In 2005-2006 Sarah Howell and I did a project involving emission-line stars. We wanted to observe some of the strange bands we had seen in a few stars in our previous classical spectrograph construction project. We used an observatory in Oklahoma at Northern Oklahoma College and tried a spectrograph available there—the SBIG (Santa Barbara Instruments Group) Self-Guided Spectrograph, or SGS. We didn’t actually use the auto-guiding capabilities of the spectrograph.

 

 

1.    Abstract

The first goal of this project was to document the setup, calibration, and operation of the Santa Barbara Instruments Group Self-Guiding Spectrograph (SBIG SGS). It was used in conjunction with a SBIG ST-8 CCD camera and a Celestron 14-inch Schmidt-Cassegrain telescope. The second goal was to gather and analyze spectral data. Our targets were emission-line objects such as Wolf-Rayet and Be stars.

Problems included equatorial drift and the misidentification of faint target stars. Drifting problems were improved by adjusting the Paramount equatorial alignment with the program Tpoint. Stars were centered on the camera’s tracking chip and then spectra were obtained with imaging chip. It was necessary to find the brighter of two images, since one image was a spurious reflection. The spectra were wavelength and flux calibrated with Desnoux’s VisualSpec program, which also enabled line identification and spectral synthesis.

The spectra obtained were analyzed with Planck curves, elemental line identification, and Doppler broadening calculations. Carbon bands in WC stars were initially mistaken as having been Doppler broadened. Published research and spectra on the stars provided a cross-check on the identification of elements. In some cases, low resolution, short exposures, and the complication of binary systems limited confirmation of astrophysical models.

2.    Objectives

1.      To setup and successfully operate an Santa Barbara Instruments Group (SBIG) Self-Guided Spectrograph (SGS) and document the processes for future users of the spectrograph.

2.      To gather data of celestial objects, especially emission-line objects like Wolf-Rayet stars and Be stars.

3.      To understand the astrophysics of these unusual stars by measuring atmospheric elemental composition, outflow wind velocities, and temperature through analyzation of the spectra.

3.    Theory

3.1.Overview

Our primary interest in this project was to understand Wolf Rayet stars. Wolf-Rayets are massive stars (more than 25 solar masses) that are at an evolutionary phase during which they undergo heavy mass loss. The most massive O stars, with a mass of 80 suns, lose much of their hydrogen to become 25 solar mass Wolf Rayet stars. They are extremely rare (hardly more than 150 are known to exist in our galaxy) as a consequence of their short lifetimes (only a few million years). It is thought that Wolf Rayet stars are a precursor  to supernovae.

 

3.2.Wolf-Rayet Star Formation – Our interest

Star formation starts when diffuse hydrogen and helium gas in the interstellar medium is compressed. This can happen from supernova shock waves, density waves in the spiral arms of galaxies, or even by the shock wave of winds from a Wolf- Rayet star.

In order to form stars in a cloud of gas gravitational forces must be greater than the thermal and other forces driving it apart. These clouds can be 100 to 10000 suns in mass. These lead to compact groups of new star birth. Modeling is complicated for as the gas collapses, thermal and magnetic forses oppose collapse. When considering the birth of a single star, the collapse brings the temperature to a point where it becomes hot, luminous, and opaque.  This happens when the hydrogen becomes ionised. The collapse phase may last from 10 to 30 years. With a surface temperature of 3,000K, the new protostar radiates at visual and infrared wavelengths, hidden by dust in the collapsing cloud. Eventually, the protostar becomes hot enough to fuse hydrogen nuclei (protons) to form helium. At this point, the star enters the main sequence.

 

Fig 1. Hertzsprung-Russell diagram of normal stars. Wolf Rayet Stars are as top left.

http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/bt2lf1509_a.jpg

 

Massive Stars, such as O and B stars,  have short lives, since the cores are larger and hotter than smaller stars like our sun. Nuclear reactions are dependant on high temperatures to overcome the electrostatic repulsion between protons before they can fuse to become helium and later, other elements. After a brief hydrogen burning main sequence phase, they form a helium burning shell. In Wolf-Rayet stars, they quickly leave the main sequence as radiation driven winds from the helium burning shell drives off the outer hydrogen layer. The core of the WR star also may be exposed by convection dredging up the layers, driving the helium (and carbon, etc.) to the surface of the star.

