The Da-Lite 48 x 60" Mylar Mirror is a highly specialized item for those installing a particular type of rear projection system. Perhaps for a point-of-purchase display, or theater production where projection space may be at a premium, this mirror may be of value to you.
The purpose of this item is that it allows you to shorten the amount of space you need to have behind your screen for throw distance from the lens of the projector to the back of the screen. The surface is composed of a thin, plastic film coated with an especially reflective material.
The fine tune feature allows you to make very small adjustments so your image matches up perfectly with your screen. This will be useful if one side of your image was off by 1/8". The fine tune function would permit you to "square it up".
Note: Projector sold separately