Add statement of need to README and front page of docs. Add link to docs to README.

Author: laldoroty

README.rst

@@ -9,6 +9,18 @@ This is lightcurve software being developed for the Roman Supernova Project Infr
 
 Currently, *phrosty* only handles the OpenUniverse sims [ref needed].  It reads images from those sims in FITS format, using the standard FITS WCSes found in the headers of those files for image alignment.  The galsim package and a particular version of the roman imsim package (https://github.com/matroxel/roman_imsim.git) provide the PSF used both for PSF photometry and for the PSF matching in the SFFT image subtraction software (Hu *et al.*, 2022, ApJ, 936, 157).
 
+Statement of Need
+-----------------
+Difference imaging analysis (DIA) is a technique in transient analysis that allows both transient detection and photometric measurements. Broadly, this method involves subtracting two images, one with a transient and a "reference" image without a transient, to isolate the object of interest. 
+
+The Nancy Grace Roman Space Telescope (_Roman_) is NASA’s first survey-oriented flagship mission. Its design is optimized for cosmological studies, which includes extraction and precision analysis of Type Ia Supernova (SN Ia) light curves. Analysis of these data pose several challenges: (1) the enormous quantity of data that will be returned on a regular basis, and (2) the spatially-dependent nature of the image properties as a function of location on the detector, which directly affects ease of (3) obtaining the required <1\% flux precision on SN Ia measurements.
+
+_Roman_'s High Latitude Time Domain Survey (HLTDS) is specifically designed for cosmological studies with SNe Ia. The core component of this survey will last two years, and have a cumulative 158 days of exposure time. The survey will yield an average of 241 Level 2 (rate files) from imaging mode observations per day, which totals to approximately 0.4 TB of data. Other DIA and forced photometry survey pipelines are insufficient to process this quantity of data. For example, the Dark Energy Survey (DES) pipeline, which uses the HOTPANTS algorithm, takes 10 minutes to create a difference image from an image from a single detector, with 8.3 million pixels per detector. Using this pipeline, it would take 80 hours to make difference images from all 241 _Roman_ HLTDS images obtained within one 24 hour period, which have nearly 17 million pixels per detector. In reality, the amount of time spent processing these data will be much more than 80 hours because (1) there are many steps associated with SN Ia light curve analysis in addition to creating the difference images, (2) data from other concurrent surveys will also be processed for transient studies, and (3) this amount of time also excludes re-processing data when pipelines or calibration information are updated.
+
+`phrosty` is a DIA and forced photometry pipeline that addresses these challenges. We integrate the saccadic fast fourier transform (SFFT) method for difference imaging analysis (DIA), which uses a flexible delta basis function that can accommodate Roman’s spatially-varying point spread function (PSF). `phrosty` uses both GPU and CPU computation; this architecture has been carefully chosen to address the challenge of processing the large quantity of survey data within a reasonable amount of time. 
+
+Because Roman software is currently under-development, there is no similar publicly available all-in-one pipeline that takes _Roman_ images as input and outputs light curves. The architecture choices made in development have substantially accelerated processing time such that `phrosty` is among the fastest astronomical DIA pipelines available. 
+
 Prerequisites and Environment
 -----------------------------
 
@@ -44,9 +56,7 @@ Currently, *phrosty* depends on the following environment variables to find data
 Running the code
 ----------------
 
-TODO.
-
-For now, see the one extant example below.
+Please see the [documentation](https://roman-supernova-pit.github.io/phrosty/).
 
 Examples
 --------

docs/index.rst

@@ -12,13 +12,15 @@ This package contains the Python software suite developed for use with the Roman
 Statement of Need
 =================
 
-TODO LAUREN
+Difference imaging analysis (DIA) is a technique in transient analysis that allows both transient detection and photometric measurements. Broadly, this method involves subtracting two images, one with a transient and a "reference" image without a transient, to isolate the object of interest. 
 
-State of the Field
-===================
+The Nancy Grace Roman Space Telescope (_Roman_) is NASA’s first survey-oriented flagship mission. Its design is optimized for cosmological studies, which includes extraction and precision analysis of Type Ia Supernova (SN Ia) light curves. Analysis of these data pose several challenges: (1) the enormous quantity of data that will be returned on a regular basis, and (2) the spatially-dependent nature of the image properties as a function of location on the detector, which directly affects ease of (3) obtaining the required <1\% flux precision on SN Ia measurements.
+
+_Roman_'s High Latitude Time Domain Survey (HLTDS) is specifically designed for cosmological studies with SNe Ia. The core component of this survey will last two years, and have a cumulative 158 days of exposure time. The survey will yield an average of 241 Level 2 (rate files) from imaging mode observations per day, which totals to approximately 0.4 TB of data. Other DIA and forced photometry survey pipelines are insufficient to process this quantity of data. For example, the Dark Energy Survey (DES) pipeline, which uses the HOTPANTS algorithm, takes 10 minutes to create a difference image from an image from a single detector, with 8.3 million pixels per detector. Using this pipeline, it would take 80 hours to make difference images from all 241 _Roman_ HLTDS images obtained within one 24 hour period, which have nearly 17 million pixels per detector. In reality, the amount of time spent processing these data will be much more than 80 hours because (1) there are many steps associated with SN Ia light curve analysis in addition to creating the difference images, (2) data from other concurrent surveys will also be processed for transient studies, and (3) this amount of time also excludes re-processing data when pipelines or calibration information are updated.
 
-TODO LAUREN
+`phrosty` is a DIA and forced photometry pipeline that addresses these challenges. We integrate the saccadic fast fourier transform (SFFT) method for difference imaging analysis (DIA), which uses a flexible delta basis function that can accommodate Roman’s spatially-varying point spread function (PSF). `phrosty` uses both GPU and CPU computation; this architecture has been carefully chosen to address the challenge of processing the large quantity of survey data within a reasonable amount of time. 
 
+Because Roman software is currently under-development, there is no similar publicly available all-in-one pipeline that takes _Roman_ images as input and outputs light curves. The architecture choices made in development have substantially accelerated processing time such that `phrosty` is among the fastest astronomical DIA pipelines available. 
 
 Contact and Support
 ===================