Orion "clear aperture" 3.6 reflector?

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

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.

Huh?????

"While the 6" found all of the galaxies, the contrast on the DGM was in
another league. It's as if someone "erased" half of the moon. The
gains in contrast meant that I could pick out smaller galaxies in the
Markarian's Chain region that were barely visible in the Meade. Indeed,
all objects were brighter in the 5.5" than in the 6".
http://www.scopereviews.com/page1f.html#3

"We looked at the double-double at 275X and all four components
were still tight little round balls. The contrast was impressive, too. You
could have easily fooled me into thinking I was looking through a world-
class 4" apochromatic refractor. I did some quick comparisons with my
Traveler. While the AP threw up a slightly brighter image, there was
little to choose between the scopes."
http://www.scopereviews.com/page1m.html#3

"First, the high level of contrast available from the OA-4 allows me to
consistently pick out features on Jupiter that are hard to find when seeing
is less than perfect, thereby acting against the larger apertures. Jupiter's
4 main moons are consistently resolved into disks under very good seeing. I
have observed numerous transit shadows, and even the transits themselves.
Seeing the giant planet's disk so starkly resolved against a black sky is a
sight to behold. Lesser telescopes smear the edges, and suffer from glare,
false color and, some fuzziness. I remember comparing the OA-4 to a C5
recently. The views of Jupiter and Saturn were worse in every way through
the Cat. In other words, the Celestron rendered the images duller, less
resolved, and to my surprise, dimmer. When I first got the telescope, I was
able to compare it with an older Televue Genesis. It was difficult to
compare the planets precisely because of the vastly different focal lengths,
but a view of the limb of the Moon told the story the Genesis displayed some
color, while the OA-4 was flawless."
http://www.cloudynights.com/reviews/oa4.htm

"Compared to a conventional 4 inch Chinese achromatic refractor, the
difference is laughable. The OA-4.0 exhibits contrast and resolution that is
nothing short of extraordinary, giving the lunar features a somewhat surreal
three-dimensional quality. The surrounding sky is absolutely black in stark
contrast to the lunar surface."
http://www.cloudynights.com/reviews/oa42.htm

"Recently I've been running comparisons between my old 4.25-inch and the OA.
Based on aperture, I expected the OA images to be 2.45 times brighter -
about one magnitude - and have 1.53 times the resolution - visibly sharper,
but not astoundingly so. But the differences are astounding, especially in
dark conditions with good seeing. The Sgr-4 shows Jupiter's two main bands,
while the OA at the same magnification shows two more, one to the North and
one to the South. With the OA you can see that Io is orange. When the Sgr-4
shows Saturn's "A" and "B" rings, and the Cassini division in the ansae, the
OA shows the "Crepe" ring as well, and the Cassini division is visible over
the whole ring."
http://www.cloudynights.com/reviews4/OTA6.htm

"At first glance, *$750* (mounted) seems like too much for a 4" Newt. Look
through one, and you won't feel that way, especially when you pit the
price/performance against the cost of an apochromat. I can offer no
negatives on the 'scope, and can recommend it without hesitation, though I'd
have to say I think the OA-5.5 or 7 would be a superlative purchase if
finances permit!"
*(2001 prices, currently $825)*
http://www.excelsis.com/1.0/entryvot...12&entryid=369

"This is my first newtonian reflector scope, and I've been into astronomy
after nearly 30 years of absence, so I at first expected I would have some
difficulty in collimating it. Fortunately, the expectation has turned out to
be the opposite. Reading the "Use and Care Manual" on his web page, the
assembly and collimation took me just half an hour. With a 4mm
Ortho(yealding 255X), I noticed color differences inside the NEB and SEB on
Jupiter. Even though Saturn was relatively low in the west I was able to see
the Cassini Division. I'm amazed that even at this magnitude, which is
claimed as the maximum on their web site, the image shows no breakdown at
all. Lunar work is beyond description. even my wife, who usually shows
little interest in stars, is amazed and lost for words."
http://www.excelsis.com/1.0/entryvot...12&entryid=369

