William Optics GT81

Worth knowing before you buy

What are the diffraction spikes like on the GT81?

The GT81 produces distinctive 6-8 spikes that William Optics states come from the soft plastic adjustment tips in the lens cells, not pinched optics. Some owners accept this as a characteristic of the design, while others find it unacceptable for a refractor, though the spikes are less apparent at the scope's typical imaging scale.

How good is the focuser for imaging?

Users report the focuser is excellent for a scope in this price range, with smooth rack-and-pinion action that works well for both visual and imaging applications. The built-in spacing eliminates the need to fiddle with back focus adjustments common with other fast astrographs.

How flat is the field for imaging across an APS-C sensor?

The integrated flattener produces good corner performance on APS-C sensors according to users who have tested it, with corner stars described as excellent. This is a major advantage over slower refractors that require external flatteners and spacing adjustments.

Is this a true APO or just a fast ED doublet?

The GT81 is a fast ED doublet (f/4.9), not a triplet APO, and shows noticeable chromatic aberration at higher magnifications for visual use.

What mount do I need for the GT81?

The GT81 weighs roughly 16-17 pounds with typical accessories, making it manageable on mid-sized mounts like a Losmandy GM-8 or comparable EQ mounts rated for 30+ pounds. Users note it's lightweight enough to avoid mount compatibility issues that plague heavier scopes.