Telescopes of Captain James Cook.

"Larry Brown" wrote in message
...
In case you haven't seen the page about Captain James Cook. It took me
over two years to research this and find out what telescopes he actually
took with him. The answer came from a ship's manifest. You can find the
pages he
http://www.antiquetelescopes.org/cook.html
I have also added a page about fake antique telescopes.
I can't help wondering if the name Captain James T Kirk was taken from
Captain James Cook. Cook was a real explorer who went where no man had
gone before, after all.



I thought the SCT principle was much younger than that...its an F6 SCT in
the article

Thomas M wrote:

I thought the SCT principle was much younger than that...its an F6 SCT in
the article

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa 1962)
using spherical optics and a large corrector plate. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"David Knisely" wrote in message
...
Thomas M wrote:

I thought the SCT principle was much younger than that...its an F6 SCT
in
the article

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa
1962)
using spherical optics and a large corrector plate. Clear skies to you.
--


Some of you have a real bad habit of splitting hairs...the concept or
priciple is the same!

"Thomas M" wrote in news:h1w1c.60320$A12.50960@edtnps84:


"David Knisely" wrote in message
...
Thomas M wrote:

I thought the SCT principle was much younger than that...its an F6
SCT
in
the article

The reflector in the article is not an SCT. It is a simple
Cassegrain, possibly Classical or Gregorian, as the Gregorians were
often resorted to during the era of the "brass" telescope (the tube
length argues for a Gregorian). The SCT is a different and much more
recent design (circa
1962)
using spherical optics and a large corrector plate. Clear skies to
you. --


Some of you have a real bad habit of splitting hairs...the concept or
priciple is the same!



So do you call your SCT a Classical Cassegrain?

Llanzlan.

Thomas M. wrote:

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa

1962)

using spherical optics and a large corrector plate. Clear skies to you.
--



Some of you have a real bad habit of splitting hairs...the concept or
priciple is the same!


No, I'm afraid that it is not really "splitting hairs" at all. Its a matter
of learning and *correctly* using the standard terminology of Astronomical
equipment which has been established at least for the past 40 years if not
longer. The principle behind the SCT is different than that of the purely
reflecting telescopes like Newtonians and Cassegrain instruments, and it was
not the principle used for the telescope noted in the article.
The first reflector telescope design was the Newtonian (invented in
1668), which uses a parabolidal primary and a smaller flat "diagonal"
secondary mirror to send the light collected by the primary mirror out a hole
in the side of the tube. The special parabolidal shape is necessary to form a
fully-corrected image. The second reflecting telescope design was the
Cassegrain (proposed by Guillaume Cassegrain in the year 1677), which uses a
convex secondary mirror to send the light back through a hole in the main
mirror. Like the Newtonian, the design is *purely* a reflector, as it uses
*only* reflective optics (a paraboloidal primary and a hyperboloidal
secondary). The Gregorian is similar to the Classical Cassegrain, but uses a
concave secondary mirror behind the primary focal point to redirect the light
through a hole in the main mirror. These telescopes are *purely* reflective
or "Catoptric" instruments, since they only use mirrors to form their primary
images (except for the eyepiece). They rely only on reflective optics to
create an on-axis image which is corrected for things like spherical aberration.
The term "SCT" stands for "Schmidt-Cassegrain Telescope" which is a
noticably-different design from the Classical Cassegrain or the Newtonian.
It falls into the class of "Catadioptric" or "mirror-lens" telescopes, which
use *both* refractive optics *and* reflective optics to form their main images
(ie: lenses *and* mirrors). It is a derrivative of the Schmidt Camera, and,
like its predecessor, the SCT uses a full-aperture 'corrector plate' out in
front of the telescope to introduce an opposite form of spherical aberration
to that produced by the spherical primary mirror. The secondary mirror then
lengthens the focal length and directs the light back through a hole in the
primary mirror. The use of spherical optics and the special corrector plate
is an attempt to deal with off-axis aberrations like Coma. Without the
corrector plate, the images could be nearly useless, so the principle is not
the same as that used for pure reflectors like Newtonians and Classical
Cassegrain telescopes. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote:

The term "SCT" stands for "Schmidt-Cassegrain Telescope" which is a
noticably-different design from the Classical Cassegrain or the Newtonian.
It falls into the class of "Catadioptric" or "mirror-lens" telescopes, which
use *both* refractive optics *and* reflective optics to form their main images
(ie: lenses *and* mirrors). It is a derrivative of the Schmidt Camera, and,
like its predecessor, the SCT uses a full-aperture 'corrector plate' out in
front of the telescope to introduce an opposite form of spherical aberration
to that produced by the spherical primary mirror. The secondary mirror then
lengthens the focal length and directs the light back through a hole in the
primary mirror. The use of spherical optics and the special corrector plate
is an attempt to deal with off-axis aberrations like Coma. Without the
corrector plate, the images could be nearly useless, so the principle is not
the same as that used for pure reflectors like Newtonians and Classical
Cassegrain telescopes.

Do all or most SCT's use a moving primary mirror for focusing?


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Starman asked:

Do all or most SCT's use a moving primary mirror for focusing?

Most of them do, although what is actually happening is that you are changing
the effective focal length of the instrument slightly and thus moving the
location of the focal point back and forth. This mirror movement can
introduce problems such as slight shifting of the image, which is one reason
many astrophotographers lock the mirrors and use an add-on external focuser.
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

you need to learn something about optical systems. You are wildly off!
sorry -

jerry



Thomas M wrote:

"David Knisely" wrote in message
...
Thomas M wrote:

I thought the SCT principle was much younger than that...its an F6 SCT
in
the article

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa
1962)
using spherical optics and a large corrector plate. Clear skies to you.
--


Some of you have a real bad habit of splitting hairs...the concept or
priciple is the same!

I think I was told once its Gregorian - note the long tube for the 4' ap at f/6.

Only the Gregorians had such low focal lengths - ???
jerry


David Knisely wrote:

Thomas M wrote:

I thought the SCT principle was much younger than that...its an F6 SCT in
the article

The reflector in the article is not an SCT. It is a simple Cassegrain,
possibly Classical or Gregorian, as the Gregorians were often resorted to
during the era of the "brass" telescope (the tube length argues for a
Gregorian). The SCT is a different and much more recent design (circa 1962)
using spherical optics and a large corrector plate. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Jerry Warner wrote:

I think I was told once its Gregorian - note the long tube for the 4' ap at f/6.

Only the Gregorians had such low focal lengths - ???
jerry

I think that some of them did have somewhat shorter focal lengths. The
Gregorian might have been easier to make, since it used only concave optics
and these were easier to test than convex surfaces. I recall a club program
put on by Dr. Eugene Rudd of UNL (Antique Telescope Society) who brought
several of these wonderful brass instruments to our meeting. One was a 3 or 4
inch Gregorian, and although the instrument looked wonderful, I understand
that the performance wasn't all that hot. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************