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[gnuastro-commits] master 2dff3ffa 1/3: Book: Example for Magnitude Limi

From: Mohammad Akhlaghi
Subject: [gnuastro-commits] master 2dff3ffa 1/3: Book: Example for Magnitude Limit
Date: Wed, 20 Apr 2022 18:53:14 -0400 (EDT)

branch: master
commit 2dff3ffab7af53490ffc148f244a9e06bcdaabd2
Author: S. Zahra Hosseini Shahisavandi <>
Commit: Mohammad Akhlaghi <>

    Book: Example for Magnitude Limit
    Until now, there wasn't any example or guide for calculating the magnitude
    in the section of "Magnitude limit of image".
    With this commit, we bring up an example in this section using required
    commands that can give a detailed oriented picture on this section. For
    this aim, we also take advantage of "astscript-ds9-region" to show the
    result on the images.
    Thanks to Sepideh Eskandarlou and Mohammad Akhlaghi for helping out to
    underestand and complete this section.
 doc/gnuastro.texi | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/doc/gnuastro.texi b/doc/gnuastro.texi
index 6f8de99b..a9446024 100644
--- a/doc/gnuastro.texi
+++ b/doc/gnuastro.texi
@@ -20419,6 +20419,42 @@ The same applies for a stacked image of the field 
compared to a single-exposure
 This concept is used by some researchers to define the ``magnitude limit'' or 
``detection limit'' at a certain S/N (sometimes 10, 5 or 3 for example, also 
written as @mymath{10\sigma}, @mymath{5\sigma} or @mymath{3\sigma}).
 To do this, they measure the magnitude and signal-to-noise ratio of all the 
objects within an image and measure the mean (or median) magnitude of objects 
at the desired S/N.
+For evaluating the Magnitude limit for an image it is required to have a good 
enough segment's output of the image.
+Then, the ``astmkcatalog'' will help to make a catalog of the segment's output 
(for clumps and objects extension) that will be written into two extensions (h1 
for objects and h2 for clumps).
+Like the following command, you can make catalog with the required column for 
calculating of magnitude limit.
+We highly recommend you add ``ra'' and ``dec'' columns if you want to have a 
glance at the objects that have satisfied the magnitude limit unless you can 
ignore the ``ra'' and ``dec''.
+$ astmkcatalog --ra --dec --magnitude --sn seg-out.fits
+@end example
+Remember this issue if you don't mention the required hdu, the 
``astmkcatalog'' consider the hdu/objects of segment's output as input for 
making a catalog and evaluating the magnitude limit.
+Finally, you can use ``asttable'' for selecting a required range of data based 
on the signal-to-noise column and calculating --median (or --mean) using the 
+The first command below shows the distribution of objects' magnitude (for 
objects if we use -h1, for clumps if we use -h2) at a determined S/N (in this 
example @mymath{5\sigma} or S/N=5).
+The second command shows the magnitude limit obtained using the median of the 
+$ asttable catalog.fits -h1 --range=sn,4.8:5.2 --column=magnitude\
+           | astarithmetic 
+@end example
+$ asttable catalog.fits -h1 --range=sn,4.8:5.2 --column=magnitude\
+           | astarithmetic --median
+@end example
+The last command that can help you to have a clear view of the magnitude limit 
is shown here.
+In here, you need to select a narrow range around the obtained magnitude 
limit, for example in the command below we consider the magnitude limit is 
obtained 26.
+If you are using -h2 please attention to the true number of columns, in this 
case you should use -c2,3 --namecol=4.
+It is very easy to take advantage of ``astscript-ds9-region'' for showing the 
magnitude of this result on the image.
+$ asttable catalog.fits -h1 --range=sn,4.8:5.2 --range=magnitude,25.9:26.1\
+           | astscript-ds9-region -c1,2 --namecol=3 \
+           --command="ds9 seg-out.fits" --width=2 \
+           --radius=1 --color=black
+@end example
 However, this method should be used with extreme care!
 This is because the shape of the object becomes important in this method: a 
sharper object will have a higher @emph{measured} S/N compared to a more 
diffuse object at the same original magnitude.
 Besides the inherent shape/sharpness of the object, issues like the PSF also 
become important in this method (because the finally observed shapes of objects 
are important here): two surveys with the same surface brightness limit (see 
@ref{Surface brightness limit of image}) will have different magnitude limits 
if one is taken from space and the other from the ground.

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