Off-Axis (Zero Obstruction) Reflector Telescopes

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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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

Chris L Peterson wrote in message
...

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

Chris,

I understand it`s your opinion, but you are kidding about this "not optical
performance" thing right? I believe the optical quality of Dodgens work has
been more than overwhelmingly demonstrated to be excellent in both optical
reports and bench tests, and user reports and reviews.
Now if you were thinking of just any old off-axis reflector, not one of my
scopes then that`s a different story!

Dan McShane


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On Sun, 3 Oct 2004 23:16:25 -0400, "Dan McShane" wrote:

Chris,

I understand it`s your opinion, but you are kidding about this "not optical
performance" thing right? I believe the optical quality of Dodgens work has
been more than overwhelmingly demonstrated to be excellent in both optical
reports and bench tests, and user reports and reviews.
Now if you were thinking of just any old off-axis reflector, not one of my
scopes then that`s a different story!

I'm not disputing the quality of your scopes. I'm just saying they are no better
than high quality scopes of other designs, including conventional Newtonians,
particularly when used with off-axis apertures.

_________________________________________________

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

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

On Mon, 4 Oct 2004 06:15:49 +0000 (UTC), (Brian Tung)
wrote:

Dan Chaffee wrote:
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 guess is that the RMS scales with the square root of the mask
diameter.

Hi Brian,

You would have to be assuming a nearly symmetrical error distribution
for this to be correct. If (as is often the case with large fast
mirrors) medium sized non-symmetrical high and low spots-- higher
order astigmatic errors--were present the sampled RMS of the masked
aperture in the best part of the mirror would have to scale
differently than other parts of the mirror, don't you think? Also,
suppose the zone of greatest error was the outer 10% of the
mirror and the masked aperture missed this completely, the
RMS of the masked aperture would be then scaled wrongly based on
the overall mirror's RMS.

Dan C.

Dan Chaffee wrote:
You would have to be assuming a nearly symmetrical error distribution
for this to be correct.

That is not necessary. But you're probably right that I'm making some
assumptions that don't always hold in the real world. For instance, if
the roughness is strictly self-similar, then RMS error goes up linearly
with the mask diameter.

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

Dan Chaffee says...

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.

Good point. I could move the mask around and do tests so as
to find the best section.

A smaller hole in the mask not only gives me a better probability
of finding an excellent section, but also allows me to try more
places on the mirror. If the hole is as large as possible, I can
only rotate it, but if it's smaller, I can vary how far off-axis
I place it.

Given the low cost of a mask, I can see myself having a collection
of them with different-sizes holes, each marked with the best place
to put it on the mirror.

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

According to Newt, one can build a 6.5 inch F11 Newt with a low profile
focuser, with 0.46 degree fully illuminated FOV and a 15% CO. The mirror would
be relatively easy to make, the optical window probably somewhat difficult but
overall it would seem to be less trouble than cutting a big mirror up and would
not suffer from the problems associated with fast mirrors such as coma.

Just something to think about...

It would have those problems associated with closed tubes. Which is why
planetary, long focal length newts have open ended tubes and avoid those
hassles. Just optimize the secondary mount instead.


john

Jon Isaacs wrote in message
...

It seems to me that the correct scope to compare an OA Newtonian with is a
equal sized Newtonian with an equal focal length, a low profile focuser
and
maybe an optical window to mount the secondary.

jon

Jon,

No actually folks who are interested in, and order these scopes, do so
almost universally for one reason; They want an APO but can`t afford to
shell out the bucks and/or want more more bang for the buck vs. APO`s.
If they really thought there was nothing special to a scope with zero
diffractive effects from CO, perfect color rendition, using ultra-high
quality optics they would no doubt be instead building longer FL scopes as
conventional newts with 15% CO.

Now for purposes of discussion, sure A/B them against whatever you want.

Dan McShane



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