<p>I would like to know how I can estimate the <strong>tangential</strong> and <strong>cross</strong> component of the galaxy's <strong>shear</strong> using <a href="https://en.wikipedia.org/wiki/Gnomonic_projection" rel="nofollow noreferrer">Gnomonic projection</a>
of the right ascension and declination the galaxies relate to the center of galaxy cluster and estimate?</p>
<p><strong>Edit.</strong> <em>Shear</em> is the second order moment of a galaxy's surface brightness. You can find more on the topic in the following <a href="http://gravitationallensing.pbworks.com/w/page/15553259/Weak%20Lensing" rel="nofollow noreferrer">wiki on weak lensing</a>.</p>

CC BY-SA 4.0
2021-03-11T12:42:32.367Z
I am not sure I understand what you are after: Do you mean you want to project the movement vectors of all galaxies in a galaxy cluster onto a plane which each galaxy then having a tangential and a radial component (the 3rd dimension we lost by projection), as well as a brightness. Once you have that, you want to somehow calculate the 2nd derivative of the brightness changes. Is this understanding correct?
B--rian
CC BY-SA 4.0
2021-03-11T12:43:19.807Z
So you basically are asking how to derive the components of optical transfer matrix, in particular the off-diagonal components, of the media between us and the observed galaxy (assuming some kind of ideal galaxy forms the original undistorted image)?
planetmaker
CC BY-SA 4.0
2021-03-11T12:46:39.073Z
@planetmaker Your interpretation does probably make more sense :-)
B--rian
CC BY-SA 4.0
2021-03-11T12:50:09.133Z
Not sure we get a (quick) answer 6,5 years after it was asked and 2 months after the OP was last seen :D
planetmaker
CC BY-SA 3.0
2014-09-05T09:59:52.043Z
Please elaborate what you mean by shear? When I read the title I thought about the shear in the galactic velocity fields and Oort's constants ...
Walter
CC BY-SA 3.0
2014-09-05T10:26:22.933Z
@Walter shear is **the second order moment of galaxy's surface brightness**. You can find more exclusive explanation [here](http://gravitationallensing.pbworks.com/w/page/15553259/Weak%20Lensing)
Dalek
<cosmology><mathematics><galaxy-cluster><gravitational-lensing><weak-lensing>
Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection
Dalek
# Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection

(Asked by: username_0 on 2014-08-11)

I would like to know how I can estimate the **tangential** and **cross** component of the galaxy's **shear** using [Gnomonic projection](https://en.wikipedia.org/wiki/Gnomonic_projection)
of the right ascension and declination the galaxies relate to the center of galaxy cluster and estimate?

**Edit.** *Shear* is the second order moment of a galaxy's surface brightness. You can find more on the topic in the following [wiki on weak lensing](http://gravitationallensing.pbworks.com/w/page/15553259/Weak%20Lensing).

**Tags:** cosmology, mathematics, galaxy-cluster, gravitational-lensing, weak-lensing
> **Comments:**
> **username_3**: Please elaborate what you mean by shear? When I read the title I thought about the shear in the galactic velocity fields and Oort's constants ...  
> **username_0**: @Walter shear is **the second order moment of galaxy's surface brightness**. You can find more exclusive explanation [here](http://gravitationallensing.pbworks.com/w/page/15553259/Weak%20Lensing)  
> **username_1**: I am not sure I understand what you are after: Do you mean you want to project the movement vectors of all galaxies in a galaxy cluster onto a plane which each galaxy then having a tangential and a radial component (the 3rd dimension we lost by projection), as well as a brightness. Once you have that, you want to somehow calculate the 2nd derivative of the brightness changes. Is this understanding correct?  
> **username_2**: So you basically are asking how to derive the components of optical transfer matrix, in particular the off-diagonal components, of the media between us and the observed galaxy (assuming some kind of ideal galaxy forms the original undistorted image)?  
> **username_1**: @planetmaker Your interpretation does probably make more sense :-)  
> **username_2**: Not sure we get a (quick) answer 6,5 years after it was asked and 2 months after the OP was last seen :D  

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