Hi there. You posted:
While reading the peterson field guide to the stars and planets, it gives
the diameter of M31 as 2 degrees and 40 arc minutes. It further states the
diameters are a guide to the size of the object when seen through a 6-8 in
reflector.
What is the normal diameter when looking through the eyepiece? Can someone
explain to me what degrees and arc minutes are?
thanks
Well, Degrees are a measure of angle rather than true physical size. The
zenith (straight up) is 90 degrees in angle above the horizon. A degree of
angular size on the sky is about twice the apparent diameter of the full moon,
or about the size of a U.S. dime held about 40.4 inches away from your eye. A
pair of binoculars might have a field of view on the sky of between 3 and 10
degrees (my 10x50's have a 7 degree field width on the sky (367 feet wide at
1000 yards away)). There really are no "normal" diameters of field with a
telescope. Most low-power telescopic fields are less than a couple of
degrees, although there are exceptions (like the so-called "rich-field"
telescopes). Higher power results in a smaller true field of view, so fields
can actually be only a small fraction of a degree when viewing objects like
planets at high power. Degrees are also subdivided into arc minutes and arc
seconds. There are 60 arc minutes in a degree and there are 3600 arc seconds
in a degree.
The Great Andromeda Galaxy has an apparent diameter of much more than two
degrees. In a simple pair of 10x50 binoculars on a clear moonless night away
from city lights, I can see the faintest outermost glow of the galaxy spanning
a length of nearly 3 full degrees, and some detailed photometry has
indicated that the galaxy may be as much as 4 full degrees in angular width.
With my 4 inch refractor, I can see the entire galaxy at 20x with my
widest-field eyepiece (a 30mm WideScan III), but in my big 10 inch Newtonian
using that same eyepiece, I can often only get about half of the galaxy in the
field at any one time. At higher powers, the field is even smaller (only two
of my 10 eyepieces give me fields of view on the sky of a degree or more when
used in my 10 inch). I usually do most of my observing with fields of view of
less than one degree. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory:
http://www.hydeobservatory.info/
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