Orion "clear aperture" 3.6 reflector?

The coma in these OA scopes would be exactly the same as the f/4 parent
mirror. If you're in the center of the FOV, it's not a problem. Off axis
is another story.

I don't think so....

Consider that this scope is essentially an F4 Newtonian with an Aperture mask,
at least to first order.

Then consider what part of the mirror contributes the coma to the image. In my
way of thinking it is the edge of the mirror since from a coma standpoint one
can turn a 10.25 inch F4 mirror into a 4 inch F10.25 mirror with a circular
mask. One will have lots of coma, one will have essentially no coma.

Now in the case of the Masked Newt or OA newt, the mirror is cut so a very
small portion of that low focal ratio portion is used while most of it in the
moderate and high focal ratio parts. This would indicate that the coma would
be much less than in an F4 mirror but significantly more than in an on-axis F10
mirror.

Rumour has it that the coma is comparable to that of an F6 or so scope, maybe
noticeable but not objectionable, certainly very few observers (save Jan Owen)
use a Paracorr at F6...

jon

Tim,

Wrong. They have more coma than a same aperture/F/# conventional newt, but
no where near what the "parent" 10" F/4 has. The OA-4 (98mm F10.4) has about
the same coma as a F/6.5 conventional newt.

Dan McShane

Tim Killian wrote in message
...
The coma in these OA scopes would be exactly the same as the f/4 parent
mirror. If you're in the center of the FOV, it's not a problem. Off axis
is another story.

Dan McShane wrote:

Tim Killian wrote in message
...

It's an off-axis Newtonian design that's been available from several
companies. In theory, you get the performance of an APO refractor
because there is no obstruction from the secondary mirror. But you also
get the coma inherent with any newtonian design and the focal ratio is
higher than most APOs these days.


Tim, reports of coma being problematic are all but nil from actual
user/owners of OANs.


This makes it primarily a visual instrument.


These are fine scope imaging wise as long as you stick to lunar,
planetary,
or solar.

RichA wrote in message
...
On Wed, 02 Feb 2005 10:47:29 -0700, Tim Killian
wrote:

It's an off-axis Newtonian design that's been available from several
companies. In theory, you get the performance of an APO refractor
because there is no obstruction from the secondary mirror.

Un, no. Any obstructed scope with an obstruction
under about 15% performs almost like it had no obstruction
but, NO reflector I've ever seen produces images like the best
apos, inch per inch, unobstructed or not. The use of a "sector"
of a larger paraboloid is interesting, but it not a perfect sub for
an apo, despite the high price.
-Rich

Rich,

The reality is that many of my customers *do* and have ordered these scopes
because they *are* a good sub for an APO and against all but the most top
shelf APOs are a tad better.
And why shouldn`t they? They do something no APO can in bringing the entire
visible spectrum to the same focal point, while also doing something no
conventional newt can by doing so with zero effects from CO.

Dan McShane

Jb2269 wrote:
Dan,
An interesting discussion on the construction of modern OAN's but
isnt this
an ancient design used at long f ratio as far back as the 1800's? I
seem to
remember a picture of one with a bipod set of legs at the eyepiece
end so it
could be proped up in one of the ATM books.
Bill Bambrick
41 N, 73 W, 95 ASL

Well the design is certainly akin to the Herschelian; though I wonder
if even Hershel was able to polish an off-axis parabola into speculum
metal. In any case, some amateur telescope makers build off-axis
scopes from time to time (in fact one is described in the latest issue
of Sky and Telescope), some of which were close to either the
Hershelian or the OA concept. I would say the real innovation here is
in the manufacturing method which is probably what makes the design
commercially feesible. In any case, when Dan mentioned that he
introduced it, he might have meant commercially; as far as I know, he
is the first to commercially offer this type of scope.

--
Bill

Subject: Orion "clear aperture" 3.6 reflector?
From: (Jon Isaacs)
Date: 2/2/2005 4:09 PM Eastern Standard Time
Message-id:


I don't think so....

Consider that this scope is essentially an F4 Newtonian with an Aperture
mask,
at least to first order.

Then consider what part of the mirror contributes the coma to the image. In
my
way of thinking it is the edge of the mirror since from a coma standpoint one
can turn a 10.25 inch F4 mirror into a 4 inch F10.25 mirror with a circular
mask. One will have lots of coma, one will have essentially no coma.

Now in the case of the Masked Newt or OA newt, the mirror is cut so a very
small portion of that low focal ratio portion is used while most of it in the
moderate and high focal ratio parts. This would indicate that the coma
would
be much less than in an F4 mirror but significantly more than in an on-axis
F10
mirror.

