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Hi, a Gaia newbie here. I want to use the optimal photometry tool on a set of HST/WFC3 images. The WFC3 handbook provides two sets of zeropoints: for a source extraction radius of 0.4 arcsec (= 10.1 pixels) and for an infinite radius. In practice, when doing aperture photometry, I would always measure the net counts within 0.4 arcsec, with the corresponding zeropoint. But what do I use with Gaia's optimal photometry? As a first guess, I would define a PSF with clipping radius = 0.4 arcsec, associated with the 0.4-arcsec zeropoint, and calculate results for the object. But is this correct? Or does the optimal photometry result already give me an (extrapolated) "infinite aperture" value (regardless of my choice of clipping radius) and therefore I should always use the infinite-aperture zeropoint? Thanks for any clarifications, roberto soria ICRAR ---- Starlink User Support list For list configuration, including subscribing to and unsubscribing from the list, see https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=STARLINK