Las Cumbres Observatory Calibrated Data Collection Overview Data Abstract ============= We obtained images of the NEO (65803) Didymos binary asteroid and supporting calibration data with the Las Cumbres Observatory global telescope (LCOGT) network through several filters. These images were obtained in support of NASA's Double Asteroid Redirection Test (DART) mission. The DART mission was a planetary defense mission designed to test and measure the deflection caused by a kinetic impactor (the spacecraft) on the orbit of Dimorphos asteroid around its primary, Didymos. These ground-based observations were used to obtain light curves of Didymos and time the mutual events of Dimorphos to determine the period change caused by the impact of the DART spacecraft. This collection consists of the Las Cumbres Observatory calibrated images and the supporting calibration frames: master bias and dark frames and the master flat field images. Data Set Overview ================= Raw image data (see data_lcogtraw collection) are calibrated through two pipelines, the observatory-wide BANZAI pipeline (McCully, C. et al. 2018. Real-time processing of the imaging data from the network of Las Cumbres Observatory Telescopes using BANZAI. Proceedings of the SPIE, 10707, 107070K) and then either the NEOExchange Solar System Pipeline (Lister, T. et al. 2021. NEOExchange - an online portal for NEO and Solar System science. Icarus, 364, 114387) or photometrypipeline (Momert, M., 2017. PHOTOMETRYPIPELINE: An automated pipeline for calibrated photometry. Astronomy and Computing, 18, pp.47–53). BANZAI is used to perform the basic calibration steps of: 1. Overscan subtraction: all images with a valid BIASSEC header keyword have a median value removed estimated from the BIASSEC region 2. Crosstalk correction (1m Sinistro imagers only): these imagers readout through 4 amplifiers and there is crosstalk between the quadrants which is removed using linear coefficients that relate each quadrant to every other quadrant. 3. Gain correction: All pixels in the frame are multiplied by the gain, using the GAIN header keyword. Thus, the science frames output by BANZAI are all in units of electrons. 4. Mosaic (1m Sinistro imagers only): If the file has multiple extensions, BANZAI produces a single mosaiced frame. This uses the DETSEC header keywords. 5. Trim: After being mosaiced, the data is trimmed to a usable region for science. This is set based on the TRIMSEC header keyword. 6. Bias Subtraction: Full frame master bias images are subtracted from each of the darks, flat field images, and science frames. The master bias frame that was taken closest in time to the current data will be used. 7. Dark Subtraction: Full-frame master dark frames, scaled to the exposure time of the frame, are subtracted from all flat-field and science images. The most recent master dark frame is used, normally this is taken on the same day. 8. Flat Field Correction: Master flat field images (normalized to unity using the inner quarter of the image) are divided out of every science frame. The most recent master flat-field image for the given telescope, filter, and binning is used. The NEOexchange or PHOTOMETRYPIPELINE pipelines both perform source detection in the images using SExtractor (Bertin, E and Arnouts, S. 1996. SExtractor: Software for source extraction. A&AS, 117, 393), astrometric fits to the detected sources based on the known positions of Gaia DR2 stars in the field using SCAMP (Bertin, E. 2006. Automatic Astrometric and Photometric Calibration with SCAMP. ASPC, 351,112), photometrically calibrates the zero point of the images based on the measured flux of field stars that have known catalog magnitudes (typically from eithe the PanSTARRS or Gaia-DR2 catalogs), and then using these astrometric and photometric solutions derives a calibrated magnitude for Didymos. The photometry tables included in Derived Data Product data_lcogtddp collection are the output of these pipelines. Naming Convention ================= Calibrated Sinistro images consist of a single FITS file containing a primary header and the four quadrants stitched together into a single image. They are named according to the following LCOGT-wide convention: ----. where: filename section Description ---------------- -------------------------------------------------------------- LCOGT site code (one from {ogg, coj, lsc, cpt, elp, tfn, ngq}) Size/class of telescope (one from {2m0, 1m0, 0m4}) telescope serial number within size class LCOGT instrument code e.g. ‘fa15’ where the first letter indicates CCD manufacturer (f=Fairchild), second letter indicates