The spacer in the GSO is to enable you to place the focal plane of the eyepiece at the requisite distance of 75mm from the CC lens.
It is at this position where, when you focus the scope, the CC will be at the correct place in the light cone from the primary to correct coma properly.
If you use the CC at a different distance, and focus the scope, the CC will not be at the optimum distance from the primary and coma correction will
not be optimized. You might get less coma correction at the edge of the field.
Since you bought the coma corrector to correct coma, you might as well use it correctly.
And there is a side benefit:
Once one eyepiece is optimized and the CC is in the right place in the light cone, all other eyepieces can simply be pulled out relative to the CC lens until in focus, and that will keep the CC
in the correct place in the light cone from the primary and result in all of your eyepieces focusing at the same place in the focuser's travel.
Since the GSO does not have a helical top like the Explore Scientific HRCC or the TeleVue Paracorr, the way to accomplish that with the GSO is to start with your eyepiece that requires
the most in-focus without the CC and optimize that eyepiece first. Every other eyepiece will need to be pulled out of the CC top to achieve focus and it really helps to add parfocalizing rings to the eyepieces
so that when they are reinserted they are already almost perfectly focused. Following that, you only will need a mm of travel in the focuser. AND, you can optimize any new eyepiece you buy simply by pulling it out of the CC top until it is in focus.
The focal plane in many brands have known positions, so if you are uncertain and you own a TeleVue, Baader, Pentax, or Explore Scientific eyepiece, we can tell you the necessary distance from the CC lens to the shoulder of the eyepiece
that is needed to optimize it.
Edited by Starman1, Today, 06:06 PM.