Quote:
Originally Posted by Saturnine
As an happy owner of an 180 Skywatcher Mak I was interested in your comparison of the 150mm Skywatcher and the Intes MK65 and was wondering if the difference in image brightness is partly due to the size of the central obstruction being smaller in the Skywatcher.
Jeff
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Hi Jeff,
I think this has a minor impact only. For example assuming the Sky Watcher has a CO of 32% to the Intes CO of 37% this would amount to a difference in light collecting surface area of only 3.84%. A 5% difference in light transmission is considered the human detectability threshold.
I would think of far more importance is the fact that the Intes uses 20 year old coating technology, on both mirrors, 20 year old coating technology on the corrector plate and its coatings are in fact 20 years old and would have without question lost some reflectance/light transmission.
If we assume the reflectivity of each mirror in the Intes has dropped from 87% when new, to 80% at 20 yrs old; and we assume transmission on the corrector has dropped from say 97% to 94% after 20 years, the overall drop in light transmission drops from 73.4% when new, to 60.1% at 20 yrs of age, or a decrease of 18.1%.
Lets assume the Skywatcher with its newer technologies has 90% reflectivity on both mirrors and a corrector plate transmission of 98%. This gives an overall light transmission for the scope of 79.4%. This is a 6% increase over the Intes when it was new and a massive increase over the Intes with its aged coatings.
Cheers,
John B
Cheers,
John B