A normal mirror, that we see for example in an elevator, is a glass sheet with thin metal coating on the backside, so light reflecting from the metal surface goes first thru the glass.
Metal mirror (aka First surface mirror) has no glass in front and is therefore free of chromatic aberration (practically speaking free, not absolutely). So I'd think a metal mirror would be ideal for use in photographic telephoto mirror lenses. But, in Wikipedia article, it looks like the mirror is made of glass with the reflective surface on the back. Or is this old news, and modern mirror-lenses actually use metal mirrors?
Why is first surface metal mirrors not used in photographic mirror-lenses?
Image by Paul Chin - Wikipedia