Daystar Filters FlatCap Imaging Diffuser FlatCap is Temporarily applied over the objective to acquire a "Flat Field" reference image which is then applied to live images taken after the FlatCap is removed. Spun Aluminum mounting ring with polymer diffuser for taking flat fields. FlatCap is sold by the inside diameter of the mounting cap. Measure the outside diameter of the front most component of your telescope or ERF, and then round up to the next larger size. For example, a telescope with a 160mm front should use model FC170, and then optionally install the included felt pads for a secure fit. Do NOT order a flatcap based on the aperture of your telescope, you must measure the outside of the front.