 

Fig 2. Hertzprung-Russell (H-R)  diagram of Wolf Rayet Stars.

 

The winds of WR stars are driven by line scattering in which the photons momentum is transferred to the (outer) hydrogen gas. These fast hot winds may create a nebula, as the supersonic pressure wave piles up the interstellar medium of gas and dust in front of it.  These winds with velocities on the order of  2000 km/sec, cause the star to lose mass at the rate of approximately  10 –5 solar masses per year.

 

Fig 3. WR136 Crescent Nebula in Cygnus

http://www.aip.de/~gallery/stars/ngc7635_noao_big.jpg

 

“Their surface composition is extremely exotic, being dominated by helium rather than hydrogen, and typically showing broad wind emission lines of elements like carbon (WC type), nitrogen (WN type), or oxygen: the products of core nucleosynthesis. The presence or absence of hydrogen, respectively, is used to distinguish the so-called ‘late’ type WN stars (WNL) from the ‘early’ (WNE) types.” (http://www.peripatus.gen.nz/Astronomy/WolRaySta.html)

After the helium burning, carbon, neon, oxygen, and silicon burn.  Eventually, a core of iron ash is created. Since iron does not liberate energy as it fuses, the iron core cannot thermally support it’s own weight.  The iron then fragments into neutrons. This collapse drives a type II supernova explosion. Type II supernovae do not show hydrogen, again because the Wolf-Rayet star blew the hydrogen off of it’s helium burning core. In the end, the massive core becomes a black hole.

Many Wolf Rayet stars are binary systems. These often occur as W + O systems. As the O star plows through the thick wind from the WR star, a shock wave can be formed, often emitting x-rays. There is almost no hydrogen left in WR atmospheres, and consequently, the WR stars have started their 2nd or 3rd nuclear fusion process, burning helium or other elements, creating elements like carbon, nitrogen, oxygen, and iron. Based on their spectra, WR stars are divided into two (though sometimes WOs are categorized in another group) classes. WN stars’ spectra (visible light) show emission lines from H, NIII (4640 A), NIV, NV, HeI, HeII, and CIV (5808 A). WCs show from H, CII, CIII (5696 A), CIV (5805 A), OV (5592 A), HeII, and HeI. No nitrogen lines are seen in WCs.

The stars are surrounded by the material of their heavy stellar winds. They blow out their envelope with speeds of about 2000 km/sec and loose large amounts of mass (about equivalent to one earth mass/year). Therefore WR spectra are unique; they are characterized by thick broadened emission lines of highly ionized elements. By the knowledge of the Doppler effect, one can derive the approximate velocities of the winds in the star from the line-profile.

 

3.6              Theory of Energy Generation in Stars

Main sequence stars start out by “burning” hydrogen, via the proton-proton cycle. This occurs via:

a)   Two hydrogen atoms (protons mass=1) undergo a fusion to produce a positron, a neutrino, and a deuterium nuclei (Mass=2=proton+neutron).

b)      The deuterium nuclei reacts with another proton  to produce Helium-3 and a gamma-ray (γ)

c)      Two helium-3 isotopes produced in steps (a) and (b) fuse to form a helium nucleus + 2  protons.

 

Fig. 4. P-P reaction.

http://www.astroclub.net/mercure/aav/bol17/evolucion.php

Later in the evolution of the star…..

2 helium nuclei (mass=4)are converted to berrylium (mass=8) + a gamma-ray(γ)

One helium nuclei (mass=4) and one berylium (mass=8) are converted to carbon +  γ

Later, the CNO cycle uses carbon as a catalyst. In heavier stars, the CNO cycle predominates.

 

3.5  Basic Spectroscopy Theory

 

            Stars (an other objects) can be analyzed by considering their radiation as a blackbody of a certain temperature.  Cold stars peak in the red part of the visible spectrum, while hotter stars radiation peaks in the blue spectrum.  These peaks are subtle, with our eyes being adapted to see white as an additive  mixture of all of the suns wavelengths of visible light as filtered by our atmosphere. Fortunately, the damaging UV through gamma rays are filtered out. In addition,  individual atoms can only emit or absorb light at certain wavelengths or energies. The light is absorbed by a photon being absorbed by a electron, raising the electron to a higher energy level, or shell,  in the atom. Hydrogen is the simplest atom spectroscopically, with only 1 electron shell, and is dominant in the visual  spectrums of many stars. The first (Lyman) hydrogen line is in the ultraviolet, but successive lines form the visible Balmer series, with energies between 1 and 3 electron volts. For hydrogen gas to show Balmer spectra, it must be at a temperature hot enough that many of the atoms must already be in the second energy level.

