The imagery in "A Valediction: Forbidding Mourning" serves to create what sort of tone for the poem?

Throughout Donne's poem "A Valediction: Forbidding
Mourning" the speaker attempts to comfort his beloved who is upset about their impending
separation.  The complex imagery helps convey this tone of consolation, as the speaker
assures his beloved that their separation is only physical; it is not spiritual.  He
uses several images of constancy and unity to convey this idea and this tone.  He
compares their love to the heavenly bodies moving around the sun--"trepidation of the
spheres"-- explaining that such a movement is "innocent" because it is constant and
circular. He will return to her just as the heavenly bodies continue their
orbit.

He later compares their separation to gold that can
be beaten to "airy thinness." Their separation is not a "breach but an expansion."  In
other words, gold is so malleable that it can be stretched quite thinly without
breaking.  This image is quite comforting, for it shows how a separation need not be a
permanent break, that the two can be connected over a wide expanse of time and
space.

Lastly, Donne uses the image of the compass to
represent the idea of  spiritual unity (the compass is joined at the top) even though
the two legs are separate, just as the two lovers are separate on
earth.

All these images combine to show that since the two
lovers' love is spiritual, it can withstand a temporary physical separation.  The images
used to depict this idea are quite reassuring in that they are heavenly, precious, and
united.