Schematic of the Fireball payload
The Fireball payload is optimized for spectral imaging of faint and diffuse sources in the ultraviolet. It requires a pointing accuracy and stability of the order of one arcsecond, achieved by the combined action of the pointed gondola and a fine-pointing system integral to the payload, which comprises:
- a 1.3-metre-diameter siderostat (flat mirror)
- a 1-metre-diameter parabolic mirror with a focal ratio of f/2.5
- a multi-object spectrograph
The siderostat at the bottom of the gondola conveys the observed image of the sky to the parabolic mirror. It is mounted on a dual-axis pivot assembly to allow pointing in elevation and fine cross-elevation control to assure the required degree of precision during observations.
The spectrograph comprises an optical train that shapes the light beam and conveys it to a set of slit masks that can be controlled from the ground. A diffraction grating (grism) then splits the light diffracted by the detector’s slits.
The detector is a delta-doped electron multiplying charge coupled device (EMCCD) fabricated by e2V with a UV anti-reflective coating provided by CalTech to obtain maximum quantum efficiency (QE) in Fireball’s wavelength window.
The spectrograph assembly sits inside a dewar. The detector is cooled to 150 K by a cryocooler to reduce electronic noise to a minimum.
Dewar housing the spectrograph and mounted on its rotation stage
The dewar is mounted on a rotation stage designed to compensate for field rotation during observations and for pendulum oscillations of the gondola platform.
The visible balance of the light from the telescope beam goes to a dichroic mirror and feeds a fine sensor with a CCD camera for use in the fine-pointing loop.
Main payload features and performance:
- Bandpass: 200 – 208 nm
- Field of view: 10 x 20 arcmin2
- Angular resolution: 5 arcsec
- Spectral resolution: 2,250
- Detector array: 1,024 x 2,048 pixels
- Pixel size: 13.5 microns
- uantum efficiency in bandpass: 70%
- Sensitivity: 5,000 LU / 20 arcsec diameter @ S/N=5 per hour
- Boresight stability: 1 arcsec rms perpendicular to optical axis, 3 arcmin rms along optical axis (field rotation).