Prior Predictive Checks
The prior predictive distributin of a Bayesian model what you get by sampling parameters directly from the priors and calculating where the model would place the data. For example, if sampling from relative astrometry, the prior predictive model is the distribution of (simulated) astrometry points corresponding to orbits drawn from the prior. For radial velocity data, these would be simulated RV points based on an RV curve drawn from the priors.
To generate a prior predictive distribution, one first needs to create a model. We will use the model and sample data from the Fit Astrometry tutorial:
using Octofitter
using CairoMakie
using PairPlots
using Distributions
astrom_like = PlanetRelAstromLikelihood(Table(;
epoch= [50000,50120,50240,50360,50480,50600,50720,50840,],
ra = [-505.764,-502.57,-498.209,-492.678,-485.977,-478.11,-469.08,-458.896,],
dec = [-66.9298,-37.4722,-7.92755,21.6356, 51.1472, 80.5359, 109.729, 138.651,],
σ_ra = fill(10.0, 8),
σ_dec = fill(10.0, 8),
cor = fill(0.0, 8)
))
@planet b Visual{KepOrbit} begin
a ~ truncated(Normal(10, 4), lower=0, upper=100)
e ~ Uniform(0.0, 0.5)
i ~ Sine()
ω ~ UniformCircular()
Ω ~ UniformCircular()
θ ~ UniformCircular()
tp = θ_at_epoch_to_tperi(system,b,50420) # reference epoch for θ. Choose an MJD date near your data.
end astrom_like
@system Tutoria begin
M ~ truncated(Normal(1.2, 0.1), lower=0.1)
plx ~ truncated(Normal(50.0, 0.02), lower=0.1)
end b
We can now draw one sample from the prior:
prior_draw_system = generate_from_params(Tutoria)
prior_draw_astrometry = prior_draw_system.planets.b.observations[4]
PlanetRelAstromLikelihood Table with 5 columns and 8 rows:
epoch ra dec σ_ra σ_dec
┌───────────────────────────────────────
1 │ 50000 -44.3638 -80.2534 10.0 10.0
2 │ 50120 -38.722 -44.9657 10.0 10.0
3 │ 50240 -32.9211 -9.49359 10.0 10.0
4 │ 50360 -26.9825 26.0178 10.0 10.0
5 │ 50480 -20.9293 61.418 10.0 10.0
6 │ 50600 -14.7856 96.5523 10.0 10.0
7 │ 50720 -8.57705 131.262 10.0 10.0
8 │ 50840 -2.33035 165.387 10.0 10.0
And plot the generated astrometry:
Makie.scatter(prior_draw_astrometry.table.ra, prior_draw_astrometry.table.dec,color=:black, axis=(;autolimitaspect=1,xreversed=true))

We can repeat this many times to get a feel for our chosen priors in the domain of our data:
using Random
Random.seed!(1)
fig = Figure()
ax = Axis(
fig[1,1], xlabel="ra offset [mas]", ylabel="dec offset [mas]",
xreversed=true,
aspect=1
)
for i in 1:50
prior_draw_system = generate_from_params(Tutoria)
prior_draw_astrometry = prior_draw_system.planets.b.observations[4]
Makie.scatter!(
ax,
prior_draw_astrometry.table.ra,
prior_draw_astrometry.table.dec,
color=Makie.cgrad(:turbo)[i/50],
)
end
fig

The heavy black crosses are our actual data, while the colored ones are simulations drawn from our priors. Notice that our real data lies at a greater separation than most draws from the prior? That might mean the priors could be tweaked.