10IntroductionJust 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..
11The StartAll 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..
12Europeans LeadJust 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..
13Why Spectroscopy?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..
14Producing A SpectrumThere are basically two ways to produce a spectrum.Using a Glass Prism.Using a Diffraction Grating(both transmission and reflective types).
15PrismsPrisms, triangular pieces of glass, were first used to produce a spectrum..
16Refracting LightA 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..
17Diffraction GratingA 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.
19Where 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 creationNote: Photons can also be created by other means.
20The electron absorbs the photon’s energy. Absorbing EnergyA photoninteracts withan atom’s orbitalelectron andraises it to ahigher energystate.The electron absorbs the photon’s energy..
21Emitting 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.
22Energy and Frequency E = h * c / l or l = h * c / E Remember F = 1 / l Where:E = Photon Energyh = Planck’s Constantc = Speed of Light= WavelengthF = Frequency.h = x Jsc = 3 X 108 m/s
23Element SpectraEach 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.
25Doppler ShiftIn 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..
26Radial VelocityIt 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..
27Doppler Equation Vr = Dl * c / l Dl is the change in wavelength due to radial motionl is the stationary wavelengthVr is the relative (radial) velocityc 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.
28Why is Doppler Shift Important? 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..
29Types of Spectra Continuous Spectrum Absorption Spectrum . Emission Spectrum
32SpectrometersYou 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..
33Definitions Spectrometer Spectroscope Spectrograph Usually is a prim based device.SpectroscopePrism or diffraction grating device, used visually.SpectrographDiffraction grating device used with the a detector, e.g., CCD or film cameras..
34Question!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?.
35Why 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..
40ExperimentBefore 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...
41Test Setup..Use the above setup to produce a continuous spectrum for experimentation
42Use the test spectrum to experiment with the processing software.
43Star 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..
49Lhires 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..
50Lhires III . Lhires – Littrow High Resolution Spectrograph Grating Assembly.Lhires – Littrow High Resolution Spectrograph
51Telescopes & Lhires III The Lhires III can be used with telescopes F/8 to F/12 and is optimized for F/10.It works well with telescopes ranging from 8” to 16”.For high resolution work on epsilon Aurigae with a 12” LX200 telescope, 8 minute exposures produce excellent Ha spectra..
54Lhires III Specs 1.0 hour exposure . Grating – Lhires III (lines/mm)Dispersion (Hα) nm/pixResolving powerRadial Velocity Km/sField of view nmAll visual domain in #imagesLimiting magnitude1.0 hour exposure200mm (8”) f/10 telescope, 30µm slit,KAF0400 camera, Signal/Noise of 100).
55Limiting Magnitude . 2400 l/mm Grating Star type B0V - CCD KAF-0400 Slit Width: 25 µm - Seeing : 4 arcsec.Resolving Power (R) : 17000Sampling (KAF-0400) : A/pixelLimiting Magnitude Limiting MagnitudeTelescope S/N=50 in 1 hr S/N=100 in 1 hrD=128 mm (5”) F/D=D=200 mm (8”) F/D=D=280 mm (11”) F/D=D=355 mm (14”) F/D=D=600 mm (24”) F/D=Note: Lower resolution gratings will allow fainter limiting magnitudes.
56Spectrograph SlitTo produce a good spectrum, light must pass through a slit.While we found the slit in the Lhires III was fine as received, if there is any doubt about the slit width or parallelism, it should be examined and adjusted..
57Slit.The Lhires III has an easy removable slit. Additional slits are available in case you need to frequently change slit width.
58is the laser wavelength: Slit MeasurementMeasure the distance “x” projected on your screen. You can then determine the slit width “a” with the formula:a = D * λ / x * lis the laser wavelength:Red laser is 655 or 671 nmGreen laser is 532 nm..
59Here are some values for a laser at 650nm Slit NumbersHere are some values for a laser at 650nmSlitwidthSampleDistance xNote that the slit adjustment is made manually. To adjust, loosen the screws, move the half-slit smoothly, and tighten back. Be careful to keep the slit parallel. This is easy to control visually, by looking at light through the slit..
