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What exactly is the "Diffraction Limited Field of View"?



 
 
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  #1  
Old July 22nd 04, 02:39 AM
Derek Overdahl
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Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

Its a term I have seen over and over again with little explanation. If I
understand it correctly this is a measurement of tolerance for accurate
collmination... is that right?

I came across a table that shows the relationship between aperture, f ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs

In looking at the full chart it appears that the longer focal length the
larger the diffraction limited field of view - really quite dramatic when
comparing an 8'' f4 to say an f8 - a factor of near 4 times.

Some questions....

So? What exactly is the difference any way? Sure an f4 is harder to
collimate but beyond that does this really change the performance of a scope
once it is done well?

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured? Take for example a
8'' f4 Newt - does this mean my collmination of the light path from the
primary to the secondary needs to be within a 100 arcseconds of where it is
supposed to be? And how does that translate to something I can relate to?

Any input would be appreciated thanks?




  #2  
Old July 22nd 04, 04:50 AM
Brian Tung
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Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

Derek Overdahl wrote:
I came across a table that shows the relationship between aperture, f ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs

In looking at the full chart it appears that the longer focal length the
larger the diffraction limited field of view - really quite dramatic when
comparing an 8'' f4 to say an f8 - a factor of near 4 times.

Some questions....

So? What exactly is the difference any way? Sure an f4 is harder to
collimate but beyond that does this really change the performance of a scope
once it is done well?

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured? Take for example a
8'' f4 Newt - does this mean my collmination of the light path from the
primary to the secondary needs to be within a 100 arcseconds of where it is
supposed to be? And how does that translate to something I can relate to?

Any input would be appreciated thanks?


The problem is that even assuming that a primary mirror in a Newtonian
telescope is perfectly figured, the image it produces is only free of
aberrations at the very center of the field of view. As you move away
from the center of the field of view, coma becomes increasingly apparent.

Coma is an aberration that makes point sources (like stars) take on a
comet-like appearance, with the tails pointing away from the field of
view. There is no coma at the center of the field of view, but coma
increases linearly with "off-axis angle"--a measure of how far the object
is away from the center of the field of view. The off-axis angle can be
thought of as the angle between where the object is as seen from the
telescope, and where the telescope is actually pointed. Therefore, it
the telescope is pointed directly at the object, the off-axis angle is
zero, and the object appears at the center of the field of view.

So when we say that coma increases linearly with off-axis angle, we mean
that if you double the off-axis angle, the coma too is doubled, and so on.
If the object is far enough away from the center of the field of view,
the coma effect is stronger than the diffraction effect that is inherent
in the formation of the image. The area of the field of view within which
coma is *less* than diffraction, so to speak, is the diffraction-limited
field.

Because the (true) field of view of a telescope plus eyepiece is generally
fairly small (typically a degree or less), the off-axis angle for any
object that you can see in the eyepiece is reasonably small. But in
fast telescopes (those with low focal ratios), coma increases so rapidly
that it becomes a major factor well within the field of view.

To give you an idea of how small an angle is required to make coma readily
apparent, a degree is 1/90 of a right angle--the angle between the zenith
and the horizon. An arcminute is 1/60 of a degree, and an arcsecond is
1/60 of an arcminute (and therefore 1/3,600 of a degree). It is a very
small angle. The fact that an 8-inch f/4 mirror has a diffraction-limited
field of only 100 arcseconds across means that you have to be pointed
within 50 arcseconds (1/72 of a degree) of an object for that object's
image to be diffraction-limited.

Fortunately, there are a number of solutions. One is to get a coma
corrector, like Tele Vue's Paracorr. This is an accessory that you fit
into the eyepiece holder like a Barlow, and you put eyepieces into it.
It largely eliminates the coma.

Another solution, if you are designing a telescope, is to use a slower
(that is, larger) focal ratio. For a given aperture, coma decreases as
the inverse square of the focal ratio, so that an 8-inch f/6 has only
16/36 = 4/9 of the coma that the 8-inch f/4 does. That is why the f/6
has a diffraction-limited field over twice as wide as the f/4 does.

