From the period starting February 25th 2010 and ending July 9th 2011, 249 hostless transients or "orphans" were discovered in the PS1 Medium Deep fields. AN orphan is defined as an object that is >3.4" away from the centre of a catalogued galaxy or point source brighter than approximately 23.5m (in any of the gP1 rP1 iP1 filters that the transient was detected in). The PS1 observations are obtained through a set of five broadband filters, which we have designated as gP1, rP1, iP1, zP1, and yP1. Although the filter system for PS1 has much in common with that used in previous surveys, such as SDSS (Abazajian et al., 2009ApJS..182..543A), there are important differences. The gP1 filter extends 20nm redward of gSDSS, paying the price of 5577Å emission for greater sensitivity and lower systematics for photometric redshifts, and the zP1 filter is cut off at 930nm, giving it a different response than the detector response which defined zSDSS. SDSS has no corresponding yP1 filter. Further information on the passband shapes is described in Stubbs et al. (2010ApJS..191..376S). The PS1 photometric system and its response is covered in detailed in Tonry et al. (2012ApJ...750...99T, Cat. J/ApJ/750/99). Photometry is in the "natural" PS1 system, m=-2.5log(flux)+m', with a single zeropoint adjustment m' made in each band to conform to the AB magnitude scale. (8 data files). <P /