Nothin unique there.... I guess these folks just need to read SAA alot more
often, and get their eyes examined ASAP!
:-)

Dan McShane

Bill,

Your welcome. I am usually at most of the summer SP`s here on the east
coast, Stellafane, Rockland SP, Connecticut, Arunah Hill Days, The Junction,
etc.

clear skies!
Dan McShane

Jb2269 wrote in message
...
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

Dan McShane wrote:

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.

It is fairly easy to figure out blur size for an off-axis segmant (OAS)
mirror with the parent mirror specs. known. Approximately, linear blur
length is reduced by the ratio of OAS mirror diameter to the parent
mirror radius (in fact, there is little difference here, since an
effective blur length in the parent mirror is truncated by
c.obstruction), while the blur width is approximately reduced by the
ratio of the diameters (OAS vs. parent mirror). In the above example,
the OA blur would have ~80% of the original comatic blur
length-wise, and ~40% of its width. Since the length/width ratio of the
comatic blur is 1.5:1, this means that the OA blur would have it
approx. 3:1.

Although the OAS aberration has identical origins to that of coma, it
is now different blur with different energy re-distribution, and should
be called differently. It can't be compared to the parent-mirror coma
directly, just as it can't be done with any other type of aberration.
The factor that has much more influence on an effective size of
aberration than reduction in the linear blur size is larger Airy disc
of the OAS. In this example, the Airy disc is 2.6 times larger than
that of the parent mirror, making relative size of the blur in the OA
as much smaller.

Considering different nature of the aberration in the OAS, its exact
effect can't be determined by direct comparison of blur sizes. However,
for a quick approximation, the increase in Airy disc size should be a
good enough basis.
In this example, with the Airy disc being 2.6 times larger than for the
f/4 parent mirror, the field quality should be similar to that of an
f/4*2.6^0.33
or f/5.5 parabola.

Vlad

"Dan McShane" wrote in message
...
RichA wrote in message
cShane

That would be fine, if they actually provided something unique, they
don't.

Huh?????

Dan,
Does the OA 3.6" provide better views than the 10" newt with an OA mask?
How do the views compare to the 80ED or 100ED?

peace,
jon

You can have a =ADNewtonian 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.

Well of course, but the lack of obstruction and diffraction spikes are
in fact the very reasons that one would consider an OA Newt over a
standard Newt. I think there's little dispute that the standard
Newtonian far and away offers the most brightness and resolution per
unit dollar of any design.

But those spikes really *can* hurt in certain situations, such as when
observing close companions to bright stars. I'm sure that observing
Sirius B would be much easier in an OA Newt than a standard one,
despite the poorer resolution.

Regarding central obstruction, many on this newsgroup and even Harold
Suiter state that the impact is negligible when it's smaller than 20%
of the primary's diameter. Then why do APOs consistently outperform
Newts inch-for-inch? There are several reasons:

- refractive surfaces vs. reflective (less light loss)
- no tube currents
- not a folded light path

Cheers,
Ritesh

On 3 Feb 2005 12:36:48 -0800, "Axel" wrote:

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.

Well of course, but the lack of obstruction and diffraction spikes are
in fact the very reasons that one would consider an OA Newt over a
standard Newt. I think there's little dispute that the standard
Newtonian far and away offers the most brightness and resolution per
unit dollar of any design.

But those spikes really *can* hurt in certain situations, such as when
observing close companions to bright stars. I'm sure that observing
Sirius B would be much easier in an OA Newt than a standard one,
despite the poorer resolution.

One of the early Sky and Tel reports of splitting Sirius was
accomplished with a C14 using a hexagonal mask that produced
diffraction spikes and made it possible (by orientating the mask)
to see the companion between the spikes. Cute trick.
-Rich

Jax wrote in message
...

