HRIV Context Images for HRII Lookback Data March 2006 Overview -------- To properly analyze the data acquired by the HRI IR Spectrometer during lookback after the impact with comet 9P/Tempel 1, the location of the IR slit with respect to the nucleus must be determined. During November 2005, Steve Collins (JPL/Deep Impact Navigation team), reconstructed attitude information for the HRI IR Spectrometer (HRII) and the HRI Visible CCD (HRIV) from 44 through 75 minutes after impact, then took each HRII observation and drew an estimate of the slit location on an HRIV context frame taken near the time of the IR observation. The context frames cover IR observations from UTC 06:28:37 through 06:59:47 on 4 Jul 2005. The following description of this process and the resulting context images was provided by Collins. Processing ---------- A combination of SAO's DS9 and JPL's Tball Navigation Simulator packages is used to produce the context frames. The software combination takes a pair of FITS images, one HRII and one HRIV, and returns a JPEG file that shows how the IR slit from first (the observation-image) lies on the second (the context-image). The tool also builds an MS-Excel readable file that contains the relative geometry data. Another way to think of the results is this: We assume that the scene looks the same at the epoch of the observation-image as it does at the epoch of the context-image. We figure out where the ends of the IR slit (and HRI framelines) fall on the scene and then compute where those scene points will map to in the context image. Because we are mapping information from one epoch to another, looking at only the changes in pointing between the two epochs, the results should be fairly insensitive to errors in the ephemeris and spacecraft clock to spacecraft ephemeris time conversions. The naming convention for the resulting JPEG files is: HIObsExpID_ObsImgNum_HVContextExpID_ContextImgNum.JPG. For example, the file name HI9010000_001_HV9010003_001.JPG indicates HRII observation 9010000_001 is superimposed on HRIV context exposure 9010003_001. For this set of context frames, a set of full-frame HRIV images at various epochs is selected to use as context-images. While it is difficult to use subframed HRIV data as the context frames, the 512x512 mode is supported and is used for a few of the IR observations right after impact. Notes of Caution ---------------- 1) The science team validated the context frames by careful comparison to the IR spectra. In particular, the team verified that the IR slit limb crossings, that these reconstructions predict, matched those observed in the spectra. 2) The context images provide an estimate of the location and size of the IR slit slit location and size on an HRIV CCD image taken near the time of the IR observation. Because the IR frames were taken at a much higher rate than the HRIV frames, the context images provide only a spatial estimate, and not a temporal estimate, of the location of the slit. For example, if the slit appears to be over ejecta in an HRIV frame, one should not assume this was the point in the evolution of the ejecta that the HRII instrument was imaging. 3) A small set of representative context frames was selected to show the general nucleus to IR slit geometry. To look at details such as the development of the impact plume, additional context-images would be needed with frames taken as close as possible to the IR exposures in question. 4) The estimated impact site** as predicted by the ephemeris is marked on each context image with a small circle. It should always be close to the impact site on the context-image, but unlike the IR slit positions, any ephemeris and sclk-scet errors will effect it to first order. In other words, the position of the IR slit relative to the nucleus will be more accurate than the accuracy of the displayed estimated impact site. ** For lookback frames, the location of the impact site was still calculated although the location was on the far side of the nucleus; the impact site circle should be ignored for these frames. Additional Technical Details ---------------------------- The attitude telemetry is interpolated to the epochs of the two images by assuming a constant angular rate (i.e., it is a first order fit), which appears to be adequate for the high rate telemetry all the way down to shield mode entry. The midpoint of an observation is used for this analysis and is calculated by extracting the spacecraft clock count at the end of an IR exposure from the SCSTOP keyword in the FITS header, converting the value to ephemeris time using the latest SPICE spacecraft clock and ephemeris time (SCLKSCET) kernel (DIF_SCLKSCET_00015_SCIENCE.TSC), then extracting half of the exposure time from the INTTIME in the FITS headers. The telemetry as it exists in the Deep Impact query server has been spacecraft ephemeris time (SCET) tagged using an older version of the SCLKSCET file, DIF_SCLKSCET.00015.TSC. Since Tball uses the SCET times, a correction of -1.783 seconds is applied to all the attitude data timetags. This makes the values consistent with DIF_SCLKSCET_00015_SCIENCE.TSC and Dan Kubicheck's (JPL/DI Navigation Team) reconstructed ephemeris to a few hundredths of a second. Description of Values Presented in the Maps ------------------------------------------- Observation Img: Provides the name of the HRII observation-image file. An old file naming convention is used. The exposure ID and image number are the 5th and 6th strings in the file name: hi_1_0173730347_061_9010000_001_rad_v01.fit ------- --- | | | HRII image number | HRII exposure ID Context Img: Provides the name of the HRIV context-image file. An old file naming convention is used. The exposure ID and image number are embedded in the file name: nhvcbxf9010003_001162216000.fit ------- --- | | | HRIV image number | HRIV exposure ID Obs Img Range: Provides range to the impact site at the epoch of HRII observation-image. Context Img Range: Provides the range to the impact site at the epoch of the HRIV context-image. IR1 and IR2: Provide the estimated position (sample and line) of each end of the observation-image IR slit in the context-image. Given in context-image pixels with origin at lower left as displayed by the DS9 image viewer. HRI ctr: Provides the center (sample and line) of the observation-image HRI frame in the context-image, given in context-image pixels with origin at lower left as displayed by the DS9 image viewer. Impact: Provides the estimated position of the impact site in the context-image, given in context-image pixels with origin at lower left as displayed by the S9 image viewer. This is computed from the interpolated attitude telemetry at the context-image epoch, and the reconstructed ephemeris kernel (currently Deep_Impact_flyby-Heliocenter2.bsp using the SCLKSCET conversion from DIF_SCLKSCET_00015_SCIENCE.TSC. No information from the observation-image is used in this computation.