Lab 6: Spectroscopy Due Monday, April 10

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1 Lab 6: Spectroscopy Due Monday, April 10 The aim of this lab is to provide you with hands-on experience obtaining and analyzing spectroscopic data. In this lab you will be using a spectrograph to obtain spectra for a pair of calibration sources and the Sun. The procedure is the same as one would use to obtain a spectrum for a fainter astronomical object. You will use calibration sources to determine the correspondence between wavelength and pixel location in the image, which will then enable you to determine the wavelengths of absorption features in the solar spectrum. Note that for this lab it is important that you take your solar spectra relatively early in the lab while there is still light outside. It is also the case that we will have two lab groups sharing the spectrograph on most evenings; the first group with all members present will start off using the spectrograph and we will alternate between groups every 30 minutes as needed. Each group should bring a flash drive to class. The Pepito Mark III spectrograph being used for this lab employs a reflection grating and one of the SBIG detcctors used at CTO. It is a multifiber spectrograph, meaning that light enters the instrument through a fiber optic bundle, with each fiber yielding a distinct spectrum similar to what was shown in class. The wavelength coverage (which you will calculate more precisely) is approximately Angstroms. Objectives: Your basic objectives for the lab are the following. 1. Characterize the spectrograph using a set of calibration sources. Specifically, you will be expected to derive a wavelength calibration, and determine the wavelength coverage and spectral resolution of the spectrograph. 2. Detect and identify absorption features in the solar spectrum. Procedure: Despite the need to obtain solar spectrum, this lab will be conducted entirely in room 7. The spectrograph is sufficiently sensitive that you will be able to obtain a solar spectrum from light coming through the windows. Note: you should read through this entire lab before beginning. 1. Familiarize yourself with the experimental setup. The equipment used for this lab consists of the spectrograph, a holder for the fiber bundle, and an arc lamp fixture with a series of tubes containing different gases that can be used for calibration. It is up to you to choose which gases to use for calibration. Figures showing examples for a few elements are shown near the end of this document. Exercise extreme caution when handling the fiber bundle, as this is the most fragile part of the instrument, was custom built in the department, and is not easily replaced. The camera for the spectrograph is controlled by the same CCDOps software as is used at the Campus Teaching Observatory for the class projects. 2. Using CCDOps, establish a link with the camera and take a short test image. You will see 7 spectra going across the center of the resultant image, each corresponding to an individual fiber in the fiber bundle. You will likely also see features in these spectra from the fluorescent lights in the room. At this point it is

2 advisable to start taking a sequence of images in focus mode as you set up the rest of the experiment 3. Carefully place the fluorescent fixture in front of the fiber bundle. Note that it does not need to be particularly close, as the arc lamps are bright. You are being provided with five tubes that contain hydrogen, helium, mercury, oxygen, and neon, respectively. One person in the group should put on a glove, place one of these tubes in the lamp, and turn on the lamp. At the end of this lab you will see figures and tables showing the major emission lines from these three elements. While looking at these figures you will want to in turn insert each of the gas tubes into the lamp (always turning it off before removing or inserting a bulb, and always using a glove), and decide on one or two tubes that you wish to use to determine the wavelength solution for the spectrograph. To be clear, what you are specifically aiming to do is determine the wavelength corresponding to each x-axis pixel location on the detection (e.g. column x=745 corresponds to what wavelength?). To do this, you will be identifying emission lines from a subset of these known gases in your image. You will record the wavelength and pixel location for each line, and then fit a line of the form: λ = a * x + b The relation between wavelength and x pixel is quite linear, so this is a good approximation. Now, to derive a reasonable solution you must have a minimum of three lines, and these lines must cover a reasonable fraction of the width of the detector (i.e. have a reasonable spread in wavelength). It is fine for these lines to come from multiple calibration sources if desired. 4. Once you have decided which gas tube(s) you wish to use, in CCDOps switch from Focus to Grab mode. Insert the tube(s) of your choosing. Turn off all other room lighting and take an exposure. Make sure that the emission lines are not saturated in the image. If they are, either shorten the exposure time or (if it is already at the minimum) decrease the amont of light from the lamp by either rotating it or moving it further away. Record the data to a flash drive. Turn off the lamp when you are finished. Also, explain in your notebook why you chose the lamp(s) which you did. 5. In addition to the calibration data, you also now need a spectrum of the sun. First, place the camera back in to Focus mode and start acquiring a series of short exposures. Have one member of the group keep an eye on the screen during the rest of this step. Next, remove one (or more) of the black window panels which cover the exterior windows to the basement and pull up the blinds. Make sure that all doors to the room are closed and all artificial lighting is turned off. Point the fiber bundle in the general direction of the open window. The best way to do this is to gently shift/rotate the holder for the end of the fiber bundle until it is pointing roughly in the right direction. You should see a spectrum brighten on the screen as you get close to the direction of the window. 6. Once you are satisfied with the position, switch back to Grab mode and adjust the exposure time until you have a bright spectrum of the sun, making sure that you are not saturating the detector. Once you have a good exposure, record this data to the flash drive as well.

