AAVSO Citizen SkySpectroscopy Workshop 07August 2009 Presented byHopkins Phoenix Observatory
HPO Research Research at the Hopkins Phoenix Observatory UBV Photon Counting Photometry (Since 1980) High Resolution Spectroscopy (Since 2008) .
HPO-1 . HPO-1 is a two-story observatory with an 8” Celestron C-8 telescope.
HPO-1 Photometry HPO UBV PMT based photon counting photometer. .
HPO-2 HPO-2 is a single story roll-off roof observatory with a 12” LX200 GPS telescope. .
HPO Spectroscopy High resolution Lhires III spectrograph .
Lhires III @ HPO Lhires III mounted on a 12” LX200 GPS. DSI Pro I for Guiding (Black) DSI Pro II for Imaging (Blue) .
Introduction Just a few years ago those who did astronomy and had a background in physics could only dream of doing astronomical spectroscopy. Even with a physics background amateur astronomers felt spectroscopy was well beyond reach. In addition to the extreme cost, it was thought very large telescopes were needed even on fairly bright stars. .
The Start All that started to change when CCD cameras became available. It started slow. Cookbook CCD cameras were among the first for amateurs. Then came web cams, modified web cams and low cost astronomical CCD cameras such as the Meade DSI series. The advantage of these devices was that they were very sensitive and ideal for astronomical applications. While most amateur astronomers used these for imaging pretty pictures, some saw the potential for more serious work. Photometry was one of the first uses. Spectroscopy is just now gaining momentum. .
Europeans Lead Just a couple of years ago some serious spectrographs for amateurs and smaller observatories came on the market. These are expensive, but not prohibitive for many amateurs. Coupled with the sensitive CCD cameras, all at once the field of spectroscopy was available to the advanced amateur. User groups started, first in Europe and are just now expanding in the USA and other countries. For the first time amateurs astronomers can contribute professional astronomical spectroscopic data. .
WhySpectroscopy? While photometry measures the brightness of stars in definite fixed bandwidths, spectrometry measures the whole visible spectrum of a star in fine detail. Photometry takes the pulse of a star whereas spectroscopy analyses the soul of the system. .
Producing A Spectrum • There are basically two ways to produce a spectrum. • Using a Glass Prism. • Using a Diffraction Grating • (both transmission and reflective types) .
Prisms Prisms, triangular pieces of glass, were first used to produce a spectrum. .
Refracting Light A sliver of light enters the prism and is delayed (refracted) at different wavelengths causing the light to emerge at different positions producing a rainbow of colors. .
Diffraction Grating A reflective diffraction grating is a reflective surface with closely spaced groves and will produce a spectrum similar to that of a prism. High quality gratings can produce much finer resolution than a prism. . Lhires III removable grating.
Where Do Photons Come From? Atoms consist of a nucleus surrounded by electrons. The electrons are in specific energy states or levels. If an electron is raised to a higher energy state it will soon fall back to its lower state and emit a photon of energy equal to the difference in the two energy states. This is the most common method of photon creation Note: Photons can also be created by other means.
Absorbing Energy A photon interacts with an atom’s orbital electron and raises it to a higher energy state. The electron absorbs the photon’s energy. .
Emitting Energy After a short time the electron falls back to its lower energy state emitting a photon with the energy of the difference between the two energy states .
Energy and Frequency E = h* c / l or l = h* c / E Remember F = 1 / l • Where: • E = Photon Energy • h = Planck’s Constant • c = Speed of Light • = Wavelength • F = Frequency . h = 6.62606896 x 10-34 Js c = 3 X 108m/s
Element Spectra Each element has a unique set of precise energy states that relate to specific spectral Lines. Like a fingerprint. By identifying a set of lines, the element that emitted or absorbed that energy can be identified.
