This makes it easier to keep your eye in the right place, and lets you go back and forth to the scope to make adjustments. I use a photo light stand and clamp to hold the card in place. The light stand has a vertical adjustment the makes it easier to set it to the right location. This telescope is out of alignment. The CenOb shadow is concentric with the CenOb, but the other rings are not concentric This telescope is properly collimated.
All rings are concentric.