21 cm "Hydrogen Emission Line " sky maps

[Alex P]

21 cm  "Hydrogen Emission Line " sky maps

For several years, I have seen radio astronomy software with "H Line" maps which disagreed with the data I was collecting.  I never understood the problem .

Only One agreed .. and to a fraction of an RA hour. ( the one published by CCERA )

 In recent SARA Topic, Rober Meade explained the Mystery : Many Charts, even labeled "H1"  were obtained from wide band continuum surveys from systems using bandwidths of 10-20  MHz. 

These surveys are too wide to selectively show only neutral hydrogen emission brightness.

 Stellarium Software "1420 MHz" and "H1" Surveys  show the same wide bandwidth issues.

 ===========================================

comments posted by Robert Meade

The survey is NOT centered about the HI line at 1420.405 (ish) MHz with BW only covering HI emission from the Milky Way as we are commonly interested in. It's literally w/ some BW about 1420 MHz.

So when you're seeing temperature values and heatmaps  .... the majority of the measured BW used to make those plots is thermal continuum emission as opposed to narrowband (and doppler spread) HI line emission which will have overall spread BW < 400 kHz for contributions from the Milky Way ..==================================================

 SO: If  you data does not 'match the map', perhaps it is not your System but the Survey being used.

 Alex Pettit

[Andrew Thornett]

I think it is amazing that we, as amateurs, can now dispute such published maps - just goes to show how much the hobby has progressed!

Andy 

Sent from Outlook for Android

---

From: 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Friday, January 2, 2026 2:14:55 PM
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: [SARA] 21 cm "Hydrogen Emission Line " sky maps

 

21 cm  "Hydrogen Emission Line " sky maps

For several years, I have seen radio astronomy software with "H Line" maps which disagreed with the data I was collecting.  I never understood the problem .

Only One agreed .. and to a fraction of an RA hour. ( the one published by CCERA )

 In recent SARA Topic, Rober Meade explained the Mystery : Many Charts, even labeled "H1"  were obtained from wide band continuum surveys from systems using bandwidths of 10-20  MHz. 

These surveys are to wide to selectively show only neutral hydrogen emission brightness.

 Stellarium Software "1420 MHz" and "H1" Surveys  show the same wide bandwidth issues.

 ===========================================

comments posted by Robert Meade

The survey is NOT centered about the HI line at 1420.405 (ish) MHz with BW only covering HI emission from the Milky Way as we are commonly interested in. It's literally w/ some BW about 1420 MHz.

So when you're seeing temperature values and heatmaps  .... the majority of the measured BW used to make those plots is thermal continuum emission as opposed to narrowband (and doppler spread) HI line emission which will have overall spread BW < 400 kHz for contributions from the Milky Way ..==================================================

 SO: If  you data does not 'match the map', perhaps it is not your System but the Survey being used.

 Alex Pettit

 

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[b alex pettit jr]

SOME are not even close .. not too challenging 

[Marcus D. Leech]

On 2026-01-02 12:46, 'Andrew Thornett' via Society of Amateur Radio Astronomers wrote:

I think it is amazing that we, as amateurs, can now dispute such published maps - just goes to show how much the hobby has progressed!

Andy 

I don't think it's a question of "disputing the published maps".  Those maps ARE correct, for the things that they're measuring.

The CCERA map was published specifically to address the typical bandwidths and *interests* of the amateur observer.

Consider how different the sky looks in the IR compared to the visible wavelengths, for example.   Those views don't
  contradict one another...

-Marcus

To view this discussion visit https://groups.google.com/d/msgid/sara-list/FRWP194MB275201F70EA211241BD34F19FFBBA%40FRWP194MB2752.EURP194.PROD.OUTLOOK.COM.

[b alex pettit jr]

Hi Marcus,

Agreed .. except many are labeled as " H1 survey " & associated with amateur level sky maps.

[b alex pettit jr]

Once beyond the " I Did It " level, more precise confirmation of the results are quite meaningful,

One chart being used for HLine is a 408M survey.

Alex

============================================================

[andrew....@googlemail.com]

I get that, Alex – I was just trying to be encouraging…..although admittedly I wasn’t too accurate…

 

I still find is amazing that we can be having this conversation – I have a book on amateur radio astronomy from the 1960s which describes hydrogen line radio observing as “only for the [really] advanced amateur” & mapping the Milky Way as an exercise for those who are at nearly professional level…..and here we are – all doing it, only a few decades later! And doing it for such a cheap cost – long gone are the $thousands required for such projects.

 

So now – we move forward – you are I are mapping the Milky Way in 3D(Wow!), demonstrating dark matter(Wow! Wow!), weighing the Milky Way(little Wow!) and maybe even contributing to the advancement of science(Wow! Wow! Wow! Wow!) – but perhaps most incredibly we can all walk in the scientific footsteps of those who came before us, and travel that journey of scientific discovery that leads us to understand the universe through eyes not our own, in wavelengths we cannot see, at distances we can hardly comprehend, over time periods that reduce our civilisation to a tiny pin-prick in the history of the universe!(Lots of Wows!)

 

……I am trying to encourage everyone here to give this all a try.

