White Paper - SX12 Tracking and Target Separation 7
https://geospatial.trimble.com
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Figure 9a. A target at a longer distance will create a smaller image on the tracker detector.
Figure 9b. The same target at a closer distance gives a larger image on the tracker detector.
The image size of the targets will decrease as the distance to the targets increases. This means that at longer
distances the angle between two prisms can decrease and the SX12 can still separate them. This is true until the
resolution of the optics and the tracker detector itself sets a lower limit for the angle separation between two
prisms.
The SX12 tracker lens is 50 mm in diameter but the entire lens surface will only receive light if the prism is big
enough. Prisms that are 25 mm or bigger will fill the entire lens and will therefore get defocus discs of the same
size, which means that the smallest prism separation (center-to-center) is the same for all prisms 25 mm or larger.
Prisms smaller than 25 mm, on the other hand, will render smaller defocus discs and can therefore be placed
closer together and still be distinguished by the tracker.
Figure 10a. When a large prism is used (above 25 mm) the light returning from the target fills the entire
instrument lens and the lens limits the amount of light that is focused towards the detector. When the prism is
25 mm or larger the image size is limited by the lens size and a larger prism would not increase the size of
the image.
