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First Results from the Echelle Spectrograph at the Trottier Observatory Howard Trottier February 2016 Overview

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First Results from the Echelle Spectrograph at the Trottier Observatory Howard Trottier February 2016 Overview First results from the echelle spectrograph at the Trottier Observatory Howard Trottier February 2016 Overview This is a report on the first results obtained at the Trottier Observatory using our new echelle spectrograph, which is designed primarily for high-resolution stellar spectroscopy. I’ve tried to write the report assuming relatively little physics and math background, intending for it to be used in some form in a second-year breadth course on observational astronomy that I’m developing for students from both the arts and the sciences. However the level of the report is somewhat uneven, with some parts using more sophisticated physics and math – though the math can usually be skipped entirely in favour of the interpretation of the results. Our spectrograph is made by a small company in France, Shelyak Instruments, and is called the “eShel” because of its “echelle” design.1 The basic design principles of echelles are reviewed later; for now, it’s enough to note that echelles are advantageous compared to other spectrometer designs because of their high resolution. Spectral resolution is characterized by the smallest possible wavelength interval δλ between two neighbouring spectral features that can be individually identified, as illustrated below for two emission lines. # Resolution is usually quoted in terms of a fractional measure R that is defined by # The eShel has a resolution R of about 10,000, meaning that it can distinguish between spectral features separated by about as little as 5,000 Angstroms/10,000 or 0.5Å, at a typical wavelength of optical light. The highest-resolution professional echelles have R above 1 The Shelyak webpage on the eShel is at http://www.shelyak.com/dossier.php?id_dossier=47. The company is owned by two of the world’s top “amateur” spectroscopists, François Cochard and Olivier Thizy. Page !1 of !57 100,000. A tradeoff for the high resolution of the eShel (as with all echelles) is its limited sensitivity, which makes it impractical to use for diffuse objects except for the brightest. I’m stunned by the variety and precision of measurements that can be obtained with relative ease using the eShel (that is, after learning how to use the equipment and software!). High- quality data for bright stars can be collected in minutes, and within about an hour for stars down to about sixth or seventh magnitude; this is very different from acquiring high-quality astronomical images, which require many nights of telescope time. And, unlike astronomical image processing, which demands many hours of effort, with lots of trial-and-error, to obtain the best possible final image, the processing of spectroscopic data requires requires relatively little time and intervention by the user, once the initial setup for a particular spectrometer/ telescope combination has been made. When the unit arrived in late June I knew next-to-nothing about how an echelle works, or how to use one, and I had only a vague sense of what could be done with it. I spent a couple of months learning how to install and calibrate the device, and how to use the data acquisition and analysis software packages. Then from late summer through early winter I spent a good deal of time taking data for a wide range of astrophysical objects, and figuring out how to squeeze as much quantitative information as I could from the results. In addition to measuring the spectra of a few star types, including Vega and the Sun (via the Moon), and closely comparing the results with data from other observatories, I obtained results for the following: the Ring Nebula’s emission spectrum; the orbital velocities2 of the stars in two spectroscopic binaries; the rotational speeds of several stars; the properties of several circumstellar disks; the spectra of stars with fast winds; and, perhaps most exciting of all, extraordinarily precise measurements of the (relative) recessional velocities of some carefully selected stars.3 I also came across some very useful on-line resources that I’ll cite as they come up; some of these, along with the results presented here, should help to suggest projects that others can undertake. And if you want to learn more about how our spectrometer and others work, and about the astrophysics of the systems you might want to study, I’ve included a bibliography of books that run from newbie (without math) to advanced undergrad/grad-level texts. Most of these books will soon be on the shelves of a bookcase at the observatory. Although the data taking and analysis are highly non-trivial procedures, they are made almost routine by some powerful (and free) software – though getting familiar with all of the features and interfaces takes awhile ;). I will eventually write a guide to the software setup and interfaces as used at our observatory,4 here I include information on the capabilities of these packages, emphasizing how the software is used in the analysis of the results. We recently took delivery of a second spectrograph, also made by Shelyak:5 it has lower resolution but higher sensitivity than the eShel, and will provide us with complementary capabilities, notably the ability to take spectra of faint extended objects and measure quantities like galactic redshifts and rotation curves! I hope to install and test the new unit in the spring. 