Set-up of our ALFOSC spectroscopic observation. The four lensed quasar images are indicated from A to D, starting from the brightest image A with i = 15.40 mag. The dashed rectangle shows the 1.0 arcsec slit. With a fixed slit position angle of 90 degrees, we can obtain spectra for the three brighter lensed quasar images in one shot. The underlaying image is from Pan-STARRS i-band observation (Chambers et al. 2016), with a size of 10 × 10 arcsec² with north up and east to the left.