Learn Portuguese (Portugal): Não vês o cometa a não ser que foques a lente. - You do not see the comet unless you focus the lens.

Foques is in the present subjunctive (for the tu form). We must use the subjunctive here because it directly follows the subordinating conjunction a não ser que (unless), which introduces a condition that hasn't necessarily happened yet.

Why does the spelling change to include a 'qu' in foques?

The infinitive verb is focar, which has a hard 'c' sound. To keep that same hard /k/ sound before the 'e' ending of the present subjunctive, the 'c' must change to 'qu'. If it were spelled 'foces', it would be pronounced with a soft 'c' (/s/ sound).

Why is it o cometa and not a cometa, since it ends in an 'a'?

Many Portuguese words of Greek origin that end in '-ta' or '-ma' are masculine, despite the typical feminine '-a' ending. Other common examples include o planeta, o problema, and o idioma.

Could I use a menos que in this sentence instead?

Yes! A não ser que and a menos que both mean 'unless' and both trigger the subjunctive. You could say Não vês o cometa a menos que foques a lente and it would mean exactly the same thing.