bvit: an imaging, photon counter for high time resolution astronomy on salt

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

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Southern African Large Telescope

J. McPhate ([email protected])

• 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 µm/acrsec. Good seeing/focus is about 1 arcsec.

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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.


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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.


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Berkeley Visible Imaging Tube


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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 night’s data is downloaded to a hard drive in the SALT control room via

BVIT’s 1 GB Ethernet connection.


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BVIT Instrument


BVIT CAD Model

Amplifier

HVPS

TDCs

LVPS

Tube, Iris, FWs, Shutter

BVIT on the bench post upgradeTube 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 - 2°C, to prevent condensation.

Tube background rate is temperature dependent, so the internal cooling lines go to tube heat sink first, then the CPU.

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BVIT Detector

• 25 mm active area, drop face sealed tube (~2 arcmin FOV at SALT plate scale)

• Z-stack of 10 µm 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


BVIT Upgrade Detector: Custom Photonis/DEP tube with UCB-SSL XDL anode and S25 photocathode

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BVIT Tube QE and Filter Throughputs


2500 3500 4500 5500 6500 7500 8500 95000.00

0.25

0.50

0.75

1.00

0%

5%

10%

15%

20%

BVIT QE and Color Filter Transmissions

B Band

V Band

R Band

H-alpha

BVIT QE

Wavelength (Å)

Filte

r Tra

nsm

issi

on

Qua

ntum

Effi

cien

cy

Red: Original BVIT tube QE (S20). Bkg ~ 50 cpsGreen: New BVIT tube QE (S25). Bkg ~ 10 kcpsBlue: 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.

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BVIT Throughput (cont)Filter m = 15 m = 18

B - band 57 kcps 3.6 kcps

V - band 35 kcps 2.2 kcps

R - band 28 kcps 1.8 kcps

H-alpha 1.6 kcps 100 cps

Open 150 kcps 10 kcps


Expected count rates for 15th and 18th 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 µsec bin.

While the detector and electronics can run at >2 MHz, the global count rate is limited to ~1 MHz by the computer’s ability to write data to disk.

Global sky background rate can be choked back without source rate reduction by stopping down the iris.

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BVIT Detector Spatial Resolution


Detector spatial resolution at nominal gain (about 5e6) is approximately 70 µm.

This is well below the required resolution for operation on SALT (220 µm/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 PinholeResolution

Hotspot Resolution

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User Interfaces


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).

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UZ For – January 2009(AM Her-type CV)


Light curve for the 18th 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.

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CN Leo – January 2009M-type Flare Star


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.

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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


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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.


bvit: an imaging, photon counter for high time resolution astronomy on salt

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