The CUBIC Calibration

The CCDs flown on CUBIC were calibrated in our laboratory cameras at a variety of X-ray line energies to obtain data on the quantum efficiency and spectral response as a function of energy. Following integration into the flight camera and completion of the flight software, the entire instrument was calibrated in our X-ray calibration/thermal vacuum test chamber. The image above is a portion of a raw calibration frame from the flight camera illuminated by X-rays from a Teflon target fluoresced by an X-ray beam. The energies of the X-rays are indicated by their color, with blue dots representing the lowest X-ray energies and red dots the highest energies. The cluster of red dots in the upper left corner is from one of the on-board Fe-55 calibration sources, which illuminate each corner of the CCD continuously for in-flight calibration. The top half of this image has a low count rate because this part of the CCD is protected by an aluminum cover from incident X-rays and will be used in flight to measure the spectrum of the particle background. The lower half of the image shows X-ray events from the Fluorine K alpha line at 677 eV, the Aluminum K alpha line at 1.49 keV (from the Al target holder and camera body), and some higher energy photons.

The above figure shows the raw event size histogram from the two CUBIC CCDs for this calibration data set. The lines are doubled in the raw histogram because the two CCDs have different baseline levels. After processing the spectra through our event recognition algorithm (selecting only single pixel events for best energy resolution) we obtain the figure below.

Blowups of the lines in this spectrum (with Gaussian fits) are available ; the energy resolution for each line is indicated.

Once the calibration data are processed, they are used to create a response matrix , which is used to convert model spectra into predicted detector outputs. The main purpose of the calibration data is production of an accurate response matrix, which allows the data collected by the instrument to be interpreted astrophysically.

More information is available on the following topics:

• Calibrations at the UW Synchrotron Radiation Center
• Flight calibration plans

Bibliography

First results of the CUBIC calibration of flight CCDs (July 1995).

CUBIC: Preflight Calibration Results and Initial Operations (August 1996).

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