The Galactic Astigmatism: Constraining the Milky Way Dark Matter Halo Using Ultra-Weak Lensing
An astigmatism is defined as an asymmetry of the front of the eye, causing light to not focus on a single point. Because the Sun is located ~8kpc off-center in the Milky Way, the integrated mass density of galactic dark matter varies significantly along different lines of sight. As a result, the Milky Way's own DM halo will act as a gravitational lens, magnifying background sources more along lines of sight toward the bulge, and less towards the anti-center. While such lensing may be very weak, it is decidedly non-zero. This ultra-weak lensing may be detected in the number density or apparent brightness of (for example) distant galaxies.
I note also, this effect has been mentioned only once in the literature that I am aware of, by Xu and Wu (1987), who speculate the brightness amplification signal may as large as 1e-6 to 1e-5, and possibly detectable in precision CMB data.
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