Parallaxes of stars are sometimes measured relative to the positions of galaxies or distant objects called quasars. why is this a good technique?

quasars are massive celestrial bodies that are far or remotely placed emitting large energy radiations.

parallax is the best way of determining distances between astronomical bodies and parallaxes of stars are measured relative to a quasar to minimise the measurement error. each observations is stimulated with the basic angle variations leading to minimum uncertainty in the results as the assumptions in the measurements are as minimal as possible.

in this image, you can see solar radiation going through ocean water.

no image, but probably absorption