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

Mike Fitterman wrote:
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.

I don't think so. Diffraction is not the same thing as scattering (as
usually used), although it may often be presented that way. Scattering
may indeed cause the background to be brighter, but it isn't caused by
diffraction effects. It is caused by, e.g., shiny internal surfaces,
surface roughness on the mirror, and so forth. These effects spray the
light around in a non-specular way, so as to uniformly illuminate the
focal plane. The scattering makes it more difficult to see dim objects
and, less obvious but just as important, planetary detail is obscured.

Diffraction, on the other hand, is caused by a limitation on the light
bundle entering the scope. It can be caused by a narrower tube, but the
light bundle is also constricted by the actual size of the objective.
The diffraction effect of the 6.5-inch off-axis mirror is no worse and
no better than that of any other 6.5-inch mirror. The difference in
diffraction is caused by the lack of a central obstruction (and to a
lesser degree, any spikes supporting that obstruction).

The best image would be that created by an infinite aperture, because
all of the light waves coming down help to create the sharp image. Any
part of the wave front that is blocked from forming an image will cause
an image degradation at the focal plane; the more that's blocked, the
greater the degradation. A 10-inch lens exhibits a smaller diffraction
effect than a 6-inch lens because it blocks less of the incoming wave
front--this in spite of the fact that it has a longer edge all the way
around than the 6-inch.

Now, put a 2-inch obstruction in the center. The obstructed 10-inch
now creates a somewhat worse image than the unobstructed, because a
further 2-inch disc of the wave front is being blocked from contributing
to the image. If you were to put a square-shaped obstruction in the
same place, it would produce about the same *level* of diffraction as
the circular obstruction, although the *shape* of the effect would be
somewhat different (four-way symmetry instead of radial symmetry).

Suppose it's a 10-inch mirror, rather than a 10-inch lens. And suppose
it's got a turned-down edge, so you mask the outer 1/2-inch out, so it's
now effectively a 9-inch mirror. It now has the diffraction pattern of
a 9-inch mirror, too, despite the fact that the tube diameter hasn't
changed at all. The effective mirror diameter is what determines the
diffraction pattern in this case. (Note the application to a 6-1/2-inch
off-axis mirror in a 10-inch tube.)

Note that I am *not* saying that your off-axis is not fantastic, or that
it is not fantastic for a design-related reason. I'm just saying that
it isn't directly related to diffraction. Perhaps the extra spacing
between the tube and the mirror reduces the effects of tube currents,
which (after all) can be quite long-lived. Or perhaps the asymmetric
nature of the mirror reduces the effect of zones.

People attribute all sorts of things to diffraction because they find it
vaguely mysterious. But to the optical engineer, diffraction is one of
the less mysterious factors. It's the other real-world things that can't
be easily modelled that are more mysterious--in particular, anything
having to do with turbulence and roughness, which often have "stochastic"
and "self-similar" written all over them.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Hi Mike and group ;

Mike wrote ,


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.


The difference between a stopped down 16 inch Dob. and a
DGM 6.5 inch reflector I beleave comes down to optic surface
smoothness and RMS wavefront to the eyepiece . Unless one has a very
high quality optic in there 16 inch Dob .with a high precision mirror
mount to boot the 16 inch mirror will never give the contrast that the
6.5 inch DGM Optics mirror will give at high power . And lets face it
thats the only reason to stop down a good 16 inch Dob. So in my way of
thinking stopping down the 16 inch is useless unless one is just going
for the best high power image possible from that particular scope . Of
course if you live somewhere with sub arc seeing 20 nights out of the
year you may want to invest in TEC'S 10 inch F 20 Mak. ,must be
wonderfull !!
I have seen an average 18 inch Dob. stopped down to 6 inchs and the
image of Mars was better at 6 inchs than 18 ,but that does not mean
its as good as a scope made for high power use . If your a freak for
the planets its best to pony up the bucks for a scope made for high
power work in the first place .

my 2 cents Leonard

Leonard says...

The difference between a stopped down 16 inch Dob. and a
DGM 6.5 inch reflector I beleave comes down to optic surface
smoothness and RMS wavefront to the eyepiece . Unless one has a very
high quality optic in there 16 inch Dob .with a high precision mirror
mount to boot the 16 inch mirror will never give the contrast that the
6.5 inch DGM Optics mirror will give at high power . And lets face it
thats the only reason to stop down a good 16 inch Dob. So in my way of
thinking stopping down the 16 inch is useless unless one is just going
for the best high power image possible from that particular scope . Of
course if you live somewhere with sub arc seeing 20 nights out of the
year you may want to invest in TEC'S 10 inch F 20 Mak. ,must be
wonderfull !!

