question on size of object

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

Okay, you know how the Earth is divided by latitude and longitude? Well,
imagine a sort of latitude/longitude system projected on the sky. The
Greenwich Meridian (0 deg. 0 hrs.) of the Sky is in Aries. There are 360
degrees until we come back there again. The moon is about 1/2 deg. wide (and
by a wonderful coicidence so is the sun). If you If you extend your arm the
width of your little finger at arms length is 1 degree. Take your first 3
fingers and hold them at arms length this is about 5 deg.

So now that we know angular distances across the sky are measured in
degrees, what about the 40 arcminutes? Each degree is divided into 60
arcminutes, and each arcminute is divided into 60 arseconds. So Mars back in
August was 25 arcseconds across (it's now about 12).

As for the eyepiece, saying what the diameter is when looking through any
kind of scope is not relevant without knowing the power being employed.
There are very few 6-8" scopes which can see the entire Andromeda galaxy at
once. My widest eyepiece (which in my 10" dob is ) provides a true field of
just over 2 degrees, so not enough to take in the entirety of M31.

You can determine the field of your eyepieces/telescope with these formulae
(you can get the numbers from the manufacturers):

Power (magnification) = focal length of scope / focal length of eyepiece
Apparent Field of View = field of view through the eyepiece alone
True Field of View = apparent field / magnification

For example, a 26mm Plossl with a 50 degree field of view, placed in a
telescope with a focal length of 1200 mm, will have a magnification of about
46x (which is 1200 / 26), and a true field of view of just over 1 degree
(1.09 degrees, which is 50 / 46).
The same 26mm Plossl eyepiece, placed in a telescope with a focal length of
2032 mm, would have a magnification of 78x, and a true field of view of 0.64
degrees.

Hope this helps

"n3drk" wrote in message
om...
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

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/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"Kilolani" wrote in message
ink.net...
Okay, you know how the Earth is divided by latitude and longitude? Well,
imagine a sort of latitude/longitude system projected on the sky. The
Greenwich Meridian (0 deg. 0 hrs.) of the Sky is in Aries. There are 360
degrees until we come back there again. The moon is about 1/2 deg. wide
(and
by a wonderful coicidence so is the sun). If you If you extend your arm
the
width of your little finger at arms length is 1 degree. Take your first 3
fingers and hold them at arms length this is about 5 deg.

So now that we know angular distances across the sky are measured in
degrees, what about the 40 arcminutes? Each degree is divided into 60
arcminutes, and each arcminute is divided into 60 arseconds. So Mars back
in
August was 25 arcseconds across (it's now about 12).

As for the eyepiece, saying what the diameter is when looking through any
kind of scope is not relevant without knowing the power being employed.
There are very few 6-8" scopes which can see the entire Andromeda galaxy
at
once. My widest eyepiece (which in my 10" dob is ) provides a true field
of
just over 2 degrees, so not enough to take in the entirety of M31.

You can determine the field of your eyepieces/telescope with these
formulae
(you can get the numbers from the manufacturers):

Power (magnification) = focal length of scope / focal length of eyepiece
Apparent Field of View = field of view through the eyepiece alone
True Field of View = apparent field / magnification

For example, a 26mm Plossl with a 50 degree field of view, placed in a
telescope with a focal length of 1200 mm, will have a magnification of
about
46x (which is 1200 / 26), and a true field of view of just over 1 degree
(1.09 degrees, which is 50 / 46).
The same 26mm Plossl eyepiece, placed in a telescope with a focal length
of
2032 mm, would have a magnification of 78x, and a true field of view of
0.64
degrees.

Hope this helps
Just a couple of added 'comments', that may help to give an idea of what the
sizes 'mean'. If you hold your fist out at arms length, this covers about 10
degrees. The width of a finger at the same distance, is about 1 degree. The
Moon is about 1/2 degree (so about the size of a small 'pea' held at arms
length). Jupiter is about the size of a small coin, 100 yards from you.
When magnifications are applied, the effect (in terms of the 'angle' the
objects take), is to divide the distance away by the magnification. So in
the case of Jupiter, applying a 100* magnification, makes it cover the angle
in the eyepiece, of the small coin, now just one yard away. With this
magnification, it is actually larger than the 'naked eye' moon, but will
often not appear so!. This is a classic 'optical illusion', which is quite
interesting to demonstrate for yourself. Look at the Moon 'naked eye', and
then look at again through a tube (a cardboard core from a kitchen towel
roll for instance). Though the Moon itself does not change size, you 'see'
it as smaller through the roll. The change is not much, but is enough, so
that if you look at a planet like Jupiter through an eyepiece (which has the
same effect on the human visual system as the cardboard tube), with the
magnification carefully selected to make it appear the same size, it still
tends to look smaller...
As a general 'rule' on apparent sizes, 1" at 100yards, is one 'arc minute'.

Best Wishes