Astronomical Spectroscopy - Physics - University of Cincinnati

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$1 Ch. 22 â Reflection and Refraction of Light 22.1 The ... - Physics$

– 34 – and argon (HeNeAr), or thorium and argon (ThAr), with the choices dictated by what lamps are available and the resolution. HeNeAr lamps are relatively sparse in lines and useful at low to moderate dispersion; ThAr lamps are rich in lines and useful at high dispersion, but have few unblended lines at low or moderate dispersions. • Spectrophotometric standard stars are stars with smooth spectra with calibrated fluxes used to determine the instrument response. Examples of such stars can be found in Oke (1990), Stone (1977, 1996), Stone & Baldwin (1983), Massey et al. (1988, 1990), and particularly Hamuy et al. (1994). Exposures should be long enough to obtain at least 10,000 e − integrated over 50Å bandpass, i.e., at least 200 e − per Å integrated over the spatial profile. The basic reduction steps are described here. For convenience to the reader, reference is made to the relevant IRAF tasks. Nevertheless, the goal is to explain the steps, not identify what buttons need to be pushed. 1. Fit and subtract overscan. Typically the overscan occupies 20-40 columns on the right side (high column numbers) of the chip. The simplest thing one can do is to average the results of these columns on a row-by-row basis, and then fit these with a low-order function, possibly a constant. An example of such a nicely behaved overscan in shown in the left panel of Figure 11. In other cases, there is a clear gradient in the bias section that should be removed by a higher-order fit. The IMACS chips, for instance, have an overscan at both the top and the right, and it is clear from inspection of the bias frames that the top overscan tracks a turn-up on the left side. Such an overscan is shown in the middle panel of Figure 11. Occasionally the overscan allows one to correct for some problem that occurred during the read-out of the chip. For instance, the GoldCam spectrometer chip suffers 10-20 e − striping along rows if the controller electronics get a bit too hot. (This situation is sometimes referred to as “banding”.) A similar problem may occur if there is radio transmission from a nearby walkie-talkie during readout. However, these stripes extend into the overscan, which can be used to eliminate them by subtracting the overscan values in a row-by-row manner rather than by subtracting a low- or high-order function. Such banding is shown in the right panel of Figure 11. In IRAF, overscan subtraction is a task performed in the first pass through ccdproc. 2. Trim. Most spectroscopic data do not extend over the entire face of the chip, and the blank areas will need to be trimmed off. Often the first or last couple of rows and columns are not well behaved and should be trimmed as well. At the very least, the
– 35 – Fig. 11.— Three examples of overscan structure. The overscan values have been averaged over the 20-40 widths of the overscan region, and are shown plotted against the read-axis. The overscan on the left is from the GoldCam spectrometer on the Kitt Peak 2.1-m and is best fit by a constant value. The overscan in the middle is from chip 3 of IMACS, and shows a pronounced upturn on the left, and is best fit with a low-order function. (Note too the bad column around pixel 420.) The overscan on the right is also from GoldCam, but during an electronics problem that introduced a number of bands with low values that were also in the data region. Subtracting the average values in the overscan row-by-row removed them to a good approximation.
Page 1 and 2: Astronomical Spectroscopy Philip Ma
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Page 21 and 22: - 21 - Fig. 7.— Multi-object mask
Page 23 and 24: - 23 - Fig. 8.— Optical layout of
Page 25 and 26: - 25 - Fig. 9.— View of the focal
Page 27 and 28: - 27 - Fig. 10.— Optical design o
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Page 33: - 33 - • The data frames themselv
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Page 51 and 52: - 51 - can be answered given a comp
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Page 55 and 56: - 55 - 3.2.6. Long-Slit Flat-Fieldi
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Page 67 and 68: - 67 - dispersion. Going to the par
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Page 75 and 76: - 75 - 500 stars. Figure 20 shows t
Page 77 and 78: - 77 - So, attempt to select a tell
Page 79 and 80: - 79 - Fig. 21.— Output from the
Page 81 and 82: - 81 - Wrap-Up and Acknowledgements
Page 83 and 84: - 83 - Massey, P., DeVeny, J., Jann

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