1   Overview¶

Ultimately, LSST Alert Production (AP) will need to perform all steps of the Level 1 pipeline automatically on each new raw image from the telescope. Details are in the LSST Data Management Science Pipelines Design (LDM-151, [swi17]) and the LSST Data Products Definition Document (DPDD, [jur16]). Briefly summarized below and in Figure 1, the first portion of these steps are:

1. Instrument Signature Removal (ISR), which includes crosstalk and linearity corrections, bad pixel masking, bias subtraction, and flat field division,
2. Point Source Function (PSF) and background characterization, which includes source detection and measurement,
3. Photometric and World Coordinate System (WCS) calibration, and
4. Image Differencing with an appropriate template (which may need constructing) to create Difference Image Analysis (DIA) source catalogs.

Figure 1 The first portion of the Nightly Processing Pipeline overview, from Figure 1 of LDM-151 [swi17]. This encompasses all of “Single Frame Processing” and part of “Alert Detection.”

A number of packages in the LSST Software Stack have been developed to perform portions of one or more of these four tasks, but they are not yet linked together in an automated way. This Prototype AP Pipeline is the first step toward the goal of an automatic Level 1 Pipeline that can process a set of raw images through each of these steps and verify that the output is what we expect.

2   Dataset¶

We use images from the High Cadence Transient Survey (HiTS, [for16]), a three-year survey from 2013–2015 which used the Dark Energy Camera (DECam) on the 4 m Blanco telescope at Cerro Tololo Interamerican Observatory (CTIO). The primary science goal of this survey is to detect and follow up optical transients that have characteristic timescales from hours to days, with a particular interest in catching early supernovae. The distribution of the HiTS survey fields in the sky is shown in Figure 2.

Figure 2 Spatial distribution of the fields observed in the 2013–2015 HiTS campaign (Figure 4 from [for16]). The fields selected for this dataset are three of the pink ones, which indicate they were visited in both 2014 and 2015.

We select HiTS fields Blind15A_38, Blind15A_39, and Blind15A_40, which are repeat observations of the same region of sky as Blind14A_07, Blind14A_08, and Blind14A_10 fields, respectively. They are all centered near an RA of 10:15:00 hr and Declination of –04:00:00 deg. Each field has 34–36 individual visits over about a month, and each DECam image consists of 62 CCDs. Most of the visits are in the g-band, some are in the r-band, and a few are in z or i.

For the purposes of this prototype pipeline, we consider two g-band visits in Blind15A_38 only. However, the rest of the visits in fields 15A_38–15A_40 have been ingested (see details in Method) and are ready for the next steps of processing if desired.

3   Method¶

To begin, we obtained the dataset by downloading raw images and corresponding master calibration files from the NOAO Science Archive. The HiTS survey data may be selected by searching for PI Förster. Additionally, the latest set of DECam defects (bad pixel masks) were obtained from the lsst-dev server at /datasets/decam/calib/bpmDes/2014-12-05/, and a set of Pan-STARRS astrometric reference catalogs were obtained from the lsst-dev server at /datasets/refcats/htm/htm_baseline.

Next, we ran four main processing steps with the LSST Stack on the data:

1. ingestImagesDecam.py, from the obs_decam package but drawing heavily on pipe_tasks,
2. ingestCalibs.py, from the pipe_tasks package,
3. processCcd.py, from the pipe_tasks package, and
4. imageDifference.py, also from the pipe_tasks package.

To streamline this process, we wrote a python script called decam_process.py to handle each step. It is available on GitHub. While it was only used on the Dataset described above, it can in principle be applied to any set of raw DECam images using the LSST Stack, as shown below.

repo = 'ingested_15A38/'  # used by ingest, ingestCalibs, processCcd
calibrepo = 'calibingested_15A38/'  # used by ingestCalibs, processCcd
processedrepo = 'processed_15A38/'  # used by processCcd, diffIm
diffimrepo = 'diffim_15A38_g/'  # used by diffIm
visits = [410927, 411033]  # used by processCcd, diffIm
# NOTE: visits assumes the first element is template and the rest are science
ccdnum = '1..62'  # used by processCcd, diffIm
# NOTE: the default '1..62' value includes all of the DECam CCDs

mkdir repo
ln -s /datasets/refcats/htm/htm_baseline repo/ref_cats

python decam_process.py ingest -f path/to/rawimages

python decam_process.py ingestCalibs -f path/to/biasesandflats -d path/to/defects

python decam_process.py processCcd

python decam_process.py diffIm

4   Results¶

The final difference images and DIA Source catalogs for the test dataset are available on the lsst-dev server at /project/mrawls/prototype_ap/diffim_15A38_g/deepDiff/v411033/. A small thumbnail preview of the difference images is shown in Figure 3.

Future work will extend this to more visits, perhaps using the 2014 visits as templates and the 2015 visits as science. This Prototype Pipeline will be used as a core component of the AP Minimum Viable System with a goal of verifying the different components of LSST image processing as we incrementally build toward a fully functional AP system.

Figure 3 Difference images for HiTS field Blind15A_38 with visit 410927 as the template image and visit 411033 as the science image. CCDs 2 and 61 are nonoperational, and a portion of CCD 31 is also not working. The other CCDs all perform as expected.