Spectra Capture and Processing
In order to obtain a spectrum the following images are necessary:
Images of the target object.
Dark frames, for the elimination of CCD's thermal noise.
Offset images for the elimination of CCD's residual noise and read noise.
Flat field images, in order to eliminate dust and vigneting. The quantum efficiency correction of the CCD pixels is preformed later, When correcting the instrumental response.
Images of a known spectrum for wavelength calibration.
Images of a star whose spectrum is very well established in flux, for the determination of the instrumental response.
The spectra processing phases:
Raw image of the Be star HD183656:
After basic reduction (offset, dark and flat):
After the geometric correction of the spectral lines inclination and of the spectrum inclination:
After sky subtraction:
After the sum (sigma clipping) of 6 of these images:
The spectrum is now ready for study. In order to benefit from all the acquired flux, a sum of the pixels of each column of the image above is preformed (binning). Since one single line is not readable, the line is enlarged . This spectral representation is one-dimensional; the pixels of each column have exactly the same value:
The spectrum for the wavelength calibration is processed in the same way. In this case a Neon lamp was used. Its emission lines correspond to 6532,88 Å and 6598,95 Å, since we are working on a Be star in the Ha line (6562,8Å):
The same procedure for the star chosen to the assessment of the instrumental response, in this case Altair:
Graphic representation of the Neon spectrum:
The graphic above, with two lines of known frequency, allow the calibration of the wavelength in abscissas. In this case a linear calibration is assumed, because the wavelength interval is very sharp. If the bandwidth is larger, more calibration lines are necessary and a polynomial fit is calculated, due to the fact that dispersion changes with wavelength.
The Neon graphic is substituted by the one of the object in study:
The flux is normalized, in ordinates, by dividing it with the average of a zone of continuum (without spectral lines):
The instrumental response is corrected. The objective of this correction is related with the different sensitivity of the CCD to different wavelengths. It is also the way to make our spectrum comparable with spectra of the same object taken in different equipment. In order to accomplish this, the spectrum of the calibration star acquired with our setup (in blue), is divided by the calibrated spectrum of the same star (in orange, taken from a database). The instrumental response is obtained (in red):
The spectrum of the object in study is divided by the instrumental response, et voilá:
Spectra of HD183656 before (blue) and after (red) the instrumental response correction.
spectra processed with Iris and VisualSpec