OPTICAL DESIGN ALERT
Measuring just the spot diameter on the corrector gives only the minimum. The baffling can intrude further, either the inside corrector baffle of some scopes, whether it flares, etc. Then the primary baffle diameter at the mirror, whether the front edge clips the light cone reducing the CA, or projects deeply into the converging cone blocking more than the secondary obstruction. See Telescope Design, Harrie Ruten, et al for an eye popping expose’.
You see all these contributors at the eyepiece where it counts. Measure the pupil with an eye loupe at the eyepiece, both the id and od of the donut.
The Questar is pretty tight so 33-34%. Some scopes are way off, with a small silvered spot, implying an illusory 28%, but the baffling produces more like 40%. Example Meade ETX 125.
Edited by dougspeterson, 23 June 2019 - 01:05 AM.