Measurements of Close Visual Binary Stars at the Observatory of Saint-Véran

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1 Page 476 Measurements of Close Visual Binary Stars at the Observatory of Saint-Véran J. Sérot J.E. Communal Abstract: This paper gives the results of observations of close visual binary stars carried out in July 2016 and August 2017 at the Observatory of Saint-Véran, in the French Alps, using 62 cm and 50 cm reflector telescopes. Measurements of 43 pairs, with separations between 0.25 and 1.25 arcsec and magnitudes up to 12.1 are listed, including O-C residuals for 8 pairs listed in the Sixth Catalog of Orbits. For 23 pairs, the image sequences have been reduced using three distinct methods : lucky stacking, auto-correlation and bispectrum analysis, providing an estimation of the applicability and the accuracy of the latter in terms of astrometry (separation and position angle) and photometry (difference in magnitudes between the two components). 1. Introduction The Observatoire de Saint-Véran (Observatory of Saint-Veran) is a French astronomical observatory located on the Pic de Château Renard in the municipality of Saint-Veran in the department of Hautes-Alpes in the French Alps. At 2930 m altitude, it is the highest observatory in Europe (Fig. 1). The observatory was built in 1974 as a branch of the Paris Observatory after a ten years campaign to choose the best location to erect a 4m class telescope. When this project was finally abandoned in due to the installation of the Canada- France-Hawaii Telescope in Hawaii -, the Astroqueyras Amateurs Association was set to take over the astronomical observational site and granted use of the 62 cm Cassegrain telescope. Since then, three other instruments have been installed at the observatory : a 20 cm F/4 Flat Field Camera and two 50 cm F/8 Ritchey- Chretien telescopes. The members of the association are conducting observational missions covering a large panel of activities : visual observation and astronomical sketching, planetary and deep sky imagery, astrometry and photometric analysis, stellar spectroscopy, supernovae and cometary searches, asteroid surveillance, exoplanetary transits, etc. But, quite surprisingly given the exceptional seeing frequently recorded at the observatory very few missions have been devoted to the observation and measurement of visual double stars. We had the occasion to spend two weeks at the observatory, in July 2016 and Aug This paper gives the results of these short campaigns. 2. Instrumentation The results reported in this paper have been obtained with two different instruments : A 62 cm Cassegain telescope (in July 2016) and a 50 cm Ritchey- Chretien telescope (in August 2017). The Cassegrain telescope (Figure 2) is installed under a 7.5 m dome (left on Figure 1). It has a focal length of 9 m (F/D = 15). The equatorial mount is controlled by a dedicated software, run from a remote room. The 50 cm Ritchey-Chretien telescope (Figure 3) is installed under a 4.5 m dome (right on Figure 1) on AP1600 equatorial mount controled using the Sky- Chart software. Two different cameras were used : A Raptor Kite EM-CCD (in August 2017) and a ZWO ASI290MM (in July 2016 and August 2017). These cameras have been described in previous papers ([1],[2]). On the Cassegrain telescope only the ASI290 camera was used. Its small pixels allowed us to work at

2 Vol. 14 No. 3 July 1, 2018 Journal of Double Star Observations Figure 1. The Observatoire de Saint-Véran, altitude 2930m prime focus of the instrument, with a plate scale of arcsec/pixel (F = 9 m, pixel size = 2.9 μm). On the RC telescope (F = 4 m), two optical configurations were used : a 2x Barlow when working with the ASI290 camera, giving a resulting focal length of 8.4 m and a plate scale of of arcsec/pixel a 10 mm projecting eyepiece with the Raptor Kite camera, giving a resulting focal length of 27.9 m and a plate scale of arcsec/pixel Three types of filters were used: a broad nm filter, a nm red filter and a nm green filter (Astronomik L, R and G, resp.). No atmospheric dispersion corrector (ADC) was used. Acquisition was carried out with the Genika software [3]. For all sessions, calibration was carried out using the sideral drift method using the dedicated module of the SpeckleToolBox software [4], the precise datation of each frame being performed by the Genika software. As reported in [2], this method provides very accurate and reproducible results both for plate scale and camera angle estimation. Figure 4 illustrates the analysis of a single sidereal drift with the tool. The graphics plots the x-y positions of the detected star for the whole sequence (450 positions here, for a sequence duration of approximately 5 s). The green line corresponds to the best linear approximation. The camera angle is deduced from the slope of this line, whereas plate scale (in arcsec/ pixel) is deduced from the mean of the differences in successive x and y positions. In each observing night, between 4 and 12 drifts were recorded and the final calibration values are obtained by computing the statistical mean of the extracted values. As an indication, Table 1 gives the values extracted from eight sideral drifts, on Figure 2. The 62 cm Cassegrain telescope Figure 3. The 50 cm Ritchey-Chretien telescope Page 477

