[Satchmo]

Hi John - I'd call fast these days F3 to F3.5. You don't use a 27mm eyepiece with scopes this fast.

With your F4.5 this would be a 6mm pupil about maximum. If you used a Paracor on an F3.5 it gives an amplification of 1.15X in focal length so the new effective ratio becomes F4. A 6mm pupil would then be achieved with a 24mm eyepiece on the F3.5/ Paracorr combo. The secondary shadow is caused by the small image of the secondary in the pupil of the eyepiece. In the case of your 18" sdm this image would be a 1.2mm black dot in the middle of the 6mm eyepiece pupil ( 19% central obstruction ) .

If this dot is blocking your eye pupil then I'd suggest your eye is opening up no where near 6mm and your pupil is cutting off a lot of the light coming from the primary mirror. If you base this test on daytime observation then it would be valid as the daytime pupil is only around 2mm.