Spectroscopic Observations of Vega and Beta Aurigae at Lookout Observatory

Darren Stroupe and Jackson Dryer

Department of Physics, University of North Carolina at Asheville December, 2019

Abstract

Spectroscopy provides important information about our universe, such as the composition and motion of astronomical bodies. The opportunity to collect meaningful spectroscopic data exists in the university’s possession of an underutilized LHIRES III spectrograph. We conducted spectroscopic observations of Vega and the eclipsing binary system, Beta Aurigae, using the spectrograph in tandem with the 14” Schmidt-Cassegrain telescope at Lookout Observatory. The spectrum of Vega is potentially useful in that it can be used as reference in defining the response of an optical system, while the spectrum of Beta Aurigae can be analyzed to find the radial velocities of the system’s components. We present here a description of our observations and analysis of the results.

Introduction

The first to photograph the spectrum of a star was Henry Draper in 1872. In his photographic plate of the spectrum of Vega the hydrogen absorption lines of the star were recorded. In 1876 William Huggins presented the first report of stellar spectroscopy, which included his observations of Sirius and Vega, to the Royal Society. He then published a paper, “On the Photographic Spectra of Stars,” in 1879 that detailed his observations of Sirius, Vega, Altair, Deneb, Spica and Arcturus. Draper continued his work in spectroscopic photography until his death in 1882. It was then that Edward Pickering of Harvard arranged with Draper’s widow, Anna, to continue Draper’s research and acquire his plates and equipment. Pickering would go on to establish, with Williamina Fleming and Annie Jump Cannon, a spectral classification system that would be adopted by the astronomy community and that lead to the conception of the Hertzsprung-Russell diagram, which plots stellar luminosity against spectral type (surface temperature) and makes possible the estimation of the ages and masses of stars.

Spectra can also be used to measure the radial velocities of astronomical bodies. The wavelength of light from objects moving away from an observer is stretched, and

2 shortened when moving toward an observer, following the Doppler effect. This effect is expressed as f = c + v f , ( c + v0 ) 0

where c is the speed of light, v and v0 the velocities of the observer and source with respect to the medium through which the light propagates (positive values if moving toward one another), with f and f0 the observed frequency and the frequency at the source, respectively. The equation can be stated in terms of wavelength and, when the velocities are much smaller than the speed of light, can be simplified such that

λ−λ 0 = Δλ ≈ v , λ0 λ0 c

where λ0 and λ are the known and observed wavelengths, respectively. Comparing the known and observed wavelengths of astronomical sources allows us to determine the velocities of those sources.

Pickering discovered a spectroscopic binary in Mizar A by noting the periodic splitting of absorption lines across a series of plates caused by the approach and recession of two stars with respect to Earth. Antonia Maury, one of the famous Harvard Computers, published work in 1898 that described the second discovery of a spectroscopic binary star system - Beta Aurigae.

We chose to acquire spectra of Vega and Beta Aurigae, not only because of their conspicuous place in the history of spectroscopy, but also because they are bright stars that were visible in the night sky at the time of observations (Table 1).

Target RA DEC Apparent Magnitude

h m s Vega 18 36 56.50 +38° 47′ 08.1″ +0.026 h m s Beta Aurigae 05 59 31.73 +44° 56′ 50.8″ +1.90

Table 1. Data for Vega and Beta Aurigae

3 Vega, the second brightest star in the northern hemisphere, is of spectral class A0 Va. It lies approximately 25 light years from the sun and had a radial velocity of +9.5 km/s (corrected for heliocentric velocity) at the time of observations. Because of its well-documented flux, Vega can provide a good reference in calculating the throughput response of an optical system. Beta Aurigae is an eclipsing binary system that appears almost edge on from our perspective. It is actually a triple star system, with a third red dwarf member. The two main components are both A-type subgiant stars with similar masses and radii that lie 81 light years from the sun, and had a corrected radial velocity of -12.6 km/s as a system at the time of observations. The main components orbit one another with a period of 3.96 days, producing relative orbital velocities large enough that radial velocities can be easily observed in high-resolution spectra.

