Why are Blue and Red shift markings on the hydrogen line graphs opposite of normal.

[Jan Lustrup]

I got this Doppler shift question on my “Amateur Radio”  facebook site yesterday. I’ve heard this explanation before but I cannot remember why.

So can someone please explain  why in  Hydrogen line graphs the lower frequency shift from 1420.405MHz is marked ” blue” and the higher frequency shift over 1420.405MHz  is marked “red”.

This is opposite of visual astronomy and normal  Doppler shift  thinking where a star moving away is red shifted in color and a start coming towards us is blue shifted in color.

 

[Jan Lustrup]

SRI Typo...my facebook site is called “Amateur Radio Astronomy” not “Amateur Radio”.

-Jan  LA3EQ

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[fasleitung3]

Jan, I am not sure whether this answers your question:

In the professional radio astronomy community, spectra are typically presented with the x axis in km/sec. The convention is that velocities towards the observer (blue shifted) have a negative sign and velocities away from the observer (red shifted) have a positive sign. For example, an x-axis could run from -500 km/s in the left of the graph to +500 km/s to the right of the graph.

Consequently, the higher frequency signal is to the left and the lower frequency signal is to the right (so a frequency axis is running from higher values to lower values). 

Unfortunately, in the amateur arena this convention is not always followed and spectra are presented with the frequency axis going from lower to higher values. This flips the spectrum compared to a presentation following the convention.

Best regards,

Wolfgang

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[Jan Lustrup]

Wolfgang,

Thank you for the explanation as this clear up things, aka frequency shift vs  velocities

Thanks , Jan.

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[Dave Typinski]

If the lower /radio frequency/ is marked "blue," that's a mistake.

It should be the negative /relative velocity/ that's marked "blue."
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Dave

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[Jan Lustrup]

Her is the image that make me wonder...
-Jan

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From: sara...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of Dave Typinski
Sent: torsdag 27. juni 2024 15:08
To: sara...@googlegroups.com
Subject: Re: [SARA] Why are Blue and Red shift markings on the hydrogen line graphs opposite of normal.

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[Dave Typinski]

Hi Jan, yes, the red and blue labels are incorrect on that plot.

Blue = distance decreasing = negative velocity = radio frequency increased

Red = distance increasing = positive velocity = radio frequency decreased
--
Dave

[Steve]

I would just add the reason that movement towards the observer is assigned a negative sign velocity is that velocity = ds/dt - where ds is the change in distance and dt is change in time. When an object is moving towards an observer the distance is getting smaller - so the change in distance is negative as you are going from larger distance to a smaller distance - hence relative velocity is negative.

As the parameter of interest in observations is velocity, this parameter is plotted as would be expected with negative values on the left and positive values on the right - that is - blue on the left, red on the right. The trick is when you label the X-axis with sky frequency instead. You have to remember the Doppler principle that towards is blue = higher sky frequency, away is red = lower frequency.

When I see such a graph it takes me a few moments to get that straight.

On 27/06/2024 7:18 pm, Jan Lustrup wrote:

 

From: sara...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of Jan Lustrup
Sent: torsdag 27. juni 2024 10:32
To: sara...@googlegroups.com
Subject: [SARA] Why are Blue and Red shift markings on the hydrogen line graphs opposite of normal.

 

I got this Doppler shift question on my “Amateur Radio”  facebook site yesterday. I’ve heard this explanation before but I cannot remember why.

So can someone please explain  why in  Hydrogen line graphs the lower frequency shift from 1420.405MHz is marked ” blue” and the higher frequency shift over 1420.405MHz  is marked “red”.

This is opposite of visual astronomy and normal  Doppler shift  thinking where a star moving away is red shifted in color and a start coming towards us is blue shifted in color.

--

Cheers
Steve Olney
(HawkRAO/HawkASTRO)
(Radio Astronomy Website - https://sites.google.com/view/hawkrao)
(Astrophotography Website - https://sites.google.com/view/hawkastro)
(Astronomy YouTube - https://www.youtube.com/@HawkASTRO)

[Stephen Thornber]

Just for fun Port is left is red and if you see it first the vessel is traveling right to left across the field of vision.  As I said just for fun 🤩 

 

Stephen G6SGA

FISTS: 18892

On 27 Jun 2024, at 21:29, Steve <hawkr...@gmail.com> wrote:



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[Mario Natali]

Hi Dave,

Thanks a lot for pointing this !
I had never thought about it and I will correct immediately.

I am am working now to the linearization of the plot and I hope that this will be included in the next major release.
Any other suggestion and correction will be much appreciated !

Rgds, Mario


Inviato da iPhone

> Il giorno 27 giu 2024, alle ore 20:10, Dave Typinski <dav...@typnet.net> ha scritto:
>
> Hi Jan, yes, the red and blue labels are incorrect on that plot.

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[Jan Lustrup]

Thanks Dave....I have written this down in my notebook...
-Jan

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