3 Page 478 Figure 4. The Drift analysis module of SpeckleToolBox at work for extraction of calibration parameters two stars, on the night of August 29, The final calibration values obtained from this data set are E = ± arcsec/pixel and Δ = ± 0.1. Each observing night, the selection of targets and their pointing was carried out using the WdsPick tool described in [5]. 3. Data reduction The data collected in July 2016 were analysed with the Reduc sotware [5] using the auto-correlation module to obtain angular separation (SEP) and position angle (PA) as described in [2]. The data collected in August 2017 were analysed using three different reduction methods: lucky stacking, auto-correlation and bispectrum analysis. Lucky stacking (LS) was here only used to obtain an estimation of the difference in magnitude (ΔM) of the two components. The n best images of each acquisition sequence (n = typically) are selected and co -added and then measurement is carried out on the composite image using a dedicated surface fitting algorithm («Surface») provided by the Reduc software. Auto-correlation-based reduction (AC) sometimes called speckle interferometry has been described in [1] and [2]. It consists in computing the power spectrum of individual images (taking the square of the modulus of the Fourier transform), summing these power spectrums and computing the inverse Fourier transform of the sum. The SEP and PA values of the measured pair are then deduced from the position of the peaks on the resulting auto-correlogram. Both the applicability and the precision of the method can be significantly improved by using deconvolution. Deconvolution consists in dividing the accumulated power spectrum by that obtained from a sequence of a single star observed under the same conditions (typically as close as possible in time and celestial position to the target star). We performed AC-based reduction both with Reduc (on the data obtained in 2016) and with Speckle- ToolBox version 1.08 (on the data obtained in 2017). Bispectrum (BS) analysis also known as triplecorrelation is a powerful technique aiming at recovering the final image phase in the Fourier domain that is lost when using classical auto-correlation techniques. This allows a representation of the original set of images to be reconstructed with atmospheric distortion removed. As for simple AC techniques, bispectrum analysis is more effective when used with a reference (single) star. The power spectrum and bispectrum are computed for the reference star and are then used to compensate the power spectrum and bispectrum of the target double star. Contrary to AC, the method allows a direct estimation of ΔM. We used the BS analysis modules provided by SpeckleToolBox version The procedure is executed in two steps. One first constructs the bispectrum from the average of the triple correlation of each image in the ensemble. This process is computationally intensive, typically taking 10 times more Table 1. Raw calibration values extracted from sideral drifts on the night of Aug 29, N: number of frames in the analyzed sequence; DIAM: RMS star diameter, DELTA: inferred camera angle ( ), and E: plate scale (arcsec/pixel) STAR N DIAM DELTA E HD HD Though, strictly speaking, the term speckle interferometry only applies when the input images show a sufficient high number of speckles, i.e. when D/R0 >> 1 ; in other cases, the technique should rather be called pixel auto-correlation. Called the reference star.

4 Page 479 Figure 5. The bispectrum analysis module of SpeckleToolBox at work CPU time than the computation of the power spectrum alone. The second step is the reconstruction of the final image from the power spectrum and bispectrum. This step, which iterates the image phase until convergence, is usually quite fast. During this step the user can apply filters and photon bias removal in the Fourier domain to improve the SNR of the final image. This second step is illustrated on Figure 5. The top image shows the reconstructed final image and the lower image the two control panels used both for computing it and for performing measurements on it. The target star here is BU Reconstruction has been performed using star UCAC as reference. On the reconstructed image, the yellow, green and pink circles respectively indicate the area from which the background level is estimated and the location of the primary and secondary component. The pair here is measured as having a separation of arcsec, a position angle of (taking into account the indicated calibration values) and a difference of magnitude of Results Due to bad weather and because the instruments were also used for other targets, we could only devote four nights to double stars observations : Jul 18-19, 2016 and Aug 28-29, Results are given in Tables 2 and 3. In Table 2, columns 1-10 respectively gives the identifier of the star in the WDS catalog [7] its discoverer code the magnitudes of the primary and secondary component, as reported in the WDS catalog the final PA and SEP measurement (in degree and arcsec, resp.) with estimated error when available the estimated difference of magnitude, when available the number of individual measurements, the date of measurements some notes, to be detailed after the table Table 3 gives detailed measurements showing results obtained with each of the three reduction methods listed in Sec. 3 for pairs observed in Measurements are given in column blocks named SEP, PA and ΔM. In each block, sub-blocks AC, BS and LS refer to results obtained with auto-correlation, bispectrum analysis and lucky stacking respectively. Individual columns named μ, σ and N respectively give the statistical mean, standard deviation and number of samples. The (Text continues on page 482) The error is obtained by dividing the standard deviation, computed from all the individual measurements by the square root of the number of measurements.