60Explore/ExperimentWhen you first receive your Lhires III it is suggested you spend several hours during the day on the bench without a telescope and get familiar with the unit. Otherwise you are most likely in for some frustration and wasted time.The built-in neon calibrator provides a simple means of experimenting..
61Confusing SpectrumThe spectrum you will see will be very confusing. This is particularly true when using a high resolution grating. Even using the neon spectrum will be a challenge to positively identify lines.The spectrum window will be narrow so you see only a small portion of the visual spectrum.At HPO using a DSI Pro II and 2,400 lines/mm grating, the window is only about 90 Å wide. The visible spectrum is over 3,000 Å wide..
62Micrometer Calibration One of the first things you should do is get a rough calibration of the micrometer. The micrometer is used to adjust the angle of the diffraction grating to allow different portions of the spectrum to be viewed.Note: The micrometer has significant backlash. Repeatability is approximate and not precise..
64Line IdentificationAs noted earlier even with the neon calibrator, positively identifying lines is a challenge.A red laser pointer will produce a single line around 6,550 Å (some red laser pointers are 6,710 Å) and once found is a good starting point. A green laser pointer is 5,320 Å.A pickle light is an excellent means of identifying where the sodium D lines are..
65Neon Calibration The Lhires III has a built-in neon calibrator. This is convenient for not only calibrating spectra and the micrometer, but, as mentioned earlier, for on the bench experimenting and focusing of the main spectrograph optics.Plan on 1.0 second exposures for the neon lines, but be sure the peak ADU counts stay well under 32,000..
66Neon CalibratorA built-in Neon calibrator that can be operated off 2-9V batteries (18V) is provided for wavelength calibration..
67Neon Spectrum Ne Ne 6,533 Å 6,599 Å Ha 6,563 Å Region Neon lines bracketing the Ha area (1 second exposure)
68Neon Line ProfileThese Ne lines bracket the hydrogen alpha wavelength..Ha6,562 ÅRegion
74Light Leak.There is significant light leak in the Lhires particularly around the grating unit. Half inch strips of metalized duct tape make a good seal.
75Focusing Problem.Focus of the Lhires III changes considerable with just a few degrees change in ambient temperature. Always check the Neon lines for sharp focusing just before imaging the program star.
76Histogram peak is 25,781 ADU counts (1.0 Sec) Out-of-Focus.Histogram peak is 25,781 ADU counts (1.0 Sec)
77Histogram peak is 27,764 ADU counts (1.0 Second) Focused.Histogram peak is 27,764 ADU counts (1.0 Second)
78Hydrogen Alpha Spectra (Line Profiles) Altair SpectrumDeneb Spectrum.Beta Lyrae SpectrumP Cygni SpectrumTaken with a Lhires III and 2,400 l/mm grating
79Imaging Techniques Star above the slit Because the slit cannot be seen the computer “+” cursor is placed over the star & slit for easy reference++Star on the slitCursor over the slit
80ExposureOnce on the star is on the slit with the imaging camera set for 1.0 second exposure, a faint spectrum should be seen.At this time an exposure can be started. Set the exposure time for 8 minutes and start.Essential all the techniques for imaging deep sky objects are used here.Dark and Flat frames for example.
82Imaging Processing Software Once the spectrum has been imaged, the fun starts. There are two major programs used for processing the spectral images, Iris and VSpec (Visual Spec). Also SpcAudace is available, but I have no experience with it.These are freeware and very powerful. They are a challenge to master, however.The goal is to create a calibrated line profile of the spectrum so that characteristics can be measured.
83IRISIris has many features, but at HPO, and many other places, we use it mainly for two preprocessing steps.Subtracting the sky.Optimizing the spectrum.Note: Iris uses signed 16 bit .fits images.This is why it is important to make surethe peak image pixel ADU counts arebelow 32,000.
