Re: [ATM] zone one challenge

[Michael Peck <mpeck1@ix.netcom.com>]

At 09:46 3/30/05, Koehler, Steve wrote:
>I believe I have found a possible source of bias in reading zone one in an
>8-zone test of an 18" f/4.5.  I have written up my ideas in the following
>report, which includes a challenge to find the null for zone one from a series
>of 15 simulated images.
>

I'm late to the party, but I wanted to do some simulations of my own. I 
tried dumping simulated images from my own Foucault simulator to files and 
converting them to an animated GIF. I found that viewing the simulation as 
a movie reduced my tendency to make a biased call, however my eyes are only 
good to about +- 10-15%, which works out to a reading precision of about +- 
0.03 mm in my simulation with a point source. With an extended source the 
same contrast sensitivity produces much worse reading precision. This does 
suggest that paying attention to the dynamics of the shadows as the knife 
moves back and forth (and presumably into and out of the light cone) does 
make a difference.

Some of the peculiar features of the intensity distributions go away when a 
point light source is used, although there is still considerable asymmetry 
between the left and right mask openings. By the way I see the same 
peculiar features in my simulations of an extended source, and Jim Burrows' 
diffract also shows them. The shading looks a little more subtle though in 
both Jim's program and mine. I guess that's just a slight difference in 
greyscale rendering, since cross section plots look very similar in mine.

>the outside for J.  This is because I use the equal area formula for the 
>center
>of the zone, and derived the zero point for the simulation from that.  Perhaps
>this demonstrates that the arithmetic mean of the inside and outside zone 
>radii
>is better for the first zone.

I'm finding that Nils Olof's formula for effective zone radius seems to 
work better than either the rms or average radius. His formula predicts 
that the stage position at the null is 0.48 mm outside the center of 
curvature. I found the average intensities were balanced at 0.485 mm. The 
stage position at the mean radius is 0.44mm and at the rms radius 0.52 mm, 
and these were clearly inside and outside the null position.

Lessons learned: Use narrower mask openings, or maybe throw away the mask. 
Use a finer slit, or better yet a point source. Pay attention to dynamics. 
Don't trust your lying eyes.

Mike Peck


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

_______________________________________________
ATM mailing list http://www.atmlist.net/