Observations

After ensuring that both target stars would be visible in the night sky, observatory time was booked for both the day before, and night of observations. Preparations made during the day preceding observations included attaching the spectral acquisition assembly to the telescope, making adjustments to guiding and main camera CCD orientations, focusing the guiding camera and spectrograph, and taking 15 bias and 15 flat frames with 4-second exposure times for a maximum pixel count of ~20,000. ° 1 All frames were taken with 1x1 binning and at a CCD temperature of -10 C.

The telescope was balanced early on the day of observations, which occurred during a new moon. Vega was one of the first stars that became visible as the sky grew dark. The telescope was pointed toward Vega, and focused manually. The spectrograph slit was then centered on the star and the spectrograph focus was checked. A neon calibration frame with an exposure time of 1 second was taken, as were 5 light exposures of 30 seconds. A second neon calibration frame was then taken, along with 5 dark frames with exposure times of 30 seconds.

The telescope was then slewed to Beta Aurigae’s position, and the process above repeated, but with light and dark frames of 120-second exposures due to the star’s lower magnitude.

1 A more detailed spectral acquisition procedure, as well as a description of the LHIRES III spectrograph and its components, can be found in the appendices at the end of this paper.

Data Reduction

Bias, flat field, dark and light frames for each target were processed with the AstroImageJ Data Reduction Facility. Because one Beta Aurigae light frame contained no image and another was slightly out of focus, only three frames were used in the final image reduction. The vertical position of spectra was slightly shifted from light frame to light frame requiring each frame to be manually aligned with the rest in the series. The reduced and position-adjusted light frames were then median combined to create master light frames (Figures 1 and 2). The neon calibration frames for each target were averaged. Spectral reduction of the light frames was performed in ISIS (Figures 3 and 4). Operations performed during data reduction included tilt and slant correction of the raw spectra, wavelength-to-pixel calibration using the neon calibration frames and a neon atlas (a custom list of neon lines that appeared in the calibration frames), and heliocentric velocity correction (SIMBAD). The Beta Aurigae spectrum was cropped and its continuum smoothed.

Results and Future Work

The master light frames of Vega and Beta Aurigae are shown in Figures 1 and 2. Vega’s spectrum displays a break at Hα, while that of Beta Aurigae displays two, indicating the observation of a spectroscopic binary pair.

Figure 1. The reduced master light frame of the spectrum of Vega. The spectrum is dispersed horizontally with a break in the center, indicating Hα absorption.

Figure 2. The reduced master light frame of the spectrum of Beta Aurigae. The spectrum is dispersed horizontally with a double break in the center, indicating Hα absorption from each component of the binary pair.

The reduced spectrum of Vega is displayed below in Figure 3. The spectrum shows many prominent absorption lines and, although it has been corrected for heliocentric velocity, is shifted redward with respect to where the Hα absorption line should fall. This suggests inaccuracy in the wavelength-to-pixel calibration.

Figure 3. The spectrum of Vega with relative intensity vs. wavelength (Å). The red line indicates the position of 6563Å.

The reduced and continuity-smoothed spectrum for Beta Aurigae is shown in Figure 4. Because we were unaware of what phase the binary system would be in at the time of observation, we were fortunate that the spectra showed the apparent radial velocities of each component. Two Hα lines were observed: one blueward and one redward of 6563Å.

8 Figure 4. The continuity-smoothed spectrum of Beta Aurigae with relative intensity vs. wavelength (Å). The two wells in the spectrum are the separate Hα lines for each star in the binary system. The red line indicates the position of 6563Å.

The wavelengths of 6558.597Å and 6567.329 Å were found at the minimums of the shifted Hα lines. Substituting these wavelengths, along with the known wavelength of Hα, into the Doppler Effect equation yields the following velocities: 175.67 km/s toward and 207.03 km/s away from earth. Antonia Maury reported a combined velocity for both stars of 240 km/s (Maury 1889), while a more recent measurement gave lower velocities of ~100 km/s for each star (Popper & Carlos 1970). Our velocities are decidedly higher than those reported, although they are not unphysical. This result, as in the cases of the spectrum of Vega, may be due to inaccurate calibration of the spectral reduction software.

