Re: ATM Anti reflection coatings and TDE (not related)

[Scott Rychnovsky <srychnov@chem.ps.uci.edu>]

>Hi everyone,
>   I have a couple of things that have been bugging me and I thought I'll
>place them before all of you :
>1) How do anti-reflection coatings improve light transmission? I know that
>they are of a thickness of some multiple of the wavelenght of light so that
>destructive interference reduces the amount reflected from the surface, but
>for the life of me I can't understand how reducing the reflection can
>improve the TRANSMISSION

I do not have a completely clear understanding, but the basic idea is that
100% of the light goes somewhere.  If it is not reflected, scattered,  or
absorbed, then it is transmitted.  Thus reducing reflection should increase
transmition as long as the other variables are held constant.


>
>2) TDE : How does it degrade the image? By definition, TDE means that part
>is slightly convex so light coming in must, after reflection, be diverging
>away from the cone that is being reflected (converging to focus). So how can
>this diverging part interfere with the converging part and spoil the image.
>Of course I know it does spoil the image because doing a star test on my
>telescope (I'm hooked. The star test is wonderful and its amazing how well
>it works. It showed me I have TDE. Thanks to all who encouraged me to try
>the star test) shows me but the explanation eludes me.
>

A TDE is particularly damaging to the view because the very gradual slop
error tends to throw the light just outside of the Airy disc.  So each
point source produces an Airy disc and a soft halo around it.  This halo
kills contrast.  A scratch, on the other hand, tends to throw light much
further away from the Airy disc, and as a result does not do as much damage
to the contrast of the image.


Scott
--
Scott Rychnovsky
srychnov@chem.ps.uci.edu