Las Campanas IMACS Derived Data Product Collection Overview

Data Abstract
=============
We obtained images of the (65803) Didymos system and supporting calibration 
images with the Inamori-Magellan Areal Camera and Spectrograph (IMACS) on 
the 6.5m Magellan-Baade Telescope at the Las Campanas Observatory (LCO). 
These images were taken in order to determine the orbit period of Dimorphos, 
the satellite of Didymos. This collection consists of photometry summary 
tables, which are a PDS4 derived product, detailing the results of 
photometric processing performed on collected images.

Data Set Overview
=================
Instrumental aperture photometry is measured in every image on the asteroid's 
and on the selected stars' positions for a set of apertures from 3 to 20 
pixels radius using the python package SEP (Barbary, K. 2016) for the initial 
detection of the brightest non-saturated stars and the subsequent background 
subtraction across the entire image, in order to measure the flux in the 
different apertures and transform directly into instrumental magnitudes.
In the case of IMACS data, 11 pixel radius apertures were used.

In order to estimate the photometry zero points of the individual images, 
different python packages are used: astroquery (Ginsburg et al. 2019) to 
query Vizier and Horizon databases in order to identify GAIA stars in the 
set of selected stars inside a 2 arcsecs tolerance radius, and to obtain the 
coordinates of the asteroid for the given date of the images; and gaiaxpy 
python package (De Angeli et al. 2023), to request and download synthetic 
photometry of GAIA stars (Gaia Collaboration et al. 2023) in Sloan-r band 
when available. Final photometry of the Didymos-Dimorphos system is estimated 
adding the zero points to measured instrumental magnitude in the corresponding 
images.

For some of the IMACS observations an additional step was carried out as 
follows. Within each pointing, a sequence of the early flat-fielded images of 
Didymos and its surrounding star field were then combined to make a 
subtraction template for the later observations at the same pointing, while 
another sequence of flat-fielded images taken later in the sequence are used 
to create a second subtraction template for the earlier observations. The 
"High Order Transform of PSF And Template Subtraction" (Hotpants) image 
subtraction software (Becker 2015) was used to identify all of the sources on 
a frame and to remove sources from the region around Didymos, minimizing 
contamination from nearby stars, galaxies, etc.

Aperture photometry was performed on Didymos and reference stars on each frame, 
using a variety of aperture sizes that were then selected based on image 
quality conditions for the night along with minimized photometric 
uncertainties. While the background sources were removed as described when 
doing the photometry for Didymos on each frame, reference stars photometry was 
done on flat-field corrected but not background-subtracted frames. Reference 
stars tied to photometric catalogs (GAIA when available and PanSTARRS if not) 
were used to calculate a zero point for each frame. All data are taken with
Sloan_r' and magnitudes are calculated for Sloan_r' primarily with
reference to GAIA.

The photometry summary tables are named according to the following convention: 

photometry_<instrument>_<yymmdd>.tab

where:
<instrument> is always imacs for this collection

<yymmdd> is the UTC year, month, day of observation

Photometry data are stored as a PDS4 ASCII fixed-width table according to the 
following format:

Julian date: Julian date at middle of exposure
Magnitude: Calibrated magnitude estimate, unitless
Uncertainty: Instrumental magnitude uncertainty, unitless
Flag: Binary flag to mark discrepant data. 0 = discrepant, 1 = non-discrepant
Filename: File name of the calibrated image where data were measured

References
==========
Barbary, K., “SEP: Source Extractor as a library”, The Journal of Open Source 
    Software, vol. 1, no. 6, 2016. doi:10.21105/joss.00058.
Becker, A., “HOTPANTS: High Order Transform of PSF ANd Template Subtraction”, 
    Astrophysics Source Code Library, 2015. ascl:1504.004.
De Angeli, F. et al., “Gaia Data Release 3. Processing and validation of BP/RP low-
    resolution spectral data”, Astronomy and Astrophysics, vol. 674, 2023. 
    doi:10.1051/0004-6361/202243680.
Gaia Collaboration et al., “Gaia Data Release 3. The Galaxy in your preferred 
    colours: Synthetic photometry from Gaia low-resolution spectra”, Astronomy 
    and Astrophysics, vol. 674, 2023. doi:10.1051/0004-6361/202243709.
Ginsburg, A. et al., “Astroquery: An Astronomical Web-querying Package in Python”, 
    The Astronomical Journal, vol. 157, no. 3, 2019. doi:10.3847/1538-3881/aafc33.