Hydrogen radio recombination lines in Cygnus X

[Eduard Mol]

So here's another interesting challenge for those of us who have dishes of 3 metre size and upwards:

A few years ago I encountered a very interesting report by the Astopeiler group on the observation of radio recombination lines (RRLs) towards various star forming regions with their 25-metre dish (https://astropeiler.de/wp-content/uploads/2014/11/Astropeiler_Story_4.pdf).  At the end of the report it is stated that "observation with typical amateur instruments with 3m diameter may not be possible". 

Recently I found a publication which had a map of the H166a line (1424.7 MHz) in the Cygnus X star forming complex (https://articles.adsabs.harvard.edu//full/1984AJ.....89...95L/0000095.000.html). Like the continuum emission in that region, the RRL emission is very widespread in an area of ~5X5 degrees in the sky, with line intensities ranging from 60 to 220 millikelvins. The wide area of emission would partially compensate for the small collecting area of the 3 metre dish, because the emission would practically fill the 5 degree wide beam (HPBW). From the map I "guesstimated" an expected average line intensity in the order of 50- 100 millikelvins. This is indeed very weak, but comparable to M33 (~100 mK) which has been observed by multiple amateurs with 3 metre dishes including myself ,so I think it's worth a shot. 

Since last week I have been collecting driftscans of Cyg X at DEC +41 in an attempt to detect H167a at 1399.4 MHz (H166a overlapped with a few persistent RFI spikes). So far the combined continuum signal of Cyg A and Cyg X shows up very consistently, as expected. 

There is a tentative peak at the expected frequency for H167a but I need more driftscans and some extra analysis to reach a better SNR and test if this signal is really the RRL or just some persistent RFI. 

[Dimitry UA3AVR]

Nice idea, Eduard ... 0.1 K at the line peak can be discernable with rather long integration.

вторник, 24 июня 2025 г. в 13:20:55 UTC+3, Eduard Mol:

[Marcus D. Leech]

On 2025-06-26 12:43, Dimitry UA3AVR wrote:

Nice idea, Eduard ... 0.1 K at the line peak can be discernable with rather long integration.

We've tried this in the past, for M17 and H167a, and got mixed results.   H166a is a problem for us, because, like Eduard, we
  have local RFI (harmonic of the ethernet clock) that overlaps with the expected position of the line :( :(

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[Anthony]

Hi Eduard,

No I was planning to use both dishes independent of each other and leverage the Ettus USRP B210 dual receivers. Then utilize, two Python scripts to extrapolate the drift scan data from both dishes independently. Although I do have an extra B210, but it would not make sense to use it.

Running the workflow

1. Physically point & lock the both, 3-meter dishes at Cygnus X.

2. RDP into my Ubuntu PC that’s hooked to the Ettus USRP B210.

3. Launch python acquire_rrl.py.

4. When it finishes the 10 - 20 minute drift scan (or Ctrl-C), after review the, rrl_scans.csv file.

5. Run the analysis script (stack_rrl.py ) on the previously mentioned .csv file to subtract baselines, stack scans, and look for the Line strength (miliKelvin or mK) if I understood Astropeiler's paper correctly.

Hope you don't mind me adding, SARA on the thread, because most probably had your same thoughts about me using an interferometer for this project.

Thanks again, Eduard for the support and encouragement! I'll share my results soon. 

On Thu, Jun 26, 2025, 3:07 AM Eduard Mol <eddiem...@gmail.com> wrote:

Hi Anthony,

Good luck with your observations, I'd be very curious to see what results you get! 

Those two dishes must be one heck of a setup... a single 3 metre dish is already a bit unwieldy for me to maintain and operate. 

Are you planning to use the two dishes in interferometer mode or just as two separate dishes? I suspect that Cyg X is too large and spread out to detect with an interferometer setup. 

Best regards,

Eduard

On Thu, 26 Jun 2025 at 01:52, Anthony <itpart...@gmail.com> wrote:

Hi Eduard,

Thanks for posting the AstroPeiler RRL report and the Cygnus X map in the SARA chat—I actually read that a few years ago and it’s great to revisit. I’m going to give it a shot with my two 3-meter dishes, using a pair of simple Python scripts—one to record the spectra and another to stack and process the scans—outside of BAA Seminar.

Once I’ve got everything in place and collected some drift scans, I’ll follow up directly with my setup details and initial results.

Thanks again for the inspiration!

Best regards,
Anthony Fuller

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[Marcus D. Leech]

On 2025-06-26 13:22, Anthony wrote:

Hi Eduard,

No I was planning to use both dishes independent of each other and leverage the Ettus USRP B210 dual receivers. Then utilize, two Python scripts to extrapolate the drift scan data from both dishes independently. Although I do have an extra B210, but it would not make sense to use it.

