Redshift distribution of the DP galaxy sample (M20) (top panel) and the conversion curve between redshift and the fibre diameter in kpc of the SDSS 3″ (red curve in the bottom panel). We mark with blue dashed line the three representative redshifts and corresponding fibre diameters: z = 0.05, z = 0.1, and z = 0.17 corresponding to a fibre diameter of 3, 6, and 10 kpc, respectively.