UltraViolet/Optical Telescope
The UltraViolet/Optical Telescope (UVOT) is the second of Swift's two narrow-field instruments. As with the XRT, once the BAT detects a gamma-ray burst (GRB), the spacecraft slews to bring the new GRB into the UVOT's field-of-view.
![UVOT installation onto the spacecraft](images/UVOT_install.jpg)
Technicians install the UVOT system on the spacecraft. (Credit: NASA)
How it Works
The UVOT's design is based on the Optical Monitor aboard XMM-Newton. It is a 30-cm reflecting telescope. Optical and ultraviolet light entering the UVOT is directed to one of two redundant CCD detectors, each with an 11-position filter wheel. These filters consist of a blocked position for detector safety; a white light filter; a field magnifier; two grisms; U, B, and V filters; two broadband UV filters centered on 180 and 260 nm; and a narrow UV filter centered on 220 nm. These filters allow observations of spectra in the blue or UV band (using the grisms) and of different colors.
![UVOT filter wheel and detector.](images/filterwheel.gif)
The UVOT filter wheels, with one in the front, one in the back. (Credit: NASA)
The UVOT further improves the BAT and XRT localizations, giving a position to 0.3 arcsecond accuracy. Upon acquiring the GRB, the UVOT performs a preprogrammed series of exposures and filter combinations. This program can be altered during Swift's mission to optimize the optical/UV observations. The filtered observations reveal the behavior of the burst and afterglow over time in different colors. In addition, if the burst is at a redshift greater than one, these observations may also provide redshift measurements.