"Dan McShane" wrote in message
...
RichA wrote in message
cShane

That would be fine, if they actually provided something unique, they
don't.

Huh?????

Dan,
Does the OA 3.6" provide better views than the 10" newt with an OA mask?
How do the views compare to the 80ED or 100ED?

peace,
jon

Hi Jon,

As far as the OA mask, provided the mirror has an excellent figure and stays
about .25" from the edge of the primary you would likely get a very similar
view. Although the caveat here is that a stand alone OA-4 doesn`t take as
long to cool and will adjust to falling temps better than the big scope.
Also most 10" conventional newts are in the F/5-6 range, not F/4, so the
masked "OANs" would be around f/13.5, as are the Orion scopes.
Haven`t been able to compare the OA-3.6/4.0 vs. the 100ED, but can comment
on the 80ED in comparison to my 90mm OAN. Essentially the OAN was superior
in every type of viewing, producing sharper images, higher contrast, and of
course perfect color rendition. Wide field views were the only situation
where the short FL refractor had an edge. However the ATS configuration
(OA-3.6) is no slouch for wide fields despite it`s longer FL. I have one of
the budget 30mm 80 degree 2" EPs which performs very well with the scope
producing a 2.35 degree FOV. And then on the flip side the longer FL allows
for higher mag. without barlows, or absurdly short FL EP`s.
On pure speculation I would suspect that the 100ED wouldn`t fair much better
assuming the same level of optics quality.

Dan McShane

Bill Ryan 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. This makes it primarily a visual
instrument.


That's unfortunate. I guess then it would make better sense to spend $350
more for their 100mm apo refractor ota.

Bill

Depends on two things:

1) The quality of each optically... The mirror in the OA "might" be
better figured than the lenses in the ED. Certainly the OA4 from DGM
Optics is very, very good (not that Orion is selling the DGM model).

2) Whether you have the $350... If you don't, and you want a high
contrast, color free telescope that performs similarly to a 100ED/F9...
well there ya go.

Realistically, it seems that at least some folks buy such scopes with
too high an expectation out of a 4" aperture (I've come full circle). My
102ED/F9 Vixen refractor is a very, very nice scope (and light weight
too, at just 8 lbs.), but it is still only a 4" aperture.

The advantages and disadvantages of small aperture, long focal length
scopes need to be experienced, and I've come to expect that the
difference between the OA and the 100ED are very minimal.

The reason I've come full circle, is that I started with a 4" F13 Mak
that was of questionable quality, and wasn't satisfied with it as an
entry level amateur astromer. I then went through a number of other
options, as well as experienced what others had purchased, only to come
back to a 4" refractor as a primary scope. I still have a 12.5"
Dobsonian when needed, but the Vixen 102ED on GP with drives is great
for planets, moon, and open cluster observing.

So too would the Orion 100ED be, if equal in quality (which it is
probably not). The Vixen is easily capable of 70x per inch under
excellent seeing.

But then, my 12.5" Dob cost me half the price of the Vixen setup, and it
can do 280x on planets at a meager 22x per inch (plus a whole lot more).
But, in the cold months, it takes hours before it's ready to go, and the
temperature drops around here are capable of out pacing the mirror.

Nothing like a lightweight, fast cooling solution. Of which, both the OA
and the refractor are capable. If the bottom line is price/performance,
the OA is likely the winner, but a refractor is generally more trouble
free. If the two perform equally. Unfortunately, there's no way to know
that (equal quality), without doing a direct comparison.

Order them both, keep the better one. You have 30 days (last I checked).

-Stephen Paul

Stephen Paul wrote in message
...
Bill Ryan 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. This makes it primarily a visual
instrument.


That's unfortunate. I guess then it would make better sense to spend
$350
more for their 100mm apo refractor ota.

