No 21 Orange colour filter to fit 1.25" filter threads (46% transmission). Reduces transmission of blue and green wavelengths. Useful on Mars to sharpen the bounderies between surface formations. Enhances cloud belts on Jupiter and is very effective at bringing out the Great Red Spot. Also useful on Saturn performing much like the No 15 but delivering higher contrast. The filter is made of top quality optical glass and is mounted in a black anodised aluminium holder and is supplied in plastic storage case.