Top panel: CFHT/AOB-KIR mosaicked image of the central part of SNR G21.5-0.9 taken in K′ band, smoothed with 2-dimensional Gaussian with a FWHM = 0.11′′. Bottom panel: the same sky region as in the top panel imaged with VLT/ISAAC in K_s band. The image shown is the averaged total intensity I (see Sect. 2.2). For both panels a star depicts the position of PSR J1833-1034, while the extent of the compact core seen with Chandra HRC (Camilo et al. 2006) is shown with contours. The observations taken with CFHT/AOB-KIR have higher resolution than the ones obtained with VLT/ISAAC, which has direct implications for a flux determination of an extended emission structure visible around the pulsar position (see Sect. 3).