The new Zenithstar 61 is the most compact telescope thus far offered by William optics. But this does not detract from its versatility. The short focal length delivers wide visual fields and almost panoramic deep-sky astro-images. At the same time the Ohara FPL-53 objective yields excellent control of false color for high power work.
The FPL-53 substrate used in its objective has dispersion characteristics not much different from that of the much vaunted fluorite crystal, and so is termed a synthetic fluorite. This permits the employment of a fairly fast f/5.9 while delivering superb chromatic aberration suppression.
The generously large illuminated circle of 41mm (43mm with the Flattener 61) practically covers the entirety of a full-frame DSLR camera sensor, a diagonal angular coverage of 6.5 degrees. Framing the scene is made easy via the rotatable L-bracket.
For the wide field astro-imager particularly, William Optics offers a dedicated field flattener--the Flattener 61--with which to achieve the sharpest images to the sensor corners. Aiding in the determination of when to check for focus drift, the focuser knob has a handy built-in thermometer. Critical focus is assessed using the included Bhatinov mask cover, and is afforded by the co-axial 10:1 fine focus knob. The sturdy rack-and-pinion focuser employs a helical rack that will not slip under heavy loads in the way a Crayford type is more likely to.
