Focus Scientific

Cheshire Eyepiece/Sight Tube Combo Collimation Tool (1.25\")

Collimating Newtonian reflector telescopes can be a daunting task, but it is crucial to achieving good performance, especially those instruments with fast focal ratios (eg. f/5).

This tool is a combination Cheshire eyepiece and sight tube. It allows for precise alignment of both the primary and secondary mirrors. It fits standard 1.25" focusers, and can be used with an optional 2" reducer adapter (2" reducer adapter sold separately).

Step One - Secondary Mirror Alignment - The sight tube portion of this tool helps you centre the secondary mirror under the focuser. The built-in crosshairs help you align the secondary mirror's extended optical axis to the centre of the primary mirror.

Step Two - Primary Mirror Alignment - The Cheshire eyepiece portion has a light window with a 45° Aluminum surface which helps you align the primary mirror to the crosshair.

Step Three - Collimation on a Star - A final star collimation should be performed to weed out any residual error. For telescopes that don't track, use Polaris because it barely moves.

Aren't Laser Collimators Better? - Lasers get you close. Cheshire eyepieces and sight tubes get you closer. It's difficult to make adjusts so that the laser hits the centre of the secondary and primary mirrors simultaneously, especially considering most reflectors don't allow for direct observation of the secondary mirror reflective surface - most astronomers simply guess. Experts advise customers using laser collimators to place a centre mark on their secondary mirrors, and over the last 17 years, we've only seen a small handful of customers willing to do this.

Laser collimators with a side-viewing inspection window are useful when collimating primary mirrors, especially of longer focal length instruments, because you can see your adjustments in real-time. Follow it up with the Cheshire to refine your alignment.