Calibration Unit
Each MODS has its own internal calibration system consisting of an integrating sphere and projection optics located in a deployable calibration tower located above the slit plane. The calibration unit and AGw occupy the same space in the instrument in front of the focal plane when “deployed”. A selection of Pen-Ray® wavelength calibration lamps and continuum lamps for spectral and imaging flat fields are mounted in the integrating sphere. In the table below, the available lamps in the Calibration Unit are summarized, as well as their primary use:
| Lamp | Description | Use |
|---|---|---|
| Hg | Mercury(Argon) Pen-Ray® Lamp | Blue channel wavelength calibration |
| Ne | Neon Pen-Ray® Lamp | Red channel wavelength calibration |
| Ar | Argon Pen-Ray® Lamp | Dual-channel wavelength calibration |
| Xe | Xenon Pen-Ray® Lamp | Dual-channel wavelength calibration |
| Kr | Krypton Pen-Ray® Lamp | Dual-channel wavelength calibration |
| QTH1 | 10W Quartz-Tungsten Halogen Lamp | Blue spectral flat field |
| QTH2 | 10W Quartz-Tungsten Halogen Lamp | Blue spectral flat field |
| VFlat | Variable-intensity Incandescent Lamp | Red spectral and Dual imaging flats |
The VFlat variable-intensity incandescent lamp is used for imaging flats in the u,g,r,i and z bands, and red spectral flat fields in the grating and prism modes. Its intensity can be varied from level 1 to through 10. The brightness of the lamp is non-linear with intensity value, and the lamp gets slightly bluer with higher intensity. The VFlat intensity is proportional to the applied voltage, and the lamp filament gets hotter with greater voltage.
To view the calibration system the AGw guide stage has to be retracted to its home position, the instrument dark hatch closed, and the calibration optics inserted into the beam, a process that requires about 30 seconds (up to 40 seconds if the guide probe is near the science field center). Special CALMODE and OBSMODE commands are provided to configure the instrument for taking internal calibrations (CALMODE) or observing the sky (OBSMODE), taking care of the fine configuration details.
Further details about the calibration modes & exposure times are reviewed on the Observing -> Calibration page.
