What does Emerson mean by "...luster of the firmament of bards and sages"?
Emerson is using a metaphor. He is figuratively speaking of all the bards and sages, that is, all the poets and philosophers, as if they are stars lighting up the entire sky like our Milky Way galaxy. Emerson is not the first writer to speak of great men of the past as if they had been transformed into eternal stars. In Shelley's famous poem Adonais he speaks of the soul of dead poet John Keats as having been transformed into a star. But Shelley got the notion from the ancient Greeks and Romans.
I am borne darkly, fearfully, afar!
Whilst, burning through the inmost veil of heaven,
The soul of Adonais, like a star,
Beacons from the abode where the Eternal are.
More important than Emerson's metaphor is the important truth he expresses.
A man should learn to detect and watch that gleam of light which flashes across his mind from within, more than the lustre of the firmament of bards and sages.
A man's own thoughts are more important--for himself at least--than all the thoughts of the bards (poets) and sages (philosophers) from the ancients up to the present. Sir Francis Bacon expresses a similar thought in one of his essays.
Write down the thoughts of the moment. Those that come unsought for are commonly the most valuable.
Emerson is simply telling us that we should pay attention to our own thoughts, impressions, observations, insights, etc. We have all, as homo sapiens, inherited marvelous brains which have been evolving over millions of years. We are all capable to forming thoughts that would be extremely valuable, not only to ourselves, but to human society. It was typical of Emerson that he urged readers to think for themselves and not to spend their lives looking back at the thoughts of other men. There is one big difference between great poets and thinkers of the past and ourselves. They are all dead and we are alive.