More Knife Edge

After trying a few more focus positions, we've been using this point, which puts the blackbody aperture about 40mm closer to the ellipsoid from the 'design' best focus position.  Here the image snaps on fairly quickly, and it's hard to say from which side (click for the magic):

After seeing this, Dana used his Zemax model to evaluate the focus for different wavelengths and positions.  Here is what he came up with (NFP = Nominal Focus Position, units are µm):

	Telescope separation	IRTF focus position delta	Entrance focus position delta	Slit focus position delta	7micron Beam size at slit	7micron Airy disk at slit	7micron Beam size at detector	7micron Airy disk at detector	12micron Beam size at slit	12micron Airy disk at slit
	7021.096	-20000	-8535	-3936	368	174	430	174	300	298
	7021.048	-10000	-6545	-3025	292	174	336	174	222	298
NFP	7021.012	-1764	-5116	-2475	236	174	266	174	165	298
	7020.960	10000	-3000	-1700	150	174	164	174	82	298
	7020.916	20000	-1290	-872	86	174	91	174	16	298
	7020.872	30000	534	-122	28	174	74	174	60	298
	7020.820	40000	2856	732	65	174	142	174	142	298
	7020.784	50000	3852	1340	122	174	200	174	200	298
	7020.736	60000	5795	2131	197	174	284	174	277	298

We're running at 5µm, and Dana designed the system for 12µm light.  His 7µm data point shows best focus for 7µm is 32mm away from the design focus, so our finding of a ~40µm difference at 5µm isn't absurd.

Having found this out, we decided to run a focus check in camera mode (knowing the dubious character of the alignment mirror) to see whether we might be fooling ourselves.  Here is what the spot looks like as we run the blackbody source over a90mm range along the Z-axis:

In this movie the BB is moved along Z toward the entrance window at 10mm/frame.  The two spots may be coalescing (or simply passing one another) as we get closer to the entrance window, but it's difficult to be quantitative.  This data is stored on the mac mini as 'Camera Mode BB Focus Run' if anyone wants to look at it more closely.