Schiefspiegler telescopes
twi...@hub.ofthe.net
need to fake it when you don't know something but you can simply come
out and ask. So I'm asking.
What are Schiefspiegler telescopes? I see that Iowa Scientific
Optical has started carrying a line of them with very high f numbers.
( I think that no one will complain about calling them very high f
numbers, the 6" is an f/30!)
I doubt many others really know about these telescopes, so please post
the info and I love getting email.
Thanks and I hope you all enjoy clear skies!
Twi...@hub.ofthe.net
Jay Reynolds Freeman
>What are Schiefspiegler telescopes? I see that Iowa Scientific
>Optical has started carrying a line of them with very high f numbers.
>( I think that no one will complain about calling them very high f
>numbers, the 6" is an f/30!)
Schiefspieglers -- sometimes waggishly referred to as "Sheepsprinkers"
by those who can never remember how to spell the real name -- are in
essence very long-focus off-axis Cassegrains -- the primary is tilted
to the side a little way, so that the secondary does not obscure the
incoming light beam, then the secondary itself is tilted some more.
With long f-numbers and the right tilts, the correction for spherical
aberration is good and the residual coma is just barely tolerable.
The optical tube assembly tends to be rather long; I think I remember
about ten times the primary diameter, maybe longer. The optical
tube assembly is also very peculiar looking -- it often resembles a
cartoon-style slap base made out of a washtub and a broom.
Those of you who want to be really technical may object that the
Schief is not an off-axis Cass but an off-axis Dall-Kirkham, and if
I remember correctly, that's true.
--
Jay Reynolds Freeman -- fre...@netcom.com -- I speak only for myself.
Mark Kaye
t> Optical has started carrying a line of them with very high f numbers.
t> ( I think that no one will complain about calling them very high f
t> numbers, the 6" is an f/30!)
Schiefspiegler is German for "oblique reflector". The design uses tilted
mirrors and spherical surfaces, the primary is concave and the secondary is
convex. A tri Schiefspielger uses a third mirror that is also slightly
concave. The design provides an unobstructed field of view with a long focal
length suitable for planetary and double star observing with high contrast. A
similar design using concave primaries and secondaries is called a Yolo.
Another reason for the long focal length is that coma would become
objectionable in shorter focal lengths.
For examples, see 1985-05p461, 1988-12p610 and 1991-01p31 of "Sky and
Telescope".
Clear skies!
MK
--
|Fidonet: Mark Kaye 1:249/109.1
|Internet: Mark...@observatory.crp.kingston.on.ca
|
| Standard disclaimer: The views of this user are strictly his own.
Brian L. Henry
There are diagrams and a good discussion of the design at
http://webspace.com/markv/schiefspiegler.shtml
--
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bhe...@wolfe.net Brian L. Henry http://www.wolfe.net/~bhenry
finger for PGP pub key Kirkland, Washington, USA, Earth space for rent
Hartmut Frommert
>What are Schiefspiegler telescopes? I see that Iowa Scientific
>Optical has started carrying a line of them with very high f numbers.
>( I think that no one will complain about calling them very high f
>numbers, the 6" is an f/30!)
>
>I doubt many others really know about these telescopes, so please post
>the info and I love getting email.
I have made a small webpage on unusual scopes, check somewhere in my homepage
at SEDS:
This page is mainly on Schiefs, and includes some images so that you can
see what they look like.
