When the poet uses the word "constellation" he is using it to mean a whole array of things -- not just one. He is saying that when you read silently to yourself, you are exposed to a variety of different sensations (because he says that it is a "sensory constellation) not just one sensation.
When you read to yourself, you experience many things. You "see" things, of course, but you also engage your other senses, he says. Lux says that you can feel things and smell things. You can hear the oats as they pour into the feed troughs. You can see how dirty the cow's haunches are, and you can probably smell it as well.
The whole point of saying that there is a "sensory constellation" is to tell you that reading can be a very vivid experience that impacts all of your senses (at least in your mind).
A constellation is of course a gathering together of stars, and it may also be understood to refer to a collection or gathering together of ideas. The two images here, that of a constellation (vastness, brilliance) and a barn (familiarity, drabness) are brought together in lines 20-31 to demonstrate how images expand far beyond immediately predictable limits. Particularly in lines 27-31, the speaker demonstrates the specific visual, tactile, and olfactory images that develop from the initial impetus provided by the single image “barn.” This development of imagery is an example of merging images of different senses to bring about a desired result.