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Acta Polytechnica CTU Proceedings 1(1): 311–315, 2014

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doi: 10.14311/APP.2014.01.0311

SiFAP: A New Fast Astronomical Photometer

Filippo Ambrosino1, Franco Meddi1, Roberto Nesci 2, Corinne Rossi1, Silvia Sclavi1, Ivan
Bruni3

1Dipartimento di Fisica Università La Sapienza, P.le A. Moro 5, 00185 Roma, Italy,
2INAF-IAPS, Tor Vergata, Roma, Italy,
3Istituto Nazionale di Astrofisica, Osservatorio Astronomico, Via Ranzani 1, 40127 Bologna, Italy.

Corresponding author: filippo.ambrosino@roma1.infn.it

Abstract

A fast photometer based on SiPM technology was developed and tested at the University of Rome “La Sapienza” and at

the Bologna Observatory. In this paper we present the improvements applied to our instrument, concerning new cooled

sensors, a new version of the electronics and an upgraded control timing software.

Keywords: instrumentation: photometer - stars: pulsars.

1 Instrument Description

The general description of our 3-channel photometer
SiFAP (Silicon Fast Astronomical Photometer) is pre-
sented in Meddi et al. (2011; 2012). We only remind
here that each channel is dedicated to a different target:
variable source (channel 0), nearby sky (channel 1) and
reference star (channel 2). A GPS receiver sends a Pulse
Per Second (PPS) to a micro Processor (µP) which in
turn produces a signal that we call Gated PPS. This
last is used both to drive two LEDs in order to have an
optical temporal marker and to synchronize our custom
electronics.

During the last year we replaced the old Hamamatsu
Multi Pixel Photon Counter (MPPC) modules with new
ones, having the sensor cooled by a built-in Peltier cell
1; we modified our electronics and to increase the unin-
terrupted acquisition time and to reduce the sampling
gate duration from 0.55 ms down to 0.1 ms. The new
block diagram of the instrument is shown in Figure 1.

The Thermo-Electric cooled system allows to reach
a fixed working temperature of -10 ◦C, with a large
reduction of the mean dark count. The S/N ratio is im-

proved by a factor 3 with respect to the previous one.
The modules have limited geometrical dimensions and
low weight so they can be directly located at the exit
pupil of the telescope. Other characteristics of the new
sensors are similar to the old ones: pixel size of 50 µm
for a total active area of 1 mm2 and Photon Detection
Efficiency with a maximum value of about 50% at wave-
length of 440 nm2.

The built-in electronics of each Hamamatsu mod-
ule can generate three types of output: pulse count
via USB interface already processed in time windows of
1 ms, analog and discriminated. We use the last one to
feed our electronics which uses a new protocol for data
exchange between a Field Programmable Gate Array
and a µP and a new data storage support. The inte-
gration time windows for this output are now 0.1 ms.
We called this electronics P3E, which stands for Pulsar
Pulse Period Extractor.

The pointing and the signal maximization proce-
dures and the fast pre-analysis to check the pulsating
behavior of the pointed source are described in Meddi
et al. (2012).

1 http://www.hamamatsu.com/resources/pdf/ssd/c11208 series kacc1176e03.pdf
2http://www.hamamatsu.com/resources/pdf/ssd/s11028 series kapd1026e04.pdf

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Filippo Ambrosino et al.

Figure 1: General block diagram of SiFAP mounted at the telescope. The GPS antenna is located outside
the dome. Hamamatsu MPPC sensors are integrated in their modules, P3E units are fed by the discriminated
output. Each couple of MPPC sensor and P3E unit is dedicated to a different target: variable source (MPPC0
and P3E0), nearby sky (MPPC1 and P3E1) and a reference star (MPPC2 and P3E2).

Figure 2: FFT applied to the whole raw data acquired by P3E0 on 2012, December 19.

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SiFAP: A New Fast Astronomical Photometer

Figure 3: Fundamental frequency and period values of the Crab pulsar signal obtained by our data with applied
the heliocentric corrections compared with those computed from JBO ephemeris. The uncertainties on the JBO
values are due to a numerical interpolation procedure. The quoted errors on our data are statistical only.

Figure 4: Crab pulsar light curve folded by the Xronos task efold for P3E0 (2012, December 19) corrected
data.

2 Data Analysis and Results

On 2012 December 19 we observed the Crab pulsar
with the Loiano telescope, for one hour. To opti-
mize the pointing of the target, we performed a quick
pre-processing analysis on a short acquisition of the

data (∼ 100 s), based on autocorrelation and FFT tech-
niques. We then analyzed the whole data set with a
FFT analysis to estimate the spin period of the pul-
sar. In Figure 2 we show the amplitude spectrum of
the Crab pulsar signal obtained from data collected

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Filippo Ambrosino et al.

by P3E0. We used the Xronos package of the High
Energy Astrophysics Science Archive Research Center
(HEASARC) to apply the heliocentric correction (task
earth2sun) and to determine the best fitting period for
the Crab pulsar signal (task efsearch).