To first order, collimation merely centers the diffraction-limited field
in the field of view. If the telescope is poorly collimated enough, the
entire diffraction-limited field will be *outside* the field of view,
resulting in poorer views.

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
  #3  
Old July 22nd 04, 06:13 AM
CLT
external usenet poster
 
Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

"Derek Overdahl" wrote in message
...
Its a term I have seen over and over again with little explanation. If I
understand it correctly this is a measurement of tolerance for accurate
collmination... is that right?


sort of.

I came across a table that shows the relationship between aperture, f

ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs


This depends on the telescope design. For a newt, coma is the main thing
killing you.

Let me also add that "diffraction limited" may have started out being used
by some as a real term, but is now usually nothing more than sales
department gas.

So? What exactly is the difference any way? Sure an f4 is harder to
collimate but beyond that does this really change the performance of a

scope
once it is done well?


Hmm, to do this without tracing any rays or using math...

Think of it this way. A sphere will have spherical aberation, but no
coma --- simply put the field stop at the radius of curvature, and the star
can't tell if it is on axis or off. Now if we go with an f/20, there is
practically no difference between the parabola and the sphere --- therefore
coma is tiny. As we move to f/8, there is a definite difference between the
sphere and the parabola --- therefore the coma is definitely noticable. But
when you go to f/4, the difference between the sphere and the parabola is
huge --- therefore, when you go off axis, the difference is going to be
huge --- this equals huge coma.

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured?


Take a degree. For reference, the moon is about a half a degree across.
There are 60 arc minutes per degree. Therefore the moon is about 30
arcminutes in size. Each arcminute contains 60 arcseconds. To get a feel for
this, look at some double stars.

Take for example a
8'' f4 Newt - does this mean my collmination of the light path from the
primary to the secondary needs to be within a 100 arcseconds of where it

is
supposed to be? And how does that translate to something I can relate to?


You are taking this in the wrong direction. Don't worry about it from that
angle (pun not intended). Learn to collimate. Then learn to collimate *very*
well. Then learn to collimate very well quickly enough that you will do so
at the start of every observing session. That way you don't have to worry
about collimation error measurements.

While you are at it, in the last stage of collimation, with the star out of
focus and viewing the diffraction rings, notice how they change shape across
the field. That will give you a quick read on what is happening across your
field of view.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

Any input would be appreciated thanks?






  #4  
Old July 22nd 04, 08:35 AM
justbeats
external usenet poster
 
Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

"Derek Overdahl" wrote in message ...
Its a term I have seen over and over again with little explanation

?
So? What exactly is the difference any way?...

How can I translate this into something that means something to me...

Any input would be appreciated thanks?


This discusses the "diffraction limited field" in the context of
newtonian reflectors and double star work. Might give you useful
pointers to more relevant information...

http://www.brayebrookobservatory.org...or_article.pdf

Cheers
Beats
  #5  
Old July 22nd 04, 03:59 PM
Chris1011
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Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

How can I translate this into something that means something to me...

It means that in an F4, outlying objetcs like the moons of Jupiter will be
little comet shaped things when the scope is perfectly collimated and Jupiter
is exactly in the center of the field.

Roland Christen
  #6  
Old July 22nd 04, 08:05 PM
misaligned astrognome
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Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?



This discusses the "diffraction limited field" in the context of
newtonian reflectors and double star work. Might give you useful
pointers to more relevant information...

http://www.brayebrookobservatory.org...or_article.pdf

Cheers
Beats



No one is going to get anything useful out of that crazy mathmatical
explanation...BUCK UP will ya
  #7  
Old July 22nd 04, 10:50 PM
Stephen Tonkin
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Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

misaligned astrognome wrote:

No one is going to get anything useful out of that crazy mathmatical
explanation.


Wrong! I did. (In fact, I get a lot of useful things from Chris Lord's
PDFs)

Best,
Stephen

Remove footfrommouth to reply

--
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+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
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  #8  
Old July 23rd 04, 02:07 AM
Derek Overdahl
external usenet poster
 
Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

Thanks to both of you (Brian and CLT) your responses where very helpful!

Sometimes I wonder about this hobby....
Which is more interesting - the tools we observe with or the targets we
observe?

