Diagonal ?

Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors. Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?


Thanks and clear sky !

Witch is the best between prism diagonal and miror diagonal and why ?

I asked this previously, and there were some very helpful answers:

http://groups.google.com/groups?hl=e....com&frame=off

Or, if the above link doesn't work, see he

http://tinyurl.com/pybz

Mark

Thank you Mark, that is very interesting. I note that for a fast F5 refractor
it is better to have a mirror diagonal. Is a mirror diagonal erect image ?




Mark De Smet wrote:

I asked this previously, and there were some very helpful answers:

http://groups.google.com/groups?hl=e....com&frame=off


Or, if the above link doesn't work, see he

http://tinyurl.com/pybz

Mark

On Mon, 06 Oct 2003 10:00:08 -0400, "André P." ...reflected:

Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors.

Do you have a refractor, and will its focusser accept 2" eyepieces
or a 2" diagonal? If so, then you'd almost have to go with a mirroring
diagonal. If not, then it's probably a 1.25" focusser, in which case you
could go with either, a mirror or a prism.

Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?

I personally prefer prisms, while others prefer mirrored diagonals, but
with my preference perhaps the result of inexperience rather than having
tested both.

Give us some more information before we go any further.

Alan

"André P." wrote in message . ..
Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors. Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?


Thanks and clear sky !

A prism diagonal works by with two refracting surfaces and one surface
where a total internal reflection takes place. A refracting surface has
1/4 the wavefront damage than a reflecting surface from the same surface
defect (with typical glass n=1.5). So a prism diagonal has 1.5 times
the surface damage as a mirror diagonal, assuming 3 surfaces of equal
quality. A mirror only has one surface that is optically accurate.

The total internal reflection reflects more light than all be the best
exotic coatings. and glass absorbs light. The net result is a prism
diagonal can deliver ~95% light throughput. A typical mirror diagonal
will reflect 88%, a diagonal with an enhanced aluminum coating can
reflect 96%. Exotic Bragg dielectric reflectors can reflect 98%.
A prism secondary can last forever, a mirror will last 5-20 years
before recoating becomes necessary (assuming scope is used regularly).

Refractor objectives are color corrected for no glass in the optical
path. A prism diagonal inserts a considerable chunk of glass into the
optical path 1.5"-2.2". This glass-path can add astigmatism due to the
converging light cone from the objective. Binocular objectives are
corrected for the prisms in the optical path, telescope objectives
are not.

I Alan,

I have the project to buy Orion 120mm ST F6, perhaps APM 102mm (China)
for observing at low power.

Thank.


Alan W. Craft wrote:
On Mon, 06 Oct 2003 10:00:08 -0400, "André P." ...reflected:


Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors.


Do you have a refractor, and will its focusser accept 2" eyepieces
or a 2" diagonal? If so, then you'd almost have to go with a mirroring
diagonal. If not, then it's probably a 1.25" focusser, in which case you
could go with either, a mirror or a prism.


Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?


I personally prefer prisms, while others prefer mirrored diagonals, but
with my preference perhaps the result of inexperience rather than having
tested both.

Give us some more information before we go any further.

Alan

I wouldn't buy a short focal length achromat. If you truly want a short
focal length refractor for a reasonable price I would step up to ED glass
and buy the Orion 80mm ED giving up some light gathering for color
correction. Otherwise I would buy the Orion 120mm F8 refractor which has
been given very good reviews also. You might want to hold out for a bit
since the rumor is Synta (who makes the Orion refractors) is coming out with
100mm, 120mm and 150mm ED refractors to complement their 80mm ED refractor
(which Orion is selling like hotcakes ...)

"André P." wrote in message
...
I Alan,

I have the project to buy Orion 120mm ST F6, perhaps APM 102mm (China)
for observing at low power.

Thank.


Alan W. Craft wrote:
On Mon, 06 Oct 2003 10:00:08 -0400, "André P."
....reflected:


Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors.


Do you have a refractor, and will its focusser accept 2"
eyepieces
or a 2" diagonal? If so, then you'd almost have to go with a mirroring
diagonal. If not, then it's probably a 1.25" focusser, in which case
you
could go with either, a mirror or a prism.


Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?


I personally prefer prisms, while others prefer mirrored
diagonals, but
with my preference perhaps the result of inexperience rather than having
tested both.

Give us some more information before we go any further.

Alan

"Chuck" wrote in message et...
I wouldn't buy a short focal length achromat. If you truly want a short
focal length refractor for a reasonable price I would step up to ED glass
and buy the Orion 80mm ED giving up some light gathering for color
correction. Otherwise I would buy the Orion 120mm F8 refractor which has
been given very good reviews also. You might want to hold out for a bit
since the rumor is Synta (who makes the Orion refractors) is coming out with
100mm, 120mm and 150mm ED refractors to complement their 80mm ED refractor
(which Orion is selling like hotcakes ...)

Orion has now posted that their 80mm ED uses FPL 53, the finest of ED glasses. How they can produce this glass for such low prices is beyond me. It will be interesting to see if their larger refractors also use FPL 53.

Cheers,
Tom Mack

I wouldn't buy a short focal length achromat. If you truly want a short
focal length refractor for a reasonable price I would step up to ED glass
and buy the Orion 80mm ED giving up some light gathering for color
correction. Otherwise I would buy the Orion 120mm F8 refractor which has
been given very good reviews also. You might want to hold out for a bit
since the rumor is Synta (who makes the Orion refractors) is coming out with
100mm, 120mm and 150mm ED refractors to complement their 80mm ED refractor
(which Orion is selling like hotcakes ...)

An important issue here is what does one want to do with the scope and how does
it fit in with ones other scopes.

Orions 80mm ED refractor is certainly a nice scope but it is 600mm FL and a 24
inch scope. The 100mm F6 and the 120mm F5 scopes are about the same length and
yet provide significanly more aperture. So if one already has a scope which is
better suited for higher power work, then it maybe that the short focal length
Achromats were chosen for widefield work where the aperture is more important
than the color correction.

Given the focal ratio of the 80ED, if Orion does comeout with 100mm, 120mm and
150 mm ED refractors and assuming the same glass, they will need to be slower
focal ratio which will make the tubes quite long.

Having owned the 120mm F8.3 scope for a year or so, I know that the 40 inch OTA
is quite a different bird than a 20 or 24 inch tube of a faster 100mm scope.

Lots of variables to consider....

jon

You are right Jon,

I stil have two others Newton scopes. A 12,5 inches Dob and a 8 inches EQ. The
two are at F6. I want a scope for widefield at low power and perhaps
terrestrial viewing. I think that Orion (or Skywatcher) 120 F5 wil be the berst
for me.


Jon Isaacs wrote:

An important issue here is what does one want to do with the scope and how does
it fit in with ones other scopes.

Orions 80mm ED refractor is certainly a nice scope but it is 600mm FL and a 24
inch scope. The 100mm F6 and the 120mm F5 scopes are about the same length and
yet provide significanly more aperture. So if one already has a scope which is
better suited for higher power work, then it maybe that the short focal length
Achromats were chosen for widefield work where the aperture is more important
than the color correction.

Given the focal ratio of the 80ED, if Orion does comeout with 100mm, 120mm and
150 mm ED refractors and assuming the same glass, they will need to be slower
focal ratio which will make the tubes quite long.

Having owned the 120mm F8.3 scope for a year or so, I know that the 40 inch OTA
is quite a different bird than a 20 or 24 inch tube of a faster 100mm scope.

Lots of variables to consider....

jon