Hydrogen line driftscan mapping

[Eduard Mol]

This week I assembled my 3 metre dish again and started doing driftscans at 1420 MHz. Everyday I point the dish 2.5 degrees higher. It’s going to take a few weeks to map the entire northern sky, but it has already been a lot of fun to see the first driftscan results come in. The plots below are from the first driftscan at 0 degrees declination. 

It’s been 8 years since my first HI detection, and seeing the “S-curve” on a driftscan plot still makes me excited!

[Alex P]

Excellent Quality&Resolution

[b alex pettit jr]

Eduard,

FFT size ?

Integration time per spectra ?

What software did you use for the blue 3D plot ?

>>  Your Data and this Plot is the best H1 presentation I've seen .

NICE !

Thanks,

Alex

[Eduard Mol]

Hi Alex, here are some extra details:

The airspy mini has a bandwidth of 6 MHz. I am recording spectra with 1024 FFT points, so 5.86 KHz/bin.

Integration time is 146 seconds/ spectrum. 

The blue plot is from my own custom python script, I am using Matplotlib for all the plotting. In this case, I added a small constant multiplied by the spectrum number to each spectrum to create the 3d- like effect. 

The waterfall plot is from the pyplot.contourf() function.

If you are interested I can send you the jupyter notebook (however, the code is still "under construction" at the moment and missing things like plotting against RA instead of time and Vlsr conversion). 

Best regards,

Eduard

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[b alex pettit jr]

Hi Eduard,

Yes, it obvious you are using an FFT above 512 ..

( and, if you are not going beyond the MW, 3MHz would be fine .. I use 1.2 )

Here's an idea

Your beamwidth is 4 degrees .. you 'should' be able to resolve amplitude changes in periods of 1/10th = 0.4 degrees.

At 240 secs per degree, you might try 30 or 60 seconds seconds ..

Your plots have a very low, flat noise floor ; increasing the angular resolution would be an interesting test.

Thanks for the offer on the *.py code, but I'll stay with Rinearn or Matlab.

Regards,

Alex

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

[Stephen Arbogast]

Hi Eduard,

Very nice  plots!  I like  Jupyter Notebooks, python ...   Please send your notebook  to my  email address.

Thanks,

Stephen

[Stephen Arbogast]

On  second  thought .....   I  think  others  would  like to see your  code.  If  you don't mind   please  share  here   with all of us.   I have   done   Computational  Physics  using  Jupyter Notebooks.  They  are  very  good for  collaboration.

Stephen

[Stephen Arbogast]

Here   are  two  examples. Computational  Physics  and  Digital  Signal  Processing...

Google  Computational  Physics  by  Stephen  Koonin.  It  is  a  classic  from  1985  in  Fortran   but  easy  to  translate into  Python.

Digital  Signal  Processing...    https://github.com/spatialaudio/digital-signal-processing-lecture

Stephen

[Eduard Mol]

Hi Alex, 

With my ~5 degree half power beam width, I am already oversampling the beam at 0.63 degrees/ spectrum. For mapping I don't see much benefit in increasing the resolution even further. Yes there are small changes in the appearance of the spectrum even with a 30 sec integration time, but translating this into a brightness distribution on the sky at a much higher resolution than the beam width would maybe not be possible. There's a reason why professional radio astronomers use aperture synthesis after all...

I know planetary imagers use techniques like wavelet sharpening and drizzle to go a bit beyond the resolution that the seeing and their equipment can offer in real-time, however I don't know much about the details behind these image processing techniques and how far one can stretch this. Has anyone here tried any of these techniques from optical astronomy on radio astronomy data?

As for the 6 MHz bandwidth: I can record at 1/2 the bandwidth, but IMO 5.8 KHz is a good enough resolution for HI work. Furthermore, I am expecting to get some data on HVCs and extragalactic sources as well, and the extra bandwidth will help with that. It's also nice to have some extra bandwidth for the continuum outside the hydrogen line, so far I have been able to detect continuum radiation from the Galactic plane near Scutum and Aquila for example. 

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[Eduard Mol]

Hi Stephen, 

I do not want to share the notebook publicly (at least not yet), because it is still unfinished and missing lots of features as mentioned in an earlier post. 

