Conveners:
Susan Nossal (nossal@wisp.physics.wisc.edu)
Mike Taylor (mtaylor@cc.usu.edu),
Tom Slanger (tom.slanger@sri.com)
Edwin Mierkiewicz (emierk@wisp.physics.wisc.edu)
2004 Wednesday 30 June 0100-0300 PM
Accurate calibration is important for comparing observations taken by different instruments, for model-data comparisons, and for acquiring long term data records. A recurring theme throughout the workshop was that calibration poses a major instrumental challenge for observers. Consistent calibration techniques are critically important when multiple observers contribute to a long term data set and when there are upgrades to the instrument(s) acquiring the data set. Participants addressed absolute and relative intensity calibration, as well as spectral calibration. The workshop included a series of short tutorial presentations describing a variety of calibration techniques used by observers in the CEDAR community and discussion of how to move forward to assist our community in optimizing calibration of optical instruments. Many of the ~50 participants were students who are working with a variety of optical instruments.
Observers at both the Arecibo Observatory and at Boston University use a low brightness Carbon-14 source to calibrate ground based optical photometers and spectrometers. However, such a radioactive source is not easily transportable. At the Arecibo Observatory, the Carbon-14 calibrated photometer is then used to cross-calibrate the Fabry-Perot by having both instruments simultaneously view the same patch of sky [R. Kerr]. A tungsten filament bulb is used by Boston University for cross-calibration of field instruments. This intermediate source is calibrated against the Carbon-14 source at the home institution and then used for calibration of the Boston University spectrograph at remote sites [J. Baumgardner]. The spectral dispersion of the spectrograph is calibrated through use of laboratory lamps [J. Baumgardner].
Accurate knowledge of the filter bandpass and transmission profile is crucial for reducing uncertainties in Carbon-14 and laboratory lamp brightness calibrations [R. Kerr and J. Baumgarder]. This information is especially important when using a continuum source to calibrate a monochromatic emission from the sky. In addition, it is important to characterize the bandpass filter shift to the blue with off-normal incidence angle.
The astronomical community commonly uses standard stars for brightness calibrations [B. Sharpee]. The calibration of these stars is tied to blackbody sources. Standard stars are used to calibrate the Keck II Echelle Spectrograph and Imager (ESI) from which aeronomers at SRI obtain the terrestrial spectra contained in the instrument's astronomical observations. When performing standard star calibrations it is crucial that the standard star fall completely within the field of view of the slit utilized by the observer so that the star's entire intensity is captured [B. Sharpee].
A nebular calibration method is used for intensity calibration of the University of Wisconsin's geocoronal, galactic, and cometary observations [E. Mierkiewicz]. Wisconsin observers point to a patch of the North American Nebula that has been calibrated at the H-alpha wavelength using Standard Stars. Calibrations at other wavelengths close to H-alpha can be estimated using knowledge of how the filter transmission and CCD quantum efficiency vary with wavelength. Transferring the calibration to H-beta utilizes the filter transmission, CCD quantum efficiency, and atomic physics H-beta/H-alpha line ratios. Nebular calibration offers the advantages of a stable calibration source, a line emission source, and being outside of the Earth's atmosphere, minimizing uncertainty due to atmospheric extinction. Disadvantages include however, that nebular calibration requires that the instrument have accurate pointing capabilities and that the nebula emit in a wavelength region close to that of the observation of interest [E. Mierkiewicz].
The Wisconsin group uses narrow laboratory lamp emissions for their characterization of the instrumental profile and calibration of spectral dispersion [E. Mierkiewicz]. A primary challenge of this method is to diffuse the lamp light so that it fills the instrument in a similar manner to the observed atmospheric emissions.
The TIDI instrument on board the TIMED satellite makes global wind measurements of the middle atmosphere. Approximately 5% of the TIDI measurement time is spent on calibration related measurements to insure instrument stability and to characterize any shifts in the instrument during the operation of the TIMED mission [R. Niciejewski]. TIDI makes these assessments by observing multiple laboratory lamps and through photon transfer tests to assess the stability of the Charge Coupled Device camera.
Several strategies were discussed to help aeronomers optimize their optical calibration methods. One is to use a clone of the Keck Spectrograph as a brightness calibration source [T. Slanger]. Atmospheric emission bands of known intensity ratios would be used to cross-calibrate a second optical instrument over a wide spectral range.
Another approach is to create a portable calibration instrument. European scientists use the Lindau Calibration Photometer for inter-calibration of instruments and cross-calibration of calibration methods [M. Taylor]. The calibration photometer uses a Fritz Peak standard source and makes calibration measurements at seven wavelengths. This photometer is easily portable and is used at annual European Optical Meetings for cross-calibration of instruments. The CEDAR community could consider development of a similar instrument.
Another suggestion made at the workshop was to install monochrometers at the Upper Atmospheric Facilities to enable measurement of filter transmissions. Accurate measurement of the filter transmission properties is required for several calibration techniques. Observers could bring their filters to these facilities to better characterize their filter properties.
Calibration was acknowledged as a continuing challenge for optical observers. Participants suggested that a discussion of calibration strategies and suggestions be incorporated into the Passive Optics Assessment and associated proposals. Most of the tutorial presentations from the Optical Calibration workshop can be found on the CEDAR website (http://cedarweb.hao.ucar.edu, click on 'Workshop', on 'Workshops', and then on 'Final Report' for OL2). We also plan to organize a follow-up workshop for next year's CEDAR meeting. Please contact us with suggestions regarding the format for this workshop and if you'd like to give a presentation.