[ATM] what did I do wrong???

[mdholm at telerama.com (Mark Holm)]

You are probably having one of two things wrong.

1. You are not really at the ROC.

2.  Your eye isn't close enough to the Ronchi screen.

Replace the Ronchi Screen with a white card (not over the LED), and 
focus the returning light on the card.  move the tester toward and away 
from the mirror till the image on the card is sharply focussed.  that 
should put you at the ROC.

Your eye needs to be right behind the Ronchi screen.  I mean only a few 
millimeters.

One more thing that is related, but not the cause of your immediate 
problem.  The Ronchi screen needs to be in front of both the LED and 
your eye in order for the test to work.

Mark Holm
mdholm@telerama.com