"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?