Also my notebooks are often very specific to one type of observation or project, so maybe not very useful for others. 

(especially now that anyone can generate code that's maybe even better than mine in under a minute using LLM coding models...)

I don't have your email address btw. if you have mine, can you please send me an email so I can send you the notebook?

Best regards, 

Eduard

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[b alex pettit jr]

Hello Eduard,

Okay. Looks like you have gone in detail through all the math :)

It appeared on your plots that the brighter of the plots had high freq/vel resolution but that the

angular /time axis showed the regions being compressed... I am not too familiar with aperture synthesis

as applied to radio astronomy, but yes, I can understand how that technique might be done.

It would be interesting to try an "optical unsharp mask" procedure on an RA data set !

Thanks,

Alex

Res with my 1.2m  @ 1.25deg/frame  =  1/10 Beam-Width

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

On Sunday, April 19, 2026 at 04:52:37 AM EDT, Eduard Mol <eddiem...@gmail.com> wrote:

Hi Alex, 

With my ~5 degree half power beam width, I am already oversampling the beam at 0.63 degrees/ spectrum. For mapping I don't see much benefit in increasing the resolution even further. Yes there are small changes in the appearance of the spectrum even with a 30 sec integration time, but translating this into a brightness distribution on the sky at a much higher resolution than the beam width would maybe not be possible. There's a reason why professional radio astronomers use aperture synthesis after all...

I know planetary imagers use techniques like wavelet sharpening and drizzle to go a bit beyond the resolution that the seeing and their equipment can offer in real-time, however I don't know much about the details behind these image processing techniques and how far one can stretch this. Has anyone here tried any of these techniques from optical astronomy on radio astronomy data?

As for the 6 MHz bandwidth: I can record at 1/2 the bandwidth, but IMO 5.8 KHz is a good enough resolution for HI work. Furthermore, I am expecting to get some data on HVCs and extragalactic sources as well, and the extra bandwidth will help with that. It's also nice to have some extra bandwidth for the continuum outside the hydrogen line, so far I have been able to detect continuum radiation from the Galactic plane near Scutum and Aquila for example. 

om.

[Eduard Mol]

Hi all, here a short progress update on the map: 

I have now driftscans between 0 and 25 degrees, and the Milky Way is now clearly visible on my preliminary map. The map below shows the intensity of the highest peak in each spectrum.

The dark spot at around 2 hours right ascension and between 10- 15 degrees declination is where the Sun was. 

I also calculated the integrated intensity over the full HI spectral line profile (after removing the bandpass and continuum background). In this map the HI emission appears less patchy and more confined to the Galactic plane:

Op zo 19 apr 2026 om 12:15 schreef 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[wallacefj]

Wonderful graphics! Thanks for sharing!

Sent from my Galaxy

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[b alex pettit jr]

Hello Eduard,

Very fine details in those plots.

In what declination increments are you shifting the antenna ?

Alex

[Eduard Mol]

Hi Alex,

Every day I point the dish 2.5 degrees (about half a beamwidth) higher.

Op di 28 apr 2026 om 23:43 schreef 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Eduard Mol]

Hi all, 

Im still collecting data, currently at +57.5 degrees declination.

In the meantime I made an animation with the first driftscan dataset at 0 degrees declination: https://youtu.be/GbvQeYiVVLc?si=q6rCuI0fOtNkPnTT

[Cathal O'Donghaile]

Really love this animation! Eduard what did you use to collect the data before feeding it into MatplotLib?

[Eduard Mol]

Hi Cathal, I’m using the IFaverage plugin for data collection.

Op zo 24 mei 2026 om 10:32 schreef Cathal O'Donghaile <cathalod...@gmail.com>

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[Eduard Mol]

Hi all, 

It’s been a while since the last time I posted sn update on this project, so here’s an update.

Last month I completed the last driftscans. It took me quite a bit of time to improve my script and process all the data. After I introduced Astropy for VLSR correction, everything broke… 

Anyway, I managed to fix it myself and have learned a lot along the way. 

Now I have a few preliminary maps. There are still some things to improve, for example there is still some uncertainty in the position of the northern scans. Also there is a problem with the driftscans near the northern celestial pole. But so far this dataset is looking very promising. 

