Off-Axis (Zero Obstruction) Reflector Telescopes

John Shakespeare says...

There is a place in Switzerland which makes Schiefspieglers
commercially. The focal ratios and prices are high, of course.

Here it is:
http://www.aokswiss.ch/d/tel/kutter.html
Looks like they top out at 150mm (6 inches).

I did some more searching and found these:

http://www.astromart.com/articles/ar...article_id=116
http://www.astromart.com/articles/ar...article_id=121
http://bhs.broo.k12.wv.us/homepage/a...k/sptbrief.htm
http://bhs.broo.k12.wv.us/homepage/a...k/stevpaul.htm
http://www.amsky.com/atm/telescopes/spscopes/spt.html
http://bhs.broo.k12.wv.us/homepage/a...vick/weird.htm

Here is my question: would I be able to get pretty much the same
effect as an off-axis zero obstruction) reflector from a 16 inch
Newtonian reflector with a 6 inch off-axis aperture stop mask?
It would be cheaper and would allow dual use as a normal 16 inch
Newtonian, but would it have the same high contrast and low
diffraction?

--
Guy Macon http://www.guymacon.com

"Guy Macon" http://www.guymacon.com wrote in message
...

John Shakespeare says...

There is a place in Switzerland which makes Schiefspieglers
commercially. The focal ratios and prices are high, of course.

Here it is:
http://www.aokswiss.ch/d/tel/kutter.html
Looks like they top out at 150mm (6 inches).

I did some more searching and found these:

http://www.astromart.com/articles/ar...article_id=116
http://www.astromart.com/articles/ar...article_id=121
http://bhs.broo.k12.wv.us/homepage/a...k/sptbrief.htm
http://bhs.broo.k12.wv.us/homepage/a...k/stevpaul.htm
http://www.amsky.com/atm/telescopes/spscopes/spt.html
http://bhs.broo.k12.wv.us/homepage/a...vick/weird.htm

Here is my question: would I be able to get pretty much the same
effect as an off-axis zero obstruction) reflector from a 16 inch
Newtonian reflector with a 6 inch off-axis aperture stop mask?
It would be cheaper and would allow dual use as a normal 16 inch
Newtonian, but would it have the same high contrast and low
diffraction?

I've owned a DGM Optics off-axis Reflector and a 16" Dob, and the Dob with
an aperture mask is good but isn't as good as far as contrast/diffraction
goes. They are however very close, but they are not the same. The biggest
notable difference was darkness of image for me. The DGM had a darker
background than the Dob. I suspect this is due to diffraction off the cut
circle. With the DGM scope the scatter caused from the edge of the scope
probably doesn't make it to the eye (or at least not noticibly) due to the
fact that mirror is only 6.5" and the opening is 10" in the scope. Just a
postulation. However, the images are fairly close otherwise. You would
need to be looking at something that was just on the edge of what can be
seen in the scope. The DGM scope just gave some awesome images for it's
size.

However, I believe the benefits of the 16" Dob are great if you intend to
look at Deep Sky objects and easily outway the mask issues. Besides, on a
good night of seeing you'll get a really good treat!

Mike.


--
Guy Macon http://www.guymacon.com

"Guy Macon" http://www.guymacon.com wrote in message
...

Here is my question: would I be able to get pretty much the same
effect as an off-axis zero obstruction) reflector from a 16 inch
Newtonian reflector with a 6 inch off-axis aperture stop mask?
It would be cheaper and would allow dual use as a normal 16 inch
Newtonian, but would it have the same high contrast and low
diffraction?

My "guess" is that diffraction effects would be the same, as a result of
creating the same aperture and central obstruction. Contrast on the other
hand is not so independent of other factors. Smoothness of the optics and
baffling are two examples of factors that are dependent on design execution.
If the two scopes in question had similar contrast to start with, then of
course.

The better bet is to improve the contrast of the 16" Newtonian, and enjoy
the higher resolution of the aperture, ignoring the spikes caused by the
secondary spider. Also keep in mind, as a friend just pointed out this
morning, you _can_ have highly detailed images in a scope with lesser
contrast, and conversly you can have lesser detailed images in a scope with
higher contrast. The two are at least partially independent. Take your
premise that the 6" OA is a "high contrast" scope as an example, which
implies that the 16" Newtonian is not. Nobdy will dispute that the 16"
Newtonian has a higher potential to show low contrast details by sheer brute
force of light gathering and resolution.

Contrast is in part, a subjectively aesthetic value, and is more important
in drawing out detail in samller apertures. That's not to say that it isn't
at all important in a larger aperture, of course it is.

On Sun, 03 Oct 2004 06:42:24 -0700, Guy Macon http://www.guymacon.com wrote:

Here is my question: would I be able to get pretty much the same
effect as an off-axis zero obstruction) reflector from a 16 inch
Newtonian reflector with a 6 inch off-axis aperture stop mask?

Identical. The systems are equivalent.


It would be cheaper and would allow dual use as a normal 16 inch
Newtonian, but would it have the same high contrast and low
diffraction?

