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actinic adjustment altitude amateur anastigmat angle angular aperture apparatus astronomical azimuth British Astronomical Association carefully carrier celestial equator centre clamp clock-driven comet dark slide Declination axis detail diffused nebula disc driving clock eclipse English Mechanic enlarged equatorial head equatorial mounting equatorial stand exposure required eyepiece eyepiece tube faint fairly bright field fixed flange focal aperture focussing screen Greenwich ground glass guide telescope hand slow motion hole inches focal length inches in diameter instrument John Evershed latitude lenses libration light longitude magnified mark meridian meteor method Milky minutes mirror Moon's mounted nebulae negative object glass observer obtained photographic magnitude piece planets Polar axis portable equatorial portrait lens position possible primary focus rackwork Reflecting Telescope reflector refractor Right Ascension roof screw sharp short exposure shutter Sidereal star images Stretcher Bars suitable Table taken telescope tube trail transit tripod stand usually visual magnitude writer