[gnuastro-commits] master 46d328cd 12/39: Book: writing a tutorial for the zero-point script; preface

[Mohammad Akhlaghi]

branch: master
commit 46d328cd7a7ce356f983c8f7b273b7c9e8e92575
Author: Elham Saremi <saremi_elham@yahoo.com>
Commit: Mohammad Akhlaghi <mohammad@akhlaghi.org>

    Book: writing a tutorial for the zero-point script; preface
    
    Until now, Sepideh has added a script for finding zero points to Gnuastro.
    
    With this commit, I started to write a tutorial for this script. In
    beginning, I wrote a short introduction that what is zero point and how we
    estimate it. In the next step, I will have an example to show how the
    script work.
---
 doc/gnuastro.texi | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/doc/gnuastro.texi b/doc/gnuastro.texi
index 3ec06fca..a2715716 100644
--- a/doc/gnuastro.texi
+++ b/doc/gnuastro.texi
@@ -629,6 +629,7 @@ MakeCatalog
 
 * Detection and catalog production::  Discussing why/how to treat these 
separately
 * Brightness flux magnitude::   More on Magnitudes, surface brightness, etc.
+* Photometric calibration zeropoint:: Calculating zeropoint of images
 * Quantifying measurement limits::  For comparing different catalogs.
 * Measuring elliptical parameters::  Estimating elliptical parameters.
 * Adding new columns to MakeCatalog::  How to add new columns.
@@ -24429,7 +24430,7 @@ It might even be so intertwined with its processing, 
that adding new columns mig
 
 
 
-@node Brightness flux magnitude, Quantifying measurement limits, Detection and 
catalog production, MakeCatalog
+@node Brightness flux magnitude, Photometric calibration of images with 
finding zeropoint, Quantifying measurement limits, Detection and catalog 
production, MakeCatalog
 @subsection Brightness, Flux, Magnitude and Surface brightness
 
 @cindex ADU
@@ -24608,8 +24609,39 @@ If you need to warp or convolve the image, do it 
@emph{before} the conversion.
 
 
 
+@node Photometric calibration of images with finding zeropoint, Quantifying 
measurement limits, Measuring elliptical parameters, Brightness flux magnitude, 
MakeCatalog
+@subsection Photometric calibration of images with finding zeropoint
 
-@node Quantifying measurement limits, Measuring elliptical parameters, 
Brightness flux magnitude, MakeCatalog
+As described in @ref{Brightness flux magnitude}, to convert astronomical data 
pixel values from counts to energy/time (physical units such as Janskys), we 
need to know the zero point of the image.
+This conversion is necessary to can compare two images independent of 
instruments that were observed with them.
+Actually, the zero point is used to calibrate an astronomical image to the 
standard state.
+
+To find zero point, it is common to use photometric systems with defined zero 
point such as some images or catalogs.
+For example, the SDSS data can be a good reference for finding zero point in 
optical and 2MASS data for near infra-red images.
+The general outline of the steps that we use to estimate zero point in an 
image is given below:
+
+@enumerate
+@item
+Download of Gaia catalog using Gnuastro’s Query program (see @ref{Query}) to 
determine correct coordinates of stars in the image.
+@item
+Select of reference image or catalog and download of it.
+@item
+Aperture photometry with MakeProfiles (see @ref{MakeProfiles}) and MakeCatalog 
(see @ref{MakeCatalog}); a complete tutorial is in @ref{Aperture photometry}.
+If an image is selected as a reference for finding zero point, aperture 
photometry should be done in the same way for it.
+@item
+Match catalogs (see @ref{Match} and also a tutorial in @ref{Matching 
catalogs}) to obtain differences of magnitudes in two catalogs and estimate 
zero point value.
+@end enumerate
+
+Clearly, all of top steps are very long and somewhat complicated.
+Fortunately, Gnuastro has an installed script, designed to simplify find zero 
point in an image based on a reference image or catalog with a defined zero 
point.
+Here we have a tutorial on how to use @command{astscript-zeropoint}.
+This tutorial is divided into two parts to cover both of using image or 
catalog as reference data.
+
+
+
+
+
+@node Quantifying measurement limits, Measuring elliptical parameters, Adding 
new columns to MakeCatalog, Invoking MakeCatalog
 @subsection Quantifying measurement limits
 
 @cindex Depth