Se l’arcobaleno sparisse, scatteremmo lo stesso una foto al cielo.

Because this sentence expresses a hypothetical, unlikely situation in the present or future (“if the rainbow were to disappear”). Italian uses the imperfect subjunctive in the “if” clause (protasis) of a second-type conditional. The structure is:

• se + imperfect subjunctive (e.g. sparisse)
• main clause with present conditional (e.g. scatteremmo)

Using the imperfect indicative (spariva) would signal a recurring or factual past action, not an unreal hypothesis.

With scattare (to take a photo), you commonly use the preposition a before the subject of the photo:

• scattare una foto a qualcuno/qualcosa → scattare una foto al cielo

You could also say una foto del cielo (“a photo of the sky”), but pairing scattare with a is more idiomatic for marking what you’re photographing.

In Italian, the masculine definite article il contracts to l’ before a vowel for euphony:

• il + arcobaleno → l’arcobaleno

This makes pronunciation smoother.

This is a second-type (contrary-to-fact) conditional for present/future time. Its characteristics:

• Hypothetical or unlikely scenario
• Protasis: se
  • imperfect subjunctive (sparisse)
• Apodosis: present conditional (scatteremmo)

Yes. If you want to talk about a real or likely possibility, use:

• protasis: se
  • present indicative
• apodosis: future tense

Example:
Se l’arcobaleno sparisce, scatteremo lo stesso una foto al cielo.
(“If the rainbow disappears, we will still take a photo of the sky.”)