Elements Hydrogen Spectrum Helium Spectrum Neon Spectrum . Sodium Spectrum Mercury Spectrum
Doppler Shift In 1842 Christian A. Doppler discovered an effect that produces a shift in frequency of sound dependent on the motion of the source. A train coming toward you has a higher frequency whistle than when the train is not moving or as the train passes. The departing train’s whistle is a lower frequency than when stationary. This is known as the Doppler Effect or Shift and applies to light as well as sound. .
Radial Velocity It is important to understand that Doppler Shift can provide a measure of radial velocity. That is motion directly to or from the observer. Tangential velocity will not change the wavelength and there will be no Doppler Shift. .
Doppler Equation Vr = Dl* c / l Dl is the change in wavelength due to radial motion l is the stationary wavelength Vr is the relative (radial) velocity c is the velocity in the medium (speed of light in a vacuum is 3 X 108 m/s) . To get just a 1% change in the frequency of light, a star has to be moving 1,864 miles per second. For a blue light bulb to look red, it would have to be flying away from you at 3/4 of the speed of light.
Why is Doppler ShiftImportant? When we know the wavelength shift we can determine the radial velocity of the gas that emitted or absorbed the energy at that wavelength. This tells us how fast the gas is coming toward or going away from us. Orbital velocities of binary stars can be determined with this among other things. .
Types of Spectra Continuous Spectrum Absorption Spectrum . Emission Spectrum
Spectrometers You may here the terms spectrometer, spectroscope and spectrograph. What are the differences? The terms can be used interchangeably, but for the purist there are slight differences. .
Definitions Spectrometer Usually is a prim based device. Spectroscope Prism or diffraction grating device, used visually. Spectrograph Diffraction grating device used with the a detector, e.g., CCD or film cameras. .
Question! When used visually the device is called a spectroscope or telescope. When a detector is connected to the spectroscope it becomes a spectrograph. Does that mean when a detector is connected to a telescope it becomes a telegraph? .
Why Hydrogen Alpha? When there is discussion of spectroscopy of stars you will here hydrogen alpha line mentioned a lot. Since stars are mainly hydrogen and since the hydrogen alpha line is the most prominent line for the element, it gets lots of study. For epsilon Aurigae other hydrogen lines are also of interest including the beta and gamma lines. The sodium D lines as well as the Potassium Ki lines are also of great interest. .
Star Analyser Low resolution spectroscopy For under $200.00 . Holder Star Analyser
Star AnalyserMounted . Star Analyser ready for mounting on a telephoto lens
Star Analyser Spectroscopy DSLR Camera Mounted Star Analyser . Telephoto Lens
Experiment Before going outside into the night, experiment inside your house to get familiar with the equipment and develop an initial technique that can be refined later. . .
Test Setup . . Use the above setup to produce a continuous spectrum for experimentation
Test Spectrum . . Use the test spectrum to experiment with the processing software.
Star Analyser Imaging • Steps: • Find a bright star or object and orient the Star Analyser so that the spectrum is horizontal. Note the source should be to the left of the spectrum. • Set the camera for maximum aperture, ISO 1600 or maximum sensitivity and for a 30 second exposure. .
Vega Spectrum Raw Spectrum . Annotated Spectrum
Alpha Lyrae Spectrum Raw Spectrum .
Robin Leadbeater Robin Leadbeater in Cubria England has perfected a means to use the Star Analyser with a DSLR camera. .
Low ResolutionEpsilon Aurigae Spectrum . Robin Leadbeater’s Star Analyser spectrum Hydrogen beta absorption line can be seen at 4,861Å Note: This is an out-of-eclipse spectrum.
Lhires III Spectroscopy While a Star Analyser can produce a low resolution spectrum showing the whole spectrum, to see details of the spectrum and make precise measurements, a higher resolution grating is needed. The Lhires III is an excellent spectrograph that can use gratings of 150, 300, 600, 1,200 and 2,400 (standard) lines/mm. .
Lhires III Grating Assembly . Lhires – Littrow High Resolution Spectrograph