SharpStar D 150mm f/2.8 HNT: An imaging telescope with sharp vision
Until comparatively recently, if you wanted to capture extended deep-sky objects such as diffuse emission nebulae and supernova remnants with your full-frame DSLR or CCD/CMOS camera, then your best option was to invest in an f/2 or f/2.8 telephoto prime lens from one of the big-name players in photography. Such high-precision solutions possess multi-element lenses, often made of exotic glass, and sometimes with aspheric shapes designed to ensure tight control of false colour, off-axis aberrations and field curvature over a large imaging sensor. Not surprisingly, lenses possessing these desirable characteristics don’t come cheap. For example, a Nikon 200mm, f/2 G ED AF-S Nikkor VR II or Canon EF 200mm, f/2 L IS USM will each lighten your bank balance by more than £5,000. While f/2 and f/2.8 prime lenses can record bright nebulae and galaxies in minute-long exposures at the high ISO sensitivities of modern DSLRs, 200 and 300mm focal-length lenses are too short to reveal fine detail in small-scale examples of these deep-sky objects, unless one is prepared to use one of the new breed of cameras employing CMOS sensors with tiny (less than 2.5 micron) pixels. Even so, most astro-imagers would prefer focal lengths of at least 400 to 500mm