The energy of an atoms radiation is related to its frequency:

 

E=hc/λ

 

where E=energy in electron volts  h=plancks constant.

Spectral lines do not necessarily correlate exactly with the corresponding atomic transition. They can be broadened or shifted from the following causes:

 

1)         Natural line width : approximately 10-12 Å- caused by uncertainty in atomic transition

2)         Doppler broadening: lines in the stars spectrum are broadened by motion of atoms in the gas of the star. Some atoms are moving toward the observer, while other atoms are receding. This effect increases as the gas temperature rises.

3)         Lines may be split by magnetic fields (the Zeeman effect)

4)         Lines may be split by electric fields.

5)         Hyperfine structure, with natural doubling of lines, as in sodium.

 

Small dwarf stars, like our sun,  have broadened spectral lines, since their high surface gravity causes the atoms and their disturbing electric charges are close together. In the rarified atmospheres of giants, spectral lines are narrower.

Example:          1.1 Magabs      =          16.6 Å width (a2 Gemini)

                        -6.3 Magabs    =          1.9Å width (HD 22385) source-Atoms Stars and Nebulae:Aller

This produces the spectroscopic basis for determining the luminosity classes of stars, with supergiants class I and main sequence stars class V. Further division is made by Ia being larger than Ib.

                       

3.6  Spectrograph operation

 

            Normally, a spectrograph needs a collimator, a grating or prism, and a camera. The collimation lens makes light rays parallel. The grating separates out the wavelengths, and the camera (with its lens) focuses the different colors to different points and translates the image into something we can understand. In the SBIG spectrograph, mirrors are used instad of lenses.

The selection of a grating for a spectrograph is a key step. A reflection or transmission grating can be used. A reflection grating is a mirror engraved with grooves at regular intervals. Transmission gratings are composed of thin transparent film engraved with grooves. When light hits a transmission grating, the film behaves like a prism, separating out the various wavelengths of light so that the density of each color can be determined. Reflection gratings behave much the same way, reflecting different wavelengths of light at different angles from the incident ray.

A CCD (Charged Coupled Device) camera is used for modern spectrographs. Whenever light hits the CCD chip of the camera, the pixels on the chip record whatever photons hit that specific area. The computer chip will then generate a picture of the photons which hit each individual pixel of the CCD.

Some important requirements for a high-quality spectrograph are the f-stop, the exit pupil, the grating, and the slit.

Light will be lost if the collimation lens’ f-stop is not lower than or equal to the f-stop of the telescope. The f-stop is equal to the focal length divided by the diameter of the telescope.

The telescope focuses the parallel light from the star into a cone that must be focused on the slit of the spectrograph. The width of the slit corresponds to the width of the resulting spectral lines. The resolution of the spectrograph depends on the number of grooves per mm on the grating, the number of grooves illuminated, and the width of the slit.

The CCD chip must be parallel to the grooves of the grating, or else the spectra will not be perfectly vertical.

All data was gathered using equipment at the Northern Oklahoma College (NOC) Observatory. Data was recorded using a Santa Barbara Instruments Group (SBIG) ST-8 Charged Coupled Device (CCD). This camera was clamped to an SBIG Self-Guiding Spectrograph (SGS), which was threaded onto the Celestron 14” Schmidt-Cassegrain mounted on a Paramount housed in the Ash dome on the NOC Enid campus.

SBIG stands for the Santa Barbara Instrument group (a company in California). The SBIG SGS has two gratings that can be used. They can be flipped so that either a 150 lines/mm (low resolution) or 600 lines/mm grating (high resolution) can be manipulated. The instrument can also be used with a slit 18 or 72 microns wide. The resolution of the SGS (with an 8” or 20 cm telescope) with the wide slit and the 600 lines/mm grating is 10 A. With the narrow slit it is 2.4 A. Our resolution should have been higher than this, since we used a 14” (about 36 cm) telescope.

