NavList:

Interesting point. The image of the Sun with shades in should be about as bright as the Full Moon (without shades). By magnitude difference that's a difference in light intensity of over 400,000. Let's round down to 100,000 just to be generous to safety. Light intensity is basically Watts per square arcsecond (or equivalent). If we double the area of the Sun, which we do by observing the direct and the reflected image at the same time, we have doubled the total arcseconds of apparent area of the Sun. But we're still on the order of 50,000 times fainter than the Sun itself, unshaded. Also, if the images are not overlapped (and they usually aren't in these observations), then the total energy striking any given square millimeter of the retina is unchanged. There's a larger total area, but the same local "heating" as when there's only one sun image.

There is a problem that sometimes arises with observations like this depending on the design of the shades. Sometimes the shades don't completely block the direct view through the horizon glass. If that's the case, you may occasionally get "zapped" with a direct bit of the Sun's face through the sextant. This won't blind you, but it's really annoying and maybe a bit scary. If you discover you have that problem using your sextant with an AH, there are simple tricks you can apply. For example, make a cardboard baffle that attaches to the outer edge of the primary shade in the horizon set. It's not pretty, but it works.

Finally, all of this assumes that the set of horizon shades is "good". I have seen a few sextants over the years where the horizon shades are considerably less dark in total compared to the direct shades. If that's the case, you would need another neutral dark shade somewhere.

Frank Reed