antireflection coating

[Michael P. Lesser]

Regarding the following 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
> Thank you
> 
> Jean Pierre Picat
> Observatoire Midi Pyrénées
> 14 Avenue Edouard Belin
> 31400 Toulouse   France
> tel (33)61332929  Fax (33) 61536722
> 

For back illuminated CCDs, various AR coatings can be applied to reduce
reflectivity in the 300 - 1000 nm range.  

An uncoated  CCD reflects 60% in the 300 - 400 nm region, dropping to slightly
more than 30% out to 1 um.

Single layer coatings can reduce reflectivity to less than a few percent
over a 100 nm range or so in the visible, but will still have nearly the same
uncoated reflectvity at the ends of the useful observing region (300 and 
1000 nm).

Two layer coatings do much better over a broad region, keeping reflectivity
below 10% from 500 - 1000 nm and to less than about 30% at 300 nm.

-Michael Lesser
Steward Observatory
University of Arizona
Tucson, AZ  85721