The SGS has a basic Fastie-Ebert design configuration, with another optical path for the tracking chip. The light for the tracking chip goes through the telescope, reflects off the slit, reflects of another front face mirror, goes through the focus achromat, reflects off more mirrors, and hits the tracking CCD chip. The light for the spectra follows a different path. It passes through the telescope, through the slit, reflects off a mirror, reflects of a spherical mirror, reflects of the grating, reflects off the same spherical mirror (at a different angle and area), and reflects of another mirror onto the large CCD chip. The spherical mirror acts as a calibration lens, collimating the light from the telescope before it hits the grating.

First, a list of astronomical targets is made. The rise and set times of these stars and their magnitude are taken into consideration. Most of the WR stars visible in the season and at the time of the experimentation are clustered in the constellation Cygnus. Cygnus is setting and very close to the horizon during the times that data can be obtained. The priority targets are the brightest of the WRs near Cygnus (6th, 7th, or 8th magnitude). It was thought that these were bright enough for the SBIG SGS since the spectrograph could be self-guided and could take long exposures.

Software for taking images and analyzing spectra must be downloaded. This project required many programs. CCDOPS is a program that captures images from the ST-8 CCD and saves them. Spectra is a simple calibration program that allows two-line calibrating and that synthesizes spectra. Visual Spec is a more comprehensive analyzing tool that can be used to manipulate spectra. SolidCapure is a program that allows the end user to take screen shots. In addition, many other drivers were needed.

 

Fig. 4 Light path through the SBIG SGS (courtesy Dale Mais)

 

 

4.    Procedure

 

Setting-Up and Using the SBIG SGS With the ST-8 CCD Camera.

1.      Remove camera adapter flange (and filter wheel if present) from the SBIG ST-8.

2.      With the camera facing up and the power port facing towards you, mount the spectrograph coupling so that the chamfered edges point to the right.

3.      Remove the lid of the spectrograph by removing the four Phillips screws around the baseplate.

4.      Reposition the camera mounting plate if required.

5.      Loosen the clamp that holds the camera (next to the hole that holds the camera (next to the hole without the SCT threads).

6.      Insert the camera so that its power cables are on the opposite end of the SGS from the LED switch on the bottom of the spectrograph.

7.      After affirming that the camera is resting securely inside the hole, tighten the clamp.

 

Using SBIG’s Spectra Program to Focus Spectrograph.

8.      Download SBIG’s CCDOps and Spectra programs. Run SBIG Driver-Checker Utility.

9.      Hook up camera to computer and power with supplied cords.

10.    Run CCDOps. Click CameraàEstablish COM Link to establish a link with the camera.

11.    Click CameraàSetup, and select the tracking CCD. Click CameraàFocus, and select 0.12 second exposure. Enable dark frames.. Select continuous focusing mode on Full-Low. A continuously updating image should appear on the screen.

12.    Dim room lights.

13.    With a dim flashlight pointed at the telescope coupling, loosen the focus-adjustment lens and slide it back and forth until the slit in the image on the screen appears sharpest and of thinnest width.

14.    Loosen the camera and rotate it until the image of the slit is as vertical as possible.

15.    Position an Hg-lamp in front of the telescope coupling and turn it on. Avoid directly looking at the lamp.

16.    Adjust micrometer to 5.44 mm (or another desired wavelength).

17.    Flip grating toggle up, away from the micrometer into low resolution mode. (The lever should be straight; if it is tipped erroneous spectra will be obtained.)

18.    In CameraàSetup in CCDOops, select imaging CCD. Using Focus mode in Full-High resolution set to 0.5 second exposures, acquire images.

19.    Select pause when a clear image is obtained and close the focus dialog box.

20.    Select Magnification 1:2 so that the whole image is visible.

21.    Select <Utility>, <Crop>, <Spectroscopy>, <Crop the image>.

22.    Run the SBIG Spectra program. Click “Load Spectra” and choose the cropped image previously saved.

23.    Adjust the horizontal scroll bar to the first desired line (we used Hg’s 5790 A line). In the Hg spectrums the pair of lines at 5790 and 5770 A is easily recognized. The line at 5460 is the brightest line and bright green (though the spectrum is in black-and-white, so no colors are visible). You may need to calibrate off this line in high resolution mode. After selecting a line, click “Mark Line 1.”

