Equipment and Calibration

My primary instrument for spectroscopy and the evaluation of the SGS is a Celestron 14. The spectrometer is linked to the telescope with a focal reducer, giving a final f6 ratio. The CCD camera attached to the spectrometer is the SBIG ST-8E with 9-^m pixel size. The SGS instrument appeared on the market during the second half of 1999 and was aimed at a subgroup of science-oriented

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Figure 11.1. The Santa Barbara Instrument Group self-guiding spectrometer showing various features of the instrument and the optical path. The yellow (lighter) path shows the route followed by light passing through the slit to the grating and ultimately to the imaging chip of the ST-7/8 camera. The red (heavier line) path is the route followed by the light not passing through the slit and ending at the guiding chip of the same camera. This allows for extended guiding of a star. Either a different star not passing through the slit or residual light of a star on the slit can be used for guiding purposes. The micrometer screw controls the spectrum region to be observed and has been modified by attaching a JMI digital focusing motor. This permits the spectral region being examined to be controlled by software on a computer.

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Figure 11.1. The Santa Barbara Instrument Group self-guiding spectrometer showing various features of the instrument and the optical path. The yellow (lighter) path shows the route followed by light passing through the slit to the grating and ultimately to the imaging chip of the ST-7/8 camera. The red (heavier line) path is the route followed by the light not passing through the slit and ending at the guiding chip of the same camera. This allows for extended guiding of a star. Either a different star not passing through the slit or residual light of a star on the slit can be used for guiding purposes. The micrometer screw controls the spectrum region to be observed and has been modified by attaching a JMI digital focusing motor. This permits the spectral region being examined to be controlled by software on a computer.

amateurs with special interest in the field of spectroscopy [1]. The instrument is shown in Figure 11.1 with a number of features pointed out and the path of light indicated. The instrument features several novel features. In conjunction with SBIG CCD camera's, the SGS is self-guiding in that it keeps the image of an object locked onto the entrance slit, which allows for long exposures to be taken. The light from the telescope reaches the entrance slit, which can be 18- or 72 |im wide. The light passes through the slit and reaches the grating and ultimately the CCD camera's imaging chip. The remaining field of view is observed on the guiding CCD chip of the camera and allows the viewer to select a field star to guide once the object of interest is centered on the slit. In this chapter, only results obtained using the 18 |im slit will be presented. The wider slit option allows the spectra of fainter objects to be obtained at the expense of resolution. This would be particularly useful to those interested in measuring the redshifts of more distant, and thus fainter objects, since the wider slit permits an additional 2 magnitudes of penetration. This is, however, at the expense of resolution.

The SGS features a dual grating carousel, which, with the flip of a lever, allows dispersions in both the low-resolution mode (~4 Angstroms/pixel, ~400 Angstroms/ mm, 150 line grating) and higher resolution mode (~1 Angstrom/pixel, ~100 Angstroms/mm, 600 line grating). In the low-resolution mode, about 3000 Angstrom coverage is obtained, whereas in the high-resolution mode, about 750 Angstroms for the ST-7 camera and twice this for the larger ST-8. More recently, grating carousels with even higher dispersions have become available (0.5 and 0.3 Angstrom/pixel, 1200- and 1800-line gratings, respectively). The particular region of the spectrum is determined by a micrometer dial and is set by the user. The overall wavelength range of the unit is from approximately 10,000 to 3500 Angstroms. Spectra are obtained using any of the usual camera control software packages such as CCDSOFT, MAXIM or CCDOPS and analyzed using the software package SPECTRA (from SBIG) or VSpec, a free spectroscopy package [2]. Wavelength calibration was carried out using emission lines from a thorium/ argon lamp. This type of emission lamp is widely used among professionals because it produces many lines across the visible spectrum. Figure 11.2 shows the author's spectroscopy setup with appropriate features labeled.

Figure 11.2. The author's spectroscopy setup showing the spectrometer-camera attached to the back of a C14. The thorium/argon lamp is used for wavelength calibration of the spectra obtained. The light from the lamp is introduced into a window on the bottom of the spectrometer. Thorium/argon lamps produce many lines throughout the visible and near infrared, allowing for easy selection of lines of known wavelength for the calibration steps.

Figure 11.2. The author's spectroscopy setup showing the spectrometer-camera attached to the back of a C14. The thorium/argon lamp is used for wavelength calibration of the spectra obtained. The light from the lamp is introduced into a window on the bottom of the spectrometer. Thorium/argon lamps produce many lines throughout the visible and near infrared, allowing for easy selection of lines of known wavelength for the calibration steps.

Calibration of spectra initially follows the usual reductions done for images. However, once an image has been dark-subtracted, spectra require their own set of calibrations to be conducted. These include wavelength and flux calibration. The flux calibration step can be looked upon as being similar to flat-fielding where you manipulate your spectrum with a spectrum you obtained of a standard star (usually Vega or another B- or-A type star). This is all accomplished with Vspec software relatively easily or IRAF, the professional standard that is much more difficult to use. A calibrated text file can be saved and imported into any of a variety of graphic spreadsheets, including Excel. Once the spectrum is expanded into a line profile, various routines can be applied that enhance the lines and make them easier to identify. Absorption and emission line identifications were carried out using tables from the Handbook of Chemistry and Physics [3] or, again using Vspec, which has an extensive line database.

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Digital Camera and Digital Photography

Digital Camera and Digital Photography

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