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Speckle Imaging with VLT/NACO No-AO Mode Telescopes and Instrumentation Speckle Imaging with VLT/NACO No-AO Mode Sridharan Rengaswamy1 object of interest. In the case of binary Holographic imaging of crowded fields Julien Girard1 stars, the binary nature can be visually was pioneered and presented by Rainer Willem-Jan de Wit1 identified from the peaks of the auto- Schödel (Schödel & Girard, 2012; Henri Boffin1 correlation. Schödel et al., 2013). Here we demon- strate how the NACO non-AO mode, Speckle interferometry has made signifi- coupled with the aforementioned speckle 1 ESO cant contributions to the field of binary image pro cessing technique, can be star research by creating several binary exploited to obtain near diffraction-limited star catalogues (e.g., Mason et al., 2013). images of objects of interest. We have Long-exposure stellar images recorded Since the energy spectrum does not developed the required pipeline data with large ground-based telescopes preserve the Fourier phase information, it reduction tool (Rengaswamy, Girard & are blurred due to the turbulent nature is not possible to obtain an image of the Montagnier, 2010) for reconstructing an of the atmosphere. The VLT employs object apart from the autocorrelation. image from the speckle data1. active and adaptive optics (AO) systems Hence there is a 180-degree ambiguity in to compensate for the deleterious the position angle of the binary star. In effects of the atmosphere in real time. 1977, Gerd Weigelt showed, experimen- Imaging binary stars The speckle imaging technique provides tally, that the phase of the complex triple an alternative way to achieve diffrac- product of the Fourier transform of the The study of binary stars is an important tion-limited imaging by post-processing images at three spatial frequencies (the branch of astrophysics. It is the most a series of short-exposure images. The third frequency being the sum of the common way by which stellar masses are use of speckle imaging with the no-AO first two) averaged over the ensemble, determined. Multi-epoch images of a mode of NACO at the VLT is demon- provides the sum of the Fourier phases binary system enable the calculation of strated. Application of this technique is of the object at those three frequencies. the orbital elements of the binary. A typical particularly suited to the J-band and This complex product, called the bi- long-exposure image does not resolve it provides versatile high angular resolu- spectrum, is immune to atmospheric tur- very close binary stars, i.e., those at sub- tion imaging under mediocre conditions bulence (as the random phase errors arcsecond separation, as the resolving and/or in imaging extended objects. introduced by the atmosphere are com- power is impaired by the atmospheric The implementation of this mode under- pletely cancelled in the bi-spectrum) turbulence. Short-exposure images lines the continuing attractiveness of and provides a way to retrieve the Fourier (freezing the atmosphere) can be post- NACO at the VLT. phases of the object. processed to resolve the binary. The bot- tom left panels of Figure 1 show images It should be noted that the phase of the of the binary star HIP24800, reconstructed Astronomical speckle imaging bi-spectrum is the same as the “closure from a series of 900 speckle images phase”, a term coined by Jennison (1958) recorded in NACO no-AO mode in Janu- The invention of speckle interferometry in radio astronomy and used first by ary 2010. The top panels show the best (Labeyrie, 1970) has revolutionised the Rogstad (1968) for optical imaging through and the worst images of the series. The field of high-resolution imaging with turbulence. Combining the phase infor- best image is identified as the one for ground-based telescopes. In speckle mation with the energy spectrum obtained which the sum of the fourth power of the interferometry, a series of short-exposure from the speckle interferometry, an mean subtracted pixel intensities is the images of the object of interest (for image of the object of interest can be highest. The bottom right panel shows example, a spectroscopic binary star) is reconstructed. This technique is known the long-exposure image, a simple mean recorded with a spectral filter whose as speckle imaging. of all the short exposures. The separation bandwidth is smaller than the mean between the components of the binary is wavelength (quasi-monochromatic condi- 161 milliarcseconds (mas) and their bright- tions). The modulus-squared Fourier The no-AO mode of NACO ness ratio is about 2.3. transform of the images averaged over the ensemble (a quantity generally referred The Very Large Telescope (VLT) instru- to as the energy spectrum) preserves ment NAOS–CONICA (NACO; Lenzen et Imaging near-Earth objects the information up to the diffraction limit