This well-corrected accessory lens preserves the high contrast and superb color correction of the main objective. While the telecompressor flattens the field to a certain degree, you may still notice some slightly elongated stars in the corners. This is less of an issue with CCD imaging since the chip size is smaller than a 35mm negative. For the very best edge star images, we recommend the prime focus 2.7" or 4" field flatteners, which have been specifically designed for each focal length of Astro-Physics StarFire refractors.
The telecompressor can only be used with Astro-Physics 2.7" and 4" focusers since it threads directly into the drawtube, thus placing the optics forward in the optical system. In addition to Astro-Physics refractors, these focusers are featured in several popular Ritchey-Chretiens (the telecompressor produces very fine results with these scopes) and are available for telescope makers (27FOCU2). Although not shown on the exploded diagram, the telecompressor is threaded for 58mm filters.
The distance from the edge of the 2" adapter to the film plane should be about 65mm to achieve 0.75x. You can decrease the distance for less compression or increase the distance for more compression. The focal length of the optic is 700mm.