24.    Adjust horizontal scroll bar to the second desired line (we used 4383 A, a blue line). It’s generally a good idea to calibrate off spectral lines that are farther apart rather than close together.

25.    Click “Mark Line 2.”

26.    Click “Expand Spectra.” The program should colorize the spectra, and you should see two yellow, one green, and one blue-violet line. There may be other lines in the spectrum due to higher orders or to the mechanical structure of the grating and spectrograph.

27.    To obtain high-resolution data, flip the grating toggle lever 180 degrees so that the 600 lines/mm grating is being used.

28.    Capture the image.

         Disassembling (Without Using Telescope).

29.    Shutdown the link with the ST-8.

30.    Unplug the power cord from the wall socket and from the port on the bottom of the ST-8.

31.    Unplug the cord that links the camera to the computer from both the camera and the computer.

         Setting-Up SGS to Telescope.

32.    If anything is attached to the telescope’s SCT-threads (eyepieces, lens cap, etc.), unscrew and remove it. Directly attach the SGS to the telescope using the SCT-threads.

33.    Put a ring of foamie on the end of the telescope to block out most of the room lights. It will serve as a dew cap during the observation session.

34.    Open CCDOps and select <Misc>, <Graphics/Comm Setup> and make sure that the interface selected is parallel.

35.    Establish a link with the ST-8.

36.    Balance the telescope on the mount.

37.    Take a calibration spectrum.

 

Taking Calibration Spectrum.

38.    Loosen the screws on the bottom of the spectrograph and slide the cover for the calibration light out of the way, exposing a hole into the SGS.

39.    Hold a mercury lamp light up to the hole, while taking an exposure (using the imaging chip).

40.    Locate the mercury line pattern.

41.    To move the mercury lines to the right or left, turn the micrometer to higher/lower number settings. When saving captured images, incorporate each micrometer setting of each image into the filename.

 

Focusing the Telescope.

42.    Swerve the telescope to a star and put it in the crosshairs.

43.    In CCDOps use the tracking chip and use Focus mode to take continuous frames using Full-Low and a short exposure and no dark frames.

44.    Locate the star on the chip and move it to the approximate center.

45.    If it is unfocused, you will see a ring. Turn the focusing knob on the telescope until the ring gets smaller and turns into a point. Also, as another reference, watch the intensity numbers; the highest is normally about 65,000. The higher the number, the better the focus generally is.

46.    Once it is focused, take the spectrograph of the telescope and put a short eyepiece on it.

47.    Adjust the length of the eyepiece until it is focused. Now, each time you start using the SGS, you don’t have to focus it, so long as the eyepiece is focused.

48.    Replace the eyepiece with the spectrograph.

 

Taking Spectra of Astronomical Objects.

49.    Open shutters of the observatory.

50.    Swerve the telescope to a very bright astronomical object.

51.    Flip on the internal LED light in the spectrograph. This allows view to see the slit on the tracking chip on the computer screen.

52.    IN CCDOops click “Setup.” A dialog box will appear. Make the cooling system active and select the tracking chip (not the imaging chip).

53.    Click on Focus. (This take continuous frames and displays them.) Set a short exposure time (up to 1 second, depending on the brightness of the object you are viewing). Don’t use dark frames and use Full-Low.

54.    Don’t panic if you don’t see a star. There could be several reasons for this. First, the tracking chip is extremely small. Use a mounted scope in sink with the telescope to locate the star. If it isn’t in the cross hairs, adjust the telescope until it is. Make sure to keep an eye on the computer screen to see if the star (or other object) crosses the tracking chip. Also, if you don’t see the slit in the image, it could be that the switch on the top of the spectrograph that blocks the light to the tracking chip is turned to where the light is blocked).

55.    Once you see the star on the chip, center it on the slit. If your mount doesn’t track well, watch the screen to see what direction the star tends to drift and place the star to one side of the slit (so that as you take the exposure of the star, it will pass briefly over the slit). Otherwise, by the time you take an exposure, the star will probably have drifted out of the slit. You may want to rotate the spectrograph, in relation to the telescope, so that any drifting you observe occurs in the plane of the slit.

56.    After you have positioned the star where you want it, make sure to always turn of the internal LED light in the spectrograph.