At least it gives us something to do when the sun is out during the day and
the clouds and tropical humidity ruin the nights.

thanks again - big help!






"CLT" not@thisaddress wrote in message
...
"Derek Overdahl" wrote in message
...
Its a term I have seen over and over again with little explanation. If I
understand it correctly this is a measurement of tolerance for accurate
collmination... is that right?


sort of.

I came across a table that shows the relationship between aperture, f

ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs


This depends on the telescope design. For a newt, coma is the main thing
killing you.

Let me also add that "diffraction limited" may have started out being used
by some as a real term, but is now usually nothing more than sales
department gas.

So? What exactly is the difference any way? Sure an f4 is harder to
collimate but beyond that does this really change the performance of a

scope
once it is done well?


Hmm, to do this without tracing any rays or using math...

Think of it this way. A sphere will have spherical aberation, but no
coma --- simply put the field stop at the radius of curvature, and the

star
can't tell if it is on axis or off. Now if we go with an f/20, there is
practically no difference between the parabola and the sphere ---

therefore
coma is tiny. As we move to f/8, there is a definite difference between

the
sphere and the parabola --- therefore the coma is definitely noticable.

But
when you go to f/4, the difference between the sphere and the parabola is
huge --- therefore, when you go off axis, the difference is going to be
huge --- this equals huge coma.

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured?


Take a degree. For reference, the moon is about a half a degree across.
There are 60 arc minutes per degree. Therefore the moon is about 30
arcminutes in size. Each arcminute contains 60 arcseconds. To get a feel

for
this, look at some double stars.

Take for example a
8'' f4 Newt - does this mean my collmination of the light path from the
primary to the secondary needs to be within a 100 arcseconds of where it

is
supposed to be? And how does that translate to something I can relate

to?

You are taking this in the wrong direction. Don't worry about it from that
angle (pun not intended). Learn to collimate. Then learn to collimate

*very*
well. Then learn to collimate very well quickly enough that you will do so
at the start of every observing session. That way you don't have to worry
about collimation error measurements.

While you are at it, in the last stage of collimation, with the star out

of
focus and viewing the diffraction rings, notice how they change shape

across
the field. That will give you a quick read on what is happening across

your
field of view.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

Any input would be appreciated thanks?








  #9  
Old July 23rd 04, 07:32 AM
CLT
external usenet poster
 
Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

"Derek Overdahl" wrote in message
...
Thanks to both of you (Brian and CLT) your responses where very helpful!

Sometimes I wonder about this hobby....
Which is more interesting - the tools we observe with or the targets we
observe?


Both! But only rarely is there an interesting fight over the targets!

;-)

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

At least it gives us something to do when the sun is out during the day

and
the clouds and tropical humidity ruin the nights.

thanks again - big help!






"CLT" not@thisaddress wrote in message
...
"Derek Overdahl" wrote in message
...
Its a term I have seen over and over again with little explanation. If

I
understand it correctly this is a measurement of tolerance for

accurate
collmination... is that right?


sort of.

I came across a table that shows the relationship between aperture, f

ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs


This depends on the telescope design. For a newt, coma is the main thing
killing you.

Let me also add that "diffraction limited" may have started out being

used
by some as a real term, but is now usually nothing more than sales
department gas.

So? What exactly is the difference any way? Sure an f4 is harder to
collimate but beyond that does this really change the performance of a

scope
once it is done well?


Hmm, to do this without tracing any rays or using math...

Think of it this way. A sphere will have spherical aberation, but no
coma --- simply put the field stop at the radius of curvature, and the

star
can't tell if it is on axis or off. Now if we go with an f/20, there is
practically no difference between the parabola and the sphere ---

therefore
coma is tiny. As we move to f/8, there is a definite difference between

the
sphere and the parabola --- therefore the coma is definitely noticable.

But
when you go to f/4, the difference between the sphere and the parabola

is
huge --- therefore, when you go off axis, the difference is going to be
huge --- this equals huge coma.

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured?


Take a degree. For reference, the moon is about a half a degree across.
There are 60 arc minutes per degree. Therefore the moon is about 30
arcminutes in size. Each arcminute contains 60 arcseconds. To get a feel

for
this, look at some double stars.

