Measurements of 60 Double Star Systems Using a Small Telescope and Four Different Methods

Transcription

1 Page 193 Measurements of 60 Double Star Systems Using a Small Telescope and Four Different Methods Shishir Deshmukh 1, Ameya Deshpande 2, Ganesh Pawar, Gauri Joshi, Sayali Bhat, Rujuta Vaze Akash Mitra Mandal Kalyan, Maharashtra, India 1. shishir.supernova@gmail.com 2. ameyaddeshpande@gmail.com Abstract: Imaging of 60 southern double stars was undertaken using an 8-inch reflector telescope and an unmodified standard DSLR camera and measurements were made using four different methods after plate solving each image. The results were then averaged and standard deviations were computed and are reported here. Introduction Double star astronomy is a potential field for amateurs. Useful astrometric observations can be made using a consumer grade Digital Single Lens Reflex (DSLR) camera. The images acquired using a DSLR camera can be processed using various software. Today, a number of open source image processing software for data reduction and processing are available. In this project, a DSLR camera was used for astrometric measurements of double stars. Stars from the southern celestial hemisphere were chosen randomly for imaging. Amateurs in Akash Mitra Mandal (an Amateur Astronomers organization) in Kalyan, Maharashtra, India took part in this project. Objectives The primary objective of this project was to measure the current Separation and Position Angle (PA) of the selected double stars in Washington Double Star (hereafter WDS) [Mason, 2016] catalog from the southern celestial hemisphere. In order to achieve greater accuracy of measurements, four different methods were employed. Each system was measured by all the four methods by one or more measurers independently. Imaging Setup For this project, Akash Mitra Mandal s 8-inch Orion F/3.9 reflector telescope was used with HEQ5 mount and Canon EOS 600 D unmodified DSLR camera. The telescope was operated from a laptop using Patrick Chavalley s free planetarium software Cartes du Ciel (v 4.0) [Chavalley, 2017]. The imaging pipeline was executed using the time unlimited demo version of Astro Photography Tool (APT), v3.31 [Incanus, Ltd. 2018]. All the images were saved in RAW format. The images were taken at ISO 800 with 15 s single exposure for most of the stars. Data Reduction and Measurements All the images were plate solved using the online utility at nova.astrometry.net [2018] and the scale of each plate was found to be arcsec/pix. Plate solved images were saved in.fits format and were used for measurements. In order to achieve greater accuracy, double stars within the resolution limit of the observation setup were measured using four independent methods. Each system was measured by one or more measurers independently. Further, it was ensured that every system is measured using all the methods. In the following sections, a brief note on each method is given. Aladin Aladin [Fernique, 2010] is an interactive sky atlas developed at CDS, Strasbourg Observatory, France. It helps the user to visualize digitized astronomical images with a capability of superimposing astronomical catalogues and databases on it. This makes it a very effective tool to identify objects in the images. In most cases, Aladin s Version 9.0 was used. First, a plate solved image in.fits format was opened, then the WDS catalog was loaded to view all the cataloged doubles in the im-

2 Page 194 Figure 1: Screenshot of measurements using Aladin age. All the entries in WDS catalog were then overlaid on the corresponding objects on the image. Using Dist tool, Position Angle (PA) and Separation were measured by dragging the cursor from the centroid of the primary star to the centroid of the secondary star. A line joining the centroids of the stars appeared and the measured values were displayed below the image window (refer to Figure 1). These measurements were then carefully noted in a spreadsheet. For better accuracy, three iterations were taken and the average value was computed. AstroImageJ AstroImageJ [Freed, 2018] is an astronomy specific extension of ImageJ software which was traditionally used in the life sciences. Collins et.al [2017] presented its astronomical capabilities. For this project, its daily build version db ImageJ 1.47i was used as it has the necessary tools to compute separation and PA. First, a plate solved image in.fits format was opened. Figure 2: Screenshot of measurements using AstroImageJ