Below are the maps for the integrated intensity and the highest peak of the hydrogen line. For the latter I experimented with different Matplotlib colormap (“magma”), I think it shows the weaker structures a bit better. 

I also made a version in polar coordinates to show an “all-sky view” of the northern sky.

Now that all the spectra are VLSR corrected, next thing I want to try is making maps for different Doppler shifts and possibly combining them into a map where the Doppler shift is shown as a color. 

Op ma 25 mei 2026 om 10:52 schreef Eduard Mol <eddiem...@gmail.com>

[b alex pettit jr]

Hello Eduard,

Excellent Results.

This quite closely matches the CCERA Chart I use as a reference :

Regards,

Alex 

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

On Sunday, July 19, 2026 at 05:04:42 AM EDT, Eduard Mol <eddiem...@gmail.com> wrote:

Hi all, 

It’s been a while since the last time I posted sn update on this project, so here’s an update.

Last month I completed the last driftscans. It took me quite a bit of time to improve my script and process all the data. After I introduced Astropy for VLSR correction, everything broke… 

Anyway, I managed to fix it myself and have learned a lot along the way. 

Now I have a few preliminary maps. There are still some things to improve, for example there is still some uncertainty in the position of the northern scans. Also there is a problem with the driftscans near the northern celestial pole. But so far this dataset is looking very promising. 

Below are the maps for the integrated intensity and the highest peak of the hydrogen line. For the latter I experimented with different Matplotlib colormap (“magma”), I think it shows the weaker structures a bit better. 

I also made a version in polar coordinates to show an “all-sky view” of the northern sky.

[b alex pettit jr]

Hello Eduard,

FYI the V5 release of IFavg has many improvements over V2.7

and is compatible with the most recent version of SDR# 

Menu : re-organization of functions

Spectral Window : expanded Y axis and XY cursor readout

Averaging : no 'last frame' carryover and faster convergence

Alex

[Dykes Cupstid]

Alex,

Please provide download link and any installation instructions that I need to be aware of. Currently  running v2.7. Thanks.

---

From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Sunday, July 19, 2026 7:16 AM
To: sara...@googlegroups.com <sara...@googlegroups.com>

Subject: Re: [SARA] Re: Hydrogen line driftscan mapping

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[b alex pettit jr]

Here's the info

( you can keep both V2.7 and V5. installed in different sub-directories if you wish )   

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step2A_Software_SDR%231921_IFavgV5.docx

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step2B_Softwware_TechInfo_IFavgV5.01_01.pdf

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step2B_Software_IFavgV5_QGuide.pdf

Alex

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

[Dykes Cupstid]

Thanks very much Alex!

---

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Sent: Sunday, July 19, 2026 10:16 AM
To: 'Dykes Cupstid' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

Subject: Re: [SARA] Re: Hydrogen line driftscan mapping

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[b alex pettit jr]

Hi Dykes,

Give me some feedback on the new rev & I'll pass it on to Dr K.

There is a Major improvement in the averaging algorithm, and the stats info on the spectral plot is useful.

V5 is the result of ~ 6 months of testing & tweaks..

Alex

[Eduard Mol]

Hi Alex, thanks for the info about the new version of IFaverage.

The data collection for this mapping project is already finished, however it may be interesting to try it out during a later project.

Best regards, 

Eduard

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[Eduard Mol]

Hi all, 

Today I was exploring the driftscan dataset a bit further, particularly what is happening outside of the hydrogen line. 

If you look for example at a waterfall plot of the raw spectra from the first driftscan at DEC = 0, you can see a horizontal bright band when the inner galactic plane crossed the beam of the dish. This means the entire background of the spectrum was raised, possibly due to broadband galactic background radiation. There was also some variation during daytime, but this could be due to RFI as well.

Next I averaged all the channels between 1418.5 - 1419.7 and 1421.3 - 1422.5 MHz, outside the frequency range of the hydrogen line. I did this for all driftscans. 

Plotting this on a map, there is not much to see initially... basically there's only the Sun visible in the lower left corner of the map, which is way brighter than any other continuum source in the sky. 

However, if we zoom in on the part between 270 and 330 degrees declination, there is a subtle but persistent hump in the continuum data which appears to follow the galactic plane...

