Describing light loss through absorption and reflection in a telescope:
While this loss may be as great as 75 percent, every effort is made to reduce the loss to a minimum.
We can see that Russian periscope sight have barely 26% light transmission.
e. The main advantages of coating are as follows: ( 1 ) Increased light transmission .
When all elements of a straight telescope are coated, except the reticle and filters, there is approximately 90 percent transmission of light instead of 60 percent, or half again as much light as would pass through the instrument were its optics uncoated. This improvement will result in a brighter image at times when light is scarce. It will be most obvious at dawn, dusk, or at night; in other words, at times of poor visibility. Targets are visible from 1/4 to 1/2 hour longer at dusk and at dawn. The range of vision at night is increased approximately 20 percent for standard binoculars. Therefore, coating is essential in night glasses or in instruments to be used under adverse conditions and now is used in all military optical systems.
The 90% figure seems a bit optimistic. But, a 50% increase in transmission would seem possible.