From: "Frank Q" 
To: "Richard Schwartz" ,
        
Reply-To: "Frank Q" 


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Hi All,

Here is a sample calculation just to get an idea of what we can expect:

Mirror is 8" f/5
Airy disk radius is 1.22 f w / D =3D 1.22 x 1.0 x 0.5E-6 / 0.2 =3D =
3microns (for green light w =3D 0.5microns)

A depression on the diagonal of 1 wave results in an optical path = difference of=20
0.5 / 0.7071 =3D 0.7microns. (=3D 1.5 wavelengths !!!)

We're looking at a significant fraction of the Airy disk and I suspect =
this would cause trouble - lots
of it.

BUT

If the diagonal were figured to an off axis hyperbola/ellipse with one =
focus corresponding to the
main mirror focus (without diagonal) and the other at the eyepiece (or =
just in front of it), then it
should not introduce any errors.

It would require a comprehensive study using many possible "bad"
= diagonal surfaces to ascertain
the consequences but from the ultra-simplistic example above, the =
diagonal would need to be flat
(not hyperbolic/elliptical) to, say, 1/15 wave.

Cheers
Frank Q


  ----- Original Message -----=20
  From: Richard Schwartz=20
  To: atm{at}shore.net=20
  Sent: Tuesday, April 01, 2003 8:04 AM
  Subject: ATM How Good Does a Diagonal Need to Be?


  I am wondering if the quality required in a diagonal depends on the =
distance from the diagonal to the focal plane.   If the diagonal is very =
close to the focal plane, perhaps error in the diagonal surface can't do =
much damage....

  . . . Richard

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Hi
All,
 
Here is a sample calculation just to =
get an idea of=20
what we can expect:
 
Mirror is 8"
f/5
Airy disk radius is 1.22 f w / D =3D =
1.22 x 1.0 x=20
0.5E-6 / 0.2 =3D 3microns (for green light w =3D =
0.5microns)
 
A depression on the diagonal of 1 wave =
results in=20
an optical path difference of 
0.5 / 0.7071 =3D 0.7microns. (=3D 1.5 =
wavelengths=20
!!!)
 
We're looking at a significant
fraction =
of the Airy=20
disk and I suspect this would cause trouble - lots
of it.
 
BUT
 
If the diagonal were figured to an off =
axis=20
hyperbola/ellipse with one focus corresponding to the
main mirror focus
(without diagonal) and the other at the eyepiece (or just in =
front of=20
it), then it
should not introduce
any errors.
 
It would require a comprehensive study =
using many=20
possible "bad" diagonal surfaces to ascertain
the consequences but=20
from the ultra-simplistic example above, the diagonal would need to be=20
flat
(not hyperbolic/elliptical) to, say, =
1/15=20
wave.
 
Cheers
Frank Q
 
 

  ----- Original Message
----- 
  From:=20
  Richard=20">mailto:richas{at}earthlink.net">Richard=20
  Schwartz 
  To:
atm{at}shore.net">mailto:atm{at}shore.net">atm{at}shore.net 
  Sent:
Tuesday, April 01, 2003 =
8:04=20
  AM
  
Subject: ATM How Good Does a =
Diagonal=20
  Need to Be?
  
  I am wondering if the quality =
required in a=20
  diagonal depends on the distance from the diagonal to the focal=20
  plane.   If the diagonal is very close to the focal plane, =
perhaps=20
  error in the diagonal surface can't do much damage....
   
  . . .=20
Richard

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