TVS Task Force

Deep and Accurate PSF Photometry in Crowded Fields, and Search for Variable Stars Task Force

Work Plan

Members:

Contributors:

Context

The Original Crowded Field Task Force (upon which this task force was founded) was established to identify, from a users perspective, all the needs and difficulties inherent in the extraction of accurate photometry from point sources in LSST crowded fields.

The proposed idea was to create a reference dataset, on which to run well-established codes such as DAOPHOT/ALLFRAME, with the following goals:<\p>

• Create a solid reference catalog;
• Test custom algorithms to find variable stars;
• Cross-check the results with those from the LSST code (which was, at that time, under development);
• Publish possible scientific results.

The chosen dataset was the DECam 2013A-0721 (PI: A. Saha), a project focused on the observation of RR Lyrae variable stars in the Galactic bulge, in the ugriz bands. Since the DECam pixel scale and the deepness of the collected images are very similar to those foreseen with LSST, this dataset proved to be ideal for our purposes.

After 1-year of experience, we were able to successfully reduce and analyse a selected fraction of the total field surveyed by the Saha project. In particular, we performed accurate DAOPHOT/ALLFRAME photometry of the field, and performed a successful search for variable stars. An open problem was the calibration of the field, with the proposed idea to calibrate the data with the DECAP survey.

We then decided to extend the analysis to the whole field (and eventually add images of the same region collected during other DECam projects), to build a deep and complete photometric database, of both static and variable stars.

Important developments that occurred during the analysis phase:

• Saha et al. (2019) published the first release of their data, focused on the characterization of the RR Lyrae stars. The authors kindly agreed to share their photometric catalogue with us, allowing us to easily calibrate our reduction.
• The LSST Scarlet deblender was made available to the community. Scarlet adopts a multi-band approach to disentangle embedded sources. The Bulge and Galactic globular cluster present the most ideal benchmark to perform a complete evaluation of its performance on stellar crowded fields.

However, since the multi-band (i.e. SED) approach of Scarlet works by adopting a prior of the sources that it is going to measure, a full test of its capabilities is needed, for example, what is the minimum number of bands that can give robust results.

Task Force (TF) Goals

Given the obtained objectives and experience we have gained, the new Crowded Field Task Force will have the following, updated goals:

• Perform a full test of the aforementioned dataset with Scarlet;
• Publish the results of our tests in a refereed paper;
• Publish any scientific result which could arise from our analysis.

Work Plan and Deliverables

The proposed deliverables are:

• Daophot/allframe catalogs on original images, to use as a reference;
• Scarlet-based photometric catalogs. The photometry will be performed by testing several techniques (aperture photometry, tessellation SExtractor-like, PSF photometry)

The proposed metrics are:

• Photometric accuracy (photometric error as a function of the magnitude) for original and de-blended images;
• Photometric depth with both original and de-blended images;
• Minimum combination of photometric bands to get a satisfactory deblending;
• Optimal combination of photometric bands for the deblending;
• Variable stars detection and light curves quality with the original and deblended images.