No 23A Light Red colour filter to fit 1.25" filter threads (25% transmission). A great filter for Mars, Jupiter and Saturn! Similar in performance to the No 15 and 21 filters but delivering higher contrast. To a certain extent it provides the higher contrast views at the expence of image brightness and performs at its best with instruments of 5" aperture or greater. Again, like the No 15 this can be a useful filter when making daylight observations of Venus and Mercury. 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.