3 Analysis: Upon completion of the steps above you have all the data required to complete this lab. For the rest of this lab you will require ds9, and can use methods of your choosing for linear least squares fitting and plotting. Note that ds9 is public software, so these steps can be done either in lab or on your own. 1. Derive the wavelength solution from the calibration data. Doing so requires the following steps: i. Open your arc lamp image(s) in ds9. ii. Using the figures from the end of this lab identify the wavelength corresponding to each line in the image that you wish to use for your wavelength calibration. It is recommended that you print and record this image in your notebook with labels pointing to the various emission lines. The best way to do so is to save a copy of the image as it appears on the screen to the flash drive for later printing. Saving either by print to file, which will yield a postscript file, or via screen capture is acceptable. iii. Determine the x location of each line. Within ds9 there is a useful too that you may wish to use to help you in deriving the location. From the toolbar click on Region -> Shape -> Projection. Next, zoom in on the feature you wish to measure, and use the mouse to draw a horizontal line across this feature. A window will pop up showing you the counts as a function of pixel. Record the approximate peak count level and your best estimate of the pixel value corresponding to the center of the peak. Be sure to estimate the uncertainty in each measurement. For at least three of the lines you should also save a copy of this figures to your flash drive to print and paste in your lab notebook iv. Using the x and wavelength data derive the best linear fit for the wavelength solution, and include a plot of this fit in your notebook. 2. Using the wavelength solution, calculate the spectral resolution of the spectrograph in Angstroms/pixel, and the total wavelength coverage. 3. Identify emission lines in the solar spectrum. i. Open your solar spectrum image in ds9 and locate features that you can find superimposed on the continuum spectrum. These will be absorption features. It is advisable to look at horizontal cross-cuts as with the calibration data as well as the original image to help identify features. You should record the pixel values for these in the same fashion as before, and should also save a copy of the 1D cross-cut plots for inclusion in your notebook. ii. For all features that you identify, use the wavelength solution from the arc lamps to derive the corresponding wavelengths. Compare these with the wavelengths of known stellar features (see figures/tables at the end of this lab) and identify which features you have found. There should be a figure in your notebook showing the spectrum and the identified features.

4 In the write-up for this lab you should discuss any issues you had, including any discrepancies between your derived wavelengths and the actual wavelengths of spectral features. Figure 1: Neon spectrum taken with the Pepito Mark III spectrograph used in the lab. Figure 2: Mercury spectrum taken with the Pepito Mark IIIs spectrograph used in the lab.

5 Figure 3: Hydrogen spectrum taken with the Pepito Mark III spectrograph used in the lab.

6 Table 1: Wavelengths of key spectral features for H, He, Hg, O, and Ne. Element Wavelength (A) Comment H Hα H Hβ H Hγ H Hδ H 3970 He weak line He weak line He medium strength line He medium strength line He strong line He weak line He strong line He medium strength line Hg Hg Hg Hg Hg Hg O [OII] O [OII] O [OIII] O [OIII] Ne Ne Ne Ne Ne Ne Ne Ne Ne Ne I Ne I Ne I Ne I Ne I Ne I Ne I Ne I Ne I Ne I Ne I Ne I

7 Hg Hg Hg Hg Hg Hg Hg Table 2: Key spectral features in the Sun. Note that you will not see all these lines, and multiplets may be blended together. Feature Wavelength (A) Ni t Fe T Ti P Fe N Fe L Ca K Ca H Hδ G(Fe/Ca) , Hγ Fe e Fe d Hβ Fe c Mg b triplet ,5172.7, Fe E He d Na D doublet , [OII] a H-alpha [OII] B