Ljubljana fiber fed fast photometer

Andrej ČadežBojan DintinjanaAnja LautarDejan ParadižDušan Ponikvar

Bled, March.2008

Steps toward timing the Crab pulsar - hardware

Telescope with robotic

guiding system

Off-axis 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

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0t i m e s

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0c o u n t r a t e H z

First Crab observation (March 2nd 2008)(before field distortion correction)

10 100

166000

168000

170000

172000

174000

Aut

o -

corr

elat

ion

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 50m fiber has an 1.7 arc seconds diameter receiving area. This area must be increased to 3 arc seconds diameter (100m) . 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 100m fiber input.

Observing Crab with a fast photometer – why it might be interesting?

Karpov et al., Astrophys Space Sci 2007

• 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 = 0.20.1 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 s on a time scale of ~1h and ~100s 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