From: Mark Holm 
To: ea8yu{at}arrl.net, atm{at}shore.net
Reply-To: Mark Holm 


Distance from focus should not affect the image formation.  This is one of
the ways that ray optics fails to predict actual performance.  Ray optics
says that distance from focus does matter because ray optics looks at the
slope errors, calculates the tilt angle of the ray and projects it to
focus.  The farther the projection distance, the greater the error.  Wave
optics says path length difference is established when the light is
reflected, and, assuming the air from the secondary to focus is uniform,
remains the same all the way to the image, no matter the distance.

Someone wrote a few weeks back that Texerau contains a statement that
secondary errors are less important than primary errors because of being
closer to focus.
  I have not read that part of the book recently to confirm it, but, if so, I
think it is an error.

Now, I have to admit there are aspects of this argument that puzzle me too,
and I am not sure I have it 100% correct.  I keep thinking I need to sit
down and write a computer program that calculates path length differences
for different diagonal situations and plots the resulting diffraction
patterns.  That would be
the most rigorous way to be sure of the predictions of wave optics.
Unfortunately I have to spend a lot of my time earning a living and doing
other necessary things.

You want filters close to focus to minimize vignetting, just as you want
the diagonal close to focus to minimize vignetting.

Mark Holm
mdholm{at}telerama.com

Goran Hosinsky wrote:

> Is the distance from focus not an important factor? I have learned that
> when using filters you should
> place them as near focus as possible, just far enough off that you
> cannot see the dust on them.
> Goran
> La Palma
>
>

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