# TDSS Target Selection

The Time-Domain Spectroscopic Survey (TDSS) is an SDSS-IV eBOSS subprogram to systematically obtain follow-up spectra of ~200,000 photometrically time-variable sources. Target selection for TDSS in SDSS-IV is detailed in Morganson et al. (2015) and MacLeod et al. (2018).

TDSS targets will come from three types of observing programs:

1. Single-Epoch Spectroscopy (SES) program encompassing ~80% of all TDSS fibers and aimed at obtaining initial single-epoch discovery spectra of photometrically variable objects
2. Few-Epoch Spectroscopy (FES) programs consisting of various smaller projects involving multi-epoch spectroscopy of special select targets (particular subclasses of quasars or stars with anticipated spectral variability)
3. Repeat Quasar Spectroscopy (RQS) also involving multi-epoch spectroscopy (but in a less subclass-biased way than FES) of quasars across a broad range of redshift, luminosity, quasar subclass, etc.

Details on target selection are provided in the references above.

## TDSS Single-Epoch Spectroscopy (SES) target selection

SES target selection, which comprises the bulk of TDSS TDSS targets, is based on a uniform algorithm. This algorithm is briefly summarized below.

Targets for TDSS SES spectra in SDSS-IV are selected based on photometric gri light curves of point sources, constructed using a combination of SDSS Data Release 9 single-epoch imaging and Pan-STARRS1 (PS1) 3$\pi$ survey multi-epoch imaging.

Photometrically variable objects are selected based on their:

1. long-timescale (~10 years observed-frame) variability
2. shorter-timescale (~2 years observed-frame) variability
3. median PS1 magnitudes

Long-timescale variability is measured as the difference in magnitude between the SDSS and median PS1 epoch magnitudes (where the SDSS magnitudes are color-corrected to match PS1 filters). Shorter-timescale variability is measured as the variance in the PS1 light curves.

The median PS1 magnitudes are included in the target selection procedure primarily to take into account the increase in variability that is required for fainter objects to enter the sample due to increased photometric uncertainties, and do not involve color information.

These three parameters for each object are input into a 3-dimensional kernel density estimator (KDE) trained on the SDSS Stripe 82 variable object and standard star catalogs. The KDE assigns a probability that each object is variable based on the efficiency E with which variable objects are selected in its region of parameter space using the training sets. Candidate variable objects with E above some threshold are then selected as “variable.” To ensure a uniform sky density of targets across the survey footprint of 10 deg$^{-2}$, this E threshold is calculated independently for different sky regions (each typically 4 deg$^2$) and is thus non-constant.

Many TDSS targets selected using the above procedure have previous SDSS spectra (~100,000 objects), or are also selected for spectroscopy in SDSS-IV by other eBOSS programs (e.g. the majority of TDSS quasars are also targeted by eBOSS). TDSS SES does not obtain new spectra of targets with previous spectra, while objects also targeted by eBOSS are `free’ and do not count towards the 10 deg$^{-2}$ sky density of TDSS fibers.

After targets are selected, an additional visual pre-screening is performed before spectra are obtained. This visual pre-screening of TDSS targets uses postage stamps of SDSS imaging to remove targets whose variability is suspect due to obvious photometry problems (e.g., lying on diffraction spikes of bright stars, lying within the isophotes of a spatially resolved galaxy, etc.). This pre-screening removes a non-negligible fraction (~1/3 in some cases) of objects to maximize the purity of the final TDSS sample.

TDSS fibers are also included as part of the SEQUELS survey in SDSS-III. Differences in the target selection between SDSS-IV and SDSS-III SEQUELS are small, and are detailed in Ruan et al. (2016).