I just bought a telescope and saw the moon but with the bright moon at the center there are two faint images of the moon above and below it. I figured out it was due to the periscope mirror. If I see my finger through that periscope it has a faint outline around it. Which is getting magnified when I try to look the moon.then I tried to see my finger from a normal mirror diagonally and I first time noticed that every mirror does the same , there is a faint outline around my finger if I see it diagonally through a mirror. So my question is, how do I fix it now?
If it was built cheaply, the "periscope"(?) might have used back-surface mirrors; normal mirrors with glass on the front and the reflective material on the back side where it's protective.
In this case, you'll get roughly a 5% reflection from the front glass surface at 45° and maybe 80% reflection from the metal on the back (only 95% is transmitted and maybe the metal is 85% reflective)
A periscope has two mirrors in a dogleg configuration, so that will produce one "ghost image" above and one below.
Our eyes are non linear, so even though the reflections are roughly 40 times dimmer than the main image they will still be irritatingly visible.
Good optics would have used front surface = first surface mirrors. Glass lenses used in telescopes have anti-reflection coatings otherwise we'd see lots of lens flare in every image from the curved surfaces.
But since mirror surfaces are flat, they make ghosts instead of flare.
I think there's not much you can do unless you replace the mirrors with front-surface mirrors.
Probably the periscope was meant to be used in daytime so the weak reflections weren't as noticeable. But in astronomy the background is black and even tiny reflections are easy to see.