Telescope magnification question

"Neil W" wrote in message
...
"Chris.B" wrote in message
...
On Mar 18, 3:03 am, Mike Williams wrote:

The objective lens in optics is the lens in a telescope or other optical
instrument that receives the first light rays from the object being
observed.

With respect, Mike

While remaining on the correct side of pedantic I'm not sure one can
agree with the above definition without reservation.

For practical purposes:

The objective is usually the largest element in a telescope's optical
system.

In a refractor it is the largest lens at the front of the instrument.

In a reflector it is the large mirror at the bottom of the tube.

In a compound instrument (Schmidt or Maksutov) there is usually a
corrector lens in front of the large mirror.

The large mirror is still the objective. The corrector lens has very
little power and can be treated as a plain piece of glass for
magnification purposes.

For simplicity the focal length is the distance from the objective
where a real image is formed by the objective.

i.e. where an image can be focussed on a piece of paper, photographic
film, sensor or ground glass.

It is also the point where an eyepiece can be focussed to obtain a
magnified image.

Magnification = Focal length of objective / focal length of eyepiece.
(in similar units)

I'm not sure if we are talking about the same things. I'm asking about
the
Object Distance and the Image Distance. Are you saying that the distance
to
the mirror is the Obect Distance? Sorry for being such a novice.


Not exactly, because some telescopes are more complicated.

In a basic refractor, the focal length of the objective lens is the same as
the distance from the lens to the focus (pretty much, ignoring questions
about "principal planes", etc). In a simple reflector like a Newtonian,
with only one concave curved mirror (plus a non-magnifying flat mirror), the
focal length is the same as the physical distance from the mirror to the
focal point (total distance, including being bent through 90 degrees by the
flat).

In cassegrain telescopes, including catadioptric telescopes like modern
Schmidt-Cass with corrector plates, there are two curved reflectors
involved, teh concave main mirror and a smaller convex secondary, and the
focal length of the system has to be designed in through detailed
calculations. In such cases the focal length of the objective is not the
focal length of the system, because it depends on the curvature of both
mirrors.

In astronomy, anything we observe is at an infinite distance, as far as
optics are concerned. Hence the equation 1/object-dist + 1/image-dist =
1/focal-length simply results in 1/object-dist = 0 so image distance = focal
length.

For a magnifying lens, you may have small object distance and large image
distance, which is why you get magnification, which depends on the actual
focal length of the magnifier. An eyepiece is a type of magnifying lens,
with the object being the real image formed by the telescope. The
magnification of the entire system (telescope plus eyepiece) is given by
f(tel)/f(eyepiece).

If we are having to explain all this, your teacher is perhaps not doing his
job sufficiently well and needs to go over it with you (or the whole class)
again. I recommend raising your hand in class and requesting this.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)