> I built a new Foucault tester last night.
<Snip>
> The problem is that the <red> LED is not bright enough.
>
> The only reason I even tried the LED was that I read someone's post
> either here or sci.astro.amateur about using an LED.
>
> I could get the image in focus, and cut into the cone of light, but
> there was NO contrast. The mirror just went uniformly dark.
> And I know that I don't have a spherical mirror from the other tests.
>
> So I'll stop by the surplus junk store, and get a small light bulb to
> use in place of the LED.
>
May I suggest using a different color LED? The human eye is not particularly sensitve to red light. Yellow and green LEDs operate in a region where the eye is *much* more responsive. Also, LEDs are pretty cheap, try boosting the current on the one you have now.
Sam Paris (Who really ought to grind a mirror of his own before being so free with advice. :-)