Spatial filters have two functions. The first is to diverge the laser beam as simple diverging lenses and objectives do, and the second is to eliminate noise caused by dust and scratches on the optics and the internal noise created in the laser cavity. Using a spatial filter will provide the ultimate in totally clean holographic images irrelevant of scratches and dust. The X and Y micrometers move the pinhole to center on the objective's central light axis, and a Z micrometer moves the objective so that the narrowest point of its focal point is located at the center of the pinhole. When this is achieved, you have a very clean beam at the plate holder from the reference beam.
Spatial filters are expensive.
You need not consider these for your initial holograms. Producing Transmission and Reflection Holograms. A 6-inch diameter mirror is recommended with a focal length of 24 inches.