Overall, acquisition of stellar spectra with the LHIRES III spectrograph was successful. The spectrum of Vega compares favorably to its known spectrum, and the velocities derived from the shifted Hα lines of the spectroscopic binary, Beta Aurigae, are not far off from those previously reported. To provide a profile of the optical system’s response, further reduction of Vega’s spectrum is still needed. Furthermore, repeated observations of the Beta Aurigae over time would enable the calculation of the pair’s physical properties and orbital dynamics. Utilizing a more user-friendly and accurate spectral reduction software, the raw spectra could doubtless produce more meaningful data. It is our hope that this introductory work proves helpful in future spectroscopic studies.

Appendix A: The Spectrograph

The LHIRES III (Littrow high resolution) spectrograph (Figure 5) is equipped with a variable slit and interchangeable gratings of different resolutions, and can be used in tandem with a telescope - or without, when acquiring lab or solar spectra. The optical path (Figure 6) of the spectrograph begins at a variable slit that is mirrored on the incoming side and angled at 10°. What light doesn’t pass through the slit is reflected back to another mirror that, in turn, reflects the light toward a guiding eyepiece or camera. The guiding eyepiece/camera is used to determine whether a target is centered in the slit, and to focus the image from a telescope at the slit. The light that does pass through the slit is diffracted and travels toward a mirror angled at 45° that then reflects the low-order diffraction toward the main lens. The spectrograph is of a compact Littrow design wherein the main lens is a doublet that both collimates the diffracted light from the slit before it reaches the grating, and then focuses the diffracted light from the grating as it returns closely along its original optical path toward the spectrograph output. By adjusting the tilt of the grating - and, therefore, the diffraction angle – with a micrometer, the wavelength range observed at the spectrograph output can be shifted either blueward, toward shorter wavelengths, or redward, toward longer wavelengths.

Slit

The slit diffracts the light from a source, creating the coherent illumination of a zero order beam at the main mirror. It can be removed from the spectrograph chassis by removing the two thumbscrews that secure the slit mount. The width of the slit is variable in that four different slits of widths 15, 19, 23 and 35 μm are arranged in a square (Figure 7a) on the slit plate, and can be changed by removing the faceplate and rotating the slit plate so that the desired slit width is visible through the circular opening, opposite the side of the face plate (Figure 7b). The choice of slit width depends on the observer’s

Figure 5. Images of the LHIRES III spectrograph. Original images from optcorp.com.

11 objective, with a larger slit width resulting in the ability to capture a larger angular diameter of the sky with increased flux, but with less dispersion and resolution. A smaller slit width results in the opposite effect. The 23 μm slit was used throughout all spectral acquisition in this paper and proved adequate for bright sources. The correct orientation of the 23 μm slit, where the number designation reads in the direction of the slit length, is shown in Figure 7a.

The outer side of the slit plate is mirrored and reflects the image of the slit and target field toward another mirror that reflects the image toward the guiding eyepiece. The guiding eyepiece is useful in ensuring that the slit remains positioned on a target. When the spectrograph is attached to a telescope and a guiding camera attached to the guiding eyepiece, the image of the slit is focused at the camera’s CCD by changing the position of the camera inward or outward with respect to the spectrograph’s chassis, allowing the telescope to be manually focused so that the reflected target field is also focused on the CCD.

Figure 6. Optical schematic of the LHIRES III spectrograph. Image from the manufacturer at http://www.astrosurf.com/thizy/lhires3/index-en.html.

a b

Figure 7. Photos of the slit showing (a) the numbered slit widths and box-arrangement of slits and (b) the mirrored side of the slit plate with the 23 μm slit just visible.

