Telescope mirrors and other mirrors used by scientists telescopes regularly do use a silver coating. See for instance here. However, aluminum coating are the norm (certainly for the large primary mirrors deployed in telescopes) because of durability reasons. I quote from the text linked to above:

The challenge with using silver as a coating material is that, unlike aluminum, it tarnishes with exposure to air, specifically to sulfur.

And

"Like the family silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly develops brown tarnish spots over time and must be regularly polished, the shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity and increasing emissivity. The observatory's engineers, however, can't just grab a cloth and some polish when the tarnish spots appear."

On you second question: a back surface reflective coating implies an additional reflective surface: the air-glass interface. This leads to increased light losses and the need for anti-reflective coatings.

Telescope mirrors and other mirrors used by scientists telescopes regularly do use a silver coating. See for instance here. However, aluminum coating are the norm (certainly for the large primary mirrors deployed in telescopes) because of durability reasons. I quote from the text linked to above:

The challenge with using silver as a coating material is that, unlike aluminum, it tarnishes with exposure to air, specifically to sulfur.

And

"Like the family silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly develops brown tarnish spots over time and must be regularly polished, the shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity and increasing emissivity. The observatory's engineers, however, can't just grab a cloth and some polish when the tarnish spots appear."

On your second question: a back surface reflective coating implies an additional reflective surface: the air-glass interface. This leads to increased light losses and the need for anti-reflective coatings.

Telescope mirrors and other mirrors used by scientists telescopes regularly do use a silver coating. See for instance here. However, aluminum coating are the norm (certainly for the large primary mirrors deployed in telescopes) because of durability reasons. I quote from the text linked to above:

The challenge with using silver as a coating material is that, unlike aluminum, it tarnishes with exposure to air, specifically to sulfur.

And

"Like the family silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly develops brown tarnish spots over time and must be regularly polished, the shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity and increasing emissivity. The observatory's engineers, however, can't just grab a cloth and some polish when the tarnish spots appear."

On you second question: a back surface reflective coating implies an additional reflective surface: the air-glass interface. This implies additional light losses and the need for anti-reflective coatings.

Telescope mirrors and other mirrors used by scientists telescopes regularly do use a silver coating. See for instance here. However, aluminum coating are the norm (certainly for the large primary mirrors deployed in telescopes) because of durability reasons. I quote from the text linked to above:

The challenge with using silver as a coating material is that, unlike aluminum, it tarnishes with exposure to air, specifically to sulfur.

And

"Like the family silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly develops brown tarnish spots over time and must be regularly polished, the shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity and increasing emissivity. The observatory's engineers, however, can't just grab a cloth and some polish when the tarnish spots appear."

On you second question: a back surface reflective coating implies an additional reflective surface: the air-glass interface. This leads to increased light losses and the need for anti-reflective coatings.