AAVSO Citizen Sky Spectroscopy Workshop 07August 2009 Presented by Hopkins Phoenix Observatory.

AAVSO Citizen SkySpectroscopy

Workshop

07August 2009 Presented

byHopkins Phoenix Observatory

Background

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HPO Research

Research at the Hopkins Phoenix Observatory

UBV Photon Counting Photometry(Since 1980)

High Resolution Spectroscopy(Since 2008)

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HPO-1

HPO-1 is a two-story observatory with an 8” Celestron C-8 telescope.

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HPO-1 Photometry

HPO UBV PMT based photon counting photometer.

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HPO-2

HPO-2 is a single story roll-off roof observatory with a 12” LX200 GPS telescope.

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HPO Spectroscopy

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High resolution Lhires III spectrograph

Lhires III @ HPO

Lhires III mounted on

a 12” LX200

GPS.

DSI Pro Ifor Guiding

(Black)

DSI Pro IIfor Imaging

(Blue).

Configuration

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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.  

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The 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.

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Europeans Lead

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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?

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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

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There are basically two ways to produce a spectrum.

1.Using a Glass Prism.

2.Using a Diffraction Grating (both transmission and reflective types) 

Prisms

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Prisms, triangular pieces of glass, were first used to produce a spectrum.

Refracting Light

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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

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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.

Physics Review

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

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A photoninteracts with

an atom’s orbital electron and raises it to a

higher energy state.

The electron absorbs the

photon’s energy.

Emitting Energy

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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

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E = h * c / or

= h * c / E

Remember F = 1 /

h = 6.62606896 x 10-34 Jsc = 3 X 108

m/s

Where:E = Photon Energyh = Planck’s Constantc = Speed of Light = WavelengthF = Frequency

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

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Hydrogen Spectrum

Neon Spectrum

Helium Spectrum

Sodium Spectrum

Mercury Spectrum

Doppler Shift

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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

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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

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is the change in wavelength due to radial motion

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 10

8 m/s)

Vr = * c /

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?

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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

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Continuous Spectrum

Absorption Spectrum

Emission Spectrum

Stellar Spectrum

Solar Spectrum

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Spectrometers

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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

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SpectrometerUsually is a prim based device.

Spectroscope Prism or diffraction grating device, used visually.

SpectrographDiffraction grating device used with the a detector, e.g., CCD or film cameras.

Question!

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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?

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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.

Low ResolutionSpectroscopy.

Star Analyser

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HolderStar Analyser

Low resolution spectroscopyFor under $200.00

Star Analyser Mounted

.Star Analyser ready for

mounting on a telephoto lens

Star Analyser Spectroscopy

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DSLR Camera

Telephoto LensMounted

Star Analyser

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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.

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Test Setup

Use the above setup to produce a continuous spectrum for experimentation

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Test Spectrum

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Use the test spectrum to experiment with the processing software.

Star Analyser Imaging

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Steps:

1.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.

2.Set the camera for maximum aperture, ISO 1600 or maximum sensitivity and for a 30 second exposure.

Vega Spectrum

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Raw Spectrum

Annotated Spectrum

Alpha Lyrae Spectrum

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Raw Spectrum

Robin Leadbeater

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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 4,861ÅNote: This is an out-of-eclipse spectrum.

High ResolutionSpectroscopy.

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.

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Lhires III

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Grating Assembly

Lhires – Littrow High Resolution Spectrograph

Telescopes & 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 H spectra.

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Lhires III Design

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Lhires III Light Path

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Lhires III Specs

Grating – Lhires III (lines/mm) 2400 1200 600 300 150 Dispersion (H) nm/pix 0.012 0.035 0.074 0.149 0.300 Resolving power 17000 5900 2800 1400 700 Radial Velocity Km/s 5 17 35 75 150 Field of view nm 8.5 25 55 110 230 All visual domain in #images 45 15 7 4 2 Limiting magnitude 5.0 6.8 7.5 8.4 9.2

. 1.0 hour exposure200mm (8”) f/10 telescope, 30µm slit, KAF0400 camera, Signal/Noise of 100)

Limiting Magnitude2400 l/mm Grating

Star type B0V - CCD KAF-0400 Slit Width: 25 µm - Seeing : 4 arcsec.

