antireflection coating

[Paul Jorden]

Response to this message:

We are designing a multifiber spectrograph for the VLT (FUEGOS) and we are
looking for spurious images and scattered light in the optical design. We
do not yet know what CCD we will be used nor what antireflection coating
will be used to enhance the quantum efficiency from 0.37 to 1 micron. We
would be interested by informations (or references), concerning different
kind of coatings, on their reflectivity in all the above spectrum and their
scattering properties. The spectrograph camera is opened at a focal ratio
f/1.5

Jean Pierre Picat
-----------------

1. a reference:
Wynne, Worswick, Lowne & Jorden. 1984. The Observatory, vol 104, p 23.
About ghost images in spectrographs, encountered as a result of reflections from 
CCDs. May not be specifically useful to your problem, but could give some 
ideas..

2. About reflection from CCDs:
It is my understanding that in the .37-.8 micron wavelength range all photons 
that are not detected (by a thinned CCD) are reflected. therefore "1-QE" = 
Reflection. This should give some idea of likely signals, if you know the qe of 
the CCD.
Even using this formula at other wavelengths will indicate the maximum amount of 
reflection. However at short wavelengths there may be absorption losses within 
the CCD.
At longer wavelengths the light suffers poorer absorption within the CCD and may 
well pass through the silicon (twice) and suffer a reflection from the front 
surface in some cases (for a thinned CCD).
Basically you want a CCD with the highest QE of course! This maximises signal 
response, and minimises unwanted reflections.

regards, Paul Jorden
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