Orion "clear aperture" 3.6 reflector?

On Sun, 06 Feb 2005 11:56:59 -0500, Stephen Paul
wrote:

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

How about collimation issues? That's a con. Or light throughput
compared to the refractor?
-Rich

I too have been looking at this scope.

So, is it the consensus of the group that their performance can match a
4" APO?

Has anyone done a side by side comparison?

TMT

RichA wrote in message
...
On Sun, 06 Feb 2005 11:56:59 -0500, Stephen Paul
wrote:

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

How about collimation issues? That's a con.

Not all refractors are perfect out the box. Objectives aren`t always squared
to the optical axis. I would cite all the problems Meade had with their ED`s
3 or 4 years ago.

Or light throughput
compared to the refractor?

"While the AP threw up a slightly brighter image, there was little to choose
between the scopes."
http://www.scopereviews.com/page1m.html#3
Which is pretty much what you could expect because the Traveller is 105mm
vs. 98mm for the OA-4. And that`s vs. a high quality APO with high quality
coatings (97% TX, objective only).

OANs have just 2 coated surfaces with enhanced 96% coatings so you`re
getting 92% TX at the focal plane.
I doubt the current crop of budget APOs have nearly as good TX%, especially
air spaced EDs.

It is a myth that the APO has any significant, if any, advantage in
throughput.

Dan McShane

-Rich

On Mon, 7 Feb 2005 21:15:42 -0500, "Dan McShane"
wrote:

Hi Dan,

OANs have just 2 coated surfaces with enhanced 96% coatings so you`re
getting 92% TX at the focal plane.

Understood and I'm sure you're using top quality coatings, but that's
the coating efficiency of freshly aluminized surfaces. According to
Bryan Greer (Protostar), all aluminum coatings begin losing
reflectivity upon leaving the chamber, falling a few percentages
after a few months. Refractors keep refracting at virtually the same
transmission unless the coatings are damaged.

One factor that can hold a high quality OAN back from
consistantly refractor-like performance is the fact that open
reflective systems are affected by thermal gradients near the optical
surfaces and in the tube. Add to that the disadvantage of the
primary at the bottom of the deck that must pierce periodic
wafting of heat fom the observer traversing the optical path,
an equal sized refractor (of any type) will, as you like to
say, put the max. energy in the diffraction disk--- for a greater
percentage of time---than a reflector with the primay at the
bottom. Getting the objective as far off the ground and as far
away from the operator as possible is an irrefutable advantage for
high power viewing.

Why don't you add an optical window? Think of the potential
of keeping the tube quiescent and the mirrors clean and bright for
years, and your next wave of SAA infomecials could cover even more
ground...

Dan C.

Dan Chaffee wrote in message
...
On Mon, 7 Feb 2005 21:15:42 -0500, "Dan McShane"
wrote:


Hi Dan,

OANs have just 2 coated surfaces with enhanced 96% coatings so you`re
getting 92% TX at the focal plane.

Understood and I'm sure you're using top quality coatings, but that's
the coating efficiency of freshly aluminized surfaces. According to
Bryan Greer (Protostar), all aluminum coatings begin losing
reflectivity upon leaving the chamber, falling a few percentages
after a few months. Refractors keep refracting at virtually the same
transmission unless the coatings are damaged.

Well let me see, just to lay down my creditials, beginning in 1984 I worked
for Balzers USA, Contraves, Corion Inc, Optical Corporation of America,
Corning Photonics Division up until April 2002. I`ve been involved with some
of the most sophisticated technologies and thin film processes there are. At
Optical Corporation of America I was involved with the development of the
first ultra-narrow, ultra hard coating bandpasses which were the prototypes
of filters used in fiberoptics Wavelength Division Multiplexers (WDM). And I
designed my own Nebula Filters.
http://users.erols.com/dgmoptics/LPRfilters.htm. So I think I can speak with
a degree of authority on the subject of optical thins that very few on this
or any other group can.

A properly applied RX% coatings *might* lose a few 1/10ths of a percentage
of RX%, NOT a few percent. Only the most poorly executed coating would lose
that much RX% in such a short period.

BTW, your statement regarding AR coatings used for refractors is not
completely accurate in that even improperly applied hard oxide coatings are
also subject TX% loss, and even some WL shift, if the coating is not dense
enough and/or a non-dense film is not identified and carefully low
temperature annealed. All it takes is a single layer applied during the
process to compromise the film.

With thin films process is everything and it applies to all optical thin
films, not just RX% coatings.

Why don't you add an optical window? Think of the potential
of keeping the tube quiescent and the mirrors clean and bright for
years, and your next wave of SAA infomecials could cover even more
ground...

SAA infomercials?

I consider it more like countering the SAA *Office of Speculative Baseless
Disinformation*.
:-)

Dan McShane

Dan C.

I remember that pickture. I always wanted to build one, since, before
the advent of dobs, it looked like a good solution to getting a
lightweight mount. It was a standard Newtonian, as I recall. I don't
remmeber how many were actually built, but there was an article on it in
an old Sky & Tel probably from the 1950's.
Bill Meyers

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