57.    Exit the focus, click “Setup,” and switch chips from the tracking to the imaging.

58.    Click on focus and take a longer exposure in Full-High mode. Check that dark-frames are enabled.

59.    If you do not see a spectrum, first check if your grating is in the position you want it (we used low-resolution mode), in between the niches. If it isn’t at the right angle, the spectrum generally disappears. If you need to fix the grating, switch chips again and check if the star has moved off of the slit. If it has, reposition, and try the process again.

60.    Save images as the default file type (.ST8) if you plan on calibrating with SBIG’s Spectra program. If you plan to do more complicated analyzation (with the program Visual Spec), your files must be saved as .FIT.

61.    To convert .ST8 files to .FIT files, load the .ST8 file in CCDOps. Click <File>, <Save As>, and change the type of file to FITS and click “Save.”

62.    Repeat for all other objects.

 

Using Visual Spec – Wavelength Calibration.

63.    Download the Visual Spec program from http://www.astrosurf.com/vdesnoux/

64.    Open Vspec, click <File>, <Open Image>, find the file you want to analyze, and open it up.

65.    Click the icon on the toolbar for <Mirrorx> (to flip the spectrum, as it is backwards—higher wavelengths are to the left side).

66.    Click the icon on the tool bar for <Object Binning>. This graphs the intensities at each pixel.

67.    Select the new window and right click. Then select <Calibrate>, <Yes>.

68.    Another toolbar appears. Locate the text boxes labeled “raie 1” and “raie 2.”

69.    Fill in the wavelengths (in Angstroms) of the two lines to be used for calibration. Many bright stars have Hydrogen alpha and beta. These are commonly used; however, one cannot make accurate Dopplar shift measurements of a star using spectral lines from its atmosphere, since these lines themselves may be Dopplar shifted. Accurate calibration of spectra may be accomplished using an Hg (Mercury) or other calibration lamp, or with telluric (atmospheric) lines from the atmosphere, on the assumption that spectral lines in the atmosphere don’t change over time. There are two molecular oxygen (O2) spectral lines at roughly 6875 A and 7605 A in the red end of the spectrum that may be used for calibration. If they are not present you may use elemental lines from the star’s atmosphere; you can identify, or compare the spectrum with your calibration spectrum. Hydrogen’s Balmer series lines are often apparent; for Sirius (a bright A star, fairly easy to calibrate), set line 1 (raie 1) to 4861 A (H-beta) and set line 2 (raie 2) to 6563 A (H-alpha).

70.    Click and hold down mouse and drag it over a line.

71.    Right-click over window and select <line 1>.

72.    Do steps 70 & 71 with second line.

73.    Right-click again and select <Calibrate>. The spectrum should now be calibrated.

74.    To label other lines, drag the mouse over one, right-click, and select <Label>.

75.    If you don’t know what a line is select <Tools>, <Elements>. Then click the box labeled <Selection>. This lists the elemental lines that are found within your selected area. (To select an area drag the mouse over that specific area of the spectrum, or enter the beginning and end wavelengths for that region of the spectrum in the Elements window).

 

Using Visual Spec – Flux Calibration.

76.    Click <Tools>, <Library>. Select the spectral class of the wavelength calibrated calibration star and drag the filename into its spectrum.

77.    Click <Operations>, <Spline Filter>. Adjust the verticle scroll tool until the spline curve has appropriately fitted the spectrum. The strength of the spline curve depends on the spectrum. Sometimes it should be set so that it only matches the curve, with no spectral lines; in spectra with particularly strong spectral lines (such as the near ultraviolet lines of the Balmer series) this step may be completely admitted, so that the spectrum to be divided by the intensity curve retains the full strength of its spectral lines. Through trial and error it becomes evident which method produces a spline curve that best fits the Planck curve for the library star’s spectrum.

78.    Click <Operations>, <Divide>. Click “Intensity” and then click OK.

79.    Click <Operations>, <Spline Filter>. Adjust the intensity of the spline curve until it matches the flux calibration spectrum. In this step, unlike step #77, it is neccesary that all elemental lines be removed, since this spectrum will be multiplied to other spectra with different spectral lines.

80.    To obtain a spectrum of your calibration star that is calibrated for instrumental response and atmosphere absorption, select the “Intensity” curve and select <Operations>, <Multiply>.

