definition?

What is the definition of diffraction limited? Limited to what? Clearity
and crispness
of a star or planet?

OncoBilly wrote:
What is the definition of diffraction limited? Limited to what? Clearity
and crispness
of a star or planet?



In lay terms, it means that since light has wave properties (with
'diffraction' being one of them) one can only figure a lens or mirror
(or any series/combination of multiple lens/mirror surfaces) to a
certain level of perfection before any more precise figuring beyond that
point becomes totally optically meaningless. I.E. you won't be able to
see the difference in clarity and/or crispness between a "complete
optical system" that is figured just to the minimum limit of optical
perfection, or a 'like' optical system that is figured to a level of
perfection far beyond what is required to meet the minimum for perfection.

Lawrence Sayre
--
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

Diffraction limited means, that the quality of the image is only limited by
the nature of light, and not by other factors, such as bad or mediocre
optics, obstruction, seeing, etc.
--
Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/


"OncoBilly" schreef in bericht
news:%GSyc.37979$%i1.10834@edtnps89...
What is the definition of diffraction limited? Limited to what? Clearity
and crispness
of a star or planet?

OncoBilly wrote:
What is the definition of diffraction limited? Limited to what? Clearity
and crispness of a star or planet?

Many things may blur the image of a star or planet, including diffraction
effects, optical quality, atmospheric effects, incomplete cooldown, etc.
Diffraction-limited means that when transient effects such as atmosphere
and cooldown are eliminated, diffraction is the limiting factor on the
sharpness of the image, not optical quality.

More at MOPFAQ (see link below).

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Diffraction-limited means that when transient effects such as atmosphere
and cooldown are eliminated, diffraction is the limiting factor on the
sharpness of the image, not optical quality.

As usual, Brian hits the nail on the head. Some random points:

1. "Diffraction limited" is a measure of optical quality, normally taken a
quarter as some reference wave length of light, normally around 500
nanometers. This means that the sum of the errors in the optics are less than
125 nm, a human hair is about 600 thicker than this.

2. It is good to remember that "diffraction limited" essentially says that for
the chosen design of the telescope, the optics are are of sufficient quality so
that the optics themselves do not limit the capabilities. If the scope is of
a poor design, an overly large central obstruction, a very fast focal ratio,
the optics can be diffraction limited but give poorer views than a scope with a
better design. "Diffraction limited" says nothing about the design, only that
the optics allow the design to meet its full potential whatever that might be.

3. Manufacturers like to use this term, however there are many ways to measure
optical quaility and whether the optics are diffraction limited and as such,
most manufacturers choose the most optimistic of those. Factors such as
surface smoothness that affect the image quality are not normally measured.

Bottomline: Diffraction limited is a term often used by Manufacturers of good
quality quality scopes (Meade, Celestron, Orion) to side step the issue of how
good the optics might be. It sort of a minimum standard and top notch
manufacturers will produce optics that far exceed this standard.

Some high end manufacturers provide test data of all sorts, P-V ratios, RMS
values, Stehl ratios, sometimes overwhelming and difficult for someone who is
not an optician to interpret.

And then some use a minimalist approach. Mirror maker Mike Spooner simply says
this: "I will not ship any mirror that I would not be happy to use myself for
the rest of my life."

Jon

On 2004-06-13, OncoBilly wrote:
What is the definition of diffraction limited? Limited to what? Clearity
and crispness
of a star or planet?
It's when the spot formed by the image of a point source such as a star
is smaller than the Airy disk that results from the diffraction of light.
Because of the wave nature of light the image of a star consists of a
central disk surrounded by fainter diffraction rings. The central disk
is called the Airy disk after Sir George Airy, who was the Royal
Astronomer in the first part of the 19th century and worked out the
mathematics to compute the diffraction structure of an image. In
addition, the wavefront error at the image plane should be less than
1/4 the wavelength of light. This is Rayleigh's criterion, after
Lord Rayleigh, another 19th century scientist. Then you have Danjon's
criteria, which says that you need to have the wave front error under
1/4 wave and the spot size less than the Airy disk at the same focus.
Danjon was a 20th century French astronomer.