Resolving Power (R) : 17000 Sampling (KAF-0400) : 0.115 A/pixel

Limiting Magnitude Limiting Magnitude Telescope S/N=50 in 1 hr S/N=100 in 1 hrD=128 mm (5”) F/D=8 6.5 5.6

D=200 mm (8”) F/D=10 6.7 5.9

D=280 mm (11”) F/D=10 7.1 6.2

D=355 mm (14”) F/D=11 7.2 6.3

D=600 mm (24”) F/D=8 8.1 7.2

Note: Lower resolution gratings will allow fainter limiting magnitudes

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Spectrograph Slit

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To 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.

Slit

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The Lhires III has an easy removable slit. Additional slits are available in case you need to frequently change slit width.

Slit Measurement

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Measure the distance “x” projected on your screen. You can then determine the slit width “a” with the

formula: a = D * λ / x *

is the laser wavelength: Red laser is 655 or 671 nm

Green laser is 532 nm.

Slit Numbers

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Here are some values for a laser at 650nm

Note 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.

Slitwidth

Distance x

Sample

Explore/Experiment

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When 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.

Confusing Spectrum

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The 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.

Micrometer Calibration

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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.

Dr. Bob’s Calibration

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in Å

Mic

rom

ete

r Sett

ing

Line Identification

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As 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.

Neon Calibration

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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.

Neon Calibrator

A built-in Neon

calibrator that can be operated off

2-9V batteries (18V) is

provided for wavelength calibration.

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Neon Spectrum

Ne6,533 Å

Ne6,599 Å

H6,563 ÅRegion

Neon lines bracketing the H area (1 second exposure)

Neon Line Profile

.These Ne lines bracket the hydrogen alpha wavelength.

H6,562 ÅRegion

Neon Lines

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Laser Pointer

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Shine the laser pointer at the telescope plastic dust cover (to reduce the intensity) on the spectrograph.

Laser Pen Line

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Laser Pen Line is at 6,550 Å (just one line – 1.0 second exposure)

Laser & Neon Lines

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Ne6,533 Å

Laser6,550 Å

Ne6,599 Å

H6,563 ÅRegion

Neon and laser lines bracketing the H area (1 second exposure)

Pickle Light

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Sodium D Lines 5,889.95 Å & 5,895.92 Å

Light Leak

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There is significant light leak in the Lhires particularly around the grating unit. Half inch strips of metalized duct tape make a good seal.

Focusing Problem

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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.

Out-of-Focus

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Histogram peak is 25,781 ADU counts (1.0 Sec)

Focused

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Histogram peak is 27,764 ADU counts (1.0 Second)

Hydrogen Alpha Spectra(Line Profiles)

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Altair Spectrum

Beta Lyrae Spectrum P Cygni Spectrum

Deneb Spectrum

Taken with a Lhires III and 2,400 l/mm grating

Imaging Techniques

Because the slit cannot be seen the computer “+”

cursor is placed over the star & slit for easy reference

+

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Star above the slit

Star on the slit Cursor over the slit

ExposureOnce 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.

Image Processing

Imaging ProcessingSoftware

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.

IRIS

Iris has many features, but at HPO, and many other places, we use it mainly for two preprocessing steps.

1.Subtracting the sky.

2.Optimizing the spectrum.

Note: Iris uses signed 16 bit .fits images. This is why it is important to make sure the peak image pixel ADU counts are below 32,000.

VSpec

Once 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.

Line Profiles

One 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.

Spectrum/Line Profile

Line Profile

Spectrum

Continuum Level

Absorption Lines

Wavelength

Pixel ADU Counts

Wavelength 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.

Atmospheric Template

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Atmospheric Calibration

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While the neon spectral allows a good wavelength calibration using atmospheric lines can calibrate the spectrum more accurately.

What isHeliocentric

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Earth’s Orbital Motion

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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.

Earth’s Rotation

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Depending on the latitude of the observer the Earth’s rotation can contribute up to 1,024 MPH to the radial velocity.

Heliocentric Calibration

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Once the profile is wavelength calibrated, a Heliocentric correction to remove the Earth’s motion must be made.