The diffraction would be identical. In a perfect system, the contrast would also
be identical. Of course, contrast is a complex thing, depending on the surface
quality of optics and on baffling. Since the two designs are mechanically
different, the contrast may be different, too. But neither design necessarily
provides better or worse contrast. However, minimizing surfaces is generally a
good thing, so the simpler design of the Newt would be easier and less expensive
to make, and would be potentially better optically. And, of course, you still
have a large aperture scope at your disposal for the vast majority of viewing
where the effects of a central obstruction are insignificant.

IMO the only reason to own an off-axis reflector is because of a personal
interest in clever optical designs, not optical performance.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On Sun, 03 Oct 2004 06:42:24 -0700, Guy Macon http://www.guymacon.com
wrote:

Here is my question: would I be able to get pretty much the same
effect as an off-axis zero obstruction) reflector from a 16 inch
Newtonian reflector with a 6 inch off-axis aperture stop mask?

Identical. The systems are equivalent.


It would be cheaper and would allow dual use as a normal 16 inch
Newtonian, but would it have the same high contrast and low
diffraction?

The diffraction would be identical. In a perfect system, the contrast
would also
be identical. Of course, contrast is a complex thing, depending on the
surface
quality of optics and on baffling. Since the two designs are mechanically
different, the contrast may be different, too. But neither design
necessarily
provides better or worse contrast. However, minimizing surfaces is
generally a
good thing, so the simpler design of the Newt would be easier and less
expensive
to make, and would be potentially better optically. And, of course, you
still
have a large aperture scope at your disposal for the vast majority of
viewing
where the effects of a central obstruction are insignificant.

IMO the only reason to own an off-axis reflector is because of a personal
interest in clever optical designs, not optical performance.


Not quite, price performance in for unobstructed aperature is a reason to
own one of these. You can get a 4" Off-Axis for well under a $1000. A 4"
refractor of similar quality starts nearly twice and possibly more.

Mike.


_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Sun, 03 Oct 2004 15:05:46 GMT, "Mike Fitterman" wrote:

Not quite, price performance in for unobstructed aperature is a reason to
own one of these. You can get a 4" Off-Axis for well under a $1000. A 4"
refractor of similar quality starts nearly twice and possibly more.

Yeah, but you can get the identical performance from a similarly or lower priced
10" Newt with an off-axis mask, and you still get the vastly greater performance
the large aperture provides when you remove the mask. That's better
price/performance in my book.

I guess if small size is a factor, as for traveling, there would be reason to
compare an off-axis reflector with a refractor. But even there, I'd look at
something like a Mak, since the effect of its small CO is extremely small.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On Sun, 03 Oct 2004 15:05:46 GMT, "Mike Fitterman"
wrote:

Not quite, price performance in for unobstructed aperature is a reason to
own one of these. You can get a 4" Off-Axis for well under a $1000. A
4"
refractor of similar quality starts nearly twice and possibly more.

Yeah, but you can get the identical performance from a similarly or lower
priced
10" Newt with an off-axis mask, and you still get the vastly greater
performance
the large aperture provides when you remove the mask. That's better
price/performance in my book.

I guess if small size is a factor, as for traveling, there would be reason
to
compare an off-axis reflector with a refractor. But even there, I'd look
at
something like a Mak, since the effect of its small CO is extremely small.

I wouldn't own one of the smaller Maks. Not without some major tweaks for
cool down anyway.

If I were to own one scope I'd probably do a 12.5" dob and deal with an
aperature mask when I had to.

However, I think owning two is really useful. A really high quality smaller
4" aperature instrument and the big light bucket like a 16" scope. Anything
else in between is slightly better, but nothing more really worth another
scope. This make for easy lightweight, quick setups, without any heavy
lifting, easily fits in any kind of vehicle etc etc as well as really seeing
detail that you wouldn't otherwise see.

Mike.


_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Yes.
Under the circumstances you state. If one already has a large newt, then an
aperture mask is an almost costless experiment. If it is a solid tube newt,
then it is best to leave a gap between the mask and the end of the tube. This
will keep minor tube currents from perculating out into the light path. They
can escape.

But, big mirrors have big thermal issues. Solving these will often make off
axis masking obsolete. Not useless, just used far less often.


john

Would a 16 inch Newtonian reflector with a 6 inch off-axis
aperture stop mask be a good choice for maximum magnification
lunar viewing in situations where seeing is ideal?

Would a 16 inch Newtonian reflector with a 6 inch off-axis
aperture stop mask be a good choice for maximum magnification
lunar viewing in situations where seeing is ideal?

No.
A 16 inch newtonian reflector would be a much better choice for maximum
magnification where seeing is ideal. No contest.

But, if the atmospheric seeing does not allow for pleasing viewing at 16", even
after all thermal issues have been optimized (scope has been cooled, boundary
layer thermals are being swept away by blower) then an aperture mask can yeild
more pleasing views. Not as high resolution as full aperture, but asthetically
prettier.

By the way, on-axis masking is also possible. It's just as painless as off axis
masking, for those circumstances where aperture masking is in fact the real
solution to sharpening the image.

john