tokens: Sky Survey (Skrutskie et al. 2006 ) with an overall accuracy of 012–015. To account for the measured absolute offsets of ∼023 and ∼057 in the NUV and FUV image astrometry, respectively, we used a grid of reference stars detected in a ∼12′′ radius around PSR B0540−69 to register the MAMA images onto the astrometry reference frame of the WFPC2 images with an accuracy of better than 001. We used the NUV and FUV images with the recalibrated astrometry as a reference for the pulsar identification. 3. Results 3.1. Imaging and Photometry Figure 1 shows the NUV- and FUV-MAMA images of the PSR B0540−69 field obtained after the processing described in the previous section. We clearly detected PSR B0540−69 in both the NUV and FUV images at a position coincident with its optical coordinates computed by Mignani et al. ( 2010b ): (0009), (005). 21 21 The errors refer to the average of the pulsar coordinates computed on four independent WFPC2 data sets, see Mignani et al. ( 2010b ) for details. Ours is the first detection ever of PSR B0540−69 in the UV, which also makes it the fifth isolated pulsar, among the ∼2300 known, that has been detected in the near-IR, optical, UV, X-rays, and γ -rays, after the Crab and Vela pulsars, PSR B0656+14, and Geminga (the fourth among radio pulsars 22 22 Geminga (PSR J0633+1746) has not yet been unambiguously detected as a radio pulsar despite many searches, see Maan ( 2015 ) and references therein. ). We also detected the PSR B0540−69 PWN in our NUV image (Figure 1 , left), with a structure and extent similar to what is observed in the optical and near-IR (Mignani et al. 2010b , 2012 ). The PWN is at most barely visible, however, in the FUV image (Figure 1 , right). The bright emission knot in the PWN detected at ∼17 southwest of the pulsar in HST /WFPC2 images (De Luca et al. 2007 ) is also visible in the STIS/NUV-MAMA observation, aligned with the major axis of the PWN. Since our paper is focused on the pulsar, a coherent multi-wavelength spectral and spatial analysis of the PWN and its features will be the subject of a subsequent paper. Figure 1. STIS NUV-MAMA (left) and FUV-MAMA (right) images (9650 s each) of the PSR B0540−69 field taken with the F25QTZ filters. The images are aligned in R.A. and decl., with north to the top and east to the left. The image size is 9′′ × 9′′. The pulsar is the source at the center of the nebula, clearly visible in the NUV-MAMA image. The intensity scale of the two images (in counts) is color-coded in the two horizontal bars at the bottom of each image. In both cases, the minimum/maximum of the scale have been adjusted to favor the identification of the pulsar and other stars in the field. For a better visualization, both images have been smoothed with a Gaussian function using a kernel of 2 pixel radius. For both the NUV and FUV images, we computed the pulsar fluxes through aperture photometry employing the tools in the IRAF 23 23 IRAF is distributed by the National Optical Astronomy
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