[gnuastro-commits] master 2b869c0b: Book: minor edits and clarifications in text of previous commit

[Mohammad Akhlaghi]

branch: master
commit 2b869c0b79941b5c2a004aa92e28fe01d395c910
Author: Mohammad Akhlaghi <mohammad@akhlaghi.org>
Commit: Mohammad Akhlaghi <mohammad@akhlaghi.org>

    Book: minor edits and clarifications in text of previous commit
    
    Until now, the newly added text (in the previous commit) had some
    confusions, typos and inconsistencies.
    
    With this commit, they were re-read and edited to avoid as many of the
    issues as possible (more will certainly come later).
---
 doc/gnuastro.texi | 48 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 16 deletions(-)

diff --git a/doc/gnuastro.texi b/doc/gnuastro.texi
index 7ee269fd..5ea07251 100644
--- a/doc/gnuastro.texi
+++ b/doc/gnuastro.texi
@@ -22030,7 +22030,7 @@ $ astarithmetic 20 24.8 mag-to-counts --quiet
 @cindex Luminosity to Apparent Magnitude
 @cindex Apparent Mangitude to Luminosity
 Convert the given apparent magnitude to luminosity (in units of the luminosity 
of a reference object).
-It takes the following three operands (in the order written on the 
command-line):
+It takes the following three operands (in the order written on the 
command-line; see example below):
 
 @enumerate
 @item
@@ -22047,13 +22047,13 @@ The Sun's absolute magnitude in various commonly used 
filters and magnitude syst
 For example the Sun's absolute magnitude in the SDSS u, g, r, i and z filters 
(with the AB magnitude system) is respectively 6.39, 5.11, 4.65, 4.53 and 4.50.
 @item
 @cindex K-Correction
-Conversion of apparent to absolute magnitude.
-At small distances or when talking about bolometric magnitudes, the distance 
modulus can be used for this.
-See @option{--distancemodulus} in @ref{CosmicCalculator basic cosmology 
calculations} for more details (and why @option{--absmagconv} is the prefered 
option in the absense of SED-based methods).
+Difference of apparent (@mymath{m}) and absolute (@mymath{M}) magnitudes: 
@mymath{m-M}.
+At small distances or when the input is bolometric (across all wavelengths), 
the distance modulus can be used for this.
+See @option{--distancemodulus} in @ref{CosmicCalculator basic cosmology 
calculations} for more details and why @option{--absmagconv} is preferred to 
the distance modulus in the absense of SED-based methods.
 @end enumerate
 
-For example, let's assume we want to obtain the luminosity of a galaxy (in 
units of solar luminosity at the same wavelength) at redshift 0.01 with 
apparent AB magnitude of 20 in the SDSS g filter.
-To do this, we need the distance modulus (which is sufficient for this 
distance) and the Sun's absolute magnitude, from Willmer 
@url{https://arxiv.org/abs/1804.07788,2018} in this filter (which is 5.11):
+For example, let's assume the apparent AB magnitude of a galaxy (after 
correcting Galactic extinction) is 20 in the SDSS g filter, and that its 
redshift is 0.01 and we need its luminosity (in units of solar luminosity) in 
the same filter.
+To do this, we need the apparent-absolute magnitude difference (using 
CosmicCalculator), as well as the Sun's absolute magnitude, from Willmer 
@url{https://arxiv.org/abs/1804.07788,2018} in this filter (which is 5.11):
 
 @example
 $ conv=$(astcosmiccal --absmagconv --redshift=0.01)
@@ -29689,35 +29689,35 @@ To be able to compare our scientific data with other 
data, optical astronomers h
 But before getting to those, let's review the following basic physical 
concepts first:
 
 @table @asis
-@item Energy (@mymath{erg}; also known as @emph{counts} or @emph{ADU}s)
+@item Energy (@mymath{erg}; also in @emph{counts} or @emph{ADU}s)
 Within the electromagnetic regime, we measure the received energy of 
astronomical source by counting the number of photons that have been converted 
to electrons (electric potential) in our detectors.
 
