WILLIAM OPTICS GUIDESTAR 61 F/5.9 APO

A nice guidescope for those longer focal length OTAs

This is a very nicely made guidescope, as you would expect for a William Optics refractor. It's focusing adjustments feel solid and they tighten up without any flexure in the light train. The image quality (mine with a ASI120MM camera) is good without noticeable flare or fringing. With my configuration, PHD guiding is regularly reporting 0.5 arcsec errors in average seeing conditions. Getting a good focus with the camera was drama-free, fine-adjusting with the front helical focuser which moved easily (with the usual caveat that the focuser needs to be locked in place each time it is adjusted to take up the backlash in its thread). The only issue I had with this g.s. is that it seems to be designed for the visual astronomy. To get a focus with the ASI120MM I found it necessary to insert a 35mm extension tube between the rear focuser, and the camera. Even with this extension tube, I have the rear drawer tube at 57mm and the front helical focuser at 4mm. This leaves the rear drawer tube with a small amount of adjustment, but there is still plenty of adjustment in the helical focuser. (A 50mm extension tube would probably win back 15mm in the drawer tube extension.) Even at these extensions, the 'scope is quite rigid. A 2"-to-1.25" adapter is required to mount the ASI120MM. I have no hesitation in recommending this guidescope, especially where you are operating a longer focal length telescope (>= 500mm?). Paul.