I have performed some experiments to determine the faintest
stars that I can see through my 70mm refractor and my 178mm
Dob at various magnifications, for objects at various heights
above the horizon. The table below summarizes the limiting
magnitude for stars more than 45 degrees above the horizon.
Note that the telescopic limiting magnitude improves very
rapidly as I move from low to medium magnification, and much
more slowly as I move to high magnification. The improvement
above the top magnifications shown is small.
|Aperture and Mag||Urban||Suburban|
The table below shows how the limiting magnitude
deteriorates for objects less than 45 degrees above
the horizon. Note that the deterioration is more
rapid at the urban site than at the suburban site.
Limiting magnitude depends on azimuth as well as
altitude; it is significantly worse towards the major
light source (in this case, downtown Boston) than towards
the darker parts of the sky. At both sites, the worst
skies are roughly ESE (towards Boston) and the best skies
NW (towards a corridor of wealthy suburbs). These readings
were done to the S, the direction that matters most for
astronomy, which is neither the worst nor the best part
of the sky.
|35-45||subtract 0.2||subtract 0.2|
|30-35||subtract 0.4||subtract 0.2|
|25-30||subtract 0.6||subtract 0.4|
|20-25||subtract 0.8||subtract 0.6|
|15-20||subtract 1.2||subtract 0.8|