84VSpecOnce the spectrum has been processed in Iris it is imported into VSpec. Here a line profile is created and wavelength calibration done.With a calibrated line profile, Doppler shifts and Equivalent Widths can be determined along with other characteristics.
85Line ProfilesOne of the nice things about CCDs is that they make creating the line profile very easy.All that needs to be done is to sum each column of pixel ADU values.That means even though a pixel may only have 25,000 ADU counts, when summed with others of that column the total can easily be in the hundreds of thousands ADU counts.
87Wavelength Calibration To be useful the line profile must be calibrated for the wavelength.While the Ne lines can be used for a good calibration, using the multiple atmospheric lines (if seen) can produce amore accurate calibration.
91Earth’s Orbital Motion Depending where the star is and where the Earth is in its orbit around the Sun, the radial velocity of the Earth toward or away from the star can vary between + 67,000 MPH going toward the star to – 67,000 MPH going away from the star..
92Earth’s Rotation.Depending on the latitude of the observer the Earth’s rotation can contribute up to 1,024 MPH to the radial velocity.
93Heliocentric Calibration Once the profileis wavelength calibrated, a Heliocentric correction to remove theEarth’s motion must be made..
94Some Important Wavelengths KI ,699 Å Ha 6,563 ÅNa D ,896 Å Hb 4,861 ÅNa D ,890 Å Hg 4,341 ÅOI ,772 Å Hd 4,102 ÅRed Laser 6,550 Å He 3,970 Å6,571 ÅGreen Laser 5,320 Å.
95Wavelength Calibration Tutorial A detailed tutorial for calibrating a spectrum around the hydrogen alpha region using Vspec can be found on the web site:.
96AnalysisThere are several ways to provide a numerical analysis of a spectral lines.Doppler shifts (for radial velocities).Equivalent Widths (EW) of lines.V/R (Violet or blue to Red EW) line ratios..
97Equivalent WidthContinuumUsing a line profile's Equivalent Widths allows an expression of the part's significance or strength. The area under the curve between the profile part and the continuum is the EW of that part. The area is equal to the Intensity (normalized to 1.0 for the continuum) times EW in angstrom (Å)..
98Out-of-Eclipse Spectroscopy of Epsilon Aurigae .
105Horn Dance . Hydrogen Alpha region of Epsilon Aurigae Left horn is the blue emission lineCenter is the main absorption lineRight horn is the red emission lineThe blue horn and absorption line remain fairly stable, but the red horn dances wildly44 Observations11 August 2008to13 April 2009.
106?? Questions ?? What is the source of the emission lines? What causes the EW of the lines to change?What causes the red horn to vary so much?What will be the effect on these due to the eclipse?What other lines will change and how?.
108Doppler Radial Velocity Radial VelocitiesFrom the following formula, if the change in wavelength is known, the Doppler shift and corresponding radial velocity of the gas can be determined.Doppler Radial VelocityV = Dl * c / l.
109Hydrogen Alpha Observational Data Date: 14/15 October 2008JD: 2,454,755LineObservedCenter Ha lStationaryHa lLlVHa Blue6, Å6, Å-2.19 Å-100 km/sHa Absorption6, Å-0.83 Å-38 km/sHa Red6, Å0.44 Å20 km/s.
110Eclipse StartLhires III.Robin Leadbeater obtained this spectrum of epsilon Aurigae in the Ki region showing evidence of the start of the eclipse.
111Potassium Ki lineThe eclipsing body has potassium in its outer edges and as it started to pass in front of the F star the potassium Ki line (7,699 Å) becomes red shifted. The amount of red shift relates to a +19 km/s radial velocity.This means the leading edge of the eclipsing body is rotating away from us at 19 km/s..
112ConclusionFor those wishing to do meaningful astronomical research, spectroscopy, both low and high resolution offers a means.While challenging, it is well within the capability of advanced amateur astronomers.Perhaps it will the data from an amateur astronomer or small observatory that provides some significant answers to the mystery of epsilon Aurigae..