Data Set Overview =================== This collection contains shift and coadd images generated by the BOPPS Infra-Red Camera (BIRC) during flight. This includes the flatfields generated from flight data, which are coadded and normalized images. The BOPPS Infrared Camera (BIRC) is a multispectral infrared imager designed to operate in 8 wavelengths between 2.5 and 5.0 μm, with each spectral width being ~ 3% of the center wavelength, and the astronomical R-band near 640 nm. BIRC was designed to measure the water and CO2 emissions from comets at 2.73 and 4.3 μm, respectively, and the water related infrared absorption feature in asteroids and the Moon from ~ 2.5 to 3.2 μm. This capability is obtained with a Teledyne H2RG cryocooled HgCdTe detector and an 80 cm telescope. The system produces an f/4 image over a field of view of 3 arcminutes, which subtends approximately 151 pixels on the 2K x 2K array, and employs shift/co-add algorithms to increase signal-to-noise for the observation of dim objects. The BIRC is comprised of two subsystems, a collimator and a camera, which are designed to relay the primary image from the 80 cm telescope to the focal plane of the Teledyne H2RG after passing a collimated beam through the cryogenically cooled nine-position filter wheel. The primary image propagates through a CaF2 window on a ‘cold box’ that contains the collimating optics. This subsystem consists of an enclosed cooled, nitrogen-purged box with a collimating mirror and three fold mirrors, all of which are coated with protected gold to reduce thermal self emission. Upon exiting the box, the collimated beam passes through another CaF2 window between it and the evacuated, cryogenically cooled nine-position filter wheel and then into a camera subassembly where the final image is formed on the detector via a small Ritchey-Critean telescope. Custom firmware provided by Teledyne Imaging Systems allows the BIRC flight software to readout a programmable area of interest, which was then defined to be the central 320 x 200 pixel region that contains the 3 arcmin field of view. It is this subframe that is generated by the BIRC instrument for all the image data. The detector performs a 'non-destructive' read of the data in time units of 3.48ms, such that the integration time for an individual image is N * 3.48ms. For any commanded integration time the detector performs two reads. The first read is after 3.48ms, for which the bias image is generated. The second read is at N * 3.48ms, for which the signal image is generated. This collection includes the SHIFTED, COADDED, and FLATFIELD image product types. Processing ========== After unpacking the image data from the telemetry files, all bias and signal image pairs are collated. The raw images are biased such that larger DN indicates a lower signal strength. The signal image is subtracted from the bias image to remove the bias contribution to the signal. This also inverts the DN values such that larger DN now indicates higher signal strength. Images are then calibrated by removing "hot" pixels and "popcorn noise", then divided by a flat field to remove fixed-pattern noise. A DN to electron algorithm was applied to convert the pixel values to units of electrons. CALIBRATED products associated with a given observation, filter, and integration time are collated in time order, as is pointing information consisting of the instantaneous shift up/down and left/right in arcseconds required to point the telescope boresight to the target. First order interpolation is used on the pointing records in order to get an estimate of telescope position for each image. A 2x2 rotation matrix was then used to transform the instantaneous shift values into row, column shifts in the image reference frame. The assumption was made that any swaying motion by the telescope (introducing tilt to the image) was negligible (being removed by the anti-pendulation flywheel on the gondola), hence the lack of a third rotation angle. The pipeline software then calculated the row,column image shifts with respect to the first image taken in the image set , shifted successive images with respect to the first, and added all images together. The image was then divided by the number of images shifted and added to produce an averaged result. This is saved to a FITS file in 32-bit floating point data type as a SHIFTED product. In a few of the observations of dim targets, the pointing information was not accurate enough to adjust for unexpected sharp changes in telescope or gondola orientation. Unfortunately, since the target was usually too dim to be seen in an individual image there was no way to easily identify at what point to apply additional shift values and by how much. To mitigate this problem the initial image set was divided into subsets (an equal number of images in each subset), and a shifted and co-added image was generated for each subset. A shift set number is included in the SHIFTED product file naming convention to identify cases where multiple SHIFTED products were created for a given observation. The shift set number is set to 0 for cases where only one SHIFTED product was created for an observation. The shift set number starts at 1 and goes to N, where N is the total number of shifted products created for an observation at a set filter and integration time. For images taken with the CO2 filters (3 and 4), where the SNR was very small, and data was taken over several minutes, the inherent drift rate of the pointing system was utilized to identify and isolate the target. This was done by subdividing the image set into several subsets, shifting and co-adding each subset, then combining the shifted images into an animated gif. The target was then identified since it was the only source in the image that moved in the same direction at a constant rate during the animation. The COADDED data product is generated by collating CALIBRATED data products associated with a given observation, filter, and integration time in time order, co-adding them, and dividing by the number of images to get an averaged result. The FLATFIELD data product is generated by collating BIAS SUBTRACTED data products associated with a given observation, filter, and integration time in time order. The pipeline software then remove hot pixels and popcorn noise. The filtered images are then co-added, divided by the number of images co-added, and normalized using the mean value of the averaged image in the 3 arcmin field of view. The FLATFIELD product is used in the generation of the CALIBRATED products. For observations conducted on targets during flight there were usually two sets of observations done per target, designated set A and set B. The object was offset in the field of view between these two sets. Set A was the initial observation of the target with the telescope pointed to the commanded target location in RA and DEC. Set B was taken with the telescope shifted by XX arcseconds (“nodded”) in elevation resulting in the target located in a different part of the image. Images from set A were then used to create the FLATFIELD products for use in set B calibrated products, and vice versa. The target was not masked for the generation of the FLATFIELD product, hence a “shadow” image of the other’s target sometimes appears in the SHIFTED or COADDED product. Data ==== FITS Images and PDS Labels -------------------------- Each shift and coadded BIRC image is stored in FITS file format with minimal FITS headers. Any associated metadata is contained in the XML PDS label associated with the FITS file. File Naming Convention ---------------------- The file naming convention for a SHIFTED, COADDED, or FLATFIELD data product is: obsd_x_n_hhmmss_t_YYYY.ext where: obsd – 4 character string identifying the type of observation. See table 5 for a short description of each of the observation types identified x – a digit identifying the shift subset number. n – single digit indicating filter used. See table 6 in the SIS for a list of filter number vs. wavelength. The filter wavelength is also noted in the XML label. hhmmss – two digit hour, two digit minute, two digit second corresponding to the timetag of the first image in the set of images used to generate the product. t – product type s – shifted and co-added c – coadded f – flat field. Normalized co-added image YYYY – the four digit integration time of the image in milliseconds, rounded to the nearest millisecond. .ext is a three character file extension. Either ‘fit’ for the fits file or ‘xml’ for the PDS4 XML label. An additional reference file ending in ‘txt’ is also created and is referenced by the XML label. This reference file contains the set of images that were used to generate the Level-2 image product. File Organization ----------------- The SHIFTED products are organized by product type, then by target, then by filter. Shift and coadd products were generated only for images that were taken during H2O or CO2 observations, or with observations where all filters were used, as these were the only observations where the telescope was commanded to a fixed RA and DEC for the entire observation. The FLATFIELD products are organized by target. There is only one flatfield created per filter and observation in each set. The flatfield in set A for a given observation was used to calibrate the images in set B for the same observation and vice versa. For example, the 'jaca' flatfields were used to calibrate the 'jacb' (Comet Jacques Co2 set A and set B) observations, and not used as flatfields for any other observations. One exception to this was the use of 1Ceres set B flatfields to calibrate the images from HD 163761. This was due to the fact that HD 163761 took only one set of observations due to time constraints on the mission.