Bill

Depends on two things:

1) The quality of each optically... The mirror in the OA "might" be
better figured than the lenses in the ED. Certainly the OA4 from DGM
Optics is very, very good (not that Orion is selling the DGM model).

2) Whether you have the $350... If you don't, and you want a high
contrast, color free telescope that performs similarly to a 100ED/F9...
well there ya go.

Realistically, it seems that at least some folks buy such scopes with
too high an expectation out of a 4" aperture (I've come full circle). My
102ED/F9 Vixen refractor is a very, very nice scope (and light weight
too, at just 8 lbs.), but it is still only a 4" aperture.

The advantages and disadvantages of small aperture, long focal length
scopes need to be experienced, and I've come to expect that the
difference between the OA and the 100ED are very minimal.

The reason I've come full circle, is that I started with a 4" F13 Mak
that was of questionable quality, and wasn't satisfied with it as an
entry level amateur astromer. I then went through a number of other
options, as well as experienced what others had purchased, only to come
back to a 4" refractor as a primary scope. I still have a 12.5"
Dobsonian when needed, but the Vixen 102ED on GP with drives is great
for planets, moon, and open cluster observing.

So too would the Orion 100ED be, if equal in quality (which it is
probably not). The Vixen is easily capable of 70x per inch under
excellent seeing.

But then, my 12.5" Dob cost me half the price of the Vixen setup, and it
can do 280x on planets at a meager 22x per inch (plus a whole lot more).
But, in the cold months, it takes hours before it's ready to go, and the
temperature drops around here are capable of out pacing the mirror.

Nothing like a lightweight, fast cooling solution. Of which, both the OA
and the refractor are capable. If the bottom line is price/performance,
the OA is likely the winner, but a refractor is generally more trouble
free. If the two perform equally. Unfortunately, there's no way to know
that (equal quality), without doing a direct comparison.

Order them both, keep the better one. You have 30 days (last I checked).

-Stephen Paul

Stephen,

I would fortify the opinion that only a top shelf 4" APO will give an
OA-4/3.6 a run the money. I think we saw this when comparing my 90mm ATS vs
Paul`s TV(101?), where the consensus seemed to be that the 90mm had a slight
edge in sharpness. I haven`t viewed the Orion 100ED, but have with the 80ED,
at last years Conjunction and as mentioned previously in this thread, the
90mm ATS was clearly superior, not just brightness, but sharpness and
contrast.
The other aspect of the 80ED that surprised me was it had a fair bit of
off-axis aberrations. Not sure if it was EP related, or just inherent of the
F/#. On the other hand it shouldn`t be surprising as many of the fast APO`s
must employ some sort of in line corrector/field flattener to produce a wide
flat FOV.

Dan

Dan McShane wrote:
Stephen Paul wrote in message
Nothing like a lightweight, fast cooling solution. Of which, both the OA
and the refractor are capable. If the bottom line is price/performance,
the OA is likely the winner, but a refractor is generally more trouble
free. If the two perform equally. Unfortunately, there's no way to know
that (equal quality), without doing a direct comparison.

Order them both, keep the better one. You have 30 days (last I checked).


Stephen,

I would fortify the opinion that only a top shelf 4" APO will give an
OA-4/3.6 a run the money.

Right, and so the question remains whether the Orion OA is better at the
eyepiece than the Orion 100ED, and whether the buyer is one who cares
more about the optical differences than the differences in design
(eyepiece position, maintenance, and mount requirements come to mind).

I hope that there's no one reading this who is interpreting my comments
as negatively or positively favoring one design over the other as a
matter of optical quality. While I do prefer the refractor for the
design, I think I'm on record as backing the OA 100% as an optically
excellent alternative to a long focus APO of equal aperture.

The OA Pros:
Excellent optical performance if will implemented.
The Cons:
Length of OTA and resultant eyepiece position. (Long focus Newts just
don't scale well, imposing aperture limits.)

-Stephen