Quote Originally Posted by McGuyver View Post
I know that this thread is a bit old but I thought of a possible answer to your question, the effective aperture size that you input for the calculation was likely incorrect. If you measured 91 inches but the actual effective aperture size according to the manufacturer is a little less such as maybe 87 inches it will then calculate properly. The full diameter of the dish may not always be effective due to certain formed edges, etc.
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I see your point, and I agree that there are problems when taking measurements of dishes. But if you measure at a point inside the edges, then you need to also measure the depth of the dish consistent with where you measure the diameter. Ie the depth would be less too, so usually, it's not going to make much difference unless the edges are significantly different from where the surface would be. It's usually easier to measure the depth by sighting across the edges, even if the edges aren't part of the actual reflector surface, and if you do that, you sort of have to measure the diameter by estimating where the surface would be.
I can usually estimate the diameter at where the surface would be, but my biggest problem is measuring the depth. My dish has a flat circular plate at the center of the dish, ie the reflective surface doesn't continue at the center, so I also have to estimate where the surface would be when measuring the depth and when measuring the focal length.
But yes, you have to try to measure to both the diameter and depth either at the actual surface, or by estimating where the surface would be, and the likelihood of measurement errors causing the problems here are pretty high. However I still think that the biggest error is because the dish is in halves, that there is nothing holding the dish in it's proper shape, and it is probably sagging significantly. Tie the 2 halves together, and the measurements will be different, and I'm guessing the new measurements will fit the equations.