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Old July 23rd 04, 07:32 AM
CLT
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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?