Movement may be sensed by an electro-optical device, such as a CCD chip, or by the human eye comparing star movement to a eyepiece reticle intersection or a reticle grid.
*3An amount of transparent material is formed above the mask or reticle, and its phase shifted to increase contrast of the pattern resolved on the wafer. This optical lithography method improves resolution.
I have since turned to auto-guiding, using a CCD thru that scope so my laptop can 'see' where the scope is and make small adjustments every second rather than me trying to catch drift movment every 20-30 seconds thru an eyepiece with reticle.
 See also: Lens, Sensor, Exposure, Light, Photon
  
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