Notes on the RSR of XERO

Notes on the RSR of XERO

During the IAYC 2013 I gave one of my participants, Alex Barr, the arduous task of determining the relative spectral response (RSR) of the CCD that is used in the Xbox Live Vision Camera. The motivation behind this is the comparison between the RGB channels of an Xbox Live Vision webcam and the standard BVR photometric filters. By knowning the spectral response of each on-chip filter in the CCD we will be able to determine the conversion factors between photometric values in the literature and catalogue data gleaned from the XERO Southern Hemisphere Survey.

In order to determine this RSR Alex observed Vega with a 70mm refracting telescope. The Xbox webcam, together with a Star Analyser 100 lines/mm grating, were attached to the telescope in order to record the spectrum of Vega. 

Figure 1: Single spectral image of Vega. Vega can be seen at the far left and the first order diffraction spectrum is the rainbow streak stretching from the centre to the very right.

After reducing, aligning and stacking the raw images we measured the intensity of the spectrum along the length of the image. A single raw frame of the spectrum is shown in Figure 1. The diffraction grating of the Star Analyser 100 splits up the wavelengths nicely. With the IR filter removed the sensitivity of the red channel is vastly increased, however there appears to be an IR leak in the green channel. This will prove to a challenge when the time for photometry comes.

The relative spectral response curve was easy to extract, callibrating it was the difficult part. We chose to use the hydrogen alpha and beta lines as reference points. These are shown in Figure 2. By using these two lines, we found a resolution of 1.5 nm/pixel. The final spectral response curves are shown in Figure 2.

Figure 2: Spectral response for the inbuilt RGB filters in an IR capable Xbox Live Vision webcam

We confirmed the spectral resolution by assuming that the wavelength range of the blue and green channels wouldn't be too different from the wavelength range of the standard Bessel B and V filters. Assuming a FWHM for each of the Blue and Green channels of 95nm and 90nm respectively we found a spectral resolution of 1.4 nm/pixel. Therefore we concluded that we had indeed calculated an accurate spectral resolution by using the hydrogen absorption lines.