Unfortunately there is also RFI, and general background drift due to temperature changes. Therefore I fitted a line through each continuum trace to atleast remove the overall slope and normalise them to the same level.

Below is a map of the normalised continuum between 270 and 330 degrees declination. The galactic plane is recognisable, even despite some RFI (for example the driftscan at 30 degrees declination was affected by RFI between 280 and 300 degrees RA). Especially the galactic plane at lower declinations and the Cygnus A/ X region at ~ 40 degrees DEC stand out.

[Stephen Arbogast]

Ah Ha... I've been wondering about what I am seeing in one of my recent drift scan at Dec = about 37 degrees using ezRA.  (pic on left)

Wondering if your explanation applies to my observation or I am seeing something else?

Stephen

[Eduard Mol]

Hi Stephen, 

This could indeed be Galactic background, but it can also be gain variations of the LNA and/or SDR, or broadband RFI. Difficult to say from a single driftscan. 

37 degrees DEC is very close to Cygnus A and Cygnus X at DEC 40. Both are strong continuum sources and they are only a few degrees apart, so we amateurs see them combined with our several degrees wide beams. 

What you could do is average the channels outside the hydrogen line for each spectrum and plot that against RA. If there are sudden jumps or spikes, it is terrestrial interference. If you see a smooth hump, it could be from the Milky Way or another continuum source passing through the beam. 

Another good way to confirm the detection is by repeating the driftscan, if you detect the same hump at the same RA over and over again (or 4 minutes per day earlier if you’re plotting against time), then its a very solid detection.

Op ma 20 jul 2026 om 23:17 schreef 'Stephen Arbogast' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Adrian]

Eduard,

 Those are fantastic observatiions outside the usual hydrogen line regular mapping scans.

Finally someone is paying attention.

Adrian

[Stephen Arbogast]

Eduard,

Thanks for your comments.  I have not seen the same vertical lines at galactic crossing from other scans so I think it is broadband RFI.

Stephen 

[Stephen Arbogast]

These results are more typical for my set up:

Discovery Dish/Feed-LNA->AirSpy R-2->SDRAngel (10 MegSamples/sec)->ezColAngel260514a.py->EzCon250711a.py

Stephen

[Alex P]

Stephen, It would be interesting to see how a Circular Patch Feed Disk Yagi performs in your environment 

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step1_Antenna_YagiSE_EnhancedRel_Fab04.pdf

Alex Pettit

[Stephen Arbogast]

Alex ,

I  am retired now  and  I don't  have a  machine shop  to build  one of your Circular Patch Feed Disk Yagis.   So I must  rely on  others  to build my antennas.  

I love  Physics/Math/EE......  I have   been  concentrating on improving  my  data  analysis skills.  Here is  a very  good course  on  Digital  Signal  Processing   In  Jupyter  Lab...    https://github.com/spatialaudio/digital-signal-processing-lecture   See   the  parts about  Stochastic  Processes.

Many  Thanks  to  you,  Ted Cline,  Eduard Mol  and others for helping  me out.

One  must remember  that the  Discovery Dish with  feed/lna  is  a minimal  system  to be  minimal  cost  and introductory,  not  advanced  amateur stuff.

And  to  whom it may  concern   I am  not receiving  email  from  from  the  forum.  Something  broken?

Stephen

[Stephen Arbogast]

In the 1960's a  local machine shop  and  an  old  welder  helped  me  build  my  ham  radio  antennas..   I  guess I am supposed  to buy  a 3-d  printer thee days...

[Stephen Arbogast]

Azimuth/Elevation  drives  for Discovery Dish   should start  shipping  in  about a  month  from  now..   I think..  I am  looking forward  to   using  SDRAngel  for  cold/hot    calibration......   

[Alex P]

Stephen,

>>>  Read the Document Attached <<<:       Tin Snips, Hand Saw & a Drill are all that is required 

The "SE" Student Edition is intended to be able to be fabricated by High School Students.

Alex  Pettit

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step1_Antenna_YagiSE_EnhancedRel_Fab04.pdf

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

On Wednesday, July 22, 2026 at 8:37:19 PM UTC-6 Stephen Arbogast wrote:

Alex ,

I  am retired now  and  I don't  have a  machine shop  to build  one of your Circular Patch Feed Disk Yagis.   So I must  rely on  others  to build my antennas.  