controller manufacturer (a=Archon) and the 2 digits indicate serial number. UTC date of the start of the night (defined to be UTC date at 1600 local time at the site). Does not change if the site observes through UTC midnight. four digit image counter for that instrument, resets at start of new night (defined as above). Counter starts at 0001 Type of file. One of {a=arc, b=bias, d=dark, e=exposure, s=standard, f=sky flat, l=lambert flat, w=lamp flat, x=experimental}) Reduction level from 00-99 with higher numbers indicating higher levels of data processing. In practice, one of {00=raw, 91=BANZAI processed, 92=NEOexchange processed} the file extension. ".fits" for fits file format, ".xml" for the PDS label. e.g. coj1m011-fa12-20210408-0176-e92.fits is a NEOexchange processed expose(sky) data product from the Siding Spring Observatory (coj), from 1-meter telescope #11 and the fa12 Sinistro instrument from the night of 2021-04-08. It is the 176th image taken by that instrument on that night. The LCOGT site codes (which are based on the IATA code of the nearest airport) are defined as follows: LCOGT Site Site code location description --------- ----------------------------------------------------------------- ogg Haleakala, Maui, HI, USA coj Siding Spring Observatory, NSW, Australia lsc Cerro Tololo Inter-American Observatory, Chile cpt South African Astronomical Observatory, Sutherland, South Africa elp McDonald Observatory, Texas, USA tfn Teide Observatory, Tenerife, Canary Islands, Spain ngq Ali Observatory, western Tibet, China sin Meta site for all Sinistro cameras in the network Master Bias, Dark and Flat Field Frames ======================================= BANZAI creates the master bias, dark and flat frames required for the reduction of science frames. For all instruments, we take many full-frame bias and dark exposures every afternoon and morning. We use a “robust standard deviation” (rstd) based on the median absolute deviation (mad - related to the std by σ≈1.4826×mad) to estimate the scatter of the distribution and mask pixels more than 3 rstd reliably and take a mean of the remaining pixels as usual. When creating the master dark frame, each individual frame is scaled by the exposure time (read from the header). Twilight flats are taken every day but not in every filter, because twilight is not long enough to take all 18 filters in a single night. Typically, a master flat field is produced about once every 3 clear days for any given filter. Flat-field images for a given filter are taken in the evening and morning twilight, to allow quality control. The frames are dithered so that we can remove stars in the combined master flat field. Each individual flat-field image is normalized to unity before combining them. The normalization is calculated finding the robust sigma clipped mean (3.5 rstd outliers are rejected) of the central region of the image. For the central region, we choose the central 25% of the field (the region has dimensions that are half of the full image). The flat-field frames are then stacked using a sigma clipped mean, similar to the master bias and dark frames, again with 3 rstd outlier rejection. The naming scheme for the master calibration frames follow the same general format as the Calibrated Images described above but the and sections are replaced by: ---binx- where for master calibration frames is one of {bpm (bad pixel mask), bias (master bias), dark (master dark), skyflat (master sky flatfield)}, (optional) and is one of {full_frame or center; 1m Sinistro imagers only), (optional) and is only present for skyflat calibration types. Some examples of these combinations are: cpt1m012-fa06-20210402-dark-bin1x1.fits and cpt1m012-fa06-20210402-bias-bin1x1.fits which are master dark and bias calibration frames for 1x1 binning from LCOGT 1m #12 in South Africa (cpt site) from 2021-04-02. References ========== E. Bertin and S. Arnouts., 1996. SExtractor: Software for source extraction. A&AS, 117:393–404, June 1996. doi:10.1051/aas:1996164. E. Bertin., 2006. Automatic Astrometric and Photometric Calibration with SCAMP. ASPC, 351:112. 2006ASPC..351..112B T. Lister, et al, 2021. NEOExchange - an online portal for NEO and Solar System science. Icarus, 364, 114387. doi:10.1117/12.2314340 C. McCully et al., 2018. Real-time processing of the imaging data from the network of Las Cumbres Observatory Telescopes using BANZAI. Proceedings of the SPIE, 10707, 107070K. doi:10.1117/12.2314340 Mommert, M., 2017. PHOTOMETRYPIPELINE: An automated pipeline for calibrated photometry. Astronomy and Computing, 18, pp.47–53. doi:10.48550/arXiv.1702.00834