It took me a while to find it - the topic of T-stops was a recent discussion
among the astrophotographers. Without going into the arithmetic, the T-stop
for the Rubinar is f/6.68. This assumes 100% reflectivity and 100%
transmission by the mirrors and lenses. FWIW, I get a 31percent central
obstruction for a primary diameter of 90mm and a central obstruction of
50mm.
John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far"
Matthias Wilke <Matthias.K.Wilke@xxxxxxxxxxxxxxxxxx> wrote:
>
>You are right, I have just looked for it. The diameter of the front element
>with 500mm and 1:5,6 should be circa 90 mm with an lens without central
>obstruction. With the Rubinar and its obstruction, the diameter is also
>circa 90 mm, so you have to substract the central obstruction to calculate
>the exact effective aperture. The central obstruction has a diameter of 50
>mm, so the obstruction is 17 percent, if I am right.
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