Re: Days Until

"Peter Duncanson (BrE)" <mail@xxxxxxxxxxxxxxxxxx> wrote
On Sun, 22 Mar 2009 19:30:46 +0000, Paul Wolff
<bounceme@xxxxxxxxxxxxxxx> wrote:

"Peter Duncanson (BrE)" <mail@xxxxxxxxxxxxxxxxxx> wrote
On Sun, 22 Mar 2009 16:15:02 +0000, Paul Wolff
"Peter Duncanson (BrE)" <mail@xxxxxxxxxxxxxxxxxx> wrote
On Sun, 22 Mar 2009 10:22:20 +0000, Paul Wolff
Chuck Riggs <chriggs@xxxxxxxxxx> wrote

This instrument is typical of what is available to today's amateur
astronomer:

http://www.telescopes.com/telescopes/reflecting-telescopes/meadeds2130ln
treflectortelescopewithlntautoaligntechnology.cfm

Gosh, how modern. I wonder what the optics are like. It looks a very
short focal length, which is more demanding on the mirror. The longer
the focal length, and the smaller the aperture, the closer even a
spherical mirror form comes to being acceptable.

View More Information > Specifications tab:

Aperture 127mm
Focal Length 1000mm
Focal Ratio f/7.7
Eyepiece 1 SP9.7mm
Eyepiece 2 SP26mm
Highest Useful
Magnification 350X
Limiting
Magnitude 13
Resolution 0.9 Arc Seconds
Resolving
Power 0.9 Arc Seconds

Thank you, I didn't pay attention to the tabs last time.

If the diameter of the tube can be taken as a measure of the diameter of
the mirror (127mm or 5 inches), it's hard to see how a one metre focal
length can be accommodated in the instrument shown there in the picture.
The eyepiece assembly looks in scale with an external tube diameter of 6
inches.

I agree.

I have no hands-on knowledge of telescopes. ("I learn it from a book.")

Perhaps the quoted one metre is the focal length of the primary mirror
considered on its own.

The telescope seems to have a secondary mirror to reflect the rays back
down to a third mirror which turns them through a right-angle and into
the eyepiece. If the secondary mirror is concave then an image will be
produced at much less than one metre.

If it's a normal Newtonian reflector, it will have an axially aligned
parabolic primary mirror at the end of the tube on the left as shown,
and a small flat secondary mirror angled at 45 degrees mounted in the
spider which is visible in the mouth of the tube on the right. There's
a hole in the tube wall under the eyepiece assembly so the observer can
look radially inwards towards the secondary, which is elliptical; the
major axis is root two times the minor axis, so it has a circular aspect
both from the primary and from the eyepiece. The secondary brings the
primary's image to a focus in a plane close to the wall of the tube, to
be examined by the high magnification eyepiece lens system. There's no
place for a third mirror.

OK.

A shorter tube more like the one shown is found in Cassegrain designs,
where the secondary reflects the rays back down the tube towards the
primary and through a hole in the centre of the primary, which of course
is in the secondary's shadow and of no use for light-gathering. I think
the secondary is usually convex, to allow it to be smaller and so to
interrupt less starlight. The eyepiece is situated behind the primary
mirror and the observer looks in the natural direction towards the
object. I presume that long focal length mirror lenses on regular
cameras are of this general sort.

I think an angled curved secondary in a Newtonian reflector would be a
real problem, even impossible, to figure and mount correctly so as to
bring all rays to one focal plane under the eyepiece.

I had wondered about that hypothetical possibility and rejected it
immediately.

There might be further convergence provided by the first element of the
eyepiece.

The eyepiece is basically a magnifying glass, capturing the divergent
rays from the image formed by the primary mirror at the focal plane, and
rendering the light rays from each distinct part of the image parallel,
so the observer's eye can view the whole image as if it is at the
relaxed distant focus of the eye.

Yes, but if the primary mirror does have a focal length of one metre the
focal plane is half a metre or so outside the telescope.

My thought is that the eyepiece optics are arranged so as to apply
convergence and thereby put the focal plane within the eyepiece assembly
before the magnifying glass elements of the eyepiece.

Then the diameter of the convergent beam on the secondary mirror would
then be abnormally great, that mirror would be large and stop off more
light, and the optics outside the tube would be very non-standard.

It would work better if the primary mirror was of a shorter focal
length, 500mm, and a divergent lens in the eyepiece mimicked the longer
focal length. Barlow lenses are standard for this:
<http://www.astunit.com/tutorials/barlow.htm>
But I'm sure Meade would say 500mm focal length and a x2 Barlow if
that's what it was. A bundled Barlow lens should be a selling point,
because removing it changes the magnification, which means each eyepiece
has two possible magnifications.

I don't think there's any more English usage here...
--
Paul
.

Relevant Pages

  • Re: Days Until
    ... which is more demanding on the mirror. ... Eyepiece 1 SP9.7mm ... The eyepiece assembly looks in scale with an external tube diameter of 6 ... Perhaps the quoted one metre is the focal length of the primary mirror ...
    (alt.usage.english)
  • Re: Days Until
    ... which is more demanding on the mirror. ... Eyepiece 1 SP9.7mm ... The eyepiece assembly looks in scale with an external tube diameter of 6 ... Perhaps the quoted one metre is the focal length of the primary mirror ...
    (alt.usage.english)
  • Re: Whats The Deal!
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    (sci.astro.amateur)
  • Re: Days Until
    ... which is more demanding on the mirror. ... Eyepiece 1 SP9.7mm ... The eyepiece assembly looks in scale with an external tube diameter of 6 ... If it's a normal Newtonian reflector, it will have an axially aligned parabolic primary mirror at the end of the tube on the left as shown, and a small flat secondary mirror angled at 45 degrees mounted in the spider which is visible in the mouth of the tube on the right. ...
    (alt.usage.english)
  • Re: Transverse Velocity of Light
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