I tested the following setups:

• Sigma SPORT 60-600mm ƒ 4.5-6.3 lens + 1.4x teleconverter on Canon EOS R6 [20 MP] = 840 mm
• Olympus ED 300/4 IS PRO + 1.4x teleconverter | Olympus OM-1 [20 MP] = 840 mm

And they pretty much performed nearly the same. Perhaps the Olympus had a tiny notch better fine details visible, but there are many parameters that could have affected the outcome; such as atmospheric flimmer, the movement of the moon at such long focal lengths, and of course any tiny movement (like foot moving on the balcony) while taking the image, and the shutter speed itself.

But largely they are playing in the same league optically, albeit the Olympus is a tiny not better and I don't have to top it down in aperture, where the Sigma needs a notch more.

The advantage of the Sigma is of course the flexibility - being able to zoom from normal 60mm to a whopping 600 mm which turns into 840 mm with a 1.4x teleconverter. While the Olympus is a fixed lens. It only gets so long, because 300mm on a Micro Four thirds camera, is always 2x the focal length = 600 mm.

The advantage of the Olympus lens is of course, that it is almost "tiny" compared to the whopping "Bigma" Zoom, the Sigma 60-600mm lens. Making it very easy to take with you. Gosh, with some easy support and the good Image stabilization, you can even get away with handheld moon photography.

AF of Olympus cameras, can sometimes miss exact focus !!

Albeit, i do notice that something in the Olympus OM-1 (but also EM1X and EM1 II, seem to have wacky AF; y missing the exact focus point (on the surface of the moon) when you aim at your object without enlargement in the finder or LCD screen.

It is better, to enlarge the frame - and THEN focus on the border of the moon, where the craters are. Now i get sharp images almost all the time.

I do not get that when i simply aim at the moon, without enlarging the frame first.

I also notice the same behavior when i took images of the (fog veiled sun); that I always got better, exact focus, when i first enlarged the surface of the sun, and THEN focused onto the border of the sun disk.