Mirror vs Prism diagonal?

Is a mirror diagonal or a prism diagonal better? Are there any inherent
differences, or are they just different means to the same end?

Mark

On Mon, 29 Sep 2003 03:18:01 GMT, Mark De Smet
wrote:


Is a mirror diagonal or a prism diagonal better? Are there any inherent
differences, or are they just different means to the same end?

Mark

Mark, as I understand it, correctly designed mirrors diagonals deliver
more light transmission than prisms - at least that's what I've
gathered. Doubtless, some exotic prism diagonals perform better than
the norm, but all else being equal, I understand that mirrors perform
better. Obviously, prism or mirror, there are considerations of
optical quality in terms of their manufacture.

Now to find a correct-image diagonal suitable for astronomy...


Regards,

Anthony

On Mon, 29 Sep 2003 03:18:01 GMT, Mark De Smet ...reflected:

Is a mirror diagonal or a prism diagonal better? Are there any inherent
differences, or are they just different means to the same end?

Mark

With a straight-through observance, we preserve the nobleness and
purity of observing the celestial object, directly, rather than gazing upon its
mere reflection as with a mirrored or, to a debatably lesser extent, prism
diagonal. It is also the preferred manner in which to star-test, for then
there are no additional elements within the optical path to introduce
their own aberrations.

However, as I quickly found out, straight-through observing is for the birds.

I'm presently awaiting a William Optics 2" mirror diagonal w/1.25"
adaptor, and simply because I don't like sitting on the ground whilst
observing, nor the idea of toy-eating trips to the chiropractor.

It's a necessary evil, I'm afraid.

I would prefer a 2" prism though, as the prism transmits more of the light,
so I've read, but while also introducing chromatic and possibly other
aberrations. Still, I'd like to have one, but 2" prism diagonals are not
nearly as common as 2" mirror and 1.25" mirror and prism diagonals,
and tend to be more costly.

Alan

Prisms introduce spherical abberation, the faster
the f/ratio, the more they introduce. At f/5 it is
significant, at f/15 negligible.

"Mark De Smet" wrote in message
...

Is a mirror diagonal or a prism diagonal better? Are there any inherent
differences, or are they just different means to the same end?
Lots of 'little' things.
Prism systems, are easier to make with good transmissions (it is easier to
make a glass-air surface, coated to give 99% transmission, than a mirror
that reflects 98% - remember there are two glass-air surfaces involved in a
prism, and only one 'mirror' surface in a mirror diagonal, so two '99%'
surfaces are needed to give the same result as a '98%' mirror).
Mirror surfaces tend to introduce some scattering.
Prisms introduce aberrations for off-axis light (gets worse the further
off-axis you are working, so generally should be avoided for low focal
ratios).
A mirror diagonal, is harder to maintain 'flat'. If the system surfaces are
ground to better than (say) 1/20th wave, a prism will largely retain this in
use. However it is easy to distort the mirror in a mirror diagonal, by
incorrect mounting.
Generally, a 'cheap' prism diagonal, will probably outperform a cheap mirror
diagonal (which is why such prism diagonals are common in the base-end
scopes), and pass more light. However with modern reflective coatings,
mirror diagonals are available with reflectivities in the order of 99% (at a
cost...), and these for most applications will give 'better' results than a
prism system. However (conversely), if dealing with light on a single, or
narrow path (inside an instrument like a spectrometer, or with a relatively
high focal ratio telescope), prisms will generally perform better here.

Best Wishes

Mark,
Neither mirror nor prism are better. If you notice, every manufacturer of
quality scopes with fast focal ratios uses mirror diagonals. Every
manufacturer that sells quality scopes with long focal ratios sells prism
diagonals. It's not an accident. You get the diagonal that matches the
scope.

Assuming the same quality of manufacture, prisms give less light scatter, a
good thing. But they also create false color in scopes with fast focal
ratios. Just like your old Jr. High experiments, when you send light into a
prism at an angle other than perpendicular, it will cause false color. Fast
telescopes send light into a diagonal prism at a steeper angle and can in
fact introduce false color into a scopes where there was none. The sharper
the angle (faster the telescope) the more false color is introduced.
Prisms are more expensive to make in larger sizes than mirrors and may
account why in 2” and larger diagonals, mirrors dominate.

