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Yes Scott it is all about signal to noise. I have found with an aperture in front of the lens it stops light not contributing to the image from being scattered inside the lens. This leads to loss of contrast. I look forward to seeing how your 300mm F2.8 performs with its own iris and with a limiting aperture in front. The bog standard 50mm at f/5.6 looks like it doing better than the sensor.
I am guiding the 300mm lens with a 1250 mm FL 90mm Mak and it does get the faint stars. Dithering between exposures really helps.
Bert
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