 81.   To calibrate other stars, select the splined flux calibration curve and click <Edit>, <Copy>. Open the wavelength-calibated spectral profiles of the chosen stars to be corrected and click <Edit>, <Paste>.

82.    To create a graph of the spectra, click <Format>, <Graphic>.

83.    On Axis X and Axis Y, click “Display X Axis” and “Display Y Axis” and adjust “Nb ticks” (Number of ticks between each axis label) and “Tick” (Number of Angstrom units between each axis label) to desired values. Click “Apply” to see what the graph will look like and click OK.

84.    Click <Tools>, <Synthesis> to see a clear synthetic depiction of the spectrum. Right-click and click “Colorer” to colorize the spectrum. Right-click and click “Export” to save it as a *.bmp image.

 

6.    Conclusions

 

The SBIG SGS was successfully assembled and calibrated in the project. Installation involved a long procedure of removing parts, installing new parts, adjusting parts, and testing parts. After all unnecessary items were detached, the ST-8 CCD could be attached to the SGS. Cords linked the computer to the camera and from the camera to a power source. After a link on the computer was made in CCDOPS, frames could begin to be taken. A calibration lamp (Mercury light) was placed in front of the SGS and calibration spectra were taken at several different micrometer settings using the low-resolution grating. Then, the grating toggle switch was turned and high-resolution spectra were obtained at several different micrometer settings.

Now that the SBIG SGS was setup, the SBIG SGS could be used with the telescope. It was threaded onto the Celestron 14” and all cords were plugged in. A link was made with the computer and calibration spectra were obtained using the flap on the bottom of the SGS. Since this was the first time this SGS was used, an easy bright star was our first target. We ran into several problems. One was that at first our grating switch was not at the correct angle so that we saw no spectrum on the CCD chip. Our next problem was that our micrometer was also at the wrong angle. This created a problem because the target area of the spectrum was not seen on the CCD, but the area of the spectrum into the deep inferred (wavelengths that the CCD is not very sensitive to) was. When these problems were finally noticed, they were corrected.

Our next problems were the most significant. It was found that the telescope was not perfectly equatorially aligned and that the SGS’s self-guiding feature had not been set up. This meant that the targets would not stay focused on the slit in the SGS even after they were maneuvered on to it. This meant that long exposures could not be taken, and so we could only obtain spectra of the very brightest stars. Further impedance was the fact that most of our chosen targets were in the Cygnus constellation and set early on the nights that data could be obtained. These two problems forced us to give up WR stars. Since the WR stars were so dim, spectra of even the brightest of them could not be obtained. Only data from low magnitude stars could be obtained. So data from these stars were obtained and analyzed using the various procedures described above.

It was found during analysis that since the slit was not vertical and since stars were positioned vertically in different places on the slit, the micrometer settings could only be used as a guide to reference the approximate wavelengths covered. Before more data is obtained, the slit should be adjusted. The auto-guiding system should be setup so that longer exposures can be taken, and the Paramount/telescope system should be adjusted so that it is more accurately equatorially aligned.

7.    Acknowledgements

     Many thanks are given to Dr. Fritz Osell of Northern Oklahoma College for letting us use his observatory, telescope and spectrograph and giving us his support.

8.    Bibliography

 

  1. Buil, Christian. Wolf-Rayet Stars. HTML file retrieved November 30, 2004:  http://www.astrosurf.com/buil/us/peculiar2/wolf.htm.
  2. Hesser, Scott. Dissecting the Universe: Astronomical Spectroscopy. Retrieved November 25, 2004: http://www.wheatoncollege.edu/academic/academicdept/astronomy/observatory/atorion/spectrometry/index.htm.

3.      P Cygni. 15 Jun. 2003. Retrieved March 20, 2005: http://www.peripatus.gen.nz/Astronomy/objPCyg.html.

  1. Santa Barbara Instruments Corporation. (2004). Overview of the SBIG Self-Guided Spectrograph Capabilities. PDF file retrieved November 30, 2004:  www.astrovid.com/technical_documents/_SBIG%20SGS%20SPECTROGRAPH.pdf.
  2. Talbot, John. A Few Wolf-Rayet Stars. Retrieved November 21, 2004: http://members.rogers.com/laserstars/amateur/table.html.
  3. W. Olcott. Book of the Stars for Young People. 1923.

 

 

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