Astro-Tech 1X Field Flattener For Astro-Tech AT90 Triplet APO Refractor

The AT90EDX and AT90CFT are triplet APO refractors with essentially perfect on-axis optics — guaranteed .95 Strehl or higher. But point a camera at them without correction and the field curves. Stars in the center are sharp. Stars at the edges are not. The scope didn't do anything wrong — a flat sensor and a curved focal plane just don't agree. That's where a field flattener earns its place in the imaging train. The AT90FF is a dedicated 1× flattener for the AT90 triplet series. It doesn't reduce the focal length or change the focal ratio — you keep the full 540mm at f/6. What it does is flatten the field across the sensor so stars are round and sharp from center to edge, all the way to the corners of a full-frame chip. The AT90EDX delivers clean, color-free optics. The AT90FF delivers that quality across your entire sensor. Dedicated — Not Generic This flattener is optically matched to the AT90EDX and AT90CFT's specific focal length and field curvature. A generic 1× flattener designed for f/5–f/7 refractors in general will sort-of work, but it won't deliver the same edge-to-edge correction as a flattener that knows the exact optical system it's sitting behind. One reviewer put…