I have seen an average 18 inch Dob. stopped down to 6 inchs and the
image of Mars was better at 6 inchs than 18 ,but that does not mean
its as good as a scope made for high power use . If your a freak for
the planets its best to pony up the bucks for a scope made for high
power work in the first place .

So if I get a very high quality 16 inch Newtonian and put in an
off-axis 6 inch stop, will it be as good as a 6 inch off-axis scope
that has the same quality of optics? It seems to me that it would.

I have seen an average 18 inch Dob. stopped down to 6 inchs and the
image of Mars was better at 6 inchs than 18 ,but that does not mean
its as good as a scope made for high power use . If your a freak for
the planets its best to pony up the bucks for a scope made for high
power work in the first place .


Guy wrote ,
So if I get a very high quality 16 inch Newtonian and put in an
off-axis 6 inch stop, will it be as good as a 6 inch off-axis scope
that has the same quality of optics? It seems to me that it would.

Hi Guy ,

With the cost and cooling issues of the larger
optic I don't see the project as worth the attempt. Others will , I
would like to see the results side by side . When I say freak for the
planets thats what I mean. Someone who wants the last drop of contrast
and sharpness possable from there planetary optic and its not going to
come from a stopped down Dob.

But new things happen every day . Leonard

The difference between a stopped down 16 inch Dob. and a
DGM 6.5 inch reflector I beleave comes down to optic surface
smoothness and RMS wavefront to the eyepiece . Unless one has a very
high quality optic in there 16 inch Dob .with a high precision mirror
mount to boot the 16 inch mirror will never give the contrast that the 6.5
inch DGM Optics mirror will give at high power .

I don't think there is any gain here, simply because the OA scope has a mirror
which is cut from essentially what must be a 16 inch F4.5 inch mirror, so
indeed that high quality large mirror must exist....

jon

Jon Isaacs wrote in message
...
The difference between a stopped down 16 inch Dob. and a
DGM 6.5 inch reflector I beleave comes down to optic surface
smoothness and RMS wavefront to the eyepiece . Unless one has a very
high quality optic in there 16 inch Dob .with a high precision mirror
mount to boot the 16 inch mirror will never give the contrast that the
6.5
inch DGM Optics mirror will give at high power .

I don't think there is any gain here, simply because the OA scope has a
mirror
which is cut from essentially what must be a 16 inch F4.5 inch mirror, so
indeed that high quality large mirror must exist....

jon

Jon,

Yes certainly the premium mirrors (CZ, Royce Optical, etc) will work, but
for instance to make the OA-6.5 we start with an 18" f/3.6, not an f/4.5,
which further raises the bar for required smoothness of figure. Try finding
someone to make a 98-99% Strehl, 1/45th wave RMS 18" f/3.6.
Most of the premium mirror makers have no interest in working at those low
f/#`s.

Dan McShane




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On Sun, 3 Oct 2004 20:01:19 -0400, "Dan McShane"
wrote:


Yes certainly the premium mirrors (CZ, Royce Optical, etc) will work, but
for instance to make the OA-6.5 we start with an 18" f/3.6, not an f/4.5,
which further raises the bar for required smoothness of figure. Try finding
someone to make a 98-99% Strehl, 1/45th wave RMS 18" f/3.6.
Most of the premium mirror makers have no interest in working at those low
f/#`s.

The main the reason some large apertures give better images when
stopped down is that the RMS wavefront contained in the masked area
is far and away better than the whole surface. My first attempt at
mirror making resulted in a 6"f 7.3 hyperboloid with a pathetic .5
wave of overcorrection. Star testing it with a 2" mask showed a
nearly perfect star test---certainly a high .90's strehl. My point
is that a small section of a much larger mirror will almost always be
DRASTICALLY better than the whole surface, unless a rolled edge
or excessive roughness is included in the section.
A champion wavefront on a large, fast mirror is not necessary for
getting a very good idea of unobstructed performance from
30-40% of the original aperture, provided the mirror's edge zone is
not included in the mask and it's not too rough. A 16" f/4.5 with
a reasonably smooth surface and overall strehl of .7-.8, if even that
good, would easily do for a 6" unobstructed mask.

Dan Chaffee

"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.