= F/8.8
Limiting Magnitude
To determine the limiting magnitude of a telescope, use the aperture in
the following formula for an approximation.
Limiting Magnitude = 7.5 + 5LOG(Aperture in cm)
Ex: Limiting Magnitude of a 114x1000mm telescope.
Limiting Magnitude = 7.5 + 5LOG(11.4cm)
Limiting Magnitude = 7.5 + (5 x 1.057)
Limiting Magnitude = 12.785
Resolving Power
To determine the resolving power of a telescope under ideal conditions,
divide the aperture into 4.56.
Resolving Power = 4.56/Aperture (in.)
Ex: Resolving Power of a 114x1000mm telescope.
Aperture (in.) = 114mm/25.4 = 4.49
Resolving Power = 4.56/4.49in.
Resolving Power = 1.02