This is an original TeleVue photographic Field Flattener for the Pronto or any scope in the f/5 to f/7 range with 2" focuser. Fits 2" focusers and accepts standard T-ring camera adpaters. Truly flattens the photgraphic field and extends focal ratio by approximately 1.15625x and focal length by 1.0833 (i.e. converts a fl 480mm f6.4 Pronto to fl 520 f/7.4). Excellent condition. $85 obo postage paid in the US.