Observing Andromeda with SDR and Small Dishes

[shinichir...@gmail.com]

Has anyone ever successfully observed the 21 cm hydrogen line from the Andromeda Galaxy (M31) using SDR and a small-aperture radio telescope? 

I’ve seen plenty of examples of detecting HI from the Milky Way, 
but M31 seems much more challenging due to its distance. 
If anyone has tried it or knows of any reference cases, 
I’d love to hear about it!

[Marcus D. Leech]

I’d love to hear about it! --

M31 has a spectrum that overlaps with that of the Milky Way, so it can be challenging.  You also need, on a small dish, an extended, multi-day integration,
  which means recording data in VSLR-corrected form.

I believe that Jason Burnfield has done this successfully on his home-made dish.

We've done this more than once on our dish, but it's 12.8m, so M31 is fairly easy.

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[Dr. Rich Russel]

Here is the link to the playlist for observing galaxies.

Rich

Galaxy Observing

Galaxy Observing

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[Eduard Mol]

I have observed a M31 and a few other galaxies with my 3 metre dish.

You can find the observation reports in the February and June SARA journals. 

There is indeed some overlap between M31’s spectrum and local the hydrogen line, but only for the most redshifted part. 

The main challenge is that the hydrogen spectrum of M31 is both faint and extremely broad (~-50 to -600 km/s), so you need good sensitivity and nearly perfect removal of the system bandpass response. 

Best regards,

Eduard

Op di 7 okt 2025 om 00:19 schreef 'Dr. Rich Russel' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Eduard Mol]

Here is my M31 detection.

This is from a series of four driftscans I did last year. 

Op di 7 okt 2025 om 08:07 schreef Eduard Mol <eddiem...@gmail.com>

[shinichir...@gmail.com]

Dear Marcus,

Thank you for the information. 
Yes, detecting M31 should be easy with a 12.8 m dish. 😊

[shinichir...@gmail.com]

Dear Rich,

Thank you for the link -  I will watch it later!

[shinichir...@gmail.com]

Dear Eduard,

Thank you for the plots! This is exactly what I was hoping to see — the two peaks correspond to the approaching and receding sides of the galaxy, 
as shown in Figure 3 of Chemin, Laurent, Claude Carignan, and Tyler Foster, H I Kinematics and Dynamics of Messier 31, ApJ, 705.2 (2009): 1395.

https://iopscience.iop.org/article/10.1088/0004-637X/705/2/1395/meta

The H I integrated profile of M31 displays the double-peak structure typical of rotating disc galaxies, 
with peak intensities of approximately 100 Jy and intermediate values around 40–50 Jy between the peaks. 
The velocity span reaches roughly 600 km/s, necessitating a bandwidth of at least 3 MHz for SDR.

Under ideal conditions, sensitivity calculations based on drift-scan observations with a small-aperture telescope 
suggest that a 5-day integration using a parabolic antenna of approximately 1.8 m aperture would yield an RMS noise level of ~15 Jy. 
The double-peak region is expected to achieve a signal-to-noise ratio (S/N) greater than 6, 
while the central region should still exceed an S/N of 2.

Note: In drift-scan observations across the antenna’s 3 dB beamwidth,
an integration loss factor b (approximately 0.8 for a typical beam) is applied to account for the beam shape.

I was truly encouraged by your 4-day drift-scan results with the 3 m dish. 
With the 1.8 m dish I have access to, and using the Airspy Mini (effective bandwidth ~5.4 MHz), 
I think M31 detection might be feasible with around 10 days of drift-scan observation—including some contingency time.

You've really inspired me to try it when I get the chance. 
Thanks a lot for sharing all this info!

S. Asayama

[kb3puw]

Eduard - If I am doing the calculations correctly, it seems the Nooelec SDR with it's 2.4MHz bandwidth limit only has a range of +/-300 KM/sec.  This means even if it were possible to detect M31 the spectra would likely be chopped off.  Do I have this right?  If so, I assume you are using an SDR with wider bandwidth - is that correct?