5 Page 480 Table 2. Final results for the four observing nights WDS NAME M1 M2 PA ( ) SEP (arcsec) ΔM N DATE NOTES A 1249AB ± ± ± a,3,L COU ± ± ± a,3,L BU ± ± ± a,3,L BU ± ± ± a,3,L A ± ± ± a,3,L HU ± ± ± a,3,L A ,6 ± 1,0 0,38 ± 0, ± a,3,L BU ± ± ± a,3,L HU ± ± ,3,R COU ± ± ,4,R HO ± ± ,R COU ± ± ,R+G COU ± ± ,4,G COU ± ± ,R COU ± ± ,4,G HU ± ± ,R+G COU ± ± ,R COU ± ± ,R COU ± ± ,4,R COU ± ± ,R COU ± ± ,R COU ± ± ,R COU2220Aa,Ab ± ± ,4,R COU ± ± ,R, A ± ± ± a,L A ± ± ± a,L A ± ± ± a,L BU 69AB ± ± ± b,5,L COU 527Aa,Ab ± ± ± b,4,L HEI ± ± ± b,L HU ± ± ± b,L A ± ± ± a,L RST ± ± ± b,L HU ± ± ± b,L HEI ± ± ± b,4,L RST ± ± ± b,L HU 490AB ± ± ± b,4,L HDS ± ± ± b,L A ± ± ± a,4,L COU ± ± ± b,5,L A 299AB ± ± ± a,L BU ± ± ± b,5,L COU ± ± ± b,L Notes for Table 2: 1: 62 cm Cassegrain + ASI290MM 2a: 50 cm Ritchey-Chrétien + Raptor Kite 2b: 50 cm Ritchey-Chrétien + ASI 290MM 3: Pair with an entry in 6th Catalog of Orbits. See Table 4 for O-C 4: Pair showing a significant displacement since last measure published in WDS. See Table 5 5: Only one measurement for ΔM, hence no associated error L, R, G: used filter (see Sec. 2)

6 Page 481 Table 3. Detailed measurements showing results obtained with each of the three reduction methods for pairs observed in 2017 NAME SEP PA Δm COMPARISON AC BS AC BS BS LS AC vs. BS μ σ N μ σ N μ σ N μ σ N μ σ N μ N SEP PA Δm Bs vs. LS A A A 299AB A A 1249AB A A A A BU 69AB BU BU BU BU COU COU COU 527Aa,Ab COU HDS HEI HEI HU HU HU 490AB HU RST RST Table 4. O-C residuals for pairs having a known orbit NAME WDS DATE O-C PA ( ) O-C SEP (") GRADE REF A Hrt2009 A 1249AB Zir2003 A Baz1989a BU Cve2006e BU Hrt2010a COU Doc2012i HU Ole2003d HU Doc2009g

