"Tenifer" wrote in message
oups.com...
On Nov 2, 10:28 pm, Chris L Peterson wrote:
On Fri, 02 Nov 2007 01:42:14 -0700, Tenifer
wrote:

Parallel light rays enter the objective at different angles and the
rays converge to a point at the focal plane and the eyepiece.
So theoretically you are not supposed to see or notice
the secondary obstructions when viewing thru the eyepice,
yet you can in a very faded sense. Anyone got any theory why?

A telescope in an afocal instrument: it has no focal point. You've got
the first part right, that parallel light rays enter the objective at
different angles, but you neglected to consider the role of the
eyepiece, which is to cause those same rays to exit in a similarly
parallel fashion. What comes out the eyepiece of an unobstructed
telescope is basically a solid cylinder of light rays, and what comes
from an obstructed telescope is a cylinder with a shadowed center
region. When you use the telescope during the day, or on a bright object
like the Moon, your eye's pupil is constricted. If it is constricted to
the point that it is smaller than the central shadow (which is common if
you are using a low power EP that itself has a large exit pupil) you'll
see the shadow, either entirely or as lowered contrast.

_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

While it is true that parallel rays exit the eyepiece but as
they pass thru the eyes. They are converged back to points
as the light beam reaches the retina. Now in the case of
8" SCT at 80X magnification using 25mm eyepiece, EP exit
pupil is 80/25mm=3.2mm. If you use this at daytime with
your pupil constricting to 2mm. The exit pupil of the eyepiece
is still larger than your pupil and the obstruction of the parallel
rays from the eyepiece is only 3.2mm/3= ~1mm. With
your eyes at 2mm and the obstruction at 1mm, the rest
of the 2.2mm light rays (or specifically the 1mm) at the
periphery of the parallel rays coming from EP can still reach
your eyes and converge to a point at the retina so
you are not supposed to notice the obstruction (because
it is our retinas that detect photons and not the cornea). Yet
there are obstruction seen. I think besides the parallel
rays entering the objective lens. The objective
lens itself can somehow inpinge on the eyepiece.
In other words. Even though you are viewing thru
the eyepiece. A part of your eyes is directly looking
at the objective lens and that's why you can notice
the central obstructive even as faded low constrast
trace at the center of the eyepiece. What do you think?

Not sure if you were actually experimenting with the 25mm eyepiece, or just
using that value as an example, but, at F10, a 25mm eyepiece gives a 2.5mm
exit pupil (eyepiece focal length divided by focal ratio of objective), or
calculated another way (which I suspect you intended), 200mm aperture / 80x
= 2.5mm (aperture divided by power) think binoculars (10x50 = 5mm exit
pupil).

In any event, your eye is a part of the focal system, and the light entering
the eye in a parallel manner is convergent on the retina. As the exit pupil
to eye pupil ratio approaches unity from either direction, the more
efficiency is obtained from the overall system.

If the eye pupil is equal to or larger then the exit pupil, more light
around the "donut hole" is available to come to focus on the retina,
diminishing the impact of the central obstruction.

If the eye pupil is smaller the the exit pupil, then not only are you losing
the light rays of the central obstruction, you are also losing light rays
from the outter region of the exit pupil. If the exit pupil exceeds the eye
pupil by a large enough margin, the central obstruction becomes obvious.

Take that to an extreme, and use a 30% CO, a 10mm exit pupil, and a 3mm eye
pupil. The central 3mm of the 10mm exit pupil will be obscured, and the eye
pupil will be passing only the rays from the obscured area (up close). As
you step away, then it is possible that some miniscule amount of the 35%
peripheral light exiting the eyepiece on any "side" of the CO will fall on
the retina and the edge of the CO will be seen as well.

The question is then, what percentage of the eye pupil can be centrally
obscured, before it becomes noticeable, and/or visible? If the exit pupil
exceeds the eye pupil, the CO effectively gets larger and larger than that
of the objective.

Either way, the 34% obstruction of a 2.5mm eye pupil is going to allow the
full complement of the remaining 1.7mm (66% unobstructed) worth of light to
enter the eye, which is plenty to form a nice bright image.


HTH,
Steve P.