 @cindex Gain
 @cindex Counts
 @cindex ADU (Analog-to-digital unit)
 @cindex Analog-to-digital unit (ADU)
-When counting the electrical potential, the optical (but also near 
ultra-violet and near infra-red) detectors do not actually count individual 
electrons but bundles/packages of electrons known as the analog-to-digital unit 
(ADU).
-The number of electrons in each ADU is known as the @emph{gain} of the 
instrument.
-The gain of the instrument is measured as part of its calibration to be able 
to convert ADUs to electron-counts.
+When counting/measuring the electric potential changes, the optical (but also 
near ultra-violet and near infra-red) detectors do not actually count 
individual electrons but bundles/packages of electrons known as the 
analog-to-digital unit (ADU).
+The number of electrons in each ADU is known as the @emph{gain} of the 
instrument and is measured as part of its calibration.
 
 @item Power (@mymath{erg/s})
-The amount of energy in a fixed interval of time (1 second) and is used in two 
contexts within astronomy (and physics in general).
+The amount of energy in a fixed interval of time (1 second) is known as power.
+Power is used in two contexts within astronomy which are listed below.
 Both have the same units of energy per time, but their difference is very 
important to understand in physical interpretation:
 
 @table @asis
 
 @item Brightness
 @cindex Brightness
-To be able to compare data taken with different exposure times, we define the 
@emph{measured power} of the source (energy divided by time) as the 
@emph{brightness}.
+The @emph{received} power from a source (the thing we measure).
+To be able to compare data taken with different exposure times, we define the 
received power of the source in a detector as the @emph{brightness}.
 
 @cindex Luminosity
 @item Luminosity
-This is the total power, in @mymath{erg/s}, the object emits in @emph{all 
directions}.
-Luminosity has the same units as brighntess, but as shown above its 
intepretations is very different: unlike brightness (a measured property), 
luminosity is an inherent property of the object that is calculated from the 
combination of multiple measurements (flux and distance).
-See the @option{mag-to-luminosity} operator of Arithmetic in @ref{Unit 
conversion operators} for more on the conversion of the observed magnitudes 
(described below) of an objec to luminosity.
-
+The total @emph{emitted} power of a source in @emph{all directions}.
 @end table
 
+Unlike brightness (a measured property), luminosity is an inherent property of 
the object that is calculated from the combination of multiple measurements 
(flux and distance; see below).
+
 @item Flux (@mymath{erg/s/cm^2})
 @cindex Flux
 To be able to compare with data from different telescopes (with different 
collecting areas), we define the @emph{flux} which is defined by dividing the 
brightness by the exposed aperture of our telescope.
@@ -29776,6 +29776,22 @@ For estimating the error in measuring a magnitude, see 
@ref{Quantifying measurem
 The equation above is ultimately a relative relation.
 To tie it to physical units, astronomers use the concept of a zero point which 
is discussed in the next item.
 
+See the @option{mag-to-luminosity} operator of Arithmetic in @ref{Unit 
conversion operators} for more on the conversion of the observed magnitudes 
(described below) of an object to luminosity.
+The received brightness of two object with the same luminosity but at 
different distances is going to be different (the closer one will be brighter).
+Therefore, astronomers have defined the following terminology to help avoid 
confusing distant-dependent and distance-independent magnitudes.
+
+@table @asis
+@item Apparent magnitude
+The apparent magnitude is directly related (through the equation above) to the 
received spectral flux density (that we measure in our detectors).
+Therefore, it depends on the distance to the object (and any absorption that 
may occur in the light path).
+@item Absolute magnitude
+Knowing the distance of an object and absorptions in the light path, we can 
obtain the luminosity.
+From the luminosity, we can measure the apparent magnitude if the object was a 
point at a nominal (or fixed, or standard, or absolute) distance.
+The magnitude at this absolute distance is known as the absolute magnitude.
+The standard (or abstract: just to help in comparisons) distance is 
historically defined as 10 parsecs.
+By reporting the magnitude at a fixed distance for all objects, the absolute 
magnitude therefore helps to compare the intrinsic (independent of distance) 
magnitude of astronomical objects.
+@end table
+
 @item Zero point
 @cindex Zero point magnitude
 @cindex Magnitude zero point