I have a refractor that has a 4 inch dia. objective lens cap.
but that lens cap has a second lens cap in the center of it

the second lens cap is only about an inch and a half across.

when would a body have use for one and not the other?

why are there two options?

https://www.cloudynights.com/a...ctor-telescope-r2219

For a minute I owned a NVG monocular, it had a cap with it with a tiny hole in it, so they could be used in daylight, or lighted conditions, I imagine for testing and the like. Maybe this is something similar? For use in the day, or something, without burning out your eyeballs? Of course I would think one would use a tinted lens.

At the risk of sounding like a dunce -and not wanting to treat you like one bendable -

do they separate? Maybe it's for the eyepiece?

the eye piece is 1 1/4 , the small one in the center would be much too large.

The small aperture is for looking at the moon, when you don't need the full objectives light gathering. Had one on my Vixen, but kind of miss it since I moved to a Stellarvue

thanks I will try that