The name "Schiefspiegler" (spell "sheef-speeg-ler"; "ee" as in "sheep") is
German and means "skew reflector". There are various types of Schiefs which
have been constructed, and even more are constructable. They have in common
that they are reflecting telescopes without the obstruction by secondary
mirrors which otherwise effects the images of stars and makes them less
sharp (i.e., diminishes the "image definition", as the Schief pioneer Anton
Kutter put it). This is achieved by inclining the primary mirror against
the symmetry line of the image, thus allowing to move the secondary mirror
(e.g., of a Cassegrain) out of the viewing line of the primary. By doing
so, there occur imaging defects at first, namely coma and astigmatism,
which can be influenced by inclining the secondary mirror also. These
defects stay managable for large focal ratios, therefore most Schiefs have
f:20 or higher. They can be managed by carefully choosing inclination
angles for the mirrors in small Schiefs; the Belgian manufacturer
Liechtenknecker has their optics in this form for up to 5-inch. Higher
apertures and/or shorter focal ratios require additional corrections (more
acurately, additional optical errors with opposite sign so that they
compensate); this may be achieved by adding a correcting lense
(Liechtenknecker does this "catadioptric" solution for Schiefs of 6 to 16-
inch aperture).
These simple possibilities lead to Schiefspiegler telescopes of Kutter type;
they consist of a primary about double as large as the secondary, and the
eyepiece/photo end. One common construction is to put the secondary mirror
and the eyepiece in quite a long but thin tube, which has an oval hole in
one side for letting in the light from the primary, which sits in a separate
thick tube mounted aside the eyepiece. This is the "open" construction which
leads to handy light-weight scopes.
Alternatively, one can build a larger case which "packs" the two mirrors and
has only an aperture hole at the "front end" (near the secondary mirror), of
about the same diameter as the primary mirror. This "closed" construction is
more massive and less handy then the open, but has the advantage of less air
turbulence, better light and dew protection, and, e.g., a "simpler" possible
use of full-aperture Solar filters.
There are other types of schiefs, one more notable being the Yolo: Here, the
mirrors are deformed by mechaincal forces to achieve an error-free optical
image. Others use three or four mirrors ("Tri" and "Tetra" Schiefs), however
they are not so common because patents prohibit a low-cost production (at
least this was written), and the scopes get more and more complicated.
As the Schiefs have a full obstruction-free aperture, their power is equal
to that of refractors of same size (even better than that of refractors
not well corrected for chromatic effects, as they are reflectors). This
means that they (as refractors of same aperture) are, from construction, as
"good" for the observation of planets, multiple stars, the Moon and the Sun
as Newtons and Cassegrains of about 1.8 times their aperture. For deep-sky
objects, this effect is less important, and they only profit from their a
bit larger free aperture, making them about as powerful as Newtons/
Cassegrains of 1.2 times their aperture. From their price, they are far
below a refractor of same size, and because they are lighter, they need a
less expensive mount.
Hope this is useful.
Hartmut Frommert | Use native OS/2 programs
<Hartmut....@uni-konstanz.de>, http://www.seds.org/~spider/
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Jay Reynolds Freeman
[...]
I have no complaint about the excellent technical information
Hartmut offers, but to avoid confusion, someone should comment that
United States use of the word "schiefspiegler" seems to be a little
more restricted; we seem generally to use the unqualified term
"schiefspiegler" to apply only to the original, two-mirror, Kutter
design (including the variants that use a weak lens as a final
corrector). The three-mirror designs are generally referred to as
"tri-schiefspieglers", and I have never heard the word
"schiefspiegler" applied to a Yolo before. If there is a general term
we use, it is probably "off-axis reflector", or "tilted-component
reflector", which is funny, because those are almost literal
translations of the German "schiefspiegler". I didn't say it made
sense...
This isn't a matter of what's right or wrong -- I am not trying to
start a flame war about definitions of terms; I just wanted to mention
how the word is commonly used so that beginners could be aware of the
possibility of confusion.
Thomas Back
Hartmut wrote --
>As the Schiefs have a full obstruction-free aperture, their power is equal
>to that of refractors of same size (even better than that of refractors
>not well corrected for chromatic effects, as they are reflectors). This
>means that they (as refractors of same aperture) are, from construction, as
>"good" for the observation of planets, multiple stars, the Moon and the Sun
>as Newtons and Cassegrains of about 1.8 times their aperture. For deep-sky
>objects, this effect is less important, and theyonly profit from their a
>bit larger free aperture, making them about as powerful as Newtons/
>Cassegrains of 1.2 times their aperture. From their price, they are far
>below a refractor of same size, and because they are lighter, they need a
>less expensive mount.....
