More on Amazon.com : http://www.amazon.com/gp/product/B004SFLCLO?tag=thefatkidin06-20 <br /> <br />Orion 8893 Field Flattener for Short Refractors <br /> <br />Eliminate field curvature in short optically fast refractor telescopes with this handy 38mm clear-aperture fully multi-coated field flattener <br />Designed for use with 400mm to 650mm focal length refractors with f5 to f75 focal ratios <br />Optimizes your fast refractor telescope for astrophotography <br />Preserves focal length attributes of the attached refractor telescope while flattening the field of view <br />Requires 55mm of refractor telescope backfocus weighs 75 oz