3 Page 195 Figure 3: Screenshot of measurements using DS9 AstroImageJ has the capability of automatically estimating the centroid. For better results and centroid selection, the Howell centroid method option under the Aperture Settings submenu of Edit was checked in. Then, depending on the disc of the star on the image, the appropriate size of the reticle was set and then a line from the centroid of primary to that of the secondary was drawn. The measured values of the system were then shown on the image (Refer to Figure 2). This process was repeated for three iterations by slightly modifying the size of the reticle for better results. These values were then carefully noted in a spreadsheet. SAOImage DS9 SAOImage DS9 (hereafter DS9) is a scientific visualization program originally developed by Mike Van Hilst in 1990 at the Smithsonian Astrophysical Observatory, Center for Astrophysics, Harvard University. It offers a number of tools for astronomical visualization and measurements [Joye, 2003; Bendo 2018]. For this project, DS9, version 7.6 b8 was used. First, a plate solved image was opened in DS9. The software has the capability of overlaying various catalogs on the image. This was done using the Catalogs submenu from the Analysis menu. The WDS catalog was then selected and all the systems listed in the WDS catalog were highlighted with a green circle on the image. Once the target system was identified, an appropriate zoom level was adjusted for selecting the centroid of the primary. Using the Ruler option from the Shape submenu of the Region menu, the measurement scale was activated. The Ruler option activates only when the Region submenu under the Edit menu is selected. The targets can also be identified with the help of celestial coordinates of the system. DS9 has a capability to find the centroid of the star automatically. This was done using the Centroid option under the Region menu. Figure 3 shows the screenshot of the measurement taken using DS9. Depending upon the orientation and flipping of image directions, 360 degrees should be deducted from the value of angle shown on measurement window to arrive at correct PA. This process was repeated three times and an average of those three iterations were taken as the final value. Thus, all the measurements were carried and were noted in a spreadsheet. Trigonometric Formula Trigonometric formulae proposed by Smolinski J. and Osborn W [Smolinsky, 2006] were used to compute separation and PA. These are as follows: ( RA RA ) ( Dec ) ( Dec Dec ) = arccos cos cos cos sin ( Dec2 Dec1) ( Dec Dec ) (( RA RA ) ( Dec )) o = 90 arctan cos 2 1 sin 2 1 cos 1 where, ρ = Separation and θ = Position Angle. After identifying the target double star system in the plate solved image, equatorial coordinates (Right Ascension and Declination) of the centroids of each component were found using AstroImageJ. As men-

4 Page 196 tioned in the earlier section, AstroImageJ has autocentroid functionality that facilitates locating the centroid of the star near the user-clicked position within a set radius of region of interest. The position of the centroid (Right Ascension and Declination) were given as input to these trigonometric formulae. Value of the separation was directly computed from the input values of coordinates, however, to arrive at the exact value of the PA, the trigonometric formula was slightly modified to account for the correction of quadrant. The process of locating the centroid was repeated three times with different values of radii. The final values of separation and PA were carefully noted in a spreadsheet. Discussion As stated earlier, each star system was measured in each method with three iterations and by one or more measurers. Separation and PA are collectively called here measurement. The average value of the three iterations was computed along with the standard deviation. It should be noted that average and standard deviation were confined to the method used. The average of measurements across the different methods was computed at a later stage. From the overall measurements of all the systems, an acceptable threshold of standard deviation was decided. For separation, threshold of 0.5 and for PA threshold of 0.8 was decided. All the measurements beyond this threshold were rejected. It was also found that values measured using different methods were in good agreement with each other. Figures 6 to 9 show the comparison of measurements using different methods in the form of column diagrams. Finally, the average values of measurements across different methods were taken and will be reported to WDS. Standard deviation for the final values of Separation and PA across different methods were also computed (refer to Table 1) and their frequency distributions are shown in the Figures 4 and 5. Further, most of the measurements were found consistent with the observations in WDS catalog. The primary objective of this project was to provide updated separation and PA of known double star systems in WDS, especially from the southern celestial hemisphere. As mentioned in WDS catalog, some of the stars measured under this project were not measured for last couple of decades. This project has provided current measurements for these stars. Further analysis of these measurements, along with the historical data, proper motion data and parallactic measurements will in turn help providing orbital or linear solution to these pairs. Secondary objective of this project was to verify the consistency of measurements with different methods. Having executed four different methods to measure doubles, it was found that the measurements across different methods were consistent with each other. In Table 1, the average values of measurements across different methods are given apart from the measurements from each method. Standard deviation for each set of measurement is also given in the adjacent column. Limitations Observations under this project were carried on a single night with a single exposure of most of the systems. Although the condition of the sky was good, some of the close pairs were discarded owing to their poor resolution. The fixed exposure of 15s turned out (Text continues on page 202) Figure 4: Frequency Distribution of normalized Standard deviation of Separation across all methods Figure 5: Frequency Distribution of normalized Standard deviation of Position Angle across all methods

5 Page 197 * Since the final values of the Position Angle (PA) of WDS (FAL 37) were 359.3, 1.1, 0.1 and across different methods, the same were taken as 359.3, 361.1, and respectively for better clarity and manifestation. Figure 6: Separation (top) and Position Angle measured using different methods (1 st set of 15 systems)

