CFHT, Instruments, Spectroscopy, Gumball, Gumball Manual.

Form GIF

Gumball: The User's Manual

Form GIF

The User's Manual for Gumball

At the beginning of 1998, an upgraded version of the Gumball calibration unit was commissioned at CFHT. Not only optomechanics and electronics components were modified or changed but a new Pegasus interface was also implemented. For an observer, the major changes include the possibility to define different exposure times for each lamp for the same calibration frame, pre-defined setups for diverse intrumental configurations to optimize the utilisation of the Gumball, and the availability of two Fabry-Perot interferometers providing regularly spaced calibration lines over large spectral ranges. The manual below describes in more detail the new Gumball and ... how to use it!

Table of Contents:

  1. Foreword
  2. Chapter 1: The Upgraded Gumball
  3. Chapter 2: Using Gumball through Pegasus
  4. Other Information


This manual presents the upgraded calibration system for the Cassegrain environment at CFHT. During 1997, major changes have been made on the calibration unit (alias "Gumball") so it can be used for all the instruments mounted at this telescope focus. For the observers, a new interface has been developed which allows more flexibility for acquisition of calibration frames. In the following, Chapter 1 briefly presents the new system, a short overview of the new interface, and the Gumball database. The interface and how to use the new Gumball are more extensively described in Chapter 2.

Chapter 1: The Upgraded Gumball

  • Purpose

  • The Cassegrain calibration system ("Gumball") was designed and fabricated by the Observatoire de Marseille as a visible spectral calibration unit for the MOS/SIS spectrograph. The system was operated from the MOS/SIS controller allowing remote utilisation of the different lamps from a Pegasus session. However, with the recent development of other instruments at CFHT like the Adaptive Optics Bonnette (AOB) and OASIS, or for planned instrument like CFHT-IR, it became evident that the actual Gumball unit was not well suited to provide calibrations for all these systems. In particular, the transmission characteristics of the optics did not allow calibration in the near-infrared domain and not enough light (or not enough spectral lines) was produced for the OASIS calibrations. Thus, the main purpose of the upgrade was to modify Gumball so that it can function as a universal system for all the instruments mounted at the Cassegrain focus without any dependance of the MOS/SIS optics and controller. To achieve this, significant modifications in the optics, electronics and software have been carried out, as described below.

  • Optomechanics

  • According to the specific needs of each instrument already in use or planned, the specific objectives during the upgrade were:

    To achieve these goals, a new optical layout and controller were designed. The transfer optics has been replaced with a wide bandwidth glass to increase the blue and near-IR transmission. All the lamp control sequences and status are now controlled via a ProLog based computer, directly connected to the Gumball. Moreover, two fixed Fabry-Perot etalons were installed to provide channelled spectra for OASIS (but available also for MOS/OSIS). Both etalons (one optimized for the blue, the other one for the red) are illuminated be a continuum sourve via two optical fibers.

    The new arrangement for the lamps inside Gumball is illustrated here. An important constraint of this design is the removal of the Helium lamp to make room for the FP etalons. Two pencil Mercury (Argon) lamps have been added to provide spectral lines in the blue. To provided a more powerful and uniform illumintation, two similar lamps are provided. For each lamp, a backside reflector can be installed to increase the light flux, if necessary.

  • Interface: An Overview

  • The ProLog computer implemented in the new Gumball controller allows a much better control of the lamps and provide a feedback on the real status of the different spectral sources. To fully exploit these advantages, a new Pegasus interface was designed. The user's interface was created on the following guidelines:

    Except for the pre-defined setups, all these options are now implemented.

  • The Database

  • During the upgrade, it was also clear that calibration spectra should be made available on-line for reference during data acquisition and reduction. As presented earlier, this database includes the most common frames taken with different instrumental configurations. This database will evolve when more and more data are being gathered...

    Chapter 2: Using Gumball through Pegasus

  • The Pegasus Interface
  • By selecting the Lamps button in a Pegasus session for a specific instrument mounted at the Cassegrain focus, the following form appears:

    Two basic modes of operation are offered:

    There are 10 spectral sources available in the upgraded Gumball:

    The Pre-defined Setups option is still not available but should be implemented soon. It will provide a quick way to get the setup (i.e. lamps + exposure times) for a specific configuration.

    At the beginning and during the exposure, the real status of the lamps is gathered by the ProLog computer. If a problem occurs (e.g. lamp could not be turned on), a red message will be displayed in the instrument status window located on the right terminal.

  • The Fabry-Perot Etalons

  • The Gumball offers the possibility to get channelled spectra from two fixed gap Fabry-Perot etalons. These etalons are illuminated by a common Xenon lamp via two 800 micron optical fibers. The light is collimated, passed through the etalon and is re-imaged onto fiber bundles which illuminate the internal sphere in Gumball. When using a dispersor, the resulting spectrum consist in a series of regularly spaced spectral lines (see this example).

    Although the FP etalons were primarly installed to be used with the small spectral ranges offered by OASIS, they can also be helpful with MOS/OSIS if necessary. However, since the etalons have not been fully characterized yet, for example we do not have data on the influence of temperature variations, normal calibration lamp exposures must also be taken during an observing run.. These lamps will allow to detect any variations in the spectral line positions produced by the etalons. Also, they will offer an absolute reference for the wavelength calibration. It is also very important to use the FP etalons in the spectral range they were designed for!

    Since data reduction using the channelled spectra will be a bit more complicated, for the time being we recommend that you use the FP etalons with MOS/OSIS in case of absolute necessity...

  • The Predefined Setups

  • Other Information

    Here is some supplementary information that can be useful about Gumball: