Description of the EPOCh Earth Observations By Drake Deming September 18, 2009 Updated by Tim Livengood, 2 September 2010, for EPOCh Earth Polar observations in 2009. EPOCh Earth Equatorial Observations in 2008 ------------------------------------------- The 2008 EPOCh Earth equatorial observations comprise imaging in seven optical narrow-band filters, as well as infrared (1.05 to 4.8 micron) spectroscopy. The optical images used a 512x512 subarray of the visible HRIV CCD detector at a scale of 0.4 arcsec per pixel, and each frame acquired by the HRII infrared spectrometer is 512 pixels (wavelength) x 128 pixels (spatial, with a scale of 2 arcsec per pixel). Because the HRI telescope used for both optical and infrared wavelengths is defocused, the spatial resolution as observed is about 4 arcsec in each case, but can be improved by deconvolution (see below). The optical filters are centered in wavelength at approximately 350, 450, 550, 650, 750, 850 and 950 nanometers, and have FWHM bandpasses of 100 nanometers. HRIV CCD images were taken hourly in all filters, for a duration of approximately 24 hours, at three epochs (18-19 March, 28-29 May, and 4-5 June, 2008, UT, see Table 1). The 450, 550, 650, and 850 nanometer filters were imaged more frequently, at a 15-minute cadence. Infrared spectroscopy was obtained every two hours, by scanning the slit of the HRII spectrometer over the Earth, using a set of three scans (south-to-north, north-to-south and south-to-north) consisting of 8 spectral images to ensure the entire disk of Earth would be covered. This set was repeated twice within a twenty-minute interval every two hours, alternating between three slower scans with longer frame exposure times and three faster scans with shorter frame durations. Each set of three scans took about five minutes to acquire. The slit of the IR spectrometer was aligned to be perpendicular to the Earth's terminator during the scans; during the March campaign the MRI CCD was used to acquire images of Earth as context for the IR spectra. The spacecraft orbit was approximately in the plane of the ecliptic; therefore the view of Earth is approximately from the equator. During the 28 May observation, the Moon transited Earth as seen from the spacecraft. Table 1. Summary of the 2008 EPOCh Earth observations. Start Date/Time End Date/Time Range Illumination (UT) (UT) (AU) Fraction --------------------- --------------------- ----- ----------- 18 Mar 2008, 18:19:19 19 Mar 2008, 18:19:17 0.183 76.7% 28 May 2008, 20:03:52 29 May 2008, 20:03:50 0.332 62.8% 04 Jun 2008, 16:57:30 05 Jun 2008, 16:57:28 0.337 61.5% The units of the archived raw optical images as well as the raw IR spectroscopy are raw data counts while the units of the archived calibrated optical images and calibrated IR spectra are Watts per square meter per steradian per micron. (Both raw and calibrated data files are archived at MAST and PDS.) The calibrated HRIV CCD images have been bias-subtracted with removal of electronic crosstalk and transfer smear and nominally flat-fielded, using flat-field calibration exposures of an integrating sphere made on the ground before launch. The CCD detector response has changed in space. The most precise photometry (e.g., for exoplanet transits) has to account for those changes, but the EPOCh team has not corrected the Earth images for changes in the CCD. The calibrated HRII spectra were linearized and dark corrected; electronic crosstalk was also removed. The CCD images were flux-calibrated using observations of standard stars, and the IR spectra were flux-calibrated using lunar observations. The EPOCh team used the optical images from 2008 to construct videos of the rotating Earth, using observations of standard stars to deconvolve the out of focus PSF of the telescope. Those videos are available on the EPOXI mission and MAST websites and may be archived at PDS. EPOCh Earth Polar Observations in 2009 -------------------------------------- The EPOCh Earth observations at high subspacecraft latitude ('polar') comprise imaging in seven optical narrow-band filters, as well as infrared (1.05 to 4.8 micron) spectroscopy. The optical images used a 512x512 pixel subarray of the visible HRIV CCD detector at a scale of 2 microradian per pixel (0.413 arcsec per pixel). Each frame acquired by the HRII infrared spectrometer is 512 pixels (wavelength) x 128 pixels (spatial, with a scale of 10 microradian per pixel or 2.06 arcsec per pixel). Because the HRI telescope used for both optical and infrared wavelengths is defocused, the spatial resolution as observed is about 4 arcsec in each case, but can be improved by deconvolution (see below). The optical filters are centered in wavelength at approximately 350, 450, 550, 650, 750, 850 and 950 nanometers, and have FWHM bandpasses of 100 nanometers. HRIV CCD images were taken hourly in all filters, for a duration of approximately 24 hours. The 450, 550, 650, and 850 nanometer filters were imaged more frequently, at a 15-minute cadence. The same method for acquiring infrared spectroscopy of the Earth in 2008 was repeated for the 2009 observations. Infrared spectroscopy was obtained every two hours, by scanning the slit of the HRII spectrometer over the Earth, using a set of three scans (south-to-north, north-to-south and south-to-north) consisting of 8 spectral images to ensure the entire disk of Earth would be covered. This set was repeated twice within a twenty-minute interval every two hours, alternating between three slower scans with longer frame exposure times and three faster scans with shorter frame durations. Each set of three scans took about five minutes to acquire. The slit The MRI medium-resolution instrument CCD was used to acquire images of Earth as context for the IR spectra. Data were collected in two epochs (27–28 March and 4–5 October, 2009, UT, see Table 2); a small amount of data (one HRIV imaging set) was collected in an aborted observation of Earth on 27 September 2009 UT that was terminated prematurely by spacecraft fault-protection systems. The view of Earth in these observations was from high subspacecraft latitude, for a view dominated by either the north or south polar region near equinox. Table 1. Summary of the 2008 EPOCh Earth observations. Start Date/Time End Date/Time Range Illum. Sub-S/C (UT) (UT) (AU) Fraction Lat (deg) ------------------ ------------------ ----- -------- --------- 27 Mar 2009, 16:19 28 Mar 2009, 16:37 0.114 76.7% +62.0 27 Sep 2009, 16:10 27 Sep 2009, 16:10 0.115 52.7% -73.8 04 Oct 2009, 09:37 05 Oct 2009, 09:57 0.114 53.1% -73.8 The units of the archived raw optical images as well as the raw IR spectroscopy are raw data counts while the units of the archived calibrated optical images and calibrated IR spectra are Watts per square meter per steradian per micron. Both raw and calibrated data files are archived at PDS. The calibrated HRIV CCD images have been bias-subtracted with removal of electronic crosstalk and transfer smear and nominally flat-fielded, using flat-field calibration exposures of an integrating sphere made on the ground before launch. The CCD detector response has changed in space. The most precise photometry (e.g., for exoplanet transits) has to account for those changes, but the EPOCh team has not corrected the Earth images for changes in the CCD. The calibrated HRII spectra were linearized and dark corrected; electronic crosstalk was also removed. The CCD images were flux-calibrated using observations of standard stars, and the IR spectra were flux-calibrated using lunar observations. The EPOCh team used the optical images to construct videos of the rotating Earth, using observations of standard stars to deconvolve the out-of-focus PSF of the telescope. Those videos are available on the EPOXI mission website and will be archived at PDS and MAST.