Used instead of an iris in an ERS to reduce the size of the beam but still maintain a circular shape. Easily fabricated from sheet metal. Make the outer dimensions the same size as a standard pattern holder, and make many with various sized holes from which to choose during focus. Certainly not as flexible as a drop-in iris, but certainly less expensive. Alternatively, an appropriately sized gobo may be purchased from many sources. Link to one manufacturer's offerings.
See also http://www.tlsinc.com/toolaperture.html.
See also http://www.tlsinc.com/toolaperture.html.