2 Velocities can only be measured along the line of sight of the object. 3 As explained in Section R4, these results show that we should be able to detect a few hot-Jupiter exoplanets! 4 See also Olivier Thizy’s blog http://observatoire-belle-etoile.blogspot.fr/2016/01/eshel-data-reduction.html. 5 The second spectrograph is the Shelyak “LISA”: http://www.shelyak.com/rubrique.php?id_rubrique=12. Page !2 of !57 Organization of the report The rest of the report is organized as follows. The next section presents more information on the eShel spectrograph, and contains: a short description of how it is set up for an observation run at our observatory; a brief outline of the basic design principles of echelle spectrometers (perhaps too brief to be all that clear or informative!); an overview of how the spectrometer is calibrated; and a brief description of the software that is for data capture and analysis, along with a list of some useful on-line resources. The report then presents a series of detailed sections that analyze and interpret data that I took for a wide variety of astrophysical objects. The results are divided into six sections: Vega as a case study (Section R1); the Ring Nebula and the Moon (R2); Spectroscopic binaries (R3); High-precision radial velocities (R4); Doppler rotational broadening (R5); and last but not least: Circumstellar disks and stellar winds (R6). Note that Section R1 on Vega introduces alot of background information that is used in the later sections. Most of the other sections include a fair amount of background information that is needed to fully analyze and interpret those results. Since I’m still learning about alot of this stuff, I hope that what I’ve written is reasonably clear and accurate, but there are bound to be mistakes, possibly including some whoppers. By all means, skip anything that is too detailed, confusing, or boring! And I welcome any and all feedback, especially any mistakes that you may find. Page !3 of !57 Introduction to the eShel Spectrograph layout at the observatory Here are some pictures that show the eShel layout when in use at the observatory. Incident starlight Fibre feed from spectrograph Mirror Guide camera control 50 μm fibre feed to spectrograph Spectrograph Acquisition unit Acquisition High camera voltage power Control unit & supply calibration sources Spectrometer Mirror hole Vega # The spectrograph is remarkably compact, and sits on top of a small dolly that is stored in the bathroom (of all places). The dolly should be wheeled into the dome area at least an hour before beginning observations, in order to equilibrate the spectrometer to the ambient temperature. A light-weight acquisition/guiding unit is attached to the periscope. The acquisition unit houses a plane mirror with a small hole: the mirror deflects almost all of the incoming starlight to a Page !4 of !57 guide camera mounted on the side of the unit, while a fibre optic cable mounted behind the hole in the mirror feeds light to the spectrometer. The spectrograph is positioned beside the telescope control panel, where the ends of the acquisition unit’s fibre optic and control cables come up through the floor, and are attached to the spectrometer and its control unit. The spectrometer’s diffraction grating disperses the light, and a CCD camera mounted on the unit images the resulting spectrum (more on the actual design of the eShel coming up). USB cables connect the equipment to the control room computer. The spectrograph has calibration sources, and the light that they produce when activated is fed to the acquisition unit at the telescope through a separate fibre optic cable. A flip mirror in the acquisition unit redirects the calibration light to the pickup fibre: this ensures that the calibration is done with light that follows the same optical path to the spectrometer as the starlight. More information on the calibration procedure is coming up. The fibre optic cable that transmits the incoming starlight is only 50 microns in diameter! The small size is necessary for the high resolution, but limits the sensitivity of the spectrometer. The above images of the Ring Nebula and Vega (the latter on a night of poor seeing!) were taken through the guide camera, and show the tiny region occupied by the hole in the mirror.
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