![]() In spectroscopy there is one additional calibration that is not done in normal CCD imaging. If you only plan to measure the Doppler shifts of features or changes in them relative to the continuum, then the flat-field calibration, while it might make the spectra more presentable, is not needed. It is also important to understand that unless you are doing spectral photometry (not a recommended project for most amateurs) that the Flat is a completely unnecessary step in reducing spectra. That polynomial of the extraction from the Flat is divided into the data. Instead it is extracted identically to the spectra and modeled with a low-order polynomial. The Flat is not directly divided into the CCD frame containing the spectra. The Flat plays a different roll in spectroscopy. The Dark will be subtracted from the spectrum to remove the background noise in each pixel. And we will apply the dark in very much the same way you would use a Dark frame for normal CCD imaging. The Flat frame is used to calibrate its relative response to other pixels.In spectroscopy the Dark is an important calibration. The Dark frame is used to tell how much noise there is in the pixel. In order to calibrate the pixel we need to know how much noise it would have gathered without the light falling on it and how it responds relative to the other pixels around it. Each pixel can be thought as a mini-detector of light with its own calibration properties. Many of the basic principles of CCD image reduction also apply to the reduction of spectra. The other goal is to dymystify the reduction of spectra and unchain the enthusiastic amateur from their black-box to let them understand and take more control of how their data is reduced. One goal of this tutorial is to de-mystify that process and make amateurs as confortable with reducing spectra as they are with reducing CCD images or photometry. But to do real science, it helps to understand exactly how the data are gathered.įor most amateurs, their spectrograph might come with some canned software that is for all intents and purposes a black box that you put the CCD images from the spectrograph in one end and get one-dimensional spectra out the other with no hint of what happens in-between. This opens up a whole new domain for citizen science. Amateur astronomers have increasingly better access to affordable spectrographs they can use with their telescope to take spectra of objects in the night sky.
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