Swift UVOT On-Orbit Calibration

The UV/Optical telescope (UVOT) aboard the Swift Gamma-ray Burst Observatory is a 30-cm aperture f/12.7 telescope with a wide (17{'} × 17{'}) field-of-view, and a micro-channel plate detector capable of detecting photons in the range from 170 to 600 nm. The instrument can be used for either photometry or spectroscopy, and carries a white light filter, a 4× field magnifier, UV and optical grisms, 3 broadband optical filters (UBV), two broadband UV filters (centered at 180 nm and 260 nm), and a narrow UV filter centered at 220 nm. In white light, the UVOT has a 5σ sensitivity to a limiting magnitude of B=24 in 1000s. In its grism mode, the instrument will be able to perform low-resolution (R ∼ 300) spectroscopy on targets down to B ∼ 17. The principal uses of UVOT will be to obtain accurate (0{''}-7.6mu. 3) positions for the gamma-ray bursts, detect the early ( ∼ 100) optical afterglows, monitor the photometric decline of their afterglows, and obtain spectroscopic and photometric redshifts for gamma-ray bursts.

We discuss the plans and status of the on-orbit calibration of the telescope, including measurements of the image quality, field distortion, photometric and spectroscopic response, and flat-field response.

This work is sponsored at Penn State by NASA's Office of Space Science through contract NAS5-00136 and at MSSL by funding from PPARC.