As Panoptic Mirror's triggered ability resolves, it allows you to create a copy of one of the instant or sorcery cards imprinted on Panoptic Mirror in the Exile zone (that's where the imprinted card is) and then cast it without paying its mana cost.

You don't pay the spell's mana cost. If the spell has X in its mana cost, X is 0. You do pay any additional costs for that spell. You can't use any alternative costs.

The triggered ability triggers only once each upkeep, not once per imprinted card. If no cards are imprinted on Panoptic Mirror when the triggered ability resolves, it does nothing.

You may imprint a card on Panoptic Mirror in response to the upkeep-triggered ability. If you do, that card is available to copy when the triggered ability resolves.

Exiling a card when the activated ability resolves is optional. If you choose to exile a card, you can exile only a card with converted mana cost equal to X.

To imprint a split card, you pay X equal to either side's converted mana cost. If the copied card is a split card, you choose which one side of it to cast, but you can't cast both sides. You can choose the other side later in the game, though.

The creation of the copy and then the casting of the copy are both optional.