........

One  must remember  that the  Discovery Dish with  feed/lna  is  a minimal  system  to be  minimal  cost  and introductory,  not  advanced  amateur stuff.

And  to  whom it may  concern   I am  not receiving  email  from  from  the  forum.  Something  broken?

Stephen

[tedcl...@gmail.com]

Hi Stephen,

The smoothest nice curve on your posted   ezCon191   plot, is the top AntBTVTMax signal.

But you posted   ezSky200   using the default AntBTVT signal (column 18).

Try plotting   ezSky200   using the nicer AntBTVTMax signal ?,

      py  ..\ezRA\ezSky.py  .  -ezSkyInput  19

AntBTVTMax is in column 19 of your   .ezb   file.

ezSky can plot any of the 0-19 columns, listed at the top of   .ezb   files,

      ezbMenu: TimeUtcMjd  RaH  DecDeg  GLatDeg  GLonDeg  VLSR  Count  AzimuthDeg  ElevationDeg  AntXTVTCmDop    AntAvg  AntMax    RefAvg  RefMax    AntBAvg  AntBMax    AntRBAvg  AntRBMax    AntRBTVTAvg  AntRBTVTMax

AntBTVTMax and AntRBTVTMax signals seem useful with weak reception.

Helpful ?

---

Ted Cline     N0RQV
TedClineGit at gmail.com

[Stephen Arbogast]

Oh, ok here it is...

[Stephen Arbogast]

A   question   for Ted  Cline....   In your  documentation  you   refer  to  raw filtering   and ant  filtering...    I am  confused..  please help.

Is   one  sample  filtering and the other frequency bin  filtering?  I am lazy,  easier  to ask  than go  through the code

Stephen

[b alex pettit jr]

Stephen,

Create Test Cases : add noise / RFI to real data sets and test the software.

Neither a narrative nor reviewing lines of code will reveal the functionality as well as will test data .

Alex Pettit

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

[Stephen Arbogast]

Alex,

I think  you and  I  have different  approaches to this  stuff.  I am  Physics  at  heart.  I think  you  are more   practical  than  I am . I  am  not  the   Engineering type.   I need  to  explore stuff.   I love   Mathematics   and Physics.

I  have no plans to build  another  antenna...     I love  ideas...    they  are cheap to  build....

Stephen

[Stephen Arbogast]

Completed   my   deck  refinishing .....  my    antennas  are  back in  operation.....

Stephen

[tedcl...@gmail.com]

There are some ezCon data flow diagrams in the downloaded

      doc/ezRA_22_ezCon.pdf

But here is perhaps a simpler diagram,

"In the beginning" there are Raw samples.

Some ezCon filters work on all these Raw samples.

Then ezCon has many ways to sort those Raw samples into Ant samples (with the hydrogen signal from the Sky)

and Reference samples.

There are principally 2 methods to highlight the Galactic hydrogen signal, AntB and AntRB.

Then pick one analysis with   -ezConAntXInput   and post process your choice.

Trim off messy frequencies, adjust with VLSR (to remove Earth and Sun velocities), and maybe again Trim off messy frequencies.

Also maybe pluck out some messy time samples (RFI).

All these are optional (with defaults).

Oh, and optionally create LOTS of plots.

ezCon has options to select spectra from various RaDec and Galactic sky locations, collected in various binary   .npz   files, to flow into ezGal and ezGLon.

My latest ezRA program data flow diagram,

Run a program with   -h    

      py   ezCon.py   -h

to see the helpscreen and all the options.

Choices that modify the frequencies have   freq    in the option name.

---

Ted Cline     N0RQV
TedClineGit at gmail.com

On Saturday, July 25, 2026 at 6:15:09 PM UTC-6 Stephen Arbogast wrote:

A   question   for Ted  Cline....   In your  documentation  you   refer  to  raw filtering   and ant  filtering...    I am  confused..  please help.

Is   one  sample  filtering and the other frequency bin  filtering?  I am lazy,  easier  to ask  than go  through the code

Stephen

...

 

[Stephen Arbogast]

Got it, this makes sense.

 Thanks