BVIT: An imaging, photon counter for high time resolution
astronomy on SALT Jason McPhate, Oswald Siegmund, Barry Welsh, John
Vallerga, Doug Rogers (Univ. of California, Berkeley Space Sciences
Laboratory) David Buckley, Amanda Gulbis, and Janus Brink (South
African Astronomical Observatory SALT) Technology and
Instrumentation in Particle Physics, June 2011 Slide 2 Southern
African Large Telescope J. McPhate ([email protected])TIPP
20112 Near copy of Hobby-Eberly Telescope at McDonald Observatory
in Texas. Located at 32.4 S, 20.8 E, 1783 m near Sutherland, South
Africa. Effective 10m diameter multi-segmented, spherical primary
mirror (91 x 1m hexagons). Telescope has a fixed zenith angle of
37, and does not move during target tracking. Scientific
instruments are housed in a prime focus payload that moves to track
targets. Plate scale at BVIT focus is 220 m/acrsec. Good
seeing/focus is about 1 arcsec. Slide 3 Berkeley Visible Imaging
Tube BVIT was installed in Auxiliary bay of the SALT prime focus
payload in January 2009. BVIT was designed to do high time
resolution astronomy (25ns precision) of variable objects At the
heart of BVIT is a visible sensitive, photon counting, imaging
sealed tube with time tagging electronics. Imaging capability
allows for simultaneous acquisition of target, sky background, and
field/comparison stars The photon counting nature allows BVIT data
to be corrected post- facto for a number of observing difficulties:
variable seeing/focus, tracking drift, telescope vignetting, etc.
Similarly, light curve time binning can be selected during post
processing to levels allowed by the source count rate. J. McPhate
([email protected])TIPP 20113 Slide 4 BVIT History Jan 09
Installed in situ in the SALT payload. Feb 09 Initial commissioning
and science observations. Mar 09 Second run of science
observations. Apr 09 thru Aug 10 SALT upgrades (improved spherical
aberration correction). Aug 10 BVIT upgraded with higher QE tube
and faster computer, while payload off telescope. Dec 10 BVIT
re-commissioning. Shutter problems prohibited full science
operations. May 11 Shutter and computer hard drive replaced. Jun 11
Re-re-commissioning and science observations, including coordinated
observations with RXTE. J. McPhate ([email protected])TIPP
20114 Slide 5 Berkeley Visible Imaging Tube J. McPhate
([email protected])TIPP 20115 Slide 6 BVIT Data Flow Light
enters the enclosure and passes through the shutter, 2 filter
wheels (color and ND), and an adjustable iris before reaching the
tube. Photons are converted to electrons by the photocathode,
amplified by the MCPs, and collected by the cross delay line anode.
Signals from the XDL anode are amplified and passed to two TDCs for
position encoding and time stamping. The TDCs accept input
alternately (ping-pong scheme) to reduce electronic deadtime. When
one TDC receives a count it passes control to the other TDC. The
TDCs calculate the X, Y position of each event, as well as the MCP
pulse size (P) and arrival time (T) in units of 25 ns clock cycles
since the last sync signal. An external sync signal resets the TDC
counters once per second to keep clock drift to a minimum. The X,
Y, P, and T for each count is passed to the onboard computer (via a
National Instruments DIO- 32HS acquisition card). The data
acquisition software uses a double buffer system to minimize data
loss while writing to disk. Data is stored on an onboard 500 GB
hard drive. Each count takes 10 bytes. Data rates to the hard drive
are limited to about 1.1 MHz. Above this rate, data is lost at
transitions between buffer writes. At the end of the night, all the
nights data is downloaded to a hard drive in the SALT control room
via BVITs 1 GB Ethernet connection. J. McPhate
([email protected])TIPP 20116 Slide 7 BVIT Instrument J.
McPhate ([email protected])TIPP 20117 BVIT CAD Model
Amplifier HVPS TDCs LVPS Tube, Iris, FWs, Shutter BVIT on the bench
post upgrade Tube heat sink ComputerDelay Cables Glycol cooling
lines Power consumption is ~64 watts, dominated by the TDCs,
computer, and LVPS. Glycol cooling is run at ambient - 2C, to
prevent condensation. Tube background rate is temperature
dependent, so the internal cooling lines go to tube heat sink
first, then the CPU. Slide 8 BVIT Detector 25 mm active area, drop
face sealed tube (~2 arcmin FOV at SALT plate scale) Z-stack of 10
m pore, 60:1 l/d MCPs. Cross delay line (XDL) anode readout.
