Stupid question about magnification

Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.

What about when there is no eyepiece? My refractor has some magnification
without one, but I have no clue what it is.

Cerdic wrote:
Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.




Magnification = Telescopes F.L. / Eyepiece F.L.

On 2006-01-18, Cerdic wrote:
Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.

What about when there is no eyepiece? My refractor has some magnification
without one, but I have no clue what it is.

The telescope makes a real image that you can see with the unaided eye -
an "aerial image". The magnification is the focal length of the telescope
divided by the distance from your eye to the aerial image.

--
The night is just the shadow of the Earth.

Cerdic wrote:
Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.


NOT the width of the eyepiece, but its focal length

NOT the width of the eyepiece, but its focal length


Right. All this stuff that now seems obvious to some of us can be very
hard for the newcomer to grasp. I have a co-worker that has a new
telescope and is asking me for advice. He's wants to know what eyepiece
to buy so he can "see farther". I've tried to explain how magnification
is telescope focal length divided by eyepiece focal length and that a
higher power doesn't really mean you'll see farther. Not sure he get's
the idea yet. Sometimes i'm amazed at the steep learning curve for new
stargazers. I wonder how i ever figured it out myself. So Cerdic, don't
worry about asking "stupid" questions. It's not easy. But over time
things do make sense.

-Florian

William Hamblen wrote:

The telescope makes a real image that you can see with the unaided eye -
an "aerial image". The magnification is the focal length of the telescope
divided by the distance from your eye to the aerial image.

I'll expand on this. Apparent size of the prime image does depend on
what distance
it is looked at from, but it is usually taken to be the least distance
of clear vision,
250mm or 10". It is the maximum apparent magnification an objective can
give:
f(mm)/250, or f"/10. An eyepiece does the trick of making it possible
to observe this image from the distance of its focal length. Thus,
eyepiece magnification is given by 250/f*
for the ep f.l. (f*) in mm. Telescope magnification is a product of the
two, objective and eyepiece magnification, M=(f/250)(250/f*)=f/f*.

Vlad

On 18 Jan 2006 02:46:54 GMT, Cerdic wrote:

Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.

What about when there is no eyepiece? My refractor has some magnification
without one, but I have no clue what it is.

Your refractor, without an eyepiece, isn't even a telescope- it's just
an objective lens. A telescope is requires two separated lenses sharing
a common focal point, and has a magnification determined by the ratio of
the individual focal lengths of those lenses. An objective alone doesn't
really have a magnification in an astronomically useful sense.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In article ,
Chris L Peterson wrote:

On 18 Jan 2006 02:46:54 GMT, Cerdic wrote:

Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.

What about when there is no eyepiece? My refractor has some magnification
without one, but I have no clue what it is.

Your refractor, without an eyepiece, isn't even a telescope- it's just
an objective lens. A telescope is requires two separated lenses sharing
a common focal point, and has a magnification determined by the ratio of
the individual focal lengths of those lenses. An objective alone doesn't
really have a magnification in an astronomically useful sense.

When looking through a telescope without an eyepiece, there is a
second lens involved: the observer's own eye lens. If you view the
image of the telescope's objective lens at a distance closer than the
focal length of the telescope, you'll get some magnification. Not
much, but still enough to see e.g. the bigger craters on the Moon, or
Jupiter's galilean satellites. Try!

--
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Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

Chris L Peterson wrote:
On 18 Jan 2006 02:46:54 GMT, Cerdic wrote:

Ok, so magnificiation is focal length of the telescope divided by the width
of the eyepiece.

What about when there is no eyepiece? My refractor has some magnification
without one, but I have no clue what it is.

Your refractor, without an eyepiece, isn't even a telescope- it's just
an objective lens. A telescope is requires two separated lenses sharing
a common focal point, and has a magnification determined by the ratio of
the individual focal lengths of those lenses. An objective alone doesn't
really have a magnification in an astronomically useful sense.

What I think Cerdic was talking about is the possible use of a refractor
with the human eye acting as the eyepiece as well as detector. The
optics here are sort of like eyepiece projection onto film. (You
can do it with an obstructed reflector, but have to keep your
eye well off-axis to stay out of the pupil shadow). The inner
parts of the beam diverging from focus are narrow enough to
be within the accomodation range of the eye to focus on the
retina. As far as I can tell, the magnification will depend a
good bit on just how far behind the focal plane your eye is.

Bill Keel

Chris L Peterson wrote:

An objective alone doesn't
really have a magnification in an astronomically useful sense.

?? Try using one same eyepiece - which, of course, has constant
magnification -
with a 500mm f.l. objective and 2000mm f.l. objective. Any difference?

Vlad