From 9043a6ed72ab950eaeb73dad94ea2ff10e679dae Mon Sep 17 00:00:00 2001 From: matthewholman Date: Mon, 30 Oct 2023 17:28:16 -0400 Subject: [PATCH] Update ephemerisgen.rst I mostly rearranged the discussion, with a couple of minor corrections. --- docs/ephemerisgen.rst | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-) diff --git a/docs/ephemerisgen.rst b/docs/ephemerisgen.rst index 8181f603..b34cb5a7 100644 --- a/docs/ephemerisgen.rst +++ b/docs/ephemerisgen.rst @@ -3,7 +3,7 @@ Ephemeris Generator ========================================================== -Sorcha's ephemeris generator is powered by `ASSIST `__, a software package for ephemeris-quality integrations of test particles, and the `REBOUND `__ N-body integrator. If the user prefers to use a different generator or provide the ephemeris output from a previous Sorcha run, they have the ability to point Sorcha to an external file to ingest instead. +Sorcha's ephemeris generator is powered by `ASSIST `__, a software package for ephemeris-quality integrations of test particles, and the `REBOUND `__ N-body integration package. If the user prefers to use a different generator or provide the ephemeris output from a previous Sorcha run, they have the ability to point Sorcha to an external file to ingest instead. .. tip:: We recommend using Sorcha's ephemeris generator for all your survey simulations. @@ -12,12 +12,9 @@ Sorcha's ephemeris generator is powered by `ASSIST `_, which has an adaptive time step, Sorcha's ephemeris generator instantiates a REBOUND n-body simulation for each individual massless synethetic object including the effects of the Sun, planets, Moon, and 16 asteroids (see the :ref:`MAP` section). It also includes the J2, J3, and J4 gravitational harmonics of the Earth, the J2 gravitational harmonic of the Sun, and general relativistic correction terms for the Sun, using the Parameterized Post-Newtonian (PPN) formulation. The positions of the massive bodies come from the latest `DE441 `_ ephemeris, provided by NASA's `Navigation and Ancillary Information Facility (NAIF) `_. We note that the coordinate frame for ASSIST+REBOUND is the equatorial International Celestial Reference Frame (ICRF). We note that this is barycentric, rather than heliocentric. The ephemeris generator translates the input barycentric or heliocentric orbits into x,y, z and velocities in the ICRF to be read into ASSIST. +The Sorcha ephemeris generator determines which objects will appear in or near the camera field of view (FOV) for any given exposure. It uses spatial indexing to speed up these calculations. It runs through the survey visits and does on-the-fly checks of where every synthetic object is near the center of each night for which there are visits and organizes those positions using the `HEALPix (Hierarchical Equal Area isoLatitude Pixelation of a sphere) `_ tesselation of the sky. Given that information, it then steps through the visits for that night, doing precise calculations for just those objects that are near the camera FOV (field-of-view) of each survey on-sky visit. Specifically, for each visit, the generator calculates the unit vector from the observatory's location to the RA/Dec location of the field center. Then it finds the set of HEALPix tiles that are overlapped by the survey vist's camera FOV (nside=64). The ephemeris generator then collects the IDs for the particles in the HEALPix tiles overlapped by the given survey visit FOV. It then does light time corrected ephemeris calculations for just those, outputting the right ascenion, declination, rates, and relevant distances, and phase angle values for each of the particles. - -The ephemeris generator runs through the survey visits and does on-the-fly checks of where every synthetic object is near the center of each night for which there are visits (like planting the pickets (vetical planks of wood) along a picket fence. Given that information, it then steps through the visits for that night, doing precise calculations for just those objects that are near the camera FOV (field-of-view) of each survey on-sky visit. - -For each survey visit, the generator calculates the location of the observatory and the topocentric unit vector to the field RA/Dec. Then it finds the set of `HEALPix (Hierarchical Equal Area isoLatitude Pixelation of a sphere) `_ tiles that are overlapped by the survey vist's camera FOV (nside=64). The ephemeris generator then collects the IDs for the particles in the HEALPix tiles overlapped by the given survey visit FOV, and do light time corrected ephemeris calculations for just those, outputting the right ascenion, declination, rates, and relevant distances, and phase angle values for each of the particles. +Because ASSIST uses REBOUND's `IAS15 integrator `_, which has an adaptive time step, Sorcha's ephemeris generator instantiates a REBOUND n-body simulation for each individual massless synthetic object including the effects of the Sun, planets, Moon, and 16 asteroids (see the :ref:`MAP` section). It also includes the J2, J3, and J4 gravitational harmonics of the Earth, the J2 gravitational harmonic of the Sun, and general relativistic correction terms for the Sun, using the Parameterized Post-Newtonian (PPN) formulation. The positions of the massive bodies come from the latest `DE441 `_ ephemeris, provided by NASA's `Navigation and Ancillary Information Facility (NAIF) `_. We note that the coordinate frame for ASSIST+REBOUND is the equatorial International Celestial Reference Frame (ICRF). The positions and velocities are barycentric within this frame, rather than heliocentric. The ephemeris generator translates the input barycentric or heliocentric orbits into x,y, z and velocities into the barycentric ICRF to be read into ASSIST. .. tip:: If using Sorcha's internal ephemeris generation mode (which is the default mode), **we recommend calculating/creating your input orbits with epochs close in time to the start of the first survey observation**. This will minimize the REBOUND n-body integrations required to set up the ephemeris generation.