Neon Calibration and Flat Field Lamps

The LHIRES III is equipped with a neon calibration lamp for wavelength-to-pixel calibration of spectra taken, and a flat field lamp to produce a uniformly illuminated field for use in removing the non-uniform effects over the area of a CCD that are produced by the optical system and dust. Both are positioned in the optical path of the spectrograph just after the slit. The spectrograph must be powered on for the lamps to operate. When the switches for both lamps are simultaneously engaged, neither lamp is illuminated, but the optical path is obstructed, which is ideal for producing dark frames.

If using the spectrograph with a telescope, neon calibration frames should be taken both before and after target spectra are taken, as some mechanical drift may occur over the course of long exposure times and slews. The two calibration frames can then be averaged at the time of image processing. When taking lab or solar spectra,

13 only one neon calibration frame is needed. If the neon lines create ghost images in subsequent exposures, taking several bias frame readouts can clear the CCD. The neon lines are produced across a limited range of ~5000-7000Å.

Flat field frames should be produced when taking spectra of astronomical objects other than the sun. Unfortunately, the flat field lamp is of no help here, as it can’t illuminate the telescope optics that precede it. In the case of night-time observations, flat field frames should be taken with the telescope pointing at a uniformly illuminated flat field. When the source is bright, and signal-to-noise high, as in the case of lab or solar spectra, flat fields are not necessarily needed. However, solar spectra do reveal dust in the optical system, and flat field frames could remedy those effects.

Main Mirror and Doublet

The main mirror reflects the image from the slit to the main lens. Adjustment of the mirror changes the position of the spectral image at the spectrograph output, and is made by loosening the main mirror adjustment screw (Figure 8) and sliding it upward or downward. The spectral image should be centered within the field of view of an eyepiece or on the CCD of a camera at the spectrograph output. An image is centered by adjusting the mirror while looking through the eyepiece if one is being used, or while acquiring a continually-refreshing image of the spectrum if using a camera. Adjustment of the main mirror should be done any time the grating is changed.

Figure 8. A photograph showing the location of the main mirror adjustment screw. Once loosened, the mirror - and position of the spectral image at the spectrograph output - is adjusted by sliding the screw up or down.

The main lens collimates the image reflected from the main mirror so that the light incident upon the grating is traveling in parallel rays, and then focuses the spectral dispersion of the grating at the spectrograph output. The focus of the main lens is changed by removing the main lens access panels on both sides of the spectrograph, finger-loosening the white nylon set screws revealed behind each access panel, and turning the black scalloped disk (Figure 9). Turning the disk shifts the position of the lens and, therefore, the lens’ focal point toward or away from the spectrograph output. When focus is gained, the nylon screws should be finger-tightened and the panels replaced. Focus can be achieved by using either the neon calibration lamp or the target spectrum as a source (Figure 10). Alternatively, when an eyepiece/camera adapter (Figure 11) is used at the spectrograph output, rough focus can be achieved by sliding the attached eyepiece, or camera (with its own 1.25” adapter), inward or outward.

Figure 9. The main lens access panel removed from one side of the spectrograph, revealing the white nylon set screw and black scalloped disk that shifts the lens for focus when turned.

Figure 10. A series of enlarged images showing H-alpha (Hα) lines in different states of focus. The lines on the left and right are slightly out of focus as can be seen by the subtle extensions of brightness from their centers. The line in the middle was brought into sharp focus by adjusting the position of the main lens.

Figure 11. Adapter with set screws that can accommodate an eyepiece or camera at the output of the spectrograph.

Gratings

The LHIRES III is equipped with reflection gratings that disperse incident light into its constituent wavelengths. The gratings act as slits do in that a monochromatic ray of light impinging upon the grating surface is diffracted in both positive and negative directions (Figure 12). In the case of the spectrograph, the coherent diffraction originating from the single slit is reflected by the main mirror and is collimated by the doublet before reaching the grating to be diffracted away from the angle of incidence.