In lay terms, it means that since light has wave properties (with
'diffraction' being one of them) one can only figure a lens or mirror
(or any series/combination of multiple lens/mirror surfaces) to a
certain level of perfection before any more precise figuring beyond that
point becomes totally optically meaningless. I.E. you won't be able to
see the difference in clarity and/or crispness between a "complete
optical system" that is figured just to the minimum limit of optical
perfection, or a 'like' optical system that is figured to a level of
perfection far beyond what is required to meet the minimum for perfection.



Ok, so diffraction is the bending of waves. To me this means that the
inherent quality
of the optics diffracts or bends light to a minimum standard. Correct? If
so, then in the case
of a refractor scope the total error of all the optics ie.
objective-EP-diagonal/barlow meets or exceeds
a limit or minimum standard of quality?

"Brian Tung" wrote in message
...
OncoBilly wrote:
What is the definition of diffraction limited? Limited to what?
Clearity
and crispness of a star or planet?

Many things may blur the image of a star or planet, including diffraction
effects, optical quality, atmospheric effects, incomplete cooldown, etc.
Diffraction-limited means that when transient effects such as atmosphere
and cooldown are eliminated, diffraction is the limiting factor on the
sharpness of the image, not optical quality.



Ok thanks. Is it meaningless to say that an objective in a refractor is
"diffraction limited to 180 power"?

OncoBilly wrote:
In lay terms, it means that since light has wave properties (with
'diffraction' being one of them) one can only figure a lens or mirror
(or any series/combination of multiple lens/mirror surfaces) to a
certain level of perfection before any more precise figuring beyond that
point becomes totally optically meaningless. I.E. you won't be able to
see the difference in clarity and/or crispness between a "complete
optical system" that is figured just to the minimum limit of optical
perfection, or a 'like' optical system that is figured to a level of
perfection far beyond what is required to meet the minimum for perfection.




Ok, so diffraction is the bending of waves. To me this means that the
inherent quality
of the optics diffracts or bends light to a minimum standard. Correct? If
so, then in the case
of a refractor scope the total error of all the optics ie.
objective-EP-diagonal/barlow meets or exceeds
a limit or minimum standard of quality?



There is no reason to restrict this issue to refractors, as all optics
are affected equally by the wave nature of light. The minimum standard
for diffraction limited optics is known as the Raleigh Criterion, or the
Raleigh Limit. If the image a scope forms at the eyepiece just barely
meets this criteria, it is claimed to offer as fine an image (resolution
wise) as can be had, and it will not be bettered by a similar scope
which is figured to a level of perfection which exceeds the Raleigh
Limit, even one that greatly exceeds it. I.E. you (and everyone else)
will never notice the difference. This of course assumes that the two
scopes being compared have otherwise identical apertures, identical
coatings, identical baffling, identical obstructions, identical levels
of correction for bringing the different spectral colors to a common
focus, identical transmission, are being used in the same place at the
same time, etc...

For example: If one Meade 8" LX-200 SCT just barely meets the Raleigh
criteria for 1/4 wave (I.E. is fully diffraction limited) at the
eyepiece (or in other words, for the total optical system), and another
otherwise identical Meade 8" LX-200 vastly exceeds this level of optical
figure quality, and is rated at a spectacularly excellent 1/8 wave at
the eyepiece, then by just looking through them, no one (assuming of
course that Raleigh is correct) would be able to pass judgment regarding
one being any better than the other, since they would both be as
optically good as they can be. Both scopes however would be obviously
better to observers (I.E. offer clearer and crisper views, to use your
terminology) than an otherwise identical Meade 8" LX-200 SCT which tests
to only 1/3 wave precision at the eyepiece.

Lawrence Sayre


--
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.

Ayn Rand (in the appendix to 'Atlas Shrugged')

OncoBilly wrote:
Ok thanks. Is it meaningless to say that an objective in a refractor is
"diffraction limited to 180 power"?

I'm not sure what would be meant by that, but sometimes people will say
that a telescope will "support powers as high as 180x"--indicating that
no significant image degradation is visible up to 180x. Maybe that is
what someone who says "diffraction limited to 180x" would mean, but that
is a poor way to put it, in my opinion.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt