  Context: We describe the new SEPIA (Swedish-ESO PI Instrument for APEX)
receiver, which was designed and built by the Group for Advanced Receiver
Development (GARD), at Onsala Space Observatory (OSO) in collaboration with
ESO. It was installed and commissioned at the APEX telescope during 2015 with
an ALMA Band 5 receiver channel and updated with a new frequency channel (ALMA
Band 9) in February 2016. Aims: This manuscript aims to provide, for observers
who use the SEPIA receiver, a reference in terms of the hardware description,
optics and performance as well as the commissioning results. Methods: Out of
three available receiver cartridge positions in SEPIA, the two current
frequency channels, corresponding to ALMA Band 5, the RF band 158--211 GHz, and
Band 9, the RF band 600--722 GHz, provide state-of-the-art dual polarization
receivers. The Band 5 frequency channel uses 2SB SIS mixers with an average SSB
noise temperature around 45K with IF (intermediate frequency) band 4--8 GHz for
each sideband providing total 4x4 GHz IF band. The Band 9 frequency channel
uses DSB SIS mixers with a noise temperature of 75--125K with IF band 4--12 GHz
for each polarization. Results: Both current SEPIA receiver channels are
available to all APEX observers.
