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Extended Objects: Europa Observations
Europa, satellite of Jupiter was observed on November 18, 2013 from 08:42:16 (airmass=1.63) to 08:48:16 (airmass=1.64) under average seeing conditions (DIMM seeing~0.6-0.8) in K1 band, and direct mode (no coronagraph, no apodizer). Eleven data cubes with a true integration time of 8.73 s were processed by M. Perrin using the recipe listed below.
Recipy used to process Europa Data
- Load Wavelength Calibration
- Subtract Dark Background
- Update Spot Shifts for Flexure
- Destripe science image
- Interpolate bad pixels in 2D frame
- Assemble Spectral Datacube
- Interpolate Wavelength Axis
- Divide by Lenslet Flat Field
- Interpolate bad pixels in cube
- Rotate North Up
- Accumulate Images
- Combine 3D datacubes
Ephemeris calculations provided by JPL & IMCCE indicate that the satellite was bright (V~5.6) with an apparent diameter of 0.9508 arcsec and observed with a phase angle of
8.9 deg, leading to an illumination fraction of 99.4%. Europa disk is
therefore covering ~10% of the detector in an area almost centered on the detector.
The last 6 images were taken with CCR at low power and visually seem to be of better quality. It is not possible to quantify the resolution on these observations and the gain with CCR at low power since Europa does not display unresolved features necessary to perform a direct measurement.
Europa was not dithered on the set of observations. We estimated the residual jittering to be 0.12 pixel in average by fitting the disk with an ellipsoid and measuring its center for each frame. Figure below shows the astrometric position of Europa (labeled from E0 to E10), plus the residual jittering measured on seven frames of HD1160, a A0 V=7.1 star observed in K1 direct imaging mode on the same night between 01:20 and 01:35 UT (airmass ~1.2, DIMM seeing~0.8 arcsec), label P0 to P5. Because the residual jittering on the “PSF” stars is slightly higher (0.31 pixel in average), we conclude that no residual jittering due to the extended angular size of Europa is apparent. It is likely that the angular resolution on these images of Europa is in fact very close to the angular resolution that we measured on the “PSF”, so 65 +/- 1 mas.
After data processing, the frames in the datacube still display some imperfections visible as 15-20 pixels of low and high intensity on Europa’s disk. Most of these bad pixels are identical on the 11 observations and are probably due to bad pixels on the detector. Additionally, some of the frames in the cube contains an oblique pattern a misalignment between the spectra position in the calibration and the actual position in the detector image. This misalignment results in a poor flux extraction when the cube is assembled.
Pixel Scale from Europa Observations.
We estimated the pixel scale on final assembled cube by fitting the observations of Europa by a sphere illuminated under the same geometry than Europa (Sub-Solar point and Sub-Earth point from the ephemeris) and degraded after convolving it by a gaussian PSF with a FWHM of 65 mas. We generated a grid of solutions varying the Minnaert coefficient which defines the center-to-limb profile from 1.1 to 1.5 (step of 0.02) and the pixel scale from 13 to 15 mas (step of 0.04). The best fit is obtained with a pixel scale estimated to 14.30 ± 0.31.
A 3-color picture (1.95, 1.99,2.18 um) included below shows variegation on Europa’s disk. A comparison with visible observations based on Galileo-Voyager spacecraft suggests an anti-correlation between the dark albedo features in visible of Europa which are bright at ~2 um.
To confirm that these spectroscopic differences are real, hence not instrument artifacts, we extracted the spectra of 3 areas which show significant variations on the complete data set of Europa. The eastern limb is bright at ~1.8um, the bright center area is bright at ~2um and an isolated patch north-west of Europa that could be a young crater. The flux of the extracted spectra is normalized after dividing it by the flux of HD1160 an A0 star without including an airmass correction.
The extracted spectra of these 3 different area on Europa are similar among all the spectro-images within 3%, confirming the stability of the instrument. The spectra of these three regions display identical absorption bands at 1.95, 2.00, 2.05 um which could be of atmospheric origin. It is however clear that spectroscopic variabilities (e.g at lambda>2.1 um) are detectable on Europa disk.
Neptune (JPL ephemeris V=7.9, angular diameter = 2.26 arcsec) was observed on Dec 11 2013 from 00:17 to 00:31. Seven frames with an exposure time of 59.65s were recorded in H direct with an airmass of 1.3 and a DIMM seeing of 0.66 arcsec. This data is still being processed.