Re: [ATM] Figuring question

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Bob May wrote:
> If you really want to make the judging of the mirror a lot
> easier, take a video camera and place it behind the KE and use
> that to see the returned light.  The whole testing will become a
> lot more stable and easier to read.  I'll also note that when you
> do the testing, you want to use the same brightness level for all
> of the zones or you will be getting different results for each of
> the zones.  The actual brightness is pretty much irrrevelant
> provided that the brightness selected is less than 1/2 of the max
> brightness - the slitless tester has a much more gentle slope of
> darkening in the upper half of the brightness curve than a slit
> tester but the darker regions are a lot more accurate.

But first you must fully calibrate the camera AND the screen that you 
view it on to make sure that zones of equal brightness are really of 
equal brightness.

.....and then you have to do that for each pair of zones, since this 
can vary over different parts of the CCD chip.

.....and the camera should image the mirror in exactly the same 
location each time you set it up so that the calibration is still valid.

.....and - I almost forgot - before you do that, you must also disable 
the camera's automatic brightness control (if that's even possible) to 
make sure the readings are taken in a range in which the camera has 
fairly low noise.  You must choose a this range.

You must also make sure your Foucault tester is capable of holding the 
camera's weight.

Sorry to sound so skeptical, but I've personally experienced cameras 
giving bogus readings ("hey wait a minute, that zone's not nulled when 
I look at it, but it looks fine on the monitor....").  Either 
calibrate them first, or trust them at your mirror's risk.

	Mike Lockwood

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