de-biasing interferometric visibilities in amber/vlti data of weak sources

De-biasing interferometric visibilities in AMBER/VLTI data of weak sources

1 INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monteporzio Catone2 Universit`a degli Studi di Roma ‘Tor Vergata’, via della Ricerca Scientifica 1, I-00133 Roma

3 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze

G. Li Causi1, S. Antoniucci2;1, E. Tatulli3

Calib (HR2379_B02, 1.75mas, K=2.24) Target (ZCMa A, K~4.2)

21th Jan 2006, UT1-UT2-UT4, AMBER MR @ 2.16µm, 50ms integration, seeing~0.6”;proposal: 76.C-0817 by Nisini B., Antoniucci S., Li Causi G. et al.

Gianluca Li Causi – INAF, Rome Astronomical Observatory - [email protected] - Grenoble, 30th Nov 2006

FRINGES FRINGES ?


Are there FRINGES on the target?Hypotheses:• ...FRINGES ARE NOT FORMED:

- ...the target has INTRINSIC NULL VISIBILITY (i.e. it is very large)- ...FIBERS ARE NOT CENTERED on the target- ...fringes do not form because of MALFUNCTIONS (vibrations, etc.)

• ...FRINGES ARE FORMED:- ...but they are UNDER THE NOISE because the FLUX IS TOO LOW- ...but the EYE CANNOT SEE THEM- ...but they are HIDDEN BY ARTIFACTS

Target (ZCMa A) enhanced

FRINGES ARE THERE, as we can see by means of an enhanced elaboration (Fourier filtering on deviations from local median; we’ll see better in the following)

...but, if we do amdlib Extraction (amdlibComputeSpectralCalibration,

amdlibComputeP2vm, amdlibExtractVis –b 1000),we obtain mean calibrated vis of:

1.14 +/- 0.11 meaningless !0.81 +/- 0.090.58 +/- 0.09

Why?FRINGES !


Let us have a look at the Fourier power spectrum of the interference channels:

Target (ZCMa A)average power spectrum

Calibratoraverage power spectrum

Darkaverage power spectrum

spurious peaks

B1

B3

B2

DC

interference peaks

B1

Targetphotometric channel

There are spurious peaks, which means spurious fringes in the image frames, even in the Dark and even in the masked DK channel !


We can see that the enhancement of one photometric channel of a dark, where we don’t expect fringes at all, shows lots of fringes, with the same period and inclination as those in the Target:

Dark P2 channel - original Dark P2 channel - enhanced

Hence the fringes found in the Target are spurious !!


Removing the artifact:Such spurious fringes are possibly caused by electromagnetic interferences on the detector itself.Perhaps they could be removed by hardware, however current and past observations are corrupted.So we have to remove them by software.NOTE: the artifact is not removed by the normal Dark frame subtraction because the fringes have

random phases.

Our approach:1. compute averaged power spectrum from all the Dark frames;2. identify artifact peaks in the frequency plane and

make a binary mask for each channel;3. Compute power spectrum of each target frame and

replace realistic power estimation to the masked regions;4. reconstruct corrected frames by inverse FT;5. re-write the AMBER fits files6. redo all the process for any fits of the observation,

including WAVE_3T, P2VM, Calib, Target, Sky and Dark themselves,re-computing the mask from their respective Dark frames

7. After this pre-processing, launch the amdlib standard processing on the corrected files.


1. Compute averaged power spectrum from all the Dark frames:

Dark of Calib, Sky and Targetinterf and phot channels

Dark of Calib, Sky, TargetDK channel

P2VM Darkinterf and photchannels


2. Make a binary mask for each channel:

MASK for Dark, Calib, Sky and Targetinterf and phot channels

MASK for Dark, Calib, Sky, TargetDK channel

P2VM Darkinterf and photchannels


3. Replace realistic power estimation to the masked regions:

Calib Target (ZCMa A) Dark

Target (ZCMa A)

baseline 1 SIGNAL

Peaks are replaced by sigma-clipped average

on each column


4. Reconstruct corrected frames by inverse FT: Target

Target (ZCMa A) - original Target (ZCMa A) - corrected

Compare enhancementsspurious fringes

real fringes


4. Check the residuals on Dark P2 channel

Dark P2 channel - original Dark P2 channel - corrected

spurious fringes

Compare enhancementsno residual spurious fringes

Now we launch the standard amdlib processing on the corrected files:

amdlib Extraction (amdlibComputeSpectralCalibration, amdlibComputeP2vm, amdlibExtractVis –b 1000),

we obtain mean calibrated vis of:

0.81 +/- 0.11 now less than unity!0.87 +/- 0.090.63 +/- 0.09


We also see that the dispersion of instrumental visibility among the files is reduced:


Calib - original Calib - corrected

Target - original Target - corrected

V2

file #

V2

file #

V2

file #

V2

file #


We applied the same pre-processing also to an observation of FU-ORI (magK=5.15, calib HD42807 magK=4.85) and we found a similar improvement in the SNR and visibility dispersion.If we put all the observations together we see how the artifact damage increase with decreasing coherent flux, with different behaviours for the first baseline and the other two.


Software availability:The software that we have developed to perform the described pre-processing, which corrects the detector fringe artifact, is available at the following web address:

http://www.mporzio.astro.it/~licausi/ADC/

SOME TECHNICAL NOTES:

1. Our filtering must be applied to frames already corrected for bad-pixels: in fact bad pixels translate into noticeable power offsets at all frequencies.

Due to the fact that we replace the power but we cannot restore the phase, these offsets prevent to remove the artifact fringes because the offsets power themselves are assigned to the artifact phases.Thus our method only works when the median power is near the noise level, that is when bad pixels are removed.

In practive, we perform a sigma-filtering on the frames before to apply the correction.

Another way could be to apply our program on the amdlibRawCalibrate output, i.e. the frames already corrected by bad pixels, flat, dark and sky, then to run amdlibExtractVis on the corrected fits using a dummy unity flat with no dark and no sky.

2. Our analysis of the extracted visibilities, of both the original and corrected files, shows that the real visibility dispersion across the files is underestimated by ExtractVis with default parameters, while the –e THEORIC parameter provides a correct error estimate.

We have checked this on the dark files, before and after correction of detector artifact: the default extraction provides errors not compatible with the expected null visibility, while the –e THEORIC setting provides a V compatible with zero.



Do we get scientific results?

Our corrected and calibrated visibilities still do not seem to be scientifically meaningful (but we’ve not used frame selection yet):

• baseline 1, the shortest, shows a V less than the other baselines, which is not easily modellable;• the value of V2 for baseline 1 is not compatible with observations of Keck Interferometer, published

in the literature (Monnier et al. 2005, ApJ 624, 832).

Keck

b1 b2 b3

de-biasing interferometric visibilities in amber/vlti data of weak sources

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