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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 <2hs.zahra@gmail.com>

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''.
+
+@example
+$astmkcatalog --ra --dec --magnitude --sn seg-out.fits +@end example + +Remember this issue if you don't mention the required hdu, the astmkcatalog'' consider the hdu/objects of segment's output as input for making a catalog and evaluating the magnitude limit. +Finally, you can use asttable'' for selecting a required range of data based on the signal-to-noise column and calculating --median (or --mean) using the astarithmetic''. +The first command below shows the distribution of objects' magnitude (for objects if we use -h1, for clumps if we use -h2) at a determined S/N (in this example @mymath{5\sigma} or S/N=5). +The second command shows the magnitude limit obtained using the median of the histogram. + +@example +$ asttable catalog.fits -h1 --range=sn,4.8:5.2 --column=magnitude\
+           | astarithmetic
+@end example
+
+@example
+$asttable catalog.fits -h1 --range=sn,4.8:5.2 --column=magnitude\ + | astarithmetic --median +@end example + +The last command that can help you to have a clear view of the magnitude limit is shown here. +In here, you need to select a narrow range around the obtained magnitude limit, for example in the command below we consider the magnitude limit is obtained 26. +If you are using -h2 please attention to the true number of columns, in this case you should use -c2,3 --namecol=4. +It is very easy to take advantage of astscript-ds9-region'' for showing the magnitude of this result on the image. + +@example +$ 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 \
+@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.