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:
<site><tel. class><tel. #>-<instrument>-<yyyymmdd>-<iiii>-<type><red. level>.<extn>
where:
filename section Description
---------------- --------------------------------------------------------------
<site> LCOGT site code (one from {ogg, coj, lsc, cpt, elp, tfn, ngq})
<tel. class> Size/class of telescope (one from {2m0, 1m0, 0m4})
<tel. #> telescope serial number within size class
<instrument> 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.
<yyyymmdd> 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.
<iiii> four digit image counter for that instrument, resets
at start of new night (defined as above). Counter starts at 0001
<type> 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})
<red level> 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}
<extn> 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 <type>
and <reduction level> sections are replaced by:
<calib type>-<readout window (Opt.)>-<readout speed (Opt.)>-bin<X binning>x<Y binning>-<filter>
where <calib type> for master calibration frames is one of {bpm (bad
pixel mask), bias (master bias), dark (master dark), skyflat (master
sky flatfield)}, <readout window> (optional) and is one of {full_frame
or center; 1m Sinistro imagers only), <readout speed> (optional) and
<filter> 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