Conveners:
Susan Nossal (nossal@wisp.physics.wisc.edu)
Mike Taylor (mtaylor@cc.usu.edu)
Tom Slanger (tom.slanger@sri.com)
2005 Thursday 30 June 0130-0330 PM
Accurate calibration is important for comparing observations taken by different instruments, for model-data comparisons, and for acquiring long-term data records. 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. This year's optical calibration workshop continued discussion of these topics addressed at the 2004 CEDAR optical calibration workshop.
The workshop began with an historical perspective by Fred Roesler. He described the development of a nebular calibration method used by Wisconsin observers for absolute and relative calibration of diffuse terrestrial, planetary, and astronomical observations. Roesler followed with a description of additional challenges associated with cross-calibration between instruments.
The Wisconsin observations are compared with the intensity of nebular sources, all of which are tied to the North American Nebula. The observations are then corrected for differences in atmospheric extinction due to the slant path of the sky observation compared with that of the nebular calibration. The primary nebular calibration sources used for Wisconsin H-alpha observations were calibrated using standard stars and corroborated using a blackbody source. More recently, observations of the interstellar medium taken as part of the Southern H-alpha Sky Survey Atlas by Gaustad et al. [2001] were compared with those taken by the Wisconsin H-alpha Mapper Fabry-Perot [Haffner et al., 2003], further corroborating the calibration. Nebular calibration offers long-term stability and like the geocorona and interstellar medium, nebulae are spatially extended line emission sources. The nebular calibration method has produced internally consistent results for Wisconsin-based geocoronal, planetary, and interstellar medium observations.
Inter-calibration of instruments participating in the NSF-CHARM geocoronal observation campaign of the early 1990s led to unforeseen challenges. Roesler made a hydrogen hollow cathode lamp placed in a diffusing box that was flown to different sites for inter-calibration using a line emission rather than a continuum emission source. A study of calibration differences that persisted after the instruments viewed the lamp indicated that careful consideration was required of both the source characteristics and the respective instruments viewing the source. Issues that arose included that the instruments saw different portions of the lamp emission spectrum depending upon whether they were single- or double-etalon FPI instruments, and the parasitic light (light from outside the filter bandpass) differed between instruments. The CHARM campaign indicated some of the subtle challenges associated with inter-calibration between optical instruments.
Brian Sharpee spoke about standard star calibration, a method that is the basis for several other types of calibration. For example, the nebular calibration method discussed by Roesler and the calibration of the Keck echelle spectrographs discussed by Tom Slanger are methods both tied to standard star calibration. Standard stars are stars with smooth spectral output and sufficient intensity to make their use as reference stars practical. Primary calibration stars have been calibrated against blackbody sources. Sharpee has been using standard stars to calibrate the Keck spectrographs. When performing this calibration it is essential that the star be fully enclosed within the field of view of the instrument.
Tom Slanger spoke about relative calibration of the Keck spectrographs using emission properties of molecular bands. The Keck instruments, located at Mauna Kea, Hawaii, are high resolution echelle spectrographs used primarily for astronomical studies. The terrestrial emissions in the Keck astronomical spectra also provide a rich resource for aeronomical studies. Slanger spoke about relative calibration of emission lines within O2 and OH bands. The intensity ratios between emission lines within these bands are known by atomic and molecular spectroscopy. Once one of the lines within each band has been calibrated using standard stars, the calibration can be transferred to other lines within the band by using information about relative line intensity within each band.
J. Baumgardner spoke at the 2004 CEDAR Optical Calibration workshop about calibration of the Boston University spectrograph using Carbon-14 and laboratory lamp sources, and at this year's workshop focused on techniques used to characterize and calibrate Boston University's all-sky imagers. Processing of the CCD observational images to correct for instrumental effects involves subtracting the CCD bias and the dark thermal noise (differs with exposure time). Flat field exposures are used to normalize the observational images in order to correct for vignetting, i.e. cutting off of light rays, within the instrument. It is a challenge to obtain a spatially and spectrally uniform flat field that fills the field of view of the instrument. At Boston University, researchers use a diffusing light box and a diffuser at the location of the filter wheel in order to create a flat field used to correct the imager data.
Researchers use a tungsten lamp source for intensity calibration of the Boston all-sky camera observations. Knowledge of the brightness versus wavelength of the tungsten lamp as well as the transmission of the interference filter are both required to accurately calibrate the all-sky images using the tungsten lamp source. It is especially important to accurately characterize the transmission properties of the interference filter as the information about filter transmission properties supplied by the manufacturer may not be sufficiently accurate to reduce calibration errors to below 10%. In addition to the filter transmission, uncertainties about tropospheric scattering and atmospheric transmission probably are the major sources of error in the calibration of the airglow's absolute intensity.
Sam Yee discussed mutual consistency of calculated dissociation and ionization rates using measurements from the Solar EUV Experiment (SEE) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Satellite (TIMED) with rates calculated using overlapping observations from other instruments. The SEE instrument observations are compared with long-term solar irradiance ultraviolet and extreme ultraviolet data from the SOLSTICE instrument on the UARS satellite and from the SORCE satellite. The TIMED/SEE derived O2 photo-dissociation rates and their response to solar activity are found to compare favorably with those derived from UARS/SOLSTICE, while those calculated from SORCE observations are 10-20% less. Atomic oxygen and N2 photo-ionization rates derived from TIMED/SEE X-ray and EUV measurements are generally higher than those calculated using EUV models. Measurement discrepancies provide an assessment of the uncertainties associated with parameters derived from SEE and other satellite observations and used in upper atmospheric modeling. The new information from TIMED/SEE suggests that ionospheric and airglow photochemistry might need to be reexamined.
Mike Taylor reported on the portable Lindau Calibration Photometer used by European scientists for inter-calibration of optical instruments. The calibration photometer uses a Fritz Peak standard source and makes calibration measurements at seven wavelengths. The photometer is easily portable and is used at annual European Optical meetings for cross-calibration of instruments.
Participants acknowledged the challenges associated with calibration and the need to optimize methods for intensity and spectral calibration and inter-calibration between instruments. We plan to continue these discussions at next year's CEDAR workshop. Please contact us with suggestions regarding the format of the CEDAR Optical Calibration Techniques and Issues workshop and if you would like to give a presentation next year. In addition, we are considering the idea of organizing a more extended optics workshop, perhaps in conjunction with the annual CEDAR workshop, where instrument characterization and calibration along with other optical aeronomy issues can be discussed at greater depth.