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5.9 Chromatic Aberration at Finite Aperture

169

The longitudinal chromatic aberration for this zone is given approximately by

L0ch ¼ L0a=sinU 0¼ 24b sin2 U 0

and hence the 0.7 zonal chromatic aberration will be given by

 

 

 

L0

2 a 2b sin2 U 0

 

 

Dn sum is z0 ¼

 

m0 =

ð chÞz¼ ð þ

zÞ

 

P

sin U

 

 

 

 

But sin U

 

p2 approximately, and the calculated marginal (D – d)

Hence

 

 

X ¼ Sm ¼ a sin2 U m0

þ b sin4 U m0

 

 

 

 

 

 

 

 

 

 

 

 

ðLch0

Þz¼ 2 X=sin2U m0

 

(5-13)

As a check on this result, we recall that the residual of

in our cemented

telescope doublet was –0.0000578 and sin U0

was 0.16659.

Therefore, we should

 

P

expect the zonal chromatic aberration to be –0.00417. By actual ray tracing we find

zonal LF0 ¼ 11:27022

zonal LC0 ¼ 11:27523

F C ¼ 0:00501

The small discrepancy is due to our having neglected the sin6 U0 term in the expression for S.

5.9.5 Paraxial D – d for a Thin Element

We can readily reduce the D – d expression to its paraxial form for a single thin lens element. In this case the length D in the paraxial region becomes

D ¼ d þ Z2 Z1 ¼ d þ

Y2

Y2

 

 

 

 

 

 

2r2

2r1

Hence

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Y2

 

1

 

 

 

 

Y2

ðD dÞ ¼

 

 

1

 

¼ 2f 0ðn 1Þ

2

r2

r1

 

 

 

 

Y2

 

 

 

Dn

 

Y2

ðD dÞ Dn ¼

 

 

 

¼

 

 

2f 0

n 1

2f 0V