Sized between the popular 4-inch and less common 5-inch refractors, the Vixen Optics ED115S ED doublet refractor is an f/7.7 instrument that uses FPL-53 low-dispersion glass to achieve near-perfect color correction.
More recent versions of the scope are renamed SD115S, but little has changed except for the original single-speed rack-and-pinion focuser that has been upgraded to a dual-speed unit. Made in Japan, the Vixen features a 50mm finder, flip star diagonal, tube rings and mounting plate, and a convenient carrying handle. An f/5.2 focal reducer is an available option.
The telescope is lighter than many 4-inch refractors, and rides with good stability on a CG-5-class German equatorial mount.
Optically, the Vixen is excellent: The star test reveals outstanding correction for spherical aberration and astigmatism, and false color becomes barely visible as a faint purple halo around Vega and Rigel at over 250x magnification. Blue wavelengths are almost perfectly corrected at the expense of a slight red excess.
The result is an instrument with razor-sharp lunar, planetary, and stellar images. Saturn snaps to focus with a well-defined Cassini Division, significant belt detail, and authentic color rendition of the planet’s delicate pastels.
The moon displays stark contrast, pitch-black shadows, and intricate detail limited only by the scope’s inherent resolution. Powers of 250x and more show no breakdown in image fidelity, with just a minute bit of red coloration when the seeing conditions turn poor.
Double stars are impressive with the telescope. Castor is a snap at 153x, with the duo displaying tight airy discs and delicate first diffraction rings. The difficult Eta Geminorum reveals its tiny companion at high power, and the companion to Sirius was once glimpsed on a night of perfect seeing.
Overall, the Vixen ED115S is a first-rate instrument.
The good: Superb optics, light weight
The bad: Expensive, mediocre focuser