NIS2 MgX 625 Å
During the conjunction and coronal transit as seen from SOHO on Nov 15, CDS observed Mercury using a selection of studies. One of those was a simple wide-slit movie observation, timed and pointed to let Mercury drift across the field of view. This was repeated three times centred on the time of closest approach to the solar limb. These data have been analysed and some preliminary conclusions derived about the characteristics of the CDS optics.
The observations were designed to show the transit across the brightest likely coronal background given by MgIX (368 Å) and MgX (625 Å). This also gave observations in both wavebands of the CDS NIS spectrometer. The exposures were designed so that the drift of Mercury between exposures (frames of the movies) would correspond to a single pixel in the image. The drift was almost all in the E/W direction at 0.1" per second. The drift N/S was very small at ~0.03 pixels per frame.
During each run of the three multi-exposure rasters, 22 frames were found to have a complete image of Mercury visible. After de-biasing the data, those frames not showing the planet, and which were also free of cosmic ray events, were averaged to form a flat field correction which was divided into the planetary observations. That has the advantage of not only accurately correcting for instrumental effects (MCP burn-in and CCD pixel sensitivities), but also eliminates the true solar coronal background variations.
Although the individual exposures of Mercury show only a very weak detection of the planet, it was possible to locate by eye the image position in each frame and to confirm that statistically the drifts from frame to frame were very close to that expected. (dX = 1.03 ± 0.03, dY = 0.07 ± 0.05 pixels per frame). The relevant frames from each raster were therefore flat-fielded, shifted by 1 pixel and averaged. Since it was necessary to use the OPS legs to relocate between rasters there were offsets between the planetary positions and these were corrected for by integral pixel shifts (both X and Y) of the averaged images. This was done for both wavelength images, although the MgIX (NIS1) image is considerably weaker than that of MgX (NIS2).
The images from the three rasters were then averaged and the result for NIS2 is shown in frame (A) of the image below (axes are in arcsec). There is clearly an elongation of the observed image at a position angle of about 45 ° which cannot be accounted for by image motion.
We already know that the PSF of the spectrometers is elongated and rotated from observations taken with the 2"x2" slit. However the elongation of that PSF is not at the angle required for these data (in fact it's about 90 ° away). We have therefore attempted to explain the Mercury observations by invoking a non-circular telescope PSF. Panel (B) in the image below shows the expected image if only the circular (dark, or darkish, see below) disk of Mercury is convolved with the observed spectrometer PSF. A telescope PSF, as shown in panel (C) is then invoked and applied to (B) and the result is shown in final panel. The image shown is the modelled data (with white contours) and the black contours are from the observed data. Although the agreement is good, it is not necessarily a unique minimisation-type solution because in addition to the shape and orientation of the PSF we have also added a 'dilution factor' in the model image to mimic stray light (and real emission if any) in order to match the model to the observations in terms of the 'depth' of the Mercury disk's image. This component and the PSF parameters are not entirely independent in terms of the match shown.
The parameters of the PSF used were a Gaussian with (x,y) FWHM of (7.0,13.5) arcsec and a rotation of 39 ° from the vertical. The dilution factor needed indicated a 59% depth for the original disk.
The next plot shows similar results from the NIS1 line. The image was much weaker and the results correspondingly less certain. However, as illustrated, a PSF of FWHM (7.0, 21.5) arcsec at an angle of 104 ° is required. The dilution factor indicated an original depth of 38% for the Mercury disk.
