Off-Axis (Zero Obstruction) Reflector Telescopes

"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.

Unfortunately, the off-axis Newtonian design incurs the cost of the
additional tube dimension one needs to get the secondary mirror out of the
light path of the primary. That means a couple extra inches in radius. A 4"
OA has a 6" diameter tube, and being F10-ish, it is also 40" long. The size
factor is not super compelling.

I think it's important to be mindful that OA scopes are specifically
designed to be color free and unobstructed. Naturally (logically), this
makes them more a competitor with the _long focus_ apochromats than any
other design. In which case the price/performance factor might be
compelling.

-Stephen Paul

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

Or an Intes Mak/Newt with less than 20% obstruction at a much lower focal
ratio

--

Thanks,

Chuck

"Stephen Paul" wrote in message
...

"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.

Unfortunately, the off-axis Newtonian design incurs the cost of the
additional tube dimension one needs to get the secondary mirror out of the
light path of the primary. That means a couple extra inches in radius. A
4"
OA has a 6" diameter tube, and being F10-ish, it is also 40" long. The
size
factor is not super compelling.

I think it's important to be mindful that OA scopes are specifically
designed to be color free and unobstructed. Naturally (logically), this
makes them more a competitor with the _long focus_ apochromats than any
other design. In which case the price/performance factor might be
compelling.

-Stephen Paul

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?


If the seeing is "ideal", then a 16 incher with an aperture mask would be a
good choice as long as the mask were not used.... : G (A Semi-Grin)

As we all know, we did go through this discussion a while back...

My understanding of the potential optical advantages of the OA Newt over a
larger Newt with an aperture mask"

1. Thermal issues, an OA Newt can have a smaller mirror which should reach
equilibrium more quickly and the OTA can be larger allowing for better cooling
and few issues with tube currents.

2. A fully illuminated FOV. Since the focuser can be on the opposite side of
the diagonal from the primary mirror, there is no difficulty fully illuminating
the FOV. It also may place less strict tolerances on the quality of the
secondary mirror.

There are other factors such as increased Coma over standard Newtonian of equal
focal ratio, (ie ~F10), the obvious one of loss of light gathering over the
unmasked large Newt, the size issue and length issues.

jon
Jon Isaacs

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

"Chuck" wrote in message
...
"Stephen Paul" wrote in message
...

I think it's important to be mindful that OA scopes are specifically
designed to be color free and unobstructed. Naturally (logically), this
makes them more a competitor with the _long focus_ apochromats than any
other design.

Or an Intes Mak/Newt with less than 20% obstruction at a much lower focal
ratio

Not sure how you conclude that the OA is competition for that scope. The
short focus Mak/Newt is a superior telescope for wide fields of view, and
for its compact size.

Stephen Paul

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.

Chris L Peterson wrote in message news:

Identical. The systems are equivalent.

1) A typical Schiefspiegler is f/20 to f/24. A 16 inch which gives
this kind of focal ratio with a 6" off axis stop needs to be f/7.5 to
f/9. A 16" with this kind of focal ratio is bigger than most folks
want to handle (or climb up to in the dark). Note also that the image
on the Schiefspiegler ends up where you can reach it without a ladder.
2) Consider the axes and bearings needed for the equatorial mounting
for a 16" scope compared to what you need for the much lighter
Schiefspiegler. This translates to a big bucks difference.
3) Consider the difficulty of generating a SMOOTH 16" f/8 paraboloid
free from zonal defects compared with the relative ease of making the
spherical surfaces of the Schiefspiegler. The smoothness of the
surface will have a huge effect on image contrast.

Clif

On 3 Oct 2004 14:04:11 -0700, (Clif) wrote:

Chris L Peterson wrote in message news:

Identical. The systems are equivalent.

1) A typical Schiefspiegler is f/20 to f/24. A 16 inch which gives
this kind of focal ratio with a 6" off axis stop needs to be f/7.5 to
f/9. A 16" with this kind of focal ratio is bigger than most folks
want to handle (or climb up to in the dark). Note also that the image
on the Schiefspiegler ends up where you can reach it without a ladder.
2) Consider the axes and bearings needed for the equatorial mounting
for a 16" scope compared to what you need for the much lighter
Schiefspiegler. This translates to a big bucks difference.
3) Consider the difficulty of generating a SMOOTH 16" f/8 paraboloid
free from zonal defects compared with the relative ease of making the
spherical surfaces of the Schiefspiegler. The smoothness of the
surface will have a huge effect on image contrast.

Clif

_Optically_ equivalent in terms of diffraction and contrast, which is how I took
the original question. Obviously they aren't mechanically equivalent! Smooth 16"
mirrors have become fairly common these days. I think the practical reality is
that there should be little contrast difference between the two systems.

As far as mounting is concerned, clearly a smaller, lighter scope will require a
lighter, less expensive mount. But either can be used on a Dob style mount
without incurring much cost (with an equatorial platform for tracking, if
desired). The primary reason for using an equatorial mount would be for imaging,
and I can think of no reason at all for using an off-axis reflector for that
application.

_________________________________________________

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

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