From 215620db15200b06787a6dd449c92142c1a2bcf9 Mon Sep 17 00:00:00 2001
From: John Franklin Crenshaw <41785729+jfcrenshaw@users.noreply.github.com>
Date: Fri, 15 Mar 2024 07:57:06 -0700
Subject: [PATCH] Update README.md

Added description of how PSF FWHM scales with airmass for extended sources.
---
 README.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/README.md b/README.md
index 951a4fa..cc17f55 100644
--- a/README.md
+++ b/README.md
@@ -187,6 +187,7 @@ NSR_\text{pt} \sqrt{\frac{A_\text{ap}}{A_\text{psf}}},
 $$
 
 where $A_\text{ap}$ is the area of the source aperture, and $A_\text{psf}$ is the area of the PSF.
+Note the PSF FWHM is assumed to scale like $\text{airmass}^{0.6}$.
 We set the proportionality constant to unity, so that when $A_\text{ap} \to A_\text{psf}$, we recover the error for a point source.
 
 We include two different models for calculating the aperture area. The "auto" method from [van den Busch (2020)](http://arxiv.org/abs/2007.01846) calculates the semi-major and -minor axes of the aperture ( $a_\text{ap}$ and $b_\text{ap}$) from the semi-major and -minor axes of the galaxy ( $a_\text{gal}$ and $b_\text{gal}$, corresponding to half-light radii):