In Figure 3 we compare the Jodrell Bank Observa-
tory (JBO) ephemeris3 prediction for the fundamental
frequency and period of the Crab pulsar signal with
the estimates of the same quantities obtained by our
barycentered data. The agreement between the two re-
sults is within 300 µHz for the fundamental frequency
and 300 ns for the spin period. Such discrepancy can
not be justified without taking into account systematic
effects, which are explained in terms of the clock drift
which depends on the temperature. This drift is esti-
mated by measuring the time interval between the two
optical markers generated by the Gated PPS signal, the
first at the beginning and the second one at the end of
the acquisition. Taking into account such effect, the
differences for the spin period are reduced below 15 ns.

Systematic uncertainties are due to i) the propa-
gation of the Gated PPS signal trough the cables and
ii) the rising edge of LEDs. Such systematic uncertain-
ties produce a total delay of about 320 ns. Finally, the
accuracy on the pulse output time of the PPS signal
which is ±0.001 ms at the rising edge of the pulse itself.
All these effects are not included in the numerical com-
putation of the uncertainty on the period obtained by
applying the barycentric correction.

The Crab light curves were obtained by using the
task efold on the barycentered data; the result for P3E0
is shown in Figure 4. The shape of the primary and the
second peak, and their flux ratio, are in good agree-
ment with the literature data (e.g. Golden et al., 2000;
Zampieri et al., 2011).

The sky and reference star measurements were pro-
cessed in the same way as the ones belonging to the
target to look for spurious signals which might interfere
with the astronomical signal from the Crab nebula: no
evidence of periodicity was detected.

3 Conclusions

We built a fast photometer able to collect data of pe-
riodic signals with high time accuracy integrating in
time windows down to 0.1 ms. With the Loiano 1.5 m
telescope we derived the period and a high S/N light
curve of the Crab pulsar with a measurement duration
of about 1 hour. In these conditions we obtained a fair
agreement with the fundamental frequency and the spin
period calculated from the JBO database.

We intend to upgrade our custom electronics aiming
at reaching shorter time sampling keeping a good S/N
ratio. Our final goal would be to compare fast optical

measurements with the γ-ray, X-ray, IR and RADIO
ones to explore more deeply the structure and the phe-
nomena occurring in the emitting regions of pulsars. In
particular, we want to study how the spin period slows
down (dissipative process) and the amount of the even-
tual phase delay among the peaks in the various band
(phase shifting process).

We need to collect high quality data with a 0.001 ms
time windows to be able to perform these kind of inves-
tigations. To this purpose we modified our GPS system
to reach a more accurate determination of the absolute
time scale (within 100 ns) by using a burst of n Gated
PPS signals instead of the single one presently used.

We started with theoretical computation of the min-
imum possible time sampling for a V ∼ 16 mag object
as a function of the telescope diameter. The results in-
dicate that it could be possible to measure objects of
the same magnitude as the Crab pulsar with similar
S/N ratio in time windows shorter than 0.1 ms. For in-
stance, with a 5 m telescope the integration time could
be reduced down to 0.01 ms.

Moreover it is mandatory to have larger telescopes
in order to have enough detectable photons and higher
resolution on the absolute timing by adopting a mili-
tary class GPS antenna which would be able to reach
at least 1 ns accuracy.

In this case it would be necessary to upgrade the op-
tical time marker by substituting standard LEDs with
LASER ones.

Acknowledgement

The authors thank the Bologna Observatory for the lo-
gistic support and the technical assistance during the
observations. The Universita’ di Roma “La Sapienza”
supported the project. This research has made use of
the SIMBAD database operated at CDS, Strasbourg,
France.

References

[1] Golden A., Shearer A., Redfern R. M., Beskin G.
M., Neizvestny S. I. et al., 2000, A&A, 363, 617

[2] Meddi F., Ambrosino F., Rossi C., Nesci R., Sclavi
S. et al., 2011, Acta Polytechnica, 51, N.6, 42.

[3] Meddi F., Ambrosino F., Rossi C., Nesci
R., Sclavi S. et al., 2012, PASP, 124, 448.
doi:10.1086/665925

[4] Zampieri L., Germana C., Barbieri C., Naletto G.,
Cade A. et al., 2011, Adv. Space Res., 47, 365.
doi:10.1016/j.asr.2010.07.016

3http://www.jb.man.ac.uk/pulsar/crab.html

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SiFAP: A New Fast Astronomical Photometer

DISCUSSION

B. ASCHENBACH: How do you explain that the
second harmonic in the amplitude spectrum is higher
than the first one?

F. AMBROSINO: The amplitude of the single har-
monic depends on the shape of the light curve; In the
case of the Crab pulsar in one period there are two emis-
sion peaks of different intensity and with a phase shift
of about 0.45.

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	Instrument Description 
	Data Analysis and Results 
	Conclusions