7 Page 482 Table 5. Pairs showing a significant displacement since their last measurement NAME WDS DATE SEP PA δ SEP δ PA DATE2 A COU 527Aa,Ab HEI HU 490AB COU COU COU COU2220Aa,Ab COU (Continued from page 479) last column block (labeled comparison) compares the results obtained for SEP and PA with the AC and BS methods on the one hand and those obtained for ΔM with the BS and LS methods on the other hand. The individual columns here give the difference between the values obtained by the corresponding methods. For PA, the maximum difference (in absolute value) is 4.6. The statistical mean of the difference is 0.40 and its standard deviation is 1.8. For SEP, the maximum difference (in absolute value, again) is 0.06 arcsec, the statistical mean is arcsec and the standard deviation is arcsec. This demonstrates that, as long as astrometry is concerned, both the accuracy and the precision of the results produced by bispectrum analysis are comparable with those obtained with «classical» auto-correlation method. For ΔM, the maximum difference is 1.3, the statistical mean is 0.21 and the standard deviation is Dispersion is clearly higher here and the mean value could indicate a systematic bias. It can be noted, however, than estimating magnitudes for very close pairs (as is the case here) is notoriously difficult because the estimation has to take into account the sky background and the presence of the diffraction rings. Typical post-reduction images (LS, AC and BS) are given on Plate 1. All LS images (left column) were computed by the Reduc software. The AC images in the first two rows (in color) were also computed by the Reduc software. All other images (AC and BS) were computed by the SpeckleToolBox software. For pairs having a known orbit, Table 4 gives the O -C residuals, computed from the ephemerides published in the 6th Catalog of Orbits [8]. Only two pairs show significant O-C values : A 1249AB and A For both of them, the deviation, which is very close to 180, is likely to be caused by a component inversion in the computed orbit. The latest measurements in the WDS catalog (240 in 2015 for A 1249AB, 187 in 2010 for A 2205) and the images given in Plate 1 tend to support this interpretation. Several of the pairs observed during this campaign show a significant displacement since their last measurement recorded in the WDS catalog. These pairs are listed in Table 5 (with columns DATE2, DATE, δ SEP and δ PA giving respectively the date of the last measurement in WDS, the date of our measurement and the difference between the measurements in SEP in PA). For HU 490AB, COU2220 and COU2426 the relatively large difference values either in SEP or PA can probably be attributed to the involved time scale (34, 33 and 26 years resp.). For COU1157, the corresponding timescale is unlikely to explain the large discrepancy observed in PA between our observation and the last recorded in the WDS (PA=145.7 in 2007). It can be noted, however, that this last measurement is given with the indication «quadrant flipped 180 from published value» and that all previous recorded observations, obtained between 1974 and 1990 give fluctuating PA values between 92.4 and For COU2124, we have no explanation for the large discrepancy observed both for PA and SEP between our observation and the two recorded in the WDS PA/ SEP=25.8 /0.31" and PA=36.7 /0.35" respectively except a misidentification. A search for pairs with characteristics compatible with our observation in the neighborhood of COU2124 within the WDS did not produce any hint for a possible confusion, however. Finally, several of the target stars were either viewed as simple or perceived as binaries but cannot be reliably measured because their separation was too close (<0.25 arcsec typically). These stars are listed in Table 6. (Text continues on page 486)

8 Page 483 Table 6. Pairs observed but for which no measure was obtained WDS NAME M1 M2 DATE NOTE COU BU 733AB BU 989AB COU HDS HU Notes for Table 6: 1. Viewed as simple 2. Viewed as elongated but no reliable measurement possible 3. No reliable measurement possible Plate Plate 1 1 Post-reduction Post-reduction images images LS AC BS COU 1157 N/A HO 99 N/A

9 Page 484 Plate 1 (continued). Post-reduction images LS AC BS A 295 A 2290 COU 247

10 Page 485 Plate 1 (continued). Post-reduction images LS AC BS HU 490AB A 1249AB A 2205

11 Page 486 (Continued from page 482) 6. Conclusion The results reported here confirm the high value of the Observatory of Saint-Véran for high resolution imaging, with reliable measurements of visual double stars close to the diffraction limit of the instruments being easily obtained. This was in fact already known in the field of planetary imaging but, as stated in the introduction, has never been demonstrated to the best of our knowledge in the field of close visual double star observation and measurement. These results also show that bispectrum analysis a technique which has very rarely been exploited in the amateur domain produces results which are, in terms of precision and accuracy, on a par with those obtained with other reference methods, such as pixel autocorrelation or speckle interferometry, at least for astrometry. Results in terms of photometry (DeltaM estimation) are also very encouraging, although some work probably remains to be done to further assess the accuracy of the measurements. Acknowledgments The authors would like to thank the AstroQueyras association for granting us observation time on the instruments and, more generally, for providing access to the observatory to the amateur community. This research has made use of the Washington Double Star and 6th Orbit catalogs maintained at the U.S. Naval Observatory. The history of measurements for COU2124 and COU1157 have been kindly provided by B. Mason. Data reduction was carried out using the Reduc software, developed and maintained by F. Losse, and the SpeckleToolBox software developed by D. Rowe. References [1] Sérot, J., Speckle Interferometry of Close Visual Binary Stars with a 280 mm Reflector and an EM- CCD, JDSO, 12(4), , [2] Sérot, J., Measurements of Close Visual Binaries with a 280 mm Reflector and the ASI 290MM Camera, JDSO, 13(2), , [3] [4] Harshaw R., Rowe D., Genet R., The Speckle Toolbox: A Powerful Data Reduction Tool for CCD Astrometry, JDSO, 13(1), 52-67, [5] Sérot, J., User s Guide to WdsPick, JDSO, 12(6), , [6] Losse, F., Reduc, v hfosaf. [7] Mason, D.B., Wycoff G.L., Hartkopf, W.I., Washington Double Star Catalog, USNO, [8] Hartkopf, W.I., Mason, D.B., Sixth Catalog of Orbits of Visual Binary Stars, USNO,