Prisms on the other hand never scatter light and therefore can produce
excellent contrast. Also, they never age. In terms of transmission, each
depends on the coatings. A multicoated prism and enhanced Aluminized mirror
can both achieve about the same 98% transmission. But that really is not
important. (WHAT DID HE SAY?)

Most of the discussions about diagonals have centered on reflectivity. Of
all the characteristics that make a good diagonal, reflectivity may be one
but not the most important. But it is the easiest characteristic to market,
96% is better than 95% right. Spend twice as much and get 2% more
light!!!! But 2% is not important.

There is a vast amount of laboratory data that proves that human beings
cannot perceive a few % of light change, yet it is the characteristic most
often quoted to define how good a diagonal is. Buy X because its 97% and
not Y because its only 95%. Yet even Meade’s and Celestron’s new ads for
their high transmission coatings state that you can not really “see” the
difference but a camera can easily “record” the difference. But you don’t
use diagonals when photographing do you. Question… Why is it that people
that demand full documentation on the optical accuracy of their $5000
telescope, only worry about the brightness of their diagonal. Isn’t
accuracy, in this case flatness, more important than brightness. Doesn’t
the difference between a 1/7th wave vs. a 1/20th wave surface will have a
greater impact on your viewing than the difference between 94% vs. 98%
transmission. The few that supply accuracy papers on their diagonals do so
before they are coated, not after and there can be a huge difference. The
papers make you feel good, but do not reflect (sorry) the actual product you
receive. As a general rile, the brighter the coatings, the more layers of
coatings had to be laid down, and the less it reflects (sorry again) the
accuracy of the original surface accuracy. Standard Aluminum coatings (88%)
have one layer, enhanced Aluminum (94%)has 3 layers of coatings. Dielectric
coatings can have up to 22 layers.

In short, buy the diagonal that fits your scope.
Buy the most accurate diagonal, not the brightest.
Look for all metal parts, a machined housing is best.
A true 90.00 degrees is important.
User collimatable is nice.
The substrate is important. Cervit, Zerodur, Astrosital and Quartz are
better than Pyrex. Pyrex is a bit better than BK7. BK7 is better way than
plate glass
Gary Hand









Mark De Smet wrote:

Is a mirror diagonal or a prism diagonal better? Are there any inherent
differences, or are they just different means to the same end?

Mark

Bear in mind, too, that a diagonal in a long-f-ratio telescope doesn't need
to be flat to 1/4 wave, because no single point in the image will cover more
than a small part of the diagonal. It is important for the diagonal to be
free of small ripples and irregularities, but overall flatness is not all
that demanding.

On 09/30/03 02:22 +0900, Gary Hand wrote:

[ SNIP ]

In short, buy the diagonal that fits your scope.
Buy the most accurate diagonal, not the brightest.

[ SNIP ]

That entire post was very educational for me, Gary. Thanks for
that.. A most excellent post, sir!

trane
--
//------------------------------------------------------------
// Trane Francks Tokyo, Japan
// Practice random kindness and senseless acts of beauty.
// http://mp3.com/trane_francks/

Excellent, informative post Gary.
DD

On Mon, 29 Sep 2003 17:22:03 GMT, Gary Hand
wrote:

Mark,
Neither mirror nor prism are better.
[much snnipped]

Excellent post Gary!

There are some other questions to consider. Will a standard aluminized
mirror diagonal deteriorate over time like the coatings on a Newtonian
mirror? Would this make the dielectric coated diagonals more
desirable? Is it worth re-coating a standard mirror diagonal?

Can you clean the prism diagonal with less concern than with a
standard mirror? The dielectric diagonal is also supposed to be easier
to clean.

BTW, many have said to never touch a standard mirror diagonal for fear
of causing sleeks but, how should you clean one if it's necessary?

Al Hall