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Russ
2002-Feb-15, 05:09 PM
CNN may have published errors about HST lenses but The Learning Channel (TLC) is hot on their tail.

I was watching a program, about "10 greatest inventions", the other day (the title escapes me) where they talked about HST's "refracting mirror". How's that for a neat trick? /phpBB/images/smiles/icon_lol.gif

I always thought mirrors reflected but now NASA has gone and made one that refracts.

"Well, blow me down." - Popeye

Wiley
2002-Feb-15, 05:37 PM
Maybe the HST uses one of them one-way mirrors? /phpBB/images/smiles/icon_smile.gif

How else could the CIA use it to spy on us?



<font size=-1>[ This Message was edited by: Wiley on 2002-02-15 15:07 ]</font>

Jigsaw
2002-Feb-16, 03:34 AM
WARNING: THE FOLLOWING POST COMES FROM A WAY NON-ASTRONOMY PERSON... /phpBB/images/smiles/icon_biggrin.gif

Just curious. Okay, so a "refracting" telescope uses lenses; a "reflecting" one uses mirrors. The Hubble has lenses, therefore it's a "reflecting" telescope. Then, mirrors don't refract? :confused: So, what about this?

http://faculty.virginia.edu/teach-present-bio/LightRefraction.html

TITLE: Concave and Convex Mirrors and Lenses
Presented to grade level 5

GOALS
Concave and Convex Mirrors and Lenses is designed to teach fifth grade level students about light relection and refraction as shown in curved mirrors and lenses...

--snip--

A refracting telescope uses a concave mirror, a plane mirror, and a convex lens to magnify images. Have the students try to arrange these three components to magnify an image.
Well? Is that wrong?

What about this?

http://caad.arch.ethz.ch/teaching/praxis/ss96/final/03/


The beam hits a tiny, almost point-size refracting-mirror-system positioned in the center of the dome...
What? If I'm gonna learn it, I oughta learn it right...

<font size=-1>[ This Message was edited by: Jigsaw on 2002-02-15 22:35 ]</font>

Kaptain K
2002-Feb-16, 07:27 AM
The Hubble telescope does not have lenses. It is all mirrors. It is a reflecting telescope. Some of the instruments may have lenses, but I don't know for sure.

_________________
When all is said and done - sit down and shut up!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-02-16 02:28 ]</font>

GrapesOfWrath
2002-Feb-16, 12:00 PM
On 2002-02-15 22:34, Jigsaw wrote:

TITLE: Concave and Convex Mirrors and Lenses
Presented to grade level 5

GOALS
Concave and Convex Mirrors and Lenses is designed to teach fifth grade level students about light relection and refraction as shown in curved mirrors and lenses...

--snip--

A refracting telescope uses a concave mirror, a plane mirror, and a convex lens to magnify images. Have the students try to arrange these three components to magnify an image.
Well? Is that wrong?
Most definitely!

In fact, just above that quote is this: "A reflecting telescope uses two convex lenses." That is clearly wrong. Also, the first part of that page, in the Specific Objectives, has it right! They did make a mistake. Good catch. Send them an email.

Donnie B.
2002-Feb-16, 03:38 PM
When you're talking about telescopes, the type of scope is generally determined by its primary device, i.e. that part of the optical path that the incoming light hits first.

If that's a lens, as in an old-fashioned spyglass, then it's a refracting telescope.

If it's a mirror, as in the Hubble, Keck, and almost all large astronomical telescopes, it's a reflecting scope.

Some reflecting scopes (indeed, most amateur reflectors) use lenses too, in the eyepiece. They're still reflectors, though, because the primary is a mirror.

And, of course, a mirror can also refract light to some degree; but this is undesirable in a telescope mirror and great pains are taken to minimize it.

Roy Batty
2002-Feb-16, 05:38 PM
Sorry I dont know much about telescopes (yet), i'd like to know how compound (Cassegrain?) ones fit in .. can the primary objective be either a mirror or a lens ?

Kaptain K
2002-Feb-17, 02:50 AM
I assume you mean the Schmidt-Cassegrain like those that Celestron and Meade have built their companies around. They are reflectors, since the concave primary mirror does the initial focusing. The secondary mirror lengthens the light path so that the focus is behind the primary so that it is accessible to eyepieces or cameras. The corrector plate (the piece of glass at the front of the 'scope is a fourth order curve (thick at the edges and in the middle, thin in between) that allows the primary mirror to be spherical.