6 Page 198 Figure 7: Separation (top) and Position Angle measured using different methods (2 nd set of 15 systems)

7 Page 199 Figure 8: Separation (top) and Position Angle measured using different methods (3 rd set of 15 systems)

8 Page 200 Figure 9: Separation (top) and Position Angle measured using different methods (4 th set of 15 systems)

9 Page 201 Table 1. Separation and Position Angle measured at the epoch using different methods and the average values across different methods with Standard Deviation Sl. WDS ID Name Comp Aladin DS9 AstroImageJ Trigonometry Avg. of all Standard methods Deviation Sep. PA Sep PA Sep. PA Sep. PA Sep. PA Sep. PA HJ 4511 AB HJ HJ 4493 AB CPO * FAL HJ BRT HJ 4431 AC HJ TOB TOB STF HJ HJ STF R TOB R 170 AB TOB 111 AC TOB R TOB TOB R 172 AC SHJ 145 AB HJ 4508 AB TOB PEL STF 1659 AB STF 1682 AB HDO TOB DUN 126 AB STF 1664 AB STF 1659 AC STF 1659 BC HDO 217 AC HJ 4548 AB S 639 AB S STF 1664 AC I 892 AB STF 1664 BC SIN 72 AC STF 1664 EF ENG 47 BC STF 1664 AE DUN 124 AB STF 1659 AE GIR 3 AC DUN 124 AC ENG 47 AB STF 1659 AD STF 1659 AF GWP GWP ENG 47 AC BUP 144 AD STF 1659 EF BU 1245 A,CD *Since the final values of the Position Angle (PA) of WDS (FAL 37) were 359.3, 1.1, 0.1 and across different methods, the same were taken as 359.3, 361.1, and respectively for better comparison.

10 Page 202 (Continued from page 196) to be too bright for some stars on the other hand, it turned out to be too faint for some of the fainter companions, hence such systems too were omitted from the measurements. Acknowledgements We are grateful to Akash Mitra Mandal (Amateur Astronomers organization) for making observations setup available to us for this project. We are also thankful to Manoj Binhani, Amogh Waghmare, Dr. Yogesh Karandikar and Rahul Puranik for their invaluable help in this project. This research has made use of the Washington Double Star catalog maintained at the U. S. Naval Observatory. We also express our gratitude to Aladin. This research has made use of "Aladin sky atlas" developed at CDS, Strasbourg Observatory, France. Further, we are also thankful to the developers of AstroImageJ software as this research has made use of it, especially, to Karen A. Collins and Dennis M. Conti for courtesy extended in guiding us. This research has made use of SAOImage DS9, developed by Smithsonian Astrophysical Observatory, hence we are thankful to them, especially, to Bill Joye for extending courtesy to us in using DS9. We also express our indebtedness to J. Smolinski and W. Osborn for the trigonometric formulae we used to measure separation and PA in this project. This research made use of data provided by Astrometry.net. We are grateful to them as well. References Bendo, George J., 2018, Introduction to Astronomy Images and the DS9 Image Viewer, School of Physics and Astronomy, The University of Manchester, DS9guide.pdf [Last accessed July 5, 2018] Chavalley Patrick, 2017, Cartes du Ciel, / Skychart, English Documentation, skychart/doku.php?id=en/documentation/start Joye, W.A., Mendel, E, 2003, New Features of SAOImage DS9, Astronomical Data Analysis Software and System XII, ASP Conference Series, H.E. Payne, R.I. Jedrzejewski, and R.N.Hook, eds, 295, p.489 Fernique, Pierre, 2010, Aladin User Manual, aladin.u-strasbg.fr/java/aladinmanual6.pdf [Last accessed July 7, 2018] Freed Rachel, 2018, Using AstroImageJ , [Online Video], 1 May 2018 Available from [Accessed: 7 July 2018] Incanus Ltd., 2018, User s Guide APT v3.54, Distinct Solutions Ltd usersguide/ [Last accessed July 7, 2018] Karen A. Collins, John F. Kielkopf, Keivan G. Stassun, and Frederic V. Hessman4, 2017, AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves, The Astronomical Journal, 153, No.2. Mason, B.D., Wycoff, G.L. and Hartkopf, W.I, 2016, The Washington Double Star Catalog, Astrometry Department, U.S. Naval Observatory, ad.usno.navy.mil/proj/wds/ nova.astrometry.net, 2018, [Last accessed July 7, 2018) Smolinski J. and Osborn W., 2006, Measurement of Double stars with a CCD Camera: Two methods, Third International Meeting of Dynamic Astronomy in Latin America, Instituto de Astronomía, UNAM, Ed. Carlos Abad, Angel Bongiovanni & Yaneth Guillén.