White Light Cameras
charged particles are responsible for the emission that the eye and cameras capture. What the eye sees is the Thompson-scattered emission from electrons that reflect the full spectrum of light emitted by the photosphere towards the observer. This emission is commonly known as continuum or white light radiation. Identical cameras with different focal length lenses are operated with different exposure times to cover the orders of magnitude dynamic range of the coronal white light emission. An example of an image composed from these cameras and image-processed, using the technique developed by M. Druckmüller, is shown:
The other component, indistinguishable to the eye, because it occupies a minute fraction of the coronal radiation, is produced by highly ionized heavier elements in the corona, with the dominant one being iron (Fe). It can only be captured by cameras or detectors retrofitted with filters that isolate the select ‘colors’ of the different stages of ionization of Fe, the dominant ones being Fe X 5303 A, Fe XI 7892 A, Fe XIII 10747 A and Fe XIV 5303 A, stripped of 9, 10, 12 and 13 electrons respectively. This can be achieved with narrow-band pass filters, tuned to the wavelength of the radiation emitted by these different charged states, or with spectrographs.
We currently have Fe XI and Fe XIV systems with 3” optics and 300 mm focal length lenses for all primary sites, and 2”/200 mm Fe systems for the secondary sites, four spectrographs, and the cameras and lenses required for the high resolution white light images. Also included in the primary sites are three Ar X, Ar XI imaging (line and continuum) pair systems.
To expand on the diagnostic potential of the imaging measurements, a dual channel imaging spectro- graph was designed for the 2015 eclipse over Svalbard to acquire resolved spectra of the Fe XI and Fe XIV lines in the spatial direction of a slit, and as a function of distance from the Sun. Resolved line spectra allow measurements of linewidths (= thermal + non-thermal motion) and Doppler shifts (= bulk motion) if present.
The design of the spectrometer consists of input optics, a mirror slit, imaging optics, two echelle gratings, output optics, two dichroic mirrors to split the output beam into two bands. Each channel consists of an echelle grating run in high order n (~40), several Schott filters with multi-layer coatings, a re-imaging lens and a CCD camera (see Figure 5). Using the echelon gratings in higher order has the advantage of increasing the slit-width resolution without narrowing the slit, as the resolution is proportional to the order number and inversely proportional to the slit-width. The spectral resolution can be improved, as well as the throughput of the system, by widening the slit. This design also takes advantage of the fact that the number of emission lines, both coronal and chromospheric, from the visible solar spectrum, in each spectrometer channel, is limited to approximately 20 to 30 lines. Spectral lines (2 to 3 in the present set-up) from identical transitions in different orders will always be present in the recorded spectrum. As the line positions are unique, each line can therefore be separated from overlapping spectra of different higher orders. This choice facilitated the extraction of the individual emission lines. With the improved resolution (presently by a factor of 40) compared to a first order spectrum, the line profile is resolved as it is spread over several tens of pixels. In addition, weaker lines are also readily detected. The design also includes an optical system which forms an image of the sky/sun/corona on the entrance slit of the spectrometer, thus enabling the eclipsed Sun, including any potential cloud coverage impinging on the spectrometer slit, to be imaged by a video camera. The design was constrained by the demand of eclipse expeditions, hence was made to be robust, light weight and transportable as carry-on, like the multi-wavelength imaging systems.
Based on this design, 4 spectrometers have been built for the 2017 total solar eclipse observations. An additional channel has been added to each system.
Each triple channel imaging spectrograph has three bandpasses to include the spectral (chromospheric, ‘cool’ coronal and ‘hot’ coronal). While the remarkable 2015 results were serendipitous, it has been our experience that CMEs are invariably captured in eclipse images, either as a CME front, or through the tracks left behind in the corona. Spectroscopic observations, complemented with imaging, will enable the characterization of the temperature structure of CMEs, and any cool material embedded within them. The choice of wavelength ranges within the three bandpasses underscores the unique traits of the design. Changes in the spectral line profiles as a function of distance should lead to inferences of the corresponding ionic temperature, and of any nonthermal motions along the line of sight, as possible signatures of coronal heating.