Question about newtonian with no obstruction

I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made such
that the secondary can be located to the side of the tube?

You'd need to have the mirror be a section of a parabola not centered on the
parabola's centre; I presume with modern grinding techniques this would not
be an insurmountable problem.

"jtaylor" ha scritto nel messaggio
...
I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made such
that the secondary can be located to the side of the tube?

Yes, indeed. I remember I saw this design on a S&T number (in 2004? 2003?).
It has the primary tilted by a certain angle so it reflects on the side of
the tube.

My single neuron is currently busy, so I can't dig deeper...

--
--
Beta Persei
45° 35' N
08° 51' E

Remove "_nospam" to reply

Yup, Orion Telescopes call it "Clear aperture" and others probably have
the same stuff under a different name.

Orion sells this scope at a greatly inflated price for what it is. I
had the same idea some time ago. I'd like to know how much better it
really performs. Supposedly there is a marked difference. I'd really
like to read an actual user's comments though.

my astronomy page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html

"jtaylor" wrote in message
...
I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror
and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made
such
that the secondary can be located to the side of the tube?

You'd need to have the mirror be a section of a parabola not centered on
the
parabola's centre; I presume with modern grinding techniques this would
not
be an insurmountable problem.
The only practical way to grind the shape, is to grind a much larger
mirror, and then cut a 'set' of smaller mirrors out of this. This is more
expensive than making a simple mirror.
What you are describing is the off-axis Newtonian. Orion do a small one,
and there a number of other companies making these. Look at:
http://users.erols.com/dgmoptics/
There are also more complex designs using extra mirrors to correct
aberrations. A web search on 'Schiefspiegler' will find a huge number.
The main reason it is perhaps less popular, is that it does cost more to
make, and for small central obstructions, the effect of the CO, is so
small as to be basically indetectable. Many Newtonian scopes have CO
percentages well below 20%, and at this level, the extra cost is normally
just not worth paying.

Best Wishes

jtaylor wrote:
I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made such
that the secondary can be located to the side of the tube?

You'd need to have the mirror be a section of a parabola not centered on the
parabola's centre; I presume with modern grinding techniques this would not
be an insurmountable problem.

You can look for "Herschelian telescope" named after William Herschel,
who used this design in his famous telescope.

http://vitruvio.imss.fi.it/foto/sima...t-200610rs.jpg

This design doesn't have a secondary at all, but suffers from important
aberrations.

Hope it helps.

jtaylor wrote:
I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror
and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made
such
that the secondary can be located to the side of the tube?

You'd need to have the mirror be a section of a parabola not centered
on the
parabola's centre; I presume with modern grinding techniques this
would not
be an insurmountable problem.

There are two ways to do what you are proposing: the first and most
common is to take a larger mirror and cut a section out and the other
is to flex the substrate during polishing so when it is released it
will take the desired shape. Both are considerably more expensive than
an on axis parabola when you consider the cost per square inch.

Ian Anderson
www.customopticalsystems.com

You must be really new to Astronomy, or at least not plugged into the amateur
community, if you don't know what Orion Telescopes recently brought out in their
catalog. Or simply check out the most recent issue of Sky and Telescope.

Suffice to say, you always sacrifice something when making adjustments to
designs. Optics is not stereo equipment, where swapping this or that can be
done easily without sacrificing much in the way of anything. But the idea has
ben done before, and will be done again.
--
Sincerely,
--- Dave
----------------------------------------------------------------------
It don't mean a thing
unless it has that certain "je ne sais quoi"
Duke Ellington
----------------------------------------------------------------------

"jtaylor" wrote in message
...
I'm newish to telescopes...

Normal newtonians (and other designs, but nevermind) have that mirror and
the spider in the way.

Has anyone ever considered/built a reflector with a main mirror made such
that the secondary can be located to the side of the tube?

You'd need to have the mirror be a section of a parabola not centered on the
parabola's centre; I presume with modern grinding techniques this would not
be an insurmountable problem.

spiral_72 wrote in message
oups.com...
Yup, Orion Telescopes call it "Clear aperture" and others probably have
the same stuff under a different name.

Orion sells this scope at a greatly inflated price for what it is. I
had the same idea some time ago. I'd like to know how much better it
really performs. Supposedly there is a marked difference. I'd really
like to read an actual user's comments though.

my astronomy page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html



http://users.erols.com/dgmoptics/OA-4%20review.htm
http://www.scopereviews.com/page1m.html#3
http://www.scopereviews.com/page1f.html#3
http://www.cloudynights.com/reviews/oa4.htm
http://www.cloudynights.com/reviews/oa42.htm
http://users.erols.com/dgmoptics/Customer.htm
http://www.excelsis.com/1.0/section....&findtext=&con
d=&range=&archive=-1&range=d
(may have to copy and paste)

Dan McShane
www.dgmoptics.com
http://groups.yahoo.com/group/offaxis/

There are number of unobstructed off-axis reflector designs, including
the Kutter Schiefspeigler, the tri-Schiefspeigler, the
quad-Schiefspeigler, the Yolo, the Herschelian off-axis, and the
Stevick-Paul.

Kutter Schiefspiegler
http://www.seds.org/~spider/scopes/kutterb.html
http://www.telescopemaking.org/schief.html
http://en.wikipedia.org/wiki/Kutter
http://www.seds.org/~spider/scopes/schiefv.html
http://bhs.broo.k12.wv.us/homepage/a...ck/trischi.htm

Yolo Reflector
http://www.telescopemaking.org/yolo.html
http://www.atmsite.org/contrib/Holm/yolo/

Stevick-Paul Reflector
http://www.amsky.com/atm/telescopes/spscopes/spt.html

Orion's Herschelian off-axis reflector
http://www.telescope.com/shopping/pr...roductID=77066

On the negative effects of obstruction and the benefits of off-axis
design, see:
http://www.seds.org/~spider/scopes/obstruct.html
http://legault.club.fr/obstruction.html

I am not sure why unobstructed off-axis designs did not prevail in the
market place over obstructed schmidt-cassegrains.

Refractors, off-course, are also unobstructed, so a 6" refractor is
roughly equal in its light reach to a 8" SCT.

- Peace Canopus56

canopus56 wrote in message
oups.com...

The original.
www.dgmoptics.com

The copy.
http://www.telescope.com/shopping/pr...D=77066&itemTy
pe=PRODUCT&iMainCat=4&iSubCat=8&iProductID=77066

- Peace Canopus56