GrapesOfWrath
2002-Feb-17, 12:18 PM
On 2002-02-16 21:50, Kaptain K wrote:
I assume you mean the Schmidt-Cassegrain like those that Celestron and Meade have built their companies around. They are reflectors, since the concave primary mirror does the initial focusing. The secondary mirror lengthens the light path so that the focus is behind the primary so that it is accessible to eyepieces or cameras. The corrector plate (the piece of glass at the front of the 'scope is a fourth order curve (thick at the edges and in the middle, thin in between) that allows the primary mirror to be spherical.
Perhaps Roy Batty's point was that the corrector plate is actually "that part of the optical path that the incoming light hits first" and does the "initial focusing."

We're going to have to talk about "most focusing" instead of "first focusing," maybe?

Roy Batty
2002-Feb-17, 01:21 PM
Thanks for the info so far. I think I need to go away & read more on this /phpBB/images/smiles/icon_smile.gif. My limited understanding so far was that compound telescopes incorporated both lens'es & mirrors.. I was curious which was the main part ie the 'most focusing/light collecting'. From Kaptain K's description it sounds like they are mainly reflectors with the lens being just a corrector to allow the mirror to be spherical (easier & cheaper to manufacture presumably?).

Russ
2002-Feb-18, 07:01 PM
Dear Jigsaw:

It would seem the author of your post got confused about his/her topic.

1) Refracting telescopes use clear glass/chrystle to bend the incomimg light to a focus.

2) Reflecting 'scopes use a mirror, usually pollished metal over glass, to bounce light to another mirror then to a lens for eyes or devices for photograph or electronic imaging. The reflector does not bend or otherwixe modify the incoming light.

As far as I am aware, the HST uses only mirrors to direct light to electroning imaging systems

Dear Kaptain K:



The corrector plate (the piece of glass
at the front of the 'scope is a fourth order curve (thick at the edges and in the middle, thin in between) that allows the
primary mirror to be spherical.

I was not aware of this. Could you point me to any doccumentation? I would have sworn that the literature that came with my LX-200 said it had a hyperbolic primary and the corrector lens on the front was to clean up the errors induced by the secondary at the edge of the image. I know Maksutov-Cassigranian have spherical primaries but that was not my understanding for the Schmidts.

Whacha got? /phpBB/images/smiles/icon_biggrin.gif

Kaptain K
2002-Feb-18, 10:27 PM
The original Schmidt camera has a spherical primary and a curved film holder at prim focus. IIRC the SC also uses a spherical primary. It is the secondary that is hyperbolic to flatten the field at the final focus.

_________________
When all is said and done - sit down and shut up!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-02-18 17:28 ]</font>

Russ
2002-Feb-18, 11:00 PM
On 2002-02-18 17:27, Kaptain K wrote:
The original Schmidt camera has a spherical primary and a curved film holder at prim focus. IIRC the SC also uses a spherical primary. It is the secondary that is hyperbolic to flatten the field at the final focus.

I didn't know this. I'll have to take some time to research the "original schmidt camera". Got any links? /phpBB/images/smiles/icon_smile.gif

_________________
Lighten up! I'm here for the fun of it.

edit to fix typos

<font size=-1>[ This Message was edited by: Russ on 2002-02-18 18:01 ]</font>

aurorae
2002-Feb-19, 08:07 PM
I was not aware of this. Could you point me to any doccumentation? I would have sworn that the literature that came with my LX-200 said it had a hyperbolic primary and the corrector lens on the front was to clean up the errors induced by the secondary at the edge of the image. I know Maksutov-Cassigranian have spherical primaries but that was not my understanding for the Schmidts.


Try
http://www.celestron.com/tb-2ref.htm
and
http://www.celestron.com/schmidt.htm

However, I don't think either of the above do a good job explaining the difference between SCT and Maks. This one does a good job with that, I think:

http://www.users.bigpond.com/telescopes/telescopes057.htm


I found the history of these scopes laid out here:
http://www.weasner.com/etx/guests/mak/MAKSTO.HTM

and this one
http://www.howstuffworks.com/telescope4.htm
explains the SCT but falls down on the Mak (saying it is "more spherical". My old geometry teacher would freak at that.) /phpBB/images/smiles/icon_wink.gif



<font size=-1>[ This Message was edited by: aurorae on 2002-02-19 15:10 ]</font>

Russ
2002-Feb-21, 07:49 PM
On 2002-02-19 15:07, aurorae wrote:

and this one
http://www.howstuffworks.com/telescope4.htm
explains the SCT but falls down on the Mak (saying it is "more spherical". My old geometry teacher would freak at that.) /phpBB/images/smiles/icon_wink.gif
<font size=-1>[ This Message was edited by: aurorae on 2002-02-19 15:10 ]</font>


As well he should! As well we all should! Actually, they made alot of gramatical errors in that dissertation.