Take for example a
8'' f4 Newt - does this mean my collmination of the light path from

the
primary to the secondary needs to be within a 100 arcseconds of where

it
is
supposed to be? And how does that translate to something I can relate

to?

You are taking this in the wrong direction. Don't worry about it from

that
angle (pun not intended). Learn to collimate. Then learn to collimate

*very*
well. Then learn to collimate very well quickly enough that you will do

so
at the start of every observing session. That way you don't have to

worry
about collimation error measurements.

While you are at it, in the last stage of collimation, with the star out

of
focus and viewing the diffraction rings, notice how they change shape

across
the field. That will give you a quick read on what is happening across

your
field of view.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

Any input would be appreciated thanks?










  #10  
Old July 23rd 04, 08:56 AM
Mike Ruskai
external usenet poster
 
Posts: n/a
Default What exactly is the "Diffraction Limited Field of View"?

On Thu, 22 Jul 2004 01:39:56 GMT, Derek Overdahl wrote:

Its a term I have seen over and over again with little explanation. If I
understand it correctly this is a measurement of tolerance for accurate
collmination... is that right?


No. See below.

I came across a table that shows the relationship between aperture, f ratio
and the resulting diffraction limited field diameter. Expert:

8" f4 ~100 arcsecs
8" f6 ~250 arcsecs

In looking at the full chart it appears that the longer focal length the
larger the diffraction limited field of view - really quite dramatic when
comparing an 8'' f4 to say an f8 - a factor of near 4 times.


Not focal length, focal ratio.

The diffraction-limited field is how much of the field has aberrations
smaller than the Airy disc.

The magnitude of image aberrations in basically all telescopes (the only
exception I can think off off hand is a Schmidt camera) increase the
further from the optical axis you go.

In Newtonians, for example, you can easily see that the further off axis
you are, the larger the effect of coma. The region of a Newtonian which
is diffraction limited basically means the region where the comatic star
image is smaller than the diffraction disc.

All else being equal, the larger the focal ratio, the less severe all
aberrations will be, which is why a f/6 will have a larger
diffraction-limited field than a f/4 of the same aperture.

How can I translate this into something that means something to me...
What is an Arcsecond and where exactly is it measured? Take for example a
8'' f4 Newt - does this mean my collmination of the light path from the
primary to the secondary needs to be within a 100 arcseconds of where it is
supposed to be? And how does that translate to something I can relate to?


As I said, it has nothing to do with collimation. An arcsecond is an
angle measure. There are 60 arcseconds in one arcminute. There are 60
arcminutes in one degree.

A degree, as you're no doubt aware, is 1/360th of the trip around a
circle. That's true no matter how large the circle is, which is why it's
used to describe astronomical sizes. The moon subtends an angle of about
30 arcminutes. That means if you imagine a circle with you at the center,
and the moon somewhere on the edge, you will sweep out an angle of one
half of a degree in moving from one edge of the moon to the other.

In describing the diffraction-limited field, the size on film (or CCD) is
typically what's talked about. If you attached a 35mm camera to an 8" f/4
Newtonian, the field captured on film will be not quite 9300 arcseconds
across (in the long dimension).

A diffraction-limited field of 100 arcseconds means roughly the middle 1%
of the image on 35mm film will be diffraction limited.

In practice, film spot sizes are larger than Airy discs, so you wouldn't
be that strict when using film.

CCD spot sizes are a single pixel (until overexposed, anyway), so are a
better fit for talking about diffraction limited fields.

On a 14mm CCD, the field will be about 3600 arcseconds across (one full
degree), so not quite 3% of the middle will be diffraction limited.

With an 8" f/6 Newtonian (focal length 1200mm, instead of 800mm), the
field on CCD would be about 2400 arcseconds. While the field is smaller,
a larger part of it (over 10%) will be diffraction limited.

For visual purposes, smaller focal ratios means stars look fuzzier closer
to the center of the FOV. You may have to go quite far from the center to
start noticing, however.




--
- Mike

Remove 'spambegone.net' and reverse to send e-mail.


 




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