EZRA question about galactic distance

[Conrad Cardano]

Hi,

I'm preparing a Powerpoint Presentation for my optical astronomy club about mapping the Milky Way using Radio Astronomy.

I like to anticipate questions long before giving a presentation.

I understand the Doppler Effect very well and its effect on calculating velocity.

How does EZRA calculate the "Gas Radius from Galactic Center"?

Thank You for reading this!

Conrad

[tedcl...@gmail.com]

Hi Conrad,

In your question, you posted an ezGal550 plot of your data.
I think you need to tune your   -ezGalVelGLonEdgeLevelL   from the default
      -ezGalVelGLonEdgeLevelL  1.01  20  160
I expect the red top edge of your ezGal540 plot needs improvement.



Maybe your question is answered at about
      https://www.youtube.com/watch?v=YXuPDJVRbd0&t=1892s
or maybe starting back at
      https://www.youtube.com/watch?v=YXuPDJVRbd0&t=1083s
where I explain the tangential velocity method.
The radius from the
      Galactic Center to the tangential gas cloud
is simply trigonometry math with
      Galactic Longitude angle
      Sun to Galactic Center distance
The math trick is that the tangential gas cloud is the fastest receding velocity measured along that line of sight,
hence the   -ezGalVelGLonEdgeLevelL   parameter to find those fastest receding velocity values in your data.



ezRA found all this knowledge near page 54 Eq3 of the old (and now dangerous ?)
      https://f1ehn.pagesperso-orange.fr/pages_radioastro/Images_Docs/Radioastro_21cm_2012b.pdf
But sadly, J-J Maintoux F1EHN has died, about Sep-2024,
      https://groups.google.com/g/sara-list/c/8GdsZhkFpVc
and so his personal website has died, so try the Wayback Machine's
      https://web.archive.org/web/20161026031905/http://f1ehn.pagesperso-orange.fr/pages_radioastro/Images_Docs/Radioastro_21cm_2012b.pdf
and his YouTube,
      https://www.youtube.com/watch?v=HGwkZY4E64k&ab_channel=JJMAINTOUX
Translation with Google may help,
      https://translate.google.com


J-J Maintoux F1EHN referred to SALSA,
      https://brage.oso.chalmers.se/salsa/welcome
with its download,
      https://raw.githubusercontent.com/varenius/salsa/main/Lab_instructions/HI/English/SALSA-HI_English.pdf


There is also this video from MIT,
      Mapping the Galaxy with Radio Astronomy
      https://www.youtube.com/watch?v=-UrzmAa62ho&ab_channel=ESGPEVSpring2015



And I like this fun Max Planck Society video (highlighting receding gas followed by approaching gas),
      https://www.youtube.com/watch?v=Q2mgpsTFuV8&t=1s&ab_channel=MaxPlanckSociety
Seen at the end of the video, the last 2 bright objects in the bottom right are the approaching Magellanic Clouds !



Helpful ?

---
Ted Cline   N0RQV
TedClineGit at gmail.com

For folks starting to explore radio astronomy,
      ezRA - Easy Radio Astronomy
      Free 1420 MHz Galactic hydrogen data collection and analysis
      Open source
      Windows and Linux
      https://github.com/tedcline/ezRA
        https://youtu.be/kHgwEbWKhzs  (13 minute Simple Overview)

On Saturday, February 7, 2026 at 9:11:07 AM UTC-7 Conrad Cardano wrote:

...

[Conrad Cardano]

Thank you, Ted!

I appreciate your advice..

Conrad

On Sunday, February 8, 2026, tedcl...@gmail.com <tedcl...@gmail.com> wrote:

Hi Conrad,

You posted an ezGal550 plot of your data in your question.

I think you need to tune your   -ezGalVelGLonEdgeLevelL   from the default
      -ezGalVelGLonEdgeLevelL  1.01  20  160
I expect the red top edge of your ezGal540 plot needs improvement.

Maybe your answer is explained at about

      https://www.youtube.com/watch?v=YXuPDJVRbd0&t=1892s
or maybe starting back at
      https://www.youtube.com/watch?v=YXuPDJVRbd0&t=1083s
where I explain the tangential velocity method.
The radius from the

      Galactic Center to the tangential gas cloud

is simply trigonometry math with
      Galactic Longitude angle
      Sun to Galactic Center distance
The math trick is that the tangential gas cloud is the fastest receding velocity measured along that line of sight,
hence the   -ezGalVelGLonEdgeLevelL   parameter to find those fastest receding velocity values in your data.



ezRA found all this knowledge near page 54 Eq3 of the old (and now dangerous ?)

      https://f1ehn.pagesperso-orange.fr/pages_radioastro/Images_Docs/Radioastro_21cm_2012b.pdf
But sadly, J-J Maintoux F1EHN has died about Sep-2024,

      https://groups.google.com/g/sara-list/c/8GdsZhkFpVc
and so his personal website has died, so try the Wayback Machine's

      https://web.archive.org/web/20161026031905/http://f1ehn.pagesperso-orange.fr/pages_radioastro/Images_Docs/Radioastro_21cm_2012b.pdf

and his YouTube,

      https://www.youtube.com/watch?v=HGwkZY4E64k&ab_channel=JJMAINTOUX
Translation with Google may help,
      https://translate.google.com


J-J Maintoux F1EHN referred to SALSA,
      https://brage.oso.chalmers.se/salsa/welcome

with the download,

      https://raw.githubusercontent.com/varenius/salsa/main/Lab_instructions/HI/English/SALSA-HI_English.pdf


There is also this video from MIT,
      Mapping the Galaxy with Radio Astronomy
      https://www.youtube.com/watch?v=-UrzmAa62ho&ab_channel=ESGPEVSpring2015



And I like this fun Max Planck Society video (highlighting receding gas followed by approaching gas),
      https://www.youtube.com/watch?v=Q2mgpsTFuV8&t=1s&ab_channel=MaxPlanckSociety

The last 2 bright objects seen at the end of the video in the bottom right are the receding Magellanic Clouds !

Helpful ?

---
Ted Cline   N0RQV
TedClineGit at gmail.com

On Saturday, February 7, 2026 at 9:11:07 AM UTC-7 Conrad Cardano wrote:

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