When using aggressive focal reduction, and/or when on SCTs the focus is located well outside the back end, it's possible for aperture reduction to occur. Here's how you can determine if this is happening with your 'live' setup.
Aim at a bright target, such as planet or 0-1 magnitude star, or distant streetlight, or sun glint during the day. Center the target. This is important! You are assessing the on-axis aperture; an off-center target is affected by the inevitable vignetting, which is asymmetric.
With a sheet of paper or cardboard large enough to fully cover the aperture, uncover just enough aperture to barely begin to show your target. Note how far inward from the aperture's edge the edge of your blocking sheet lies. This is the width of the annulus at the aperture's edge which is not contributing to axial image formation.
For some scopes the aperture's edge is not clearly defined at the front end (a Newtonian being a good example.) In such case--indeed, in all cases, for best accuracy--do this. Measure the blocker's distance from any convenient reference, such as the OTA's inner wall, then double it. Subtract this from the diameter of the reference you used; the result is the actual working aperture.
As you can appreciate, your target should have very high contrast. This is well met at night, but can be problematic during the day. In the latter situation, sun glint from a metal roof or car windshield a kilometre or more distant can serve well enough.
In any case, you should be well overexposing to begin with. You will be blocking the light down to the point of allowing only a very tiny fraction admitted by a little slice at the alerture's edge, perhaps only 1/10 of 1%. The stronger the signal to start, the surer you are of finding the working aperture's edge, and not having the target fade out prematurely.
I prefer this technique to the one where you throw a point source target out of focus and poke in from the aperture edge some object, noting when it becomes visible on the defocused disk of light. Diffraction requires to throw the target so far out of focus that this can actually change the working aperture. Better to test in focus.
An easy way to assess aperture reduction
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