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A cheap split beam setup for H1 and H2 setups.

A split beam setup normally requires two spatial filters and a variable beam splitter. Saxby describes how you can make a bypass H1 and H2 hologram simply by inserting a ND filer into half of the beam after the spatial filter. This is a very nice soloution indeed.

However, path matching may be difficult in a H2 bypass configuration. A soloution that works nice is to include two cheap mirrors half way into the beam at a small matching distance just after the spatial filter as show in the picture below.  This splits the beam into two parts with induvidual direction adjustment. The H2 reference beam can be lowered in intensety by a cheap ND filer from Edmund.

Any problems with the path length matching can then be corrected by adjusting the distance between the two mirrors. Since the beams basically travel in the same direction, this setup is still very stable. Exposures of several minutes should be possible with this setup if you have a reasonable stable table The beam paths are shown below together with the shutter, mirrors and  ND filter at the Brewster angle.