Who coverest thyself with light as with a garment - Referring to the first work of creation Genesis 1:3, "And God said, Let there be light, and there was light." He seemed to put on light as a garment; he himself appeared as if invested with light. It was the first "manifestation" of God. He seemed at once to have put on light as his robe.
Who stretchest out the heavens like a curtain - As an expanse spread over us. The word used here means a curtain or hanging, so called from its tremulous motion, from a word meaning to tremble. Thus it is applied to a curtain before a door; to a tent, etc. It is applied here to the heavens, as they seem to be "spread out" like the curtains of a tent, as if God had spread them out for a tent for himself to dwell in. See the notes at Isaiah 40:22.

Who coverest thyself with light - Light, insufferable splendor, is the robe of the Divine Majesty. Light and fire are generally the accompaniments of the Supreme Being, when he manifests his presence to his creatures. He appeared thus to Abraham when he made a covenant with him, Genesis 15:17; and to Moses when he appointed him to bring the people out of Egypt, Exodus 3:2; and when he gave him his law on Sinai, Exodus 19:18. Moses calls God a consuming fire, Deuteronomy 4:24. When Christ was transfigured on the mount, his face shone like the sun, and his garment was white as the light, Matthew 17:2. And when the Lord manifests himself to the prophets, he is always surrounded with fire, and the most brilliant light.
Bishop Lowth has some fine remarks on the imagery and metaphors of this Psalm. The exordium, says he, is peculiarly magnificent, wherein the majesty of God is described, so far as we can investigate and comprehend it, from the admirable construction of nature; in which passage, as it was for the most part necessary to use translatitious images, the sacred poet has principally applied those which would be esteemed by the Hebrews the most elevated, and worthy such an argument; for they all, as it seems to me, are taken from the tabernacle. We will give these passages verbally, with a short illustration: -
הוד והדר לבשת hod vehadar labashta.
"Thou hast put on honor and majesty."
The original, לבשת, is frequently used when speaking of the clothing or dress of the priests.

עטה אור כשלמה oteh or cassalmah.
"Covering thyself with light as with a garment."
A manifest symbol of the Divine Presence; the light conspicuous in the holiest is pointed out under the same idea; and from this single example a simile is educed to express the ineffable glory of God generally and universally.
נוטה שמים כיריעה noteh shamayim kayeriah.
"Stretching out the heavens like a curtain."
The word יריעה, rendered here curtain, is that which denotes the curtains or uncovering of the whole tabernacle. This may also be an allusion to those curtains or awnings, stretched over an area, under which companies sit at weddings, feasts, religious festivals, curiously painted under, to give them the appearance of the visible heavens in the night-season.

Who coverest thyself with light as with a garment,.... Referring, as Aben Ezra and Kimchi think, to the light, which was first created; and indeed this was commanded out of darkness by God the Word, or by the essential Word of God. Light is expressive of the nature of God himself, who is light, and in him is no darkness at all, and who dwells in light (h) inaccessible, and so may be said to be clothed with it; which is applicable to Christ as a divine Person, 1-John 1:5. and to whom this term "light" well agrees; Light being one of the names of the Messiah in the Old Testament, Psalm 43:3, and is often given him in the New Testament, as the author of the light of nature, grace, and glory, John 1:9. He is now possessed of the light and glory of the heavenly state, of which his transfiguration on the mount was an emblem, when his face shone like the sun, and his raiment was as the light, Matthew 17:2.
Who stretchest out the heavens like a curtain; alluding to the firmament or expanse, which, being spread out like a curtain, divided between the waters and the waters, Genesis 1:6. Heaven is represented as a tent stretched out, with curtains drawn around it, to hide the dazzling and unapproachable light in which the Lord dwells, Isaiah 40:22 and it is as a curtain or canopy stretched out and encompassing this earth; the stretching of it out belongs to God alone, and is a proof of the deity of Christ, to whom it is here and elsewhere ascribed, Job 9:8. Here Christ dwells invisible to us at present; he is received up into heaven, retained there, and from thence will descend at the last day; and in the mean while is within the curtains of heaven, unseen by us.
(h) "Pura in luce refulsit alma parens", Virgil. Aeneid. 2. "Et paulo post, pallas insedit, nimbo effulgens".

light--is a figurative representation of the glory of the invisible God (Matthew 17:2; 1-Timothy 6:16). Its use in this connection may refer to the first work of creation (Genesis 1:3).
stretchest out the heavens--the visible heavens or sky which cover the earth as a curtain (Isaiah 40:12).

Light - With that first created light, which the psalmist fitly puts in the first place, as being the first of God's visible works.