Dear Hartmut,
This is one excellent post! One of the truly knowledgeable writings
on the Internet (dealing with telescopes/optics). I would like to add
some information about the above. Reflective surfaces suffer ~16 times
greater in terms of scatter in the diffraction image as they do in a
single refractive surface. What this means in real terms is the mirrors
in a Schief better be very, very smooth if they want to compete with the
contrast and lack of scatter of a good (Zeiss APQ) Apochromatic refractor.
The Apo refractor is not perfect, however. Spherochromatism, glass
homogeneity, fifth order zonal, tertiary spectrum, and sensitive element
alignment and cell design limits the perfection of even this design.
There are no perfect telescopes (except maybe an ion beam figured
single element off-axis parabola).
Sincerely,
Thomas M. Back
Mark Kaye
t> Optical has started carrying a line of them with very high f
t> numbers. ( I think that no one will complain about calling them very
t> high f numbers, the 6" is an f/30!)
Steve Johnston
> I would like to add some information about the above. Reflective
> surfaces suffer ~16 times greater in terms of scatter in the
> diffraction image as they do in a single refractive surface. What
> this means in real terms is the mirrors in a Schief better be very,
> very smooth if they want to compete with the contrast and lack of
> scatter of a good (Zeiss APQ) Apochromatic refractor...
In TM Magazine there was a head-to-head comparison of commercial
and non-commercial apochromatic refractors with a 12.5 inch Tri-
Schiefspiegler at the Texas Star Party. The Tri-Schiefspiegler gave
the better images. Homemade for about $500, compared to vastly
more expensive commercial apos. The Tri-Schiefspiegler was judged
somewhat better even when stopped down to 7 inches. The experiment
has been done. The Schiefspiegler won.
By the way, I made that Tri-Schiefspiegler. There is an article on
it in the special T-S issue (no. 28?).
--- Steve Johnston
Thomas Back
Steve wrote --
am...@cleveland.Freenet.Edu (Thomas Back) wrote:
> I would like to add some information about the above. Reflective
> surfaces suffer ~16 times greater in terms of scatter in the
> diffraction image as they do in a single refractive surface. What
> this means in real terms is the mirrors in a Schief better be very,
> very smooth if they want to compete with the contrast and lack of
> scatter of a good (say a Zeiss APQ) Apochromatic refractor...
> In TM Magazine there was a head-to-head comparison of commercial
>and non-commercial apochromatic refractors with a 12.5 inch Tri-
>Schiefspiegler at the Texas Star Party. The Tri-Schiefspiegler gave
>the better images. Homemade for about $500, compared to vastly
>more expensive commercial apos. The Tri-Schiefspiegler was judged
>somewhat better even when stopped down to 7 inches. The experiment
>has been done. The Schiefspiegler won.
> By the way, I made that Tri-Schiefspiegler. There is an article on
>it in the special T-S issue (no. 28?).
--- Steve Johnston
Oh Please Steve!
I know a number of people that looked through all the
Schiefs/refractors and Gregory's 8" f/15 SK-16/FK-52 Apo
doublet and they all said the image of Saturn was better in
the Apo -- Sue French said she could trace the Encke division
all-the-way around the ring, she could not do that in the Schief.
An employee of Optical Research Associates (and friend), which makes
the best optical design software in the world said that Gregory's
view of Saturn was the best in his life -- bar none! All that with
a system that has three degrading flat mirrors! If you don't want
to believe this, consider the kind comments of friends and your
own bias. And believe me, Gregory's design is not perfect, and a
true test would be with a stright tube assembly, not one with as
many mirrors as your Schief. I'm sorry, but inch for inch NOTHING beats
a superior designed Apochromat. That is the real world. I have nothing
against Schief's but the twenty or so I've looked through don't compare
with my well-figured 7.1" f/9 EDT Astro-Physics, and I feel this
telescope is far from perfection.
Sincerely,
Thomas M. Back