A candle or an electronic flash unit creates light at the narrow end of a cone.  A magnifying glass or a telescope bends existing light at the wide end of a light cone into the cone.  In all cases the light is most intense at the narrow end of the cone. The magnifying glass intensifies ordinary sunlight at the wide end of the cone down into a spot at the narrow end of the cone,- strong enough to start a fire in a pile of dry leaves.  The Prime Focus of a telescope is BRIGHT ENOUGH TO MELT GLASS when the telescope is pointed at the daylight sun.  I shudder to think of what it would do to someone's eye.
For a brighter image, move the Flat towards the focal point, away from the Main Mirror,- but remember you shrink the light cone's Tail when you do this.  The Tail must reach beyond the side of the telescope.