Chapter 3: Introducing the SBIG SGS

I relied on two articles to familiarize myself with the SGS: Backyard Spectroscopy by Maurice Gavin and The SGS Operating Instructions by Alan Holmes. Before using the SGS, I advise becoming familiar with these two documents. They cover such topics as the optical pathway, the scientific potential and limits of the device, and how make initial adjustments. In this chapter I comment on a few of the topics discussed, or not discussed, in these articles.

3.1 The Tilt Problem

Spectra created by the SGS do not run horizontally along the chip. In Figure 3.1, the spectrum of a bright star is shown. In this spectrum, the spectral lines are aligned vertically; however, the overall spectrum is tilted away from the horizontal. This is a result of the spectrometer’s optical design.

Figure 3.1

The tilt is not a problem when analyzing a point source like this star. Our software can easily work around the tilt so long as the spectral lines are aligned vertically. Unfortunately, the tilt is a problem when examining extended objects, such as nebulae or galaxies. The spectra of extended objects are taken over a large portion of the entrance slit. They are often referred to as two-dimensional spectra because the intensity of the spectral line changes at different vertical positions. Our software can examine only horizontal sections of an image. In order to examine one particular region of an extended object we need to rotate that spectrum so that it is perfectly horizontal. However, by rotating it, we also rotate the spectral lines away from their vertical alignment. Figure 3.2 is a spectrum of the Orion nebula. In this image, we can also see spectra of two of the Trapezium stars inside the nebula. These stars provide us with a nice reference for the tilt of the spectrum. This spectrum has been rotated; notice that although the stellar spectra in the image are horizontal, the spectral lines are tilted. I have included a right-angled line for reference.

Figure 3.2

Ideally, it would be best to rotate a spectrum like this and then skew the image so the spectral lines were again vertical. The software we use does not have such an option. In Chapter 7, I examine the Orion Nebula. The methods I use are tedious, but I manage to work around the tilt problem. However, I would suggest that before anyone began an intensive study of extended object spectra, they find a way to easily work around this tilt, such as using a program with a skewing function.

3.2 Changing Gratings

In the previous discussion, I emphasized the need for spectral lines to be vertical. When setting up the spectrometer, the camera is rotated so that these spectral lines are vertical to the chip. However, this alignment is good for only one grating. If the camera is aligned for the low-resolution chip, the lines of the high-resolution chip will be tilted. Figure 3.3a is a mercury lamp with the low-resolution grating. In figure 3.3b, I have switched to the high-resolution grating.

Figure 3.3a

Figure 3.3b

I have also found that spectral lines do not keep their alignment across the entire spectral region. Figure 3.4a is an argon lamp with the high-resolution grating at 6500 angstroms. I have rotated the camera so the spectral lines are vertical. Later that night I changed to a different region of the spectrum. Figure 3.4b is argon at 4000 angstroms. The lines are no longer aligned.

Figure 3.4a

Figure 3.4b

There are two solutions to these problems: rotate the camera before you change gratings, or rotate the image with your software. (Remember, it is impossible to get spectral lines vertical and the spectrum horizontal at the same time, but this does not matter for point source spectra.) At first, I was rotating the camera before I changed gratings or greatly changed my spectral region. This is a waste of time, especially if you want to change gratings during an evening of observing. Most SGS users adjust the camera so their spectral lines are vertical with the low-resolution grating. Some of these users have stated that they have not opened the unit in over a year.

3.3 Wavelength Calibration

In order to calibrate spectra, you need two reference lines of known wavelength. These lines can be easily recognized lines in the object’s spectra, such as hydrogen lines. However, if this method is used you cannot measure Doppler shifts. Other reference sources can be a nearby fluorescent lamp or discharge tube. Mercury lines from local light pollution can also work, but I have found that this requires at least a three-minute exposure at Wheaton. Maurice Gavin uses a small mercury lamp attached to the opaque window on the bottom of the spectrometer.

I use a discharge lamp as a reference spectrum. After taking a spectrum of an object, I take another exposure and hold the discharge lamp in front of the telescope. For low resolution, I use a mercury tube. There are only a few intense lines and they are easily recognizable. The high-resolution grating has a narrower spectral range than the low-resolution grating, roughly 750 angstroms compared to 3200. I find that mercury does not have enough prominent spectral lines to guarantee two lines in the high-resolution range, so at high resolution I use argon instead. You can see this in Figure 4.3b.

I was unable to find any charts that identified the intense lines in the argon spectrum, so I made my own. Appendix C is the spectrum of argon from 3700 to 10500 angstroms. This was a difficult and time-consuming task. However, I find this reference chart to be a very valuable asset.

3.4 Order Overlap

The orders of a spectrum produced by a grating naturally overlap. Professional spectrometers usually have an order filter to eliminate the overlapping order. In order to keep costs down, SBIG did not install an order filter in the SGS. Because the SGS gratings are blazed for the first order spectrum, the overlapping second order is extremely faint and does not create problems. The only time I have had issue with this overlap is when taking spectra of the daytime sky, where the blue end of the second order washes out red end of the spectra. Because the sky is so intensely blue, the blue end of the second order is just strong enough to become noticeable.

3.5 Software

The spectrometer comes with a program called SPECTRA, which allows you to calibrate the spectra and then export data in text or spreadsheet format. I was initially reluctant to use this software because it does not give you the option to specify your own calibration lines. SPECTRA comes with preset reference lines and only these can be used, which makes high-resolution calibration difficult. In addition, there are no reference lines in the infrared. Even aside from this drawback, SPECTRA is overall a very limited package, and I feel it is insufficient for the work that will be done at Wheaton. Instead of SPECTRA, I began using a program called Visual Spec. This program will be discussed in Chapter 6.

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