Rumour has it that the coma is comparable to that of an F6 or so scope, maybe
noticeable but not objectionable, certainly very few observers (save Jan
Owen)
use a Paracorr at F6...

jon
*************************************
I use a Paracorr with any Newt faster than F/6, but at F/6 or slower i am fine
as wine!!!

Chas P.

Bill McHale wrote in message
oups.com...

Jb2269 wrote:
Dan,
An interesting discussion on the construction of modern OAN's but
isnt this
an ancient design used at long f ratio as far back as the 1800's? I
seem to
remember a picture of one with a bipod set of legs at the eyepiece
end so it
could be proped up in one of the ATM books.
Bill Bambrick
41 N, 73 W, 95 ASL

Well the design is certainly akin to the Herschelian; though I wonder
if even Hershel was able to polish an off-axis parabola into speculum
metal. In any case, some amateur telescope makers build off-axis
scopes from time to time (in fact one is described in the latest issue
of Sky and Telescope), some of which were close to either the
Hershelian or the OA concept. I would say the real innovation here is
in the manufacturing method which is probably what makes the design
commercially feesible. In any case, when Dan mentioned that he
introduced it, he might have meant commercially; as far as I know, he
is the first to commercially offer this type of scope.


Hi Bill,

Your clarifications of my remarks are correct. I didn`t mean to say I
invented the design, only that I introduced the design which is the same the
Orion scope. A more true name for the "cut it from a larger parent" variant
really should be a "decentered newt".

I believe the Herschellian is actually a tilted component design. If you
take a long FL spherical primary and tilt it a few degrees off-axis
abberations will be acceptable enough to use as a high power planetary
scope. Although there is still some residual astigmatism, as is the case
with most TCT`s, and F/# `s greater than 20 or so.

Dan

Jb2269 wrote in message
...
Dan,
An interesting discussion on the construction of modern OAN's but isnt
this
an ancient design used at long f ratio as far back as the 1800's? I seem
to
remember a picture of one with a bipod set of legs at the eyepiece end so
it
could be proped up in one of the ATM books.
Bill Bambrick
41 N, 73 W, 95 ASL

Hi Bill,

They definately would be a long tubed beast! A better version of a tilted
component design might be a Yolo. But the trick there is in the secondary,
which must warped to act as corrective element for the astigmatism
introduced when you tilt the primary.
That is one of the beatiful things about the true off-axis design; no
corrective secondary required.

Dan

On Wed, 2 Feb 2005 16:39:48 -0500, "Dan McShane"
wrote:


RichA wrote in message
.. .
On Wed, 02 Feb 2005 10:47:29 -0700, Tim Killian
wrote:

It's an off-axis Newtonian design that's been available from several
companies. In theory, you get the performance of an APO refractor
because there is no obstruction from the secondary mirror.

Un, no. Any obstructed scope with an obstruction
under about 15% performs almost like it had no obstruction
but, NO reflector I've ever seen produces images like the best
apos, inch per inch, unobstructed or not. The use of a "sector"
of a larger paraboloid is interesting, but it not a perfect sub for
an apo, despite the high price.
-Rich

Rich,

The reality is that many of my customers *do* and have ordered these scopes
because they *are* a good sub for an APO and against all but the most top
shelf APOs are a tad better.
And why shouldn`t they? They do something no APO can in bringing the entire
visible spectrum to the same focal point, while also doing something no
conventional newt can by doing so with zero effects from CO.

Dan McShane

That would be fine, if they actually provided something unique, they
don't. Those scopes have long focal ratios. You can have a Newtonian
made with a long focal ratio too. The obstruction (if the scope is
for visual use) will be miniscule and the only thing you'll see are
diffraction spikes. But the resolution and contrast will be as good
as that oddball from Orion.
Also, WHY spend the money on a 3.6" reflector to function as
"pseudo-apo" when you can BUY a 100mm apo refractor for the same
price from Orion??
-Rich

"Mark" wrote in message
oups.com...
Actually it's more like $999 for the 100mm APO OTA.

Actually at $649 for the 3.6" and ADD $350 DOES add up to $999


http://www.telescope.com/shopping/pr...D=54904&itemTy
pe=PRODUCT&iMainCat=4&iSubCat=13&iProductID=54904

Dan,
Thanks for the historical notes. I hear the design you manufacture gives high
quality images and hope to check it out at a star party one day.

Bill Bambrick
41 N, 73 W, 95 ASL