SSO Transmission Grating Spectrograph (TGS) User s Guide

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1 SSO Transmission Grating Spectrograph (TGS) User s Guide The Rigel TGS User s Guide available online explains how a transmission grating spectrograph (TGS) works and how efficient they are. Please refer to it if you are not familiar with TGS systems. The Sierra Stars Observatory TGS uses the same 600-line/mm TGS filters as the University of Iowa s Rigel Telescope in Arizona, which is part of SSON. This user s guide contains information specific to using the SSO TGS system to help you get the best results for your astronomy spectroscopy projects. To create a TGS jobs schedule for the SSO telescope you need to consider two factors to achieve satisfactory results: The frequency of the spectrum you want to focus on. The exposure time you need to set to get an appropriate signal to noise ratio (SNR) result for your spectrum. Determine the Correct Focus Offset for Best Resolution for a Desired Wavelength The TGS filter projects a spectrum with a curved plane on to the CCD chip in the camera. Because the projected plane is curved only part of the spectrum is in the best focus position on the CCD chip to achieve the best resolution of the spectrum. The higher-frequency part of the projected spectrum (blue end) attains best resolution at a focus position closer to the true best focus of the star or object you image. The lower frequencies of the spectrum (red end) are best resolved at focus positions farther away from the focus position of the higher-frequency part of the spectrum. You can use the spectrum profile graphs of the star Theta Aurigae in Figures 1 through 9 as a guide to determine the best focus offset to use for the part of the spectrum you want to emphasize most. Theta Aurigae is a type A0 star with a spectrum similar to the star Vega. It has strong Hydrogen Ballmer lines that show up in the spectra to make it easy to see how changing the focus position affects the spectrum profile achieved. Each focus offset setting shows a calibrated profile of the raw spectrum and a normalized calibrated profile. They also show an image of the actual spectrum measured cropped from the original FITS file. The profile graphs were created using the software program RSpec. Figure 1 shows the profiles of Theta Aurigae at the best focus position (the zero or starting position before any offset). Below the profiles are color spectra synthesized from the actual spectra taken. The top profile is calibrated using a non-linear calibration routine without any additional processing. The bottom image is a normalized presentation of the same spectrum data. The synthesized colored continuum (spectrum) below the top profile in Figure 1 tends to overpower the Hydrogen Ballmer absorption bands. This is partly due to the difference in sensitivity (quantum efficiency) of the camera CCD chip over the range of 3800 to 8000 angstroms and the color of the star.

2 You can see that the intensity (brightness) of the spectrum in the profile (left side of the graph) varies from about 13,000 to 48,000 counts (higher numbers are brighter). The bottom profile is the same spectrum normalized. Normalization is a software process that adjusts the data so the intensity (brightness) of the continuum is equalized to more clearly show the absorption bands. You can see a big difference this makes in the synthesized spectrum as well as the profile. The Hydrogen Ballmer features show more prominently in the normalized profile. Figure 1 Figures 2 through Figure 9 show how the resolution of the spectrum of the star changes from the 100 focus offset position to the 850 focus offset position. The lower-number focus offsets resolve the higher frequency part of the spectrum while the higher-numbered focus offsets resolve the lower-frequency

3 part of the spectrum. You can use this information to choose which setting to choose for the features in the spectrum you are most interested in imaging. You can schedule two or three image setting to get a resolution of the entire visible-light spectrum. Figure 2 -- Focus Offset = 100

4 Figure 3 -- Focus Offset = 200

5 Figure 4 -- Focus Offset = 300

6 Figure 5 -- Focus Offset = 400

7 Figure 6 -- Focus Offset = 500

8 Figure 7 -- Focus Offset = 600

9 Figure 8 -- Focus Offset = 750

10 Figure 9 -- Focus Offset = 850

11 Determine the Correct Exposure Time The SSO TGS spreads the spectrum light of stars and other objects over half the width of the CCD chip. This greatly attenuates the signal of the star compared to the original point source limiting how faint of a spectrum you can image in a reasonable exposure time. The graph in Figure 10 below shows the approximate exposure times (integration times) required to achieve a desired signal to noise ratio (SNR) of a spectrum of a star at a given magnitudes on the SSO TGS. Assuming a minimum SNR of 100 for a spectrum image and a maximum exposure time of 300 seconds you can get spectra of stars down to 12+ magnitude in a single exposure. You can stack spectra images to achieve a higher SNR or image fainter stars. You can use the graph to figure out what exposure time you need to set for your TGS images. Figure 10 Relationship of SNR to Exposure Time The graph is guide to help you get in the correct range for choosing appropriate exposure times. Selecting exposure times for bright stars can get a little tricky. The reason is the window between getting the highest SRN in your image data and oversaturating the spectrum data can be small for bright objects. Also, and this is important to keep in mind, the greater the focus offset you choose (higher numbers) the more the spectrum is spread out vertically spreading the light in the spectrum and reducing the overall SNR.

12 Analyzing Your TGS Image Data The TGS images you receive contain spectra of objects you chose to image. There is a wealth of information about the elemental and chemical composition of your object in your spectrum data. The information is in the form of absorption lines (dark lines) and emission lines (bright lines) that stand out in contrast to the continuum of the spectrum. The standard method of reading and analyzing spectra is to measure the darkening and brightening caused by the absorption and emission lines and plot the results in a graph called a profile. Figures 1 through 9 are examples of spectrum profile graphs. To interpret the information in the profiles you need to calibrate them to match the lines with wavelengths of elements and molecules that create the absorption and emission lines. You can download commercial and free spectrum analysis software such as RSpec, Visual Spec, ISIS and others to create spectrum profiles of your data. A Practical Example Figures 11 shows how changing the focus offset setting affects spectrum profiles of the star Beta Lyra. It has an interesting spectrum with strong emission lines in the hydrogen and helium wavelengths. Unlike absorption lines that are darker than the continuum (because they absorb some light at their wavelengths) emission lines are brighter than the continuum. Beta Lyra is a type B star that is much brighter and larger than the sun. It emits light predominately in the ultraviolet wavelengths. It is also part of a binary system with another B type star that orbits so close it is referred to as semi-detached. The stars exchange gas creating an accretion disk that surrounds the system. The strong ultraviolet light from the stars excites the hydrogen and helium gas in the accretion disk ionizing the atoms causing them to emit photons at specific wavelengths. These are the emission lines that appear in the spectrum. The spikes of the emission lines in spectrum profiles rise in the opposite direction of absorption lines. In the following examples emission lines go up compared to the continuum while absorption lines go down. The example profiles in Figure 11 were created with the software program RSpec from spectrum images taken with the SSO TGS system. It shows calibrated, normalized profiles for focus offsets of 100, 300, and 600. As described previously lower-number focus offsets best resolve the higher frequency part of spectra while higher-number focus offsets best resolve the lower-frequency part of spectra. This is readily apparent as you can see that the 100 focus offset displays highest resolution at the high frequencies, the 300 offset displays higher resolution in the mid-range, and the 600 offset displays higher resolution in the lower-range frequency. Each of these profiles emphasizes the information in the spectrum within a limited range. You can see that abortion and emission lines are sharper and more pronounced in the wavelength range with the highest resolution for each focus offset. The bottom profile in Figure 11 is a composite of the three profiles above created by combining (stacking) the spectrum images with the image processing software MaxIM DL using a median combine process. The profile shows good resolution throughout the overall range of the spectrum although it doesn t match the precision of the best resolution of the individual focus offset data.

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