Some Important Wavelengths

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KI 7,699 Å H 6,563 ÅNa D1 5,896 Å H 4,861 Å

Na D2 5,890 Å H 4,341 Å

OI 7,772 Å H 4,102 Å

Red Laser 6,550 Å H 3,970 Å 6,571 Å

Green Laser 5,320 Å

Wavelength CalibrationTutorial

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A detailed tutorial for calibrating a spectrum around the hydrogen alpha region using Vspec can be found on the web site:

http://www.hposoft.com/HaCalibration.pdf

Analysis

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There are several ways to provide a numerical analysis of a spectral lines.

1.Doppler shifts (for radial velocities).

2.Equivalent Widths (EW) of lines.

3.V/R (Violet or blue to Red EW) line ratios.

Equivalent Width

.Using 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).

Continuum

Out-of-EclipseSpectroscopy

ofEpsilon Aurigae.

Star System

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Sodium D Lines

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Hydrogen Alpha

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Hydrogen AlphaAnalysis.

Summary Table

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Emissive Absorption EmissiveUT Blue Horn Red HornDate Center Center Center 2008 EW Å EW Å EW Å VR 08/11 0.424 6,561.40 -1.009 6,563.11 0.001 6,564.77 424.00008/22 0.273 6,561.52 -1.056 6,563.10 0.000 N/A 9999.99909/03 0.292 6,561.33 -0.904 6,563.11 -0.023 6,564.76 -12.69609/05 0.342 6,561.51 -0.887 6,563.15 -0.118 6,565.29 -2.89809/22 0.265 6,561.12 -0.993 6,562.85 0.059 6,564.59 4.49209/29 0.163 6,560.71 -1.327 6,565.50 0.009 6,564.12 18.11110/12 0.225 6,560.52 -1.003 6,562.14 0.138 6,563.48 1.63010/14 0.378 6,560.62 -1.127 6,561.98 0.108 6,563.25 3.50010/15 0.343 6,561.28 -1.002 6,562.98 0.328 6,564.78 1.04610/19 0.256 6,561.66 -1.088 6,563.41 0.080 6,564.76 3.20010/19 0.268 6,561.32 -1.070 6,562.95 0.220 6,564.72 1.21810/21 0.342 6,561.32 -1.011 6,563.01 0.223 6,564.65 1.53410/21 0.262 6,561.50 -1.015 6,563.16 0.130 6,564.83 2.02310/24 0.396 6,560.14 -0.881 6,561.61 0.275 6,563.02 1.44010/26 0.341 6,561.41 -1.051 6,563.07 0.207 6,564.56 1.64710/28 0.359 6,561.30 -1.025 6,562.98 0.243 6,564.59 1.47710/30 0.305 6,561.31 -0.992 6,563.06 0.256 6,565.06 1.19111/01 0.136 6,561.50 -1.046 6,562.94 0.091 6,564.20 1.495

Equivalent Width Plot

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Horn Dance

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Hydrogen Alpha regionof

Epsilon Aurigae

Left horn is the blue emission line

Center is the main absorption line

Right horn is the red emission line

The blue horn and absorption line remain fairly

stable, but the red horn dances wildly

44 Observations

11 August 2008to

13 April 2009

?? Questions ??

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1. What is the source of the emission lines?

2. What causes the EW of the lines to change?

1. What causes the red horn to vary so much?

2. What will be the effect on these due to the eclipse?

3. What other lines will change and how?

WavelengthAnalysis

Radial Velocities

.Doppler Radial Velocity

V = * c /

From the following formula, if the change in wavelength is known, the Doppler shift and corresponding radial velocity of the

gas can be determined.

Hydrogen Alpha Observational Data

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LineObserved

Center HStationary

H

V

H Blue 6,560.62 Å 6,562.81 Å -2.19 Å -100 km/s

H Absorption 6,561.98 Å 6,562.81 Å -0.83 Å -38 km/s

H Red 6,563.25 Å 6,562.81 Å 0.44 Å 20 km/s

Date: 14/15 October 2008JD: 2,454,755

Eclipse Start

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Robin Leadbeater obtained this spectrum of epsilon Aurigae in the Ki region showing evidence of the start of the eclipse.

Lhires III

Potassium Ki line

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The 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.

Conclusion

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For 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.

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