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[Eduard Mol]

Hi Paul

Eduard - If I am doing the calculations correctly, it seems the Nooelec SDR with it's 2.4MHz bandwidth limit only has a range of +/-300 KM/sec.  This means even if it were possible to detect M31 the spectra would likely be chopped off.  Do I have this right?  If so, I assume you are using an SDR with wider bandwidth - is that correct?

I am indeed using a different SDR- an airspy mini with 6 MHz bandwidth. 

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[kb3puw]

Does this airspy work with SDR#/IFaverage on Windows (10, 11)?

[kb3puw]

Also is HackRF One a good choice that will work with SDR#/IFAverage for doing these wider bandwidth measurements?

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[Eduard Mol]

Dear dr. Asayama, 

Thanks, I would really like to see the results of this project! 

As far as I am aware there are no amateur detections of M31 with a dish smaller than 2m. Maybe Job Geheniau detected it with his 1.8m but I am not sure. 

However, given the results Jason and I were able to get with a handful of driftscans with our 2.5 and 3 metre dishes I expect that it should be possible, and your calculations seem to support that. 

The main challenge is bandpass correction. The M31 HI profile is very broad and weak. Over the course of the driftscan the bandpass response of the system changes slightly due to things like changes in temperature. The french ham radio operator JJ Maintoux really pioneered methods for extragalactic hydrogen observation with his 3-metre dish, but sadly he passed away some time ago and his articles are no longer online. It was his work that inspired me and Jason to try detecting M31 and other nearby galaxies as well. We more or less independently developed a method for bandpass correction with driftscans, by averaging two sets of spectra recorded about 20 minutes before and after the transit and subtracting this from the on-target spectrum. This way you compensate at least to some extent for changes in gain and temperature which may have occurred during the driftscan. I have also placed my SDR on a large block of aluminium to keep its temperature a bit more stable. 

By the way, it was nice to meet you in person at EUCARA last month ;-)

Best regards, 

Eduard

Op di 7 okt 2025 om 09:40 schreef shinichir...@gmail.com <shinichir...@gmail.com>

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[Eduard Mol]

Hi Paul,

Yes, the airspy works fine with IFaverage- in fact that’s exactly what I am using to record my spectra. Nit sure about the HavkRF though, I I never used it- maybe others have dome experience with this unit?

Op di 7 okt 2025 om 19:48 schreef kb3puw <kb3...@gmail.com>

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[kb3puw]

Thanks for the info Eduard - I am ordering the Airspy R2 which should be better for me because it has a USB wire instead of hanging off the PC.   Hopefully it is as good as the mini.  I suppose I will need to externally power the Nooelec LNA?  This device does not supply 5V through the coax?

[Jason Burnfield]

Even though it is lower amplitude than m31, I found that m33 was easier to detect because of its narrower spectrum (due to it's more "face on" orientation). My initial detection with my homemade setup is actually tattooed on my arm!

[Jason Burnfield]

https://www.radio-astronomy.org/pdf/hydrogen%20line%20galaxy%20observation%20tutorial.pdf

[shinichir...@gmail.com]

Dear Jason,

Thank you for the presentation — your handmade telescope’s successful detection of M33 is remarkable!

On Tuesday, October 7, 2025 at 8:06:29 PM UTC+1 Jason Burnfield wrote:

https://www.radio-astronomy.org/pdf/hydrogen%20line%20galaxy%20observation%20tutorial.pdf

[Captain Anne Flint]

Hi Jason, Thanks for the link - Wende 

On Oct 7, 2025, at 2:16 PM, shinichir...@gmail.com <shinichir...@gmail.com> wrote:

Dear Jason,

Thank you for the presentation — your handmade telescope’s successful detection of M33 is remarkable!

<Screenshot 2025-10-07 at 20.13.36.png>

On Tuesday, October 7, 2025 at 8:06:29 PM UTC+1 Jason Burnfield wrote:

https://www.radio-astronomy.org/pdf/hydrogen%20line%20galaxy%20observation%20tutorial.pdf

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<Screenshot 2025-10-07 at 20.13.36.png>

[Stephen Arbogast]

I am using an AirSpy R2  and  yes  it  does support  bias tee...   

[kb3puw]

Thanks Steve - I have one on order - will see how it goes.

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