Figure 12. A ray of monochromatic light, incident at an angle α from the grating normal, reflects at an angle β0 from the normal and is diffracted at positive and negative angles β1 and β−1 . Subscripts

indicate order of diffraction and d is the groove spacing of the grating. I mage: Palmer 2005

Parallel rays incident upon the grating surface as in Figure 13, have traveled different path lengths after diffraction, and interfere constructively when the path difference is equal to an integer multiple of the wavelength of the incident light. This relation is expressed by the diffraction equation,

d (sinα + sinβ) = mλ , (1)

17 where d is the groove spacing, sinα + sinβ , the total path difference, m the order of diffraction, and λ the wavelength of incident light. It should be noted that the inverse of d is equal to the groove density of a grating. Also worth noting is that one result of the diffraction equation,

sinα + sinβ = mλ < 2 , (2) d

requires that, for a given spacing and order, there is a limit to the longest possible wavelength that will be diffracted. Similarly, for a given spacing and wavelength, there is a limit to the number of orders produced. Furthermore, it can be worked out that different orders of different wavelengths can overlap.

Figure 13. Parallel rays incident at an angle α diffract at an angle β and travel a path difference of

sinα + sinβ . I mage: Palmer 2005

When m = 0, β =− α , and a zero order reflection occurs. Reflection can be found at the 0 mm setting on the spectrograph micrometer at which the normal of the grating surface is parallel to the incoming and outgoing optical path. When an order of diffraction other than m = 0 reflects back along its incoming optical path, β = α , so that

mλ sinθB = 2d , (3)

where θ is the blaze angle and θ = α = β. This is the Littrow design as is found in B B the LHIRES III. The blaze angle is the angle of elevation of the grating facet with respect to the grating plane (Figure 14). Gratings can be manufactured with grating angles to optimize the efficiency of diffraction at desired wavelengths. Two of the three spectrograph gratings available for this paper have groove densities of 150 and 600 rulings, or grooves, per millimeter (g/mm), and are blazed at angles of 2° 8’ and 8° 37’, respectively (Jenkins 2011). These blaze angles render the gratings most efficient at first-order optical wavelengths ~500nm. This can be confirmed by substituting these angles and d for each grating into the Littrow equation above.

Figure 14. An illustration of a diffraction grating element showing the normal to the grating (GN) and normal to the grating facet (FN). The blaze angle θB is the angle of elevation of the facet surface with

respect to the grating plane. I mage: Palmer 2005

The third grating has a groove density of 2400 g/mm, but is a holographic, or interference, grating. Holographic gratings diffract light in the same fundamental manner as blazed gratings, but with some differences. One difference is in the manufacturing of the gratings. Whereas blazed grating masters are cut with a ruling diamond, holographic grating masters are created by introducing a standing wave

19 interference pattern produced by laser beams over a light-sensitive material that rests upon the grating substrate. This process results in a sinusoidal groove pattern baked into the material that traverses the substrate. The difference in blazed and holographic grating profiles is shown in Figure 15. Other differences include grating efficiency and how the different surfaces scatter light. While the subject of holographic gratings deserves more attention - see Christopher Palmer’s Diffraction Grating Handbook for much more information – it was sufficient for the spectral analyses in this paper that the 2400 g/mm grating was, like the blazed gratings, also optimized for the optical range of light.

Figure 15. The groove profiles of a blazed grating (top) and a holographic grating (bottom). I mage: Palmer 2005

The diffraction angle β can be found in terms of wavelength by rearranging the diffraction equation so that

−1 mλ (4) β (λ) = sin ( d − sinα) .

The diffraction angle increases with longer wavelengths of a given order and with a grating of a given spacing. Conversely, larger diffraction angles produce longer wavelengths. This is manifested in the relationship between the spectrograph grating and micrometer (Figure 16). The gratings are fixed within a metal framework that allows them to swivel around a central point that lies just behind and at the center of the grating surface. The grating position is spring-loaded so that at rest the angle between the normal to the grating surface and the optical path is at its greatest. The tilt of the grating is set by the micrometer, which comes into contact with a metal bar

20 inside the grating assembly. Setting the micrometer to smaller values by turning it clockwise moves the bar downward in the Figure, and decreases the angle between the grating normal and optical path. Therefore, smaller micrometer values produce a smaller diffraction angle and move the diffraction of shorter wavelengths into the optical return path. Similarly, larger micrometer values allow the return of longer wavelength diffraction along the optical axis. By turning the micrometer through a range of values, one can scan through the spectral dispersion of a source.

