Swift's Ultraviolet/Optical Telescope (UVOT)For details of UVOT Data Analysis, see our Data Analysis page: Swift Documentation section.
UVOT RationaleGround observations of GRBs have shown that optical afterglows typically decline in brightness as t -1.1 to t -2.1. Therefore, rapid response is required to observe these counterparts and determine their redshift while they are still bright. The UVOT is uniquely capable for afterglow studies. It has UV capability which is not possible from the ground. It cannot be clouded out. It is also much more sensitive than any other quick reaction telescope. The UVOT also enables optimal ground based observations by providing rapid optical images of the GRB field so that any optical or IR counterpart can be quickly identified and studied. Stars in the FOV of the UVOT will provide an astrometric grid for the GRB field.
Table 1. A basic description of the UVOT specifications.
|Detector Operation||Photon counting|
|Field of View||17 x 17 arcmin|
|Detection Element||2048 x 2048 pixels|
|Telescope PSF||2.0 arcsec @ 350 nm|
|Wavelength Range||170-650 nm|
|Sensitivity||B = 24 in white light in 1000 s|
|Pixel Scale||0.48 arcsec|
|λ / dλ (grism)||~350|
|Brightness limit||V = 7.4 mag|
|Camera Speed||11 ms|
UVOT GRB ObservationsThe UltraViolet and Optical Telescope (UVOT) is a diffraction-limited 30 cm (12" aperture) Ritchey-Chretien reflector, sensitive to magnitude 24 in a 17 minute exposure. Table 1 and Fig. 1 describe the basic specifications and layout of the instrument.
Figure 1. A schematic of the UVOT layout, which is a 30 cm Ritchie telescope. The path of light through the telescope is denoted by arrows. First the light travels through an open door down the baffle until it is redirected into the detector. The long baffle reduces stray light background. The detector sits in front of the detector power supply between two filter wheels. Behind the detector the processing electronics are housed in front of the telescopes power supply, which is connected to two separate Digital Electronics Modules, one prime the other redundant.
Immediately after a GRB is detected and located by the BAT, the spacecraft will slew to point both the UVOT and the XRT at the GRB location. The spacecraft's 20–70 second time-to-target means that about ~ 100 GRBs per year (about 1/3 of the total) will be observed by the narrow field instruments during the gamma ray emission. As of the end of January, 1999, only one GRB has been successfully detected optically during the burst—GRB990123 which reached magnitude 9.
When a new GRB is acquired by the spacecraft, the UVOT will go through a predetermined program of exposure times and filter combinations. The initial images will be immediately sent to the ground for use as a finding chart by ground-based observers, and for comparison to archival observations of the same patch of sky to detect a variable source that could be the optical counterpart. The filtered observations will give the temporal behavior as a function of wavelength. If the GRB is at a distance greater than z ~ 1, the filtered observations will also measure the redshift of the GRB.
Figure 2. Here is an optical image of a GRB afterglow, two days after it went off. This observation was made using a large ground-based telescope (the 4.2 meter William Herschel Telescope, observation by Paul Groot) when it was around magnitude 20. This image is cropped to 2 arcminutes (compared to 17 arcmin for the UVOT FOV). The size of the 5 arcsecond diameter position determination from the XRT is shown as a green circle. The UVOT will be able to determine the location of any afterglow it sees to an accuracy of a few tenths of an arcsecond.
UVOT Technical DescriptionThe UVOT design is illustrated in Figure 1. It is a 30 cm diameter modified Ritchey-Chrétien telescope with an f/2.0 primary that is re-imaged to f/13 by the secondary. This results in pixels that are 0.5 arcsec over 17 arcmin square FOV. The filter wheel includes a 4x magnifier that results in 0.12 arcsec pixels for near diffraction limited imaging. The optics used for Swift UVOT are the flight spares from the XMM-Newton Optical Monitor (OM). The telescope structure, baffle, and thermal designs are also from the XMM-Newton project.
Figure 3. UVOT Filter wheel assembly with detector.
The detectors are copies of two micro-channel plate intensified CCD (MIC) detectors from the XMM-Newton OM design. They are photon counting devices capable of detecting very low signal levels, allowing the UVOT to detect faint objects over 170-650 nm. The design is able to operate in a photon counting mode, unaffected by CCD read noise and cosmic ray events on the CCD. The UVOT can autonomously determine the spacecraft drift using guide stars in the FOV. The UVOT design includes the XMM-Newton OM 11 position filter wheel in front of the detectors. The two grisms can be used to obtain low resolution spectra of the brightest bursts with mB< 17.
UVOT OperationOnce Swift has slewed to a new burst, the UVOT acquires a 100 s exposure of the target field. The 2x2 arcmin portion of the frame surrounding the XRT GRB position is compressed and telemetered to the ground within 50 s. The Swift Operations Center will automatically post this image to the GCN. During subsequent ground contacts, the full frame of the finding chart image is telemetered, as well as the observing sequence of image and event data for the suite of imaging and grism filters in the wheel. Images will generally contain at least 15 serendipitous stars listed in existing astrometric catalogs, allowing sub-arcsec positioning. Unlike the XMM-Newton OM, the UVOT will autonomously reduce the high-voltage on the MCPs when bright (mB <10) stars or Earthlight (25° of the limb) are in the FOV. This change removes the XMM-Newton requirement of preplanning UVOT observations. The UVOT can use a fast mode to produce high temporal resolution light curves (10 msec resolution).
UVOT PerformanceThe UVOT will detect a mB = 24 point source in 1000 s using the open filter. A comparable 30 cm ground-based telescope is limited to 20th mag due to sky brightness and seeing. Observing from space, the UVOT has very low sky brightness, better spatial resolution and a zero read-noise detector, making it competitive with a 4 m ground-based telescope. We derive the UVOT sensitivity limit by using the transmission and quantum efficiencies measured for the XMM-Newton flight model (see table below). Given an mB = 24 source with a spectrum like an A0 star, the signal-to-noise ratio is 4.3 in 1000 s. Coincidence losses will start to degrade performance at count rates of ~ 10 cts/s/pixel.
Table 2. UVOT sensitivity limits, based on the most recent (pre-launch) calibration products from MSSL. The counts are for an AOV star (Vega).
For V = 20, in 1000 s get:
|Sensitivity to Ly-alpha cutoff|
|UVM1-UVW2||z ~ 1.5|
|UVW1-UVM1||z ~ 2|
|U-UVW1||z ~ 2.7|
|B-U||z ~ 3.5|
UVOT effective areas are approximated, below, assuming manufacturer's specifications or, in the case of the grisms, an identical spectral distribution to the well-calibrated Optical Monitor (OM) grisms on-board XMM-Newton (see the OM calibration status). The overall normalization has been adjusted to the expected UVOT performance. Note that the UVOT UV sensitivity is expected to be a significant improvement over the OM sensitivity. Note that second-order structure in the curves are governed by cubic spline interpolation over a small number of provided data points. Effective area sampling and uncertainties will be improved using ground calibrations and spectrophotometic standards once in orbit.
Figure 4. Approximated UVOT broad band filter effective areas.
Figure 5. Approximated UVOT optical (VGRISM) and UV (UGRISM) grism effective areas. This figure, which was based on ground calibration, has been removed because flight calibration has shown the U grism sensitivity to differ substantially from the ground calibration result. A grism effective area plot based on flight data will be placed here as soon as it is available.
Further detail on the development and performance of the UVOT is available from the instrument teams:
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