Definition of aperture.

On Tue, 9 Sep 2003 13:20:49 -0400, "Bruce Sidell" wrote:

So I read the umpteenth misspelling of aperture today, and
it occurs to me that aperture means opening. So I look it up
in the Random House Unabridged and get :
1. an opening, as a hole, slit, crack, gap, etc.
2. Also called aperture stop. Optics.- an opening,
usually circular, that limits the quantity of light that
can enter an optical instrument.
Why then do we refer to mirror diameter as aperture?
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

In the sense of (1) you do indeed have a telescope with a 12" aperture. But in
the sense of (2), which is a technical definition used in optics, you have a 10"
aperture because it is the 10" diameter of the mirror that limits the total
quantity of light your system gathers. It is also the 10" aperture that limits
the resolution of your instrument, according to another rule of optics not
stated in the Random House definition.

Using definition (1) what would you say was the aperture of a Newtonian with its
diagonal and EP supported by a rod extending from the mirror support? Infinite?

_________________________________________________

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

"Bruce Sidell" wrote in message
...
So I read the umpteenth misspelling of aperture today, and
it occurs to me that aperture means opening. So I look it up
in the Random House Unabridged and get :
1. an opening, as a hole, slit, crack, gap, etc.
2. Also called aperture stop. Optics.- an opening,
usually circular, that limits the quantity of light that
can enter an optical instrument.
Why then do we refer to mirror diameter as aperture?
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

If you had no tube, but an open design, would you have an infinite aperture?
Conversely, if you have a 2" focuser, why don't you have a 2" aperture?
Etok



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Hi there Bruce. You posted:

1. an opening, as a hole, slit, crack, gap, etc.
2. Also called aperture stop. Optics.- an opening,
usually circular, that limits the quantity of light that
can enter an optical instrument.
Why then do we refer to mirror diameter as aperture?
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

The opening of the tube is 12 inches, but the mirror is what will be doing the
optical work. Light coming into the tube on its inside edge may or may not
hit the mirror. Its what actually impacts the mirror that is important, and
thus, the mirror (in the case of a simple reflector) is the aperture of the
telescope. If you had a "tubeless" telescope, again, the only light involved
in the final image (other than any light from the environment) came from the
mirror, and thus the mirror acts as the aperture stop. In the case of the
Schmidt-Cassegrain however, the mirror diameter may *not* be the apeture. For
a proper design, the primary has to be just a bit larger than the "entrance
aperture" which is the diameter of the corrector plate. This "entrance pupil"
out front is then the limiting factor and becomes the "aperture". This irks
me a little when Orion telescopes markets their StarMax 90mm EQ calling it a
90mm scope, when in fact, its true aperture (the clear diameter of the
meniscus corrector plate out front) is only 87.5mm (I measured mine).
Similarly, if we stop a telescope down using a smaller opening or "stop" over
the front of the telescope, the diameter of the stop becomes the aperture.
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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Bruce Sidell:
So I read the umpteenth misspelling of aperture today, and
it occurs to me that aperture means opening. So I look it up
in the Random House Unabridged and get :
1. an opening, as a hole, slit, crack, gap, etc.
2. Also called aperture stop. Optics.- an opening,
usually circular, that limits the quantity of light that
can enter an optical instrument.
Why then do we refer to mirror diameter as aperture?
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

*****

The answer is in this part of the definition, considered in terms of a
reflecting telescope:

an opening, usually circular, that limits the quantity
of light that can enter an optical instrument.

Your mirror limits the quantity of light that your telescope can
gather, thus, the mirror's diameter defines the aperture.

Davoud

--
usenet *at* davidillig dawt com

In article , Bruce Sidell
wrote:

Don't I have a 12"aperture?

You may, but if so I recommend you consult a proctologist.

--
Joe Bergeron

http://www.joebergeron.com

"Bruce Sidell" wrote in message
...
So I read the umpteenth misspelling of aperture today, and
it occurs to me that aperture means opening. So I look it up
in the Random House Unabridged and get :
1. an opening, as a hole, slit, crack, gap, etc.
2. Also called aperture stop. Optics.- an opening,
usually circular, that limits the quantity of light that
can enter an optical instrument.
Why then do we refer to mirror diameter as aperture?
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

Hmmm, that is not a particularly good dictionary. Believe it or not,
dictionaries are not all alike.

In optics, "aperture" has a specific meaning which they did not quite get.
Actually two of them; in photography it often means f-ratio. But the
meaning we have in mind is the size of whatever limits the incoming beam of
light. This could be a stop, a lens, or even the edge of a mirror.

My 10" mirror sits in a 12" ID tube. Don't I have a 12" aperture?

Well, ok. It's a 12".
But the focal length hasn't change, so now you've just altered the f/ratio and
now you have to get a Paracorr.


john

hello
"Bruce Sidell" a écrit dans le message news:
...
My 10" mirror sits in a 12" ID tube. Don't I have a 12"
aperture?

I hope that no manufacturer reads this...he could have the idea to
advertise his 10" telescopes as 12" ones! :-)
(impossible ? take the computer screens manufacturers who give
the total size of the tube instead of the size of the visible window!)

;-)