4.1. WFPC2 observations and astrometry
The WFPC2 observations used to study 53W002 at the highest possible resolution were the same as those discussed in Section 2.1.1, except that 53W002 itself was put inside the PC. The PC noise is largely limited by read-noise and dark-current, and large-scale gradients in the flat-fielded images are 3% of sky (cf. W94a, Windhorst et al. 1998; W98). For the interpretation below, one needs to know the exact location in the PC-images of 53W002's highest-resolution VLA source, which has 0".15 FWHM (W91). W98 show that, after correcting the WFPC2 positions for a small systematic VLAFK5-HSTGSSS offset, the high resolution VLA 8.44 GHz (W91) and 15 GHz (S97; S97) images coincide with 53W002's optical center - and likely its AGN (Section 4.2.1) - to within 3 PC pixels.