Hall of Mirrors - Vane 3

Note sliver of 2nd bright reflection of open end of focuser tube.

On a GSO RC6, there is a very thin angle at which this 2nd reflection can be viewed. This reflection can be made identical at each vane (and all around the perimeter of the OTA for that matter). This symmetrical sliver only occurs when the primary mirror is very tightly aligned with the secondary mirror.

Even a very small adjustment to one of the primary pull screws will make one sliver bigger or smaller than the sliver at the opposite vane. Or one or more vanes won't precisely bisect the sliver. There is a lot of ability to judge the exactness of the alignment and perfect it.