Download A. Čadež, B. Dintinjana, A. Lautar, D. Paradi , D. Ponikvar

Ljubljana fiber fed fast
photometer
Andrej Čadež
Bojan Dintinjana
Anja Lautar
Dejan Paradiž
Dušan Ponikvar
Bled, March.2008
Steps toward timing the Crab
pulsar - hardware
Telescope
with robotic
guiding
system
Offaxis
fiber
pickup
in the
image
plane
Photon
counting
detector
Event
time
tagging
device
(CAEN)
GPS
clock
The
inevitable
computer
Optics
Focal plane optics
Pointing errors (error-signal)
Focusing on the fiber with a flat
mirror
A rough
focus is
obtained,
sufficient
to start
finding
fiber
position
with
respect to
CCD field
of view.
Guiding on the fiber
1: find the star in the field of the camera
2: choose it and by clicking it, send it to initial position (still
in the field of camera)
3: check initial position and send it to the fiber – a new blue
image indicates the field of view with star on fiber
4: new picture is taken in blue – the expected image turns
red
5: choose blue and corresponding red stars to calculate
corrections to guide the telescope
Fine tuning:
• Autocollimator focus needs to be fine tuned on stars. This is done by
scanning the signal with respect to the position of guiding stars on
the CCD image. After the centroid is found, the telescope focusing is
adjusted to highest fiber signal. The new position of the fiber plane is
calculated and the fiber plane repositioned to align the fiber focal
plane with CCD focal plane. Focusing is completed when the ratio
star count rate/sky count rate is the same as in the CCD image.
• Image distortion must be taken into account when positions of off
axis guide stars are used with respect to positions of guide stars in
the original image with the object in the field of view. Image
distortion moves the position of a star by up to 10arc seconds at the
edge of the field (15arc min off axis). Image distortion errors were
handled in two steps: 1) CCD was displaced off axis to bring fiber
position closer to optical axis ; 2) Focal plane field distortion is
determined by comparing image and catalogue position data using
Fitsblink software.
Focusing on fiber – fiber plane scan
Scan - focusing
• Field
distortion
Field distortion
correction
10th magnitude star signal
count rate Hz
500
400
300
200
100
time s
0
200
400
600
800
1000
1200
First Crab observation (March 2nd 2008)
(before field distortion correction)
174000
Auto - correlation
172000
170000
168000
166000
10
100
time delay [ms]
Conclusions
• The Ljubljana fiber fast photometer performs as expected: the SPAD
count rate is comparable to the photon count rate determined from
CCD images, thus no appreciable light losses in the fiber have been
detected. A need to compare the signal with Asiago.
• Telescope pointing errors are appr. 0.3 arc sec rms with autoguding
correction arriving every 30sec. Some excursions up to 1.5 arc
seconds, are due to loss of autoguider correction signal. Field
distortion correction is expected to fix the problem.
• CAEN electronics performs as specified, the maximum count rate is
not a limiting factor for stars fainter then 8th magnitude (for VEGA).
• Outstanding problems: the 50mm fiber has an 1.7 arc seconds
diameter receiving area. This area must be increased to 3 arc
seconds diameter (100mm) . Optical coupling to a larger fiber was
tested, but losses were unacceptable. Other solutions under
consideration: a) tapered fiber coupling b) focal reducer lens before
the fiber c) use a SPAD with a 100mm fiber input.
Observing Crab with a fast photometer
– why it might be interesting?
Karpov et al., Astrophys Space Sci 2007
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it is the brightest pulsar seen in optical, it is nearby and young
one of the main properties of the Crab emission is the very high stability
of its optical pulse shape despite the secular decrease of the luminosity,
related to the spin rate decrease (Pacini 1971; Nasuti et al. 1996)
at the same time pulsars in general and Crab itself are unstable
it has been found early that the variations of the Crab optical light
curve, in contrast with the radio ones, are governed by Poissonian
statistics (Kristian et al. 1970)
a number of observations show the absence of non-stationary effects in
the structure, intensity and the duration of the Crab optical pulses, and
the restrictions on the regular and stochastic fine structure of its pulse
on the time scales from 3 μs to 500 μs (Beskin et al. 1983; Percival et
al. 1993), the fluctuations of the pulse intensity (Kristian et al. 1970)
small changes of the optical pulse intensity, synchronous with the giant
radio pulses, have been detected (Shearer et al. 2003)
the evidence for the short time scale precession of the pulsar has been
detected by studying its optical light curve (Čadez et al. 2001)
Optical spectrum of pulsar is pure
power law
Carramiñana, Čadež & Zwitter (2000)
•Stroboscope adapted to LFOSC
at 2.1m.
•The two pulses have same
spectrum: index  = 0.20.1 for
5000-7500Å.
•No absorption feature at 5900Å.
•The underlying nebular spectrum.
Nebular emission lines are excited by
leptons generated at the pulsar and moving
along the magnetic field lines
Movie outside Powerpoint
Pulsations in slow motion
(Vidrih, Carraminana, Čadež 2002)
Pulse shapes
(HST – Dolan, Galičič, Kitt Peak – Fordham et al.)
Changing pulse shapes?
(Karpov et al. 2007)
• Pulse shape is exhibiting changes on a few microsecond scale in a
few days
Timing noise
(Karpov et al. 2007)
• Timing noise of a few ms on a time scale of ~1h and
~100ms on a time scale of days has been measured
Does the pulsar free-precess?
(Čadež, Calvani, Carraminana, Galičič,Vidrih 1996-2003)
•
Pulsar stroboscopic phase photometry and HST data (over 10 years of data
span) show evidence of enhanced phase noise at 0.01711 and 0.0133 Hz