al., 2003) was built at the end of the of the telescope. Deconvolving this aver- 1990s and its deformable mirror has a The no-AO mode of NACO also makes age with a similarly obtained average of total of 185 actuators (Rousset et al., the imaging of fast-moving near-Earth an isolated single star, close in time and 2003). The no-AO mode of NACO (Girard objects at high angular resolution feasi- angular distance to the target, removes et al., 2010), essentially bypasses the ble. The near-Earth passage of the aster- the effects of the atmosphere and the adaptive optics module NAOS and uses oid 2005 YU55 in November 2011 pro- tele scope on the energy spectrum, leaving CONICA’s “burst mode” to record a vided an excellent opportunity to test behind the energy spectrum of the object series of short-exposure images, exploit- this. As the adaptive optics could not lock of interest. A subsequent Fourier trans- ing the detector’s windowing capability. onto this fast-moving object, data were form of the energy spectrum provides the There are different ways in which these recorded in the no-AO mode in the diffraction-limited autocorrelation of the short-exposure images can be processed. Ks-band. Figure 2 shows the resulting 12 The Messenger 155 – March 2014 - ". 3 QDCTBDCTMQDFHRSDQDC - ". ll3 QDCTBDCTMQDFHRSDQDC average reconstructed image of the A@CHL@FDADRSHL@FD A@CHL@FDADRSHL@FD asteroid. The nominal angular resolution of 60 mas at the Ks-band corresponds to a linear resolution of 95 metres at the dis- tance (329 900 km) of the asteroid. The dimensions of the asteroid were much larger than this resolution and thus the technique allowed the asteroid to be spa- QDBNM HL@FD LD@MHL@FD QDBNM HL@FD LD@MHL@FD tially resolved. The cross-sections of the image, subjected to an edge enhance- ment operator (Prewitt edge enhancement BRDB BRDB operator of the IDL software) to retrieve @Q @Q the asteroid dimensions, indicated a size of (261 ± 20) × (310 ± 30) metres. For comparison, tri-axial diameters of 337 × @QBRDB @QBRDB 324 × 267 metres, with uncertainties of 15 metres in each dimension, were esti- Figure 1. Speckle reconstructed images based on top panels show the worst and best images of the mated for the size of the asteroid from NACO no-AO observations of the binary HIP 24800 recorded series. The lower right panels show the the Keck adaptive optics system, six in J- (left) and Ks- (right) bands (low left panels). The mean image. hours before our observations. Account- ing for the time difference, and the 18-hour rotation period of the asteroid, NQHFHM@K our results are in good agreement with DCFDlDMG@MBDC those of the Keck AO system, despite the mediocre observing conditions (see- X ing of 1.2 to 1.5 arcseconds at 550 nm @Q SQ and atmospheric coherence time of two milliseconds). These observations dem- @QAH X BRDB onstrate the robustness of the no-AO @Q MRHS mode and the speckle imaging technique. SD (M Imaging circumstellar envelopes around evolved stars l l L2 Pup (HD 56096, HR 2748) is a semi- l l l @QBRDB 7lRHYDL regular pulsating red giant star in the constellation of Puppis with an angular Figure 2. Reconstructed image of asteroid 2005 image. The green line indicates the cross-section diameter of 17 milliarcseconds. Figure 3 YU (left) and its horizontal cross-section (right). The after applying the edge detection operator. The blue 55 shows speckle-reconstructed images red line indicates the cross-section before applying lines indicate the size. an edge detection operator to the reconstructed of L2 Pup at 2.27 μm observed with the (CD@K/2% ,@QBG2DOS Figure 3. Speckle image reconstruction of the circumstellar envelope of L2 Pup, observed in March and September 2012. The theoretical (ideal) PSF is shown for comparison. BRDB @Q l l @QBRDB The Messenger 155 – March 2014 13 Telescopes and Instrumentation Rengaswamy S. et al., Speckle Imaging with VLT/NACO No-AO Mode no-AO mode. The nominal resolution at )'*R this intermediate band is about 60 mas. While the stellar continuum is unresolved, the extended circumstellar emission from this star is visible in the recon- (#$ +/2% structed images. We have suppressed the features with intensity values less than 5% of the maximum intensity in gener ating this figure. The circumstellar envelope appears enlarged in the second reconstruction, obtained six months after the first one. This is perhaps due to the 61 140-day pulsation period of the star or due to its high mass-loss rate. The panel on the left is the ideal point spread func- tion at 2.27 μm. The middle and right BRDB panels indicate speckle reconstructions @Q obtained from the March and September 2012 data, respectively. These images 61 serve first and foremost as extremely val- uable information on the dynamical evolution of the circumstellar envelope at high angular resolution.
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