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1.25" Star diagonals, 45 degree & correct view
I just thought I would mention this. The standard 1.25 inch star diagonal appears to have a clear aperture around 28mm. What I mean is that the diagonal prism occupies virtually the whole of the inside of the 1.25 inch tube.
As a SCT owner, my poor little brain was objecting to the mirror reversed image (say for the Moon, star charts etc.) and so I bought first one of those 'correct view' upright 45 degree units, and then a similar 90 degree unit. Both perform well, but the catch is that due to the different prism, both have a clear aperture of only18mm. (My measurement are pretty crude- holding a steel ruler against the end of the tube and squinting down it - so I could stand being corrected by a mm.)
Anyway the point that I am making is that here about 5cm or more from focus you have an 18mm aperture stop. No problem for medium or higher magnification, but low and very low powers (certainly 25mm to 40mm eyepieces) would likely suffer noticable vignetting around the edges.
If someone is thinking of purchasing one of these 'correct views', please look at the physical diameter of the stop on the eyepieces you propose using, the distance to the forward surface of the prism, and nature of the narrowing cone of your lightpath and do your sums.
For quite a different example, this is a little like the focal reducers where, at F6.3, you can't take advantage of (say) a 40mm eyepiece. Firstly you might not be able to quite reach focus, but in any event, the vignetting will be very obvious.
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