Illustration of the four-point dither subtraction to remove the background. Two pairs of the largest separated dithers (dither 1–2, dither 2–1; and dither 3–4, dither 4–3), are subtracted from one another to remove the background. This leaves a clean positive PSF and a negative one. Due to the size of the PSF in channel 4, the negative and positive signal overlap, and therefore a smaller aperture is used to extract the spectrum.