Figure 16. Schematic of the LHIRES III spectrograph showing the interior, including the doublet (lower center) and grating (lower right). Dotted lines trace the optical path. Image from the manufacturer at http://www.astrosurf.com/thizy/lhires3/index-en.html.

Spectral dispersion, a measure of the angular spread in wavelength of diffracted light, is the derivative of the diffraction angle with respect to wavelength:

dβ m D = dλ = dcosβ (5)

21 (Palmer 2005). Dispersion increases with increasing order and decreases with increasing grating spacing. Additionally, Eqn.4 shows that a higher groove density (1/d) will produce a wider spacing of wavelengths of a given order. As a result, changing out the LHIRES III gratings will increase or decrease the spectral resolution around a given central wavelength, with the 150 g/mm grating giving the lowest and the 2400 g/mm grating giving the highest resolution.

One LHIRES III grating can be replaced with another by first turning the micrometer to ~20mm so that it is not engaging the spring-loaded grating assembly or obstructing the path of the grating as the grating is being removed. Four black thumbscrews must then be removed (Figure 16), and the grating slid from the spectrograph’s grating cavity. Each grating is stored in a protective metal box. The replacement grating should be slid from the box and placed in the spectrograph’s grating cavity, while the grating that was removed from the spectrograph should be placed in the vacated protective box. Care should be taken to keep the gratings dust-free. Make sure the black plastic cap at the bottom of the micrometer that prevents metal-on-metal contact between the micrometer and grating bar remains fixed in its position (Figure 18). The thumbscrews should be replaced and finger-tightened.

Figure 17. A photo of the outer grating assembly showing the four black thumbscrews that secure the grating within the spectrograph.

Figure 18. A photo of the grating cavity. The arrow indicates the position of the cap fixed at the end of the micrometer.

Appendix B: Stellar Spectra Acquisition

Apparatus

● Celestron Edge HD 1400 Telescope ● LHIRES III spectrograph ● Atik 460EX Camera with USB cable (guide camera) ● Atik One 6.0 Camera (main camera) ● Artemis Capture image acquisition software ● TheSkyX software

Procedure

● Set the grating tilt of the spectrograph by taking a solar or lab spectrum to locate and center wavelength or wavelength range of interest.

● Remove any attachments from the telescope (e.g. camera, autofocuser, autofocuser-to-telescope adapter, eyepiece). ● Screw the eyepiece/spectrograph adapter onto the telescope. ● Connect the spectrograph to the telescope so that the slit is parallel to the direction of RA. ● Remove the two plastic protective caps from the spectrograph input and output. ● Attach the camera adapters to the spectrograph output and guiding eyepiece. ● Connect the main and guide cameras to the spectrograph. ● Secure the assembly to the telescope with the Velcro strap. ● Connect the cameras’ power and USB cables.

● Connect the guide camera USB cable to a laptop with acquisition software for the guiding camera installed. ● Open the guiding camera acquisition software and set image capture to continuous acquisition (loop) with a ~0.5 second exposure time and 3x3 binning. ● Using the refreshing image in Artemis and a diffuse light source, adjust the guide camera mirror so that the slit image is centered in the CCD (this only needs to be done once and can be done before connecting the spectrograph to the telescope). ● Rotate the guide camera to align the slit image horizontally across the CCD. ● Adjust the guide camera (in or out) to bring the slit into focus. ● Mark the slit position and center of slit on the computer screen with the edge and corner of a post-it note (Figure 19). ● Stop continuous acquisition.