Original tube (Jan 09) manufactured at UCB with blue S20
photocathode Upgrade tube (Aug 10) manufactured by Photonis/DEP,
S25 (SuperGen II) photocathode J. McPhate
([email protected])TIPP 20118 BVIT Upgrade Detector: Custom
Photonis/DEP tube with UCB-SSL XDL anode and S25 photocathode Slide
9 BVIT Tube QE and Filter Throughputs J. McPhate
([email protected])TIPP 20119 Red: Original BVIT tube QE
(S20). Bkg ~ 50 cps Green: New BVIT tube QE (S25). Bkg ~ 10 kcps
Blue: 2cn Photonis XDL S25 tube, selected against because of higher
background rate. Bkg ~ 60 kcps. There is also an open position on
the color filter wheel. ND filter selection: Open, 0.3, 1.0, 3.0,
4.0 Iris can be used to decrease the ratio of sky background counts
to source counts being written to disk. Slide 10 BVIT Throughput
(cont) Filterm = 15m = 18 B - band57 kcps3.6 kcps V - band35
kcps2.2 kcps R - band28 kcps1.8 kcps H-alpha1.6 kcps100 cps Open150
kcps10 kcps J. McPhate ([email protected])TIPP 201110
Expected count rates for 15 th and 18 th magnitude, flat spectral
sources. To avoid detector burn-in and excessive local gain sag,
sources brighter than ~100 kcps should not be observed without use
of special observing techniques (telescope defocus, lower tube high
voltage). This corresponds to a SNR of 3 in a 100 sec bin. While
the detector and electronics can run at >2 MHz, the global count
rate is limited to ~1 MHz by the computers ability to write data to
disk. Global sky background rate can be choked back without source
rate reduction by stopping down the iris. Slide 11 BVIT Detector
Spatial Resolution J. McPhate ([email protected])TIPP 201111
Detector spatial resolution at nominal gain (about 5e6) is
approximately 70 m. This is well below the required resolution for
operation on SALT (220 m/arcsec, ~1 arcsec seeing). Furthermore,
the resolution degrades gracefully, allowing operation at approx.
1/5 nominal gain. This will extend tube lifetime, and improve local
count rate limits by roughly 5X. At modal gain, the resolution is
dominated by photocathode to MCP gap charge spreading (indicated by
the better resolution demonstrated by a hotspot produced on the
MCP) Imaged Pinhole Resolution Hotspot Resolution Slide 12 User
Interfaces J. McPhate ([email protected])TIPP 201112
Hardware control GUI: LabView VI controls most of the components
via a small USB DIO card from National Instruments. Filter wheels
(Finger Lakes Instr.) have their own USB controllers. The software
has several safeguards built in. It monitors the HV current draw on
the tube. If this exceeds a programmable limit, the HV is turned
off. It will also turn off the HV is a hardware high count rate
limit is exceeded (about 3 MHz). Data acquisition GUI: Visual C
heritage software from the UCB group. This comes in two varieties.
The first gives a real-time display of the image being acquired
(shown above) and is useful for acquiring field verification
images. The second is a streamlined version that streams the data
to hard drive with minimal graphical update (no real-time image of
PHD). Slide 13 UZ For January 2009 (AM Her-type CV) J. McPhate
([email protected])TIPP 201113 Light curve for the 18 th
magnitude eclipsing binary cataclysmic variable system UZ For. Data
is in counts per 0.5 second bin. Material from the secondary,
M-type star in this system is accreting onto the primary white
dwarf, but only at the magnetic poles. The interesting two step
profiles of the eclipse ingress and egress are due to the nature of
how the hot accretion spots on the white dwarf are obscured by the
inclined orbit of the companion M-type star. The data acquired with
BVIT is at higher time resolution than previously measured on this
source and allows better characterization of these shoulder
features. Slide 14 CN Leo January 2009 M-type Flare Star J. McPhate
([email protected])TIPP 201114 Data is in counts per 1
second bin. We were fortunate to catch two small flares within a
single observation on this star. Temporal expansion of the larger
flare. This is the same data but now in 100 ms bin. Note the small
precursor and the significant structure in the main flare. Data
such as these are important for determining if micro-flares (such
as these) are important heating mechanisms for the coronae of stars
like our Sun. Resolving this question requires time resolution <
1 second. Slide 15 Future Plans Analysis of recently acquired data
using upgraded BVIT. Move towards making BVIT a user facility
instrument at SALT Improve user interface and integrate more
completely with data acquisition software. Provide a portable data
software pipeline for users (currently written in IDL). Possible
second generation (BVIT 2) instrument GaAsP photocathode (better
QE) Cross-strip anode readout (faster) More specifically designed
electronics (smaller, lower power) Better optical design (possibly
with grism or simultaneous multi-color capability) Faster write to
disk speeds J. McPhate ([email protected])TIPP 201115 Slide
16 Acknowledgements The authors are indebted to the engineering and
IT staff at SALT for many hours of support without which the
success of BVIT would not be possible. This includes (but is not
limited to) Ockert Strydom, Charl Du Plessis, Hamish Whittal, Simon
Fishley, Garith Dugmore, Eben Wiid, Peter Menzies, and Ardisha
Pancham. We would like to thank the following students and staff at
UC Berkeley who worked on the design of BVIT and subsequent data
processing: Rahul Barwani, Mike Quinones, Johathan Wheatley, Navid
Radnia, and David Anderson. This work was support by NSF grants
AST-0352980 and DBI- 0552-099 to the University of California,
Berkeley. J. McPhate ([email protected])TIPP 201116
