Download SWIFT UVOT USERS GUIDE - Mullard Space Science Laboratory

Swift User Manual
1.1.3.4
Swift-UVOT-302-R03
7
Time
The ICU has two 1pps inputs: A and B. The two 1pps signals are both on, but a software-controlled switch in the
ICU selects between A and B. Interrupts are generated once per second in the ICU from the selected 1pps signal.
Onboard time is managed using two components: spacecraft clock and a UT correlation factor (UTCF). The
spacecraft clock is the spacecraft’s internal clock used for the majority of onboard functions (e.g., all CCSDS
secondary header time tags and management of stored command processing functions). The spacecraft clock is set
at initial power-on and is nominally run at 1 Hz rate without adjustments. The UTCF is a bias that is adjusted such
that the sum of the spacecraft clock with the UTCF yields a time that is as close as possible to UTC.
The spacecraft transmits time to all instruments. Spacecraft time and a Universal Time Correlation Factor are
transmitted over the 1553 bus once every second and are valid at the next one pulse per second.
1.2
Telescope Module
1.2.1
TMPSU
The telescope module power supply (TMPSU) converts the spacecraft power bus to power rails within the telescope
module. One set of rails powers the blue digital and analogue electronics and high voltages. The analogue
electronics, in turn, controls the high voltages and powers filter wheel fine sensor LED and flood LEDs. The other
set power the mechanisms and filter wheel coarse sensor. The integral ICB interface provides the channel for control
of the coarse sensor, the flood LED’s, the analogue and digital electronics and the return of current, high voltage and
fine sensor status values. Additionally the main s/c power, routed via the TMPSU, is used to drive the heaters.
1.2.2 Detector System
1.2.2.1
Overview
There are two detector assemblies. Each detector assembly consists of detector window that is slightly figured, a
S20 photocathode, three Micro-Channel Plates (MCPs), a phosphor screen, tapered fiber-optics, and a CCD (see
Figure 6. The photocathode is optimized for the UV and blue wavelengths. Although there are three separate MCPs,
MCP2 and MCP3 are butted up against each other. The CCD has 385 x 288 pixels, 256 x 256 of which are usable
for science observations. Each pixel has a size of 4 x 4 arcsec2 on the sky affording a 17 x 17 arcmin2 FOV. The first
MCP pore sizes are 8 m with distances of 10 m between pore centers. The second and third MCPs have pore sizes
of 10 m with distances of 12 m between pore centers.
Photons arriving from the BSM enter the detector window and strike the photocathode. Electrons discharged from
the photocathode are then amplified by the first MCP creating an electron cloud. This electron cloud is further
amplified by the combined second and third MCPs creating a larger electron cloud. This larger electron cloud then
illuminates the phosphor screen. The photons created from the phosphor screen then travel to the CCD via the fiberoptics. This combination of MCPs and CCD provides an amplification of ~106 of the original signal. The registering
of photons is achieved by reading out the CCD at a high frame rate and calculating the photon splash’s position by
means of a centroiding algorithm. The centroiding algorithm also affords a large format to the CCD by sub sampling
each of the 256 x 256 CCD pixels into 8 x 8 virtual pixels, thus providing an array of 2048 x 2048 virtual pixels with
a size of 0.5 x 0.5 arcsec2 on the sky. Faint residuals of a pattern formed by creating the 8 x 8 virtual pixels are
removed by ground processing. Unlike most UV or optical telescopes, because of UVOT’s high frame CCD read out
rate, the UVOT can function in a photon-counting mode
As with all photon-counting detectors, there is a maximum count rate threshold. The frame rate of the UVOT
detectors is 10.8 ms for a full 17 x 17 arcmin2 frame; therefore, for count rates above ~10.8 counts/s, assuming a
point source, a count rate correction needs to be applied. A CCD dead time correction also needs to be applied
during the data processing. Because the local sensitivity of the photocathode can be permanently depressed, care
must be taken when observing bright objects. This is accomplished through autonomous operations diminishing the
time spent on these bright sources. The detector’s dark noise is extremely low (approximately 4x10-5 counts/s/pixel)
and can be ignored when compared to other sources of background noise.