mawyatt wrote:Say I want to step 20.15625 microns (8 and 1/16 step at 2.5 microns per step). Could Zerene actually interpret this larger step with this level of precision?
I'm not sure exactly what question you're asking, so let me provide an analogy.
The optical measurement is like a nearly frictionless spring scale with major tick marks from say 0 to 500 and 10 minor ticks per major tick -- so 5000 ticks total. The spring is interchangeable so you can have pretty much any range you want, but since it's a spring, you're never quite sure what its spring constant is, or even whether it's really constant. Spelling out the analogy, "major tick marks" are full pixels on sensor, "minor tick marks" are sub-pixel interpolation, and "spring" is the lens with its attendant magnification and possible distortions.
So, if you want to measure 20 pounds (or ounces, or grams, or microns), first you have to pick the appropriate spring, and then you can measure to a precision of 20/5000 = 0.004 pounds (or ounces, or grams, or microns). But you don't know anything about accuracy or linearity unless you have calibrated the spring.
In the optical system, there is the additional wrinkle that the alignment step is really trying to match the phase of two complex waveforms. It cannot determine matched phase with perfect accuracy, so there's also some lower limit that depends on the bandwidth of the signals. In other words, if the image is sufficiently fuzzy, then the alignment cannot be determined accurately to 1/10 of a pixel due to pixel noise, vignetting, and so on. In the spring scale, a corresponding problem would be some slight remaining friction and environmental noise that would establish a lower limit on reproducibility.
You've asked specifically about 20.15625 microns. Because of the "5000 ticks" issue, the measurement could distinguish between 20.152, 20.156, and 20.160 units, but those last 2 1/2 digits of your number would be out of reach.
Specifically with microns, and assuming 400 nm light at NA 0.8, the signal to be phase matched would have wavelength 500 nm = 0.500 microns on subject. I don't really know where the limit is for this sort of computational phase matching, but if it's say 1/100 cycle, then that would be another limit around the 5 nm level.
Whether the limit is 5 or 10 or 20 nm I don't know, but I'm quite confident that it's way smaller than the 312 nm that you pushed back against. Take a look at the "Actual Movement at Finest Microsteps" graph, image 6 at
http://www.photomacrography.net/forum/v ... hp?t=27549, and note that the scale of that graph is 500 nm per coarse grid interval.
sushidelic wrote:mawyatt wrote:Anyway, I can't see any need for my use beyond what the THK rails can provide with the Stackshot controller
+1
+1 again. Microstep control of the fine focus knob of a microscope at 100 microns/turn is definitely overkill. On the other hand, it's simple and cheap overkill, especially if you can live with the limitations of a rig like
the one that nanometer shows.
--Rik