● Turn on the spectrograph neon calibration lamp. ● Take a ~1 second exposure light frame from the main camera in SkyX and note the position of neon calibration lines. ● While taking repeated images to check adjustments, align (rotate) the main camera CCD so that the spectral dispersion is displayed across the computer screen horizontally, with bluer wavelengths toward the left and redder wavelengths toward the right. Adjusting the micrometer clockwise (inward

24 toward lower readings and shorter wavelengths) should shift the spectrum rightward on screen. ● Adjust the main mirror so that image is centered in the camera CCD (this needs to be done if the grating is changed). ● Zoom in on one of the neon calibration lines and focus the spectrograph so that the line appears as narrow as possible (this needs to be done any time the grating or grating tilt is changed). ● Balance the telescope. ● If the wavelength of interest wasn’t centered before attaching the spectrograph assembly, the telescope will first need to be balanced, the grating tilt set to center wavelength by using the solar spectrum, and the main camera CCD aligned as described above, after which the telescope will again need to be checked for balance.

● Take bias frames: 15 frames taken once per observation run. ● Take flat field frames: 15 frames with ~4 second exposure time and max count ~20,000.

● Set the guiding camera acquisition software image capture to continuous acquisition. ● Slew to a reference star so that it appears within the refreshing guiding camera mirror image. ● In the guiding camera acquisition software, adjust the exposure time to a time appropriate for the brightness of the star (long enough to produce an image, but short enough to avoid overexposure) and 3x3 binning. ● Focus the telescope manually so that the guide camera mirror image of the reflected star is as point-like as possible. ● Center the reference star within the slit using the SkyX manual “move” controls (Figure 20). ● Refine the spectrograph focus by ensuring that the pixel count reaches a maximum at the center of the height of the spectrum (making sure there is no black streak running horizontally through the center of the spectrum – the equivalent of a donut in normal imaging) and the spectrum’s edges are as sharp as possible.

● Turn on the neon calibration lamp and take a neon calibration frame with ~1 second exposure time (neon calibration frames should be taken both before and after spectra are taken and averaged at the time of image processing). ● Turn off the neon calibration lamp.

25 ● Take several bias frames immediately after neon calibration to clear the CCD (there is no need to save these bias frames). ● Ensure that the target is still centered within the slit (when taking long exposures or for a target positioned closer to the horizon, the target may have to be re-centered within the slit between exposures). ● Take at least 3 light frames of the reference star spectrum (exposure times and number of exposures will vary with the brightness of the target). ● Simultaneously engage both the calibration and flat lamp switches on the spectrograph to block the optical path and take 5 dark frames with the same exposure time and at the same CCD temperature as the light frames. ● Turn the flat lamp switch off and take another neon calibration frame with ~1 second exposure time. ● Turn off the neon calibration lamp. ● Take several bias frames to clear the CCD.

● Slew to the target star and manually center the star within the slit. ● Take spectra of target star in the same manner as outlined above for the reference star.

26 Figure 19. Marking the edge and center of the slit image in the guiding camera for reference to ensure that the target remains in the slit.

Figure 20. Target star centered in the slit as indicated by its position at the corner of the post-it.

Figure 21. Photo of the spectral acquisition apparatus at Lookout Observatory.

References

1. Maury, A. C., “The K Lines of β Aurigae.” 1898, The Astrophysical Journal, 8, 173.

2. Popper, D. M., Carlos, R., “Radial Velocities from Recent Spectrograms of U Ophiuchi and β Aurigae.” 1970, Publications of the Astronomical Society of the Pacific, 82, 487.

3. Jenkins, Benjamin G., "A Study of the Lhires III Spectrograph on the Hard Labor Creek Observatory 20 inch Telescope." Thesis, Georgia State University, 2011. https://scholarworks.gsu.edu/phy_astr_theses/12/.

th 4. Palmer, C., “Diffraction Grating Handbook (6 Edition).” 2005. http://optics.sgu.ru/~ulianov/Students/Books/Applied_Optics/E. Loewen Diffraction Grating Handbook %282005%29.pdf