The new breed of super-fast photographic refractor is rapidly carving out for itself a very targeted need in amateur astronomy. Totally transportable, maintenance free and capable of very large fields at high resolution, this type of telescope is becoming ubiquitous at dark site and sky camp locations. However, the steep photonic gymnastics required of these systems demands much from the optical system. Fast optical systems are not necessarily compatible with the provision of a large flat achromatic focal plane required by imaging devices.
At f/3.8, this 100mm telescope is capable of grabbing a large slice of sky. It delivers this faultlessly via a system of no less than five optical components. Two of these are ‘magic’ lenses (my term, not theirs). Adjacent to a companion at the telescope’s aperture there sits an SD (Super-low Dispersion) lens. Light passing down the tube then meets a clutch of three further components, within which is an ED (Extra-low Dispersion) lens. This design delivers a highly corrected flat-field imaging circle of up to 70mm (60 percent illuminated), capable of filling the 645 (medium) format camera.
The helical focuser offers 31.7mm and 50.2mm push fits, in addition to M84, M60 and M42 photo threads. To facilitate precision focusing, the focuser also incorporates a Vernier scale – continued contemporary use of which will surely warm Tycho Brahe’s heart, as its inventor.
Objective lens design: SD apochromatic, fully AS coatings, 5 group-5 elements
Effective aperture: 100mm
Focal length: 380mm
Focal ratio: f/3.8
Resolving power: 1.16 arcseconds
Optical tube length: 497mm
Optical tube weight: 4.5kg
For more information: www.vixenoptics.co.uk