 

AND if anyone would like to get help on your journey or just talk to others about their (proposed or actual) projects on H-Line then there is a joint BAA/SARA members H-Line meeting on Monday night. Log in details =

 

Microsoft Teams: (UTC+0) 19:00 on 5 Jan 2026:

Join the meeting now

Meeting ID: 360 212 052 639 56

Passcode: Jr36E8dw

 

Any problems joining contact me on andrew (at) thornett (dot) net or +447770841767.

 

Drop me an email if you would like to be added to the mailing list for future meetings.

 

Andy

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[b alex pettit jr]

[Stephen Arbogast]

I  agree with  everyone  here.  Students  would  ask me  'Why must we make so many  measurements  and do statistics  stuff'.  I answered with  ... Science is a collaboration  between  many people   making measurements in different   ways.  We  must   state  how we  made the  measurements  along  with  our   results.

Example   ..  I have    been making    measurements  of    Hydrogen  Line ..  for  a while now     with    various  hardware  and software...    I  recently  settled  on  the  Discovery Dish  with   H Line  feed/lna  for   hardware.  I  keep my  sampling rate  for my  AirSpy R2/mini at  2.4 Mhz    so I am above  the  Nyquist   rate  for  a  bandwidth  of 1.2 Mhz  

I  use  various software  packages, won't mention names,  some are experimental, some are  released software  and get different  results   with   each one.  All  give  the basic  pattern  but  my problem  is   signal  strength.  With my own gnu radio  software   I can add a block  to  multiply  my  raw data  and  get  a good  plot    with ezCon.xxx.py          others software   my raw  data i  very   weak.  

So in  summary without  going  to  details  against my own  advice have others  had problems  with very   small   signal  strength  from an  AirSpy  R2?

[b alex pettit jr]

.Hello Stephen,

We are using nearly the Same Hardware .  ( Disco Feed & AirSpy min )

as I recently purchased the Kraken Disco Feed & Nooelec Dish.   

I use SDR#>IFavg>HL3D>Rinearn Software &  in Less than a Month, here are the results 

==============================================================================

You have been developing custom code for a while ...

                     May I suggest

1) Begin with Software that is robust & verified, 

2) Optimize your Hardware

3) Then write/use Custom Code ..

1) Optimize the Hardware using existing Software ( SDR#  IFavg HL3D ) to provide Calibrated, Normalized, Scaled Data

    This will allow  you to Compare your Results with various Configurations & Other Systems ( mine for example )

2) The files created by the SIH sequence can be used as the basis for further processing with your Preferred Software for filtering/averaging/etc.

     Or  you can then implement Acquisition Code which emulates the SIH sequence .. as  you will have past data as a basis to eval its performance

========================================

               ==   Final Thoughts   ==

I am aware you prefer to develop your own Software Tools ...

But there is an Advantage to Standardization =

I just processed this data set from nearly  4 Years Ago

Alex Pettit

===================================================================================

On Friday, January 2, 2026 at 09:58:14 PM EST, 'Stephen Arbogast' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

................................

[b alex pettit jr]

                                            Even Easier :

Use SDR#'s spectrum display, point the dish @  Dec+20dg RA0615  or Dec+40 RA2030

& do whatever is required of the Hardware  config to get a H1pk <> ColdSky delta of ~ 1.4 dB  ( use the cursor readouts )

[Stephen Arbogast]

Yes,   I think  I have  discovered the problem and not  software it is my hardware .  I set  a  level of RF  output  at  1420.405 Mhz  using my   Tiny Spectrum Analyzer   about   10 meters  from  my dish  and wow  very low  level  compared to when  I did  this  a year ago.   I plan  to take my  Discovery Dish  HL feed apart and check  the  connection from  dipole  to  lna  input  and then the  coaxial  cable from lna  output  to  my AirSpy  SDR.    Yes,  lots of noise from  environment  with this  amateur  test setup.

[b alex pettit jr]

Excellent .

FYI : before every drift scan recording session, I verify the cold sky level =  what it has always been ( for that particular antenna )

& That is why I was recommending SDR#  for measuring performance in dB 

Alex

[Stephen Arbogast]

No   coax  is perfect   ....    I  replaced  the coax from  my  Discovery Dish  HL LNA  with  same thing  and ran a  24 hour  drift  scan.   Woh,  what a  difference. I  collected data  and  did  a  short   post processing.   Keep in  mind I am running  Ted Cline's pre-release test  ezRA  software  so  I can compare  it with  before replacing  the coax.   Here is  a result...   attached.    I am not  saying   that  Discovery Dish has bad   products.  My point  is to compare signal strength  with old  coax   I could  barely  see  the  Doppler  before.

[Stephen Arbogast]

oops..  wrong plot...   try  this one....

[Alex P]

I will again recommend  the SDR# > IFavg > HL3D > Rinearn processing sequence :

Of the various software tools available, it is one of the most basic but powerful.

It

1) takes the IFavg file set, analyzes the drift and slope of each file at ( user specified ) non H Line influenced frequencies and normalizes all files to that value and corrects the slope.

2) changes the amplitudes to ( calibrated & verified ) dB values referenced to the Cold_Sky level,

3) converts the set to a compatible graphics file for the Rinearn Graphics Plot package ..

Alex Pettit

=============================================================================================================