Running the workflow

1. Physically point & lock the both, 3-meter dishes at Cygnus X.

2. RDP into my Ubuntu PC that’s hooked to the Ettus USRP B210.

3. Launch python acquire_rrl.py.

4. When it finishes the 10 - 20 minute drift scan (or Ctrl-C), after review the, rrl_scans.csv file.

5. Run the analysis script (stack_rrl.py ) on the previously mentioned .csv file to subtract baselines, stack scans, and look for the Line strength (miliKelvin or mK) if I understood Astropeiler's paper correctly.

Hope you don't mind me adding, SARA on the thread, because most probably had your same thoughts about me using an interferometer for this project.

Thanks again, Eduard for the support and encouragement! I'll share my results soon.

A few random thoughts:

The two dish approach theoretically improves sensitivity in this case by sqrt(2).

You're likely to need to "stack" many many observations to be able to "see" the line.   We did a 1 hour *tracked* measurement last year, and came up
  with data that could be described as ambiguous at best.

I'll note that Harvard had success with these in the 1970s (or was it 1980s?) with their 18m dish, and one could hunt down their paper on the topic.

Jodrell bank using the Lovell (70m) telescope had some very early measurements of many of the RRLs from many different sources.  Their Tsys was
  rather higher than what we're used to today, as I recall.

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[Anthony]

Thank you, Marcus! I appreciate your input and information very much! 

I figured that would be the case, stacking over many days, weeks or perhaps more. 

I am excited, Eduard brought this up. 🙂

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[Eduard Mol]

Hi Anthony, i can share my data collection/ processing workflow here as well. Hope you don’t mind the rather lengthy post :-)

The dish is pointed stationary at declination +41, so that the Cygnus X complex passes through the beam every night with the Earth’s rotation. 

I have set up the IF average plugin with SDR# to integrate and save a spectrum every minute. 

Before starting the data processing I first just plot the raw spectra to check for RFI or other problems. I also average all the spectral channels to get the continuum signal and plot this against time; this is mostly just check the system performance and pointing. 

A 30 minute block of spectra centered on the continuum peak (RA 20:30) is averaged as the “on target” result. Two additional 30 minute blocks of “off-target” spectra centered at 50 minutes before and after the continuum peak are averaged and divided from the “on-target” spectrum to remove the SDR bandpass response. If necessary, residual slope or curve is removed with a linear or second order fit. The frequency axis is of course converted to Vlsr. 

I expect that multiple transits need to be recorded and averaged to get good results.

In fact, the whole procedure described above is what Jason and I developed to record galaxy spectra. 

Some additional notes: 

It is very important that your system is stable; especially temperature changes will not only change the gain of the LNA and receiver but also slightly distort the bandpass response. Ideally one would place the whole receiver chain in a temperature controlled box but that can be quite impractical. I have now just wrapped the LNAs and even the filter in insulation material to atleast slow down and minimise temperature changes. The SDR is indoors, attached to a big slab of aluminium with thermal paste and cooled with an old PC fan. This seems to work OK so far. 

RFI is another potential issue: at my location daytime data are pretty much useless but at night it is reasonably quiet except for a persistent spike at 1400 MHz. 

By the way, do you have an estimate for your system temperature? Mine is around 100- 130K according to the Y- method and SNR method described in this Astropeiler document: 

https://astropeiler.de/wp-content/uploads/2017/12/Part2_The_3-Meter_Dish_Astropeiler_Stockert_Characterisation_and_Observations.pdf. In theory I should be able to get it below 100K but in practice this will be difficult with the feed supports and trees/ buildings nearby all acting as thermal noise sources..

I saw in your recent thread about your interferometer that you had difficulty with detecting weaker continuum sources, so this could be an issue. 

Best regards,

Eduard

Op do 26 jun 2025 om 19:34 schreef Anthony <itpart...@gmail.com>

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[Alex P]

Hello Eduard,

That looks like RG303 cable into the SDR .

Belden RG-303/U 

Frequency  Insertion Loss (Attenuation)

1000 MHz 15 dB/100ft

3000 MHz 28 dB/100ft

Times Microwave LMR240 

Frequency  Insertion Loss (Attenuation)

1000 MHz   8 dB/100ft

3000 MHz  14 dB/100ft

Cheers,

Alex Pettit

 

[Eduard Mol]

Hi Alex, 

Yep, that’s a lossy SMA cable ;-)

It’s only 50 centimeters though, the other 10 metres of cable from the shed to the scope is RG213.

Best regards, 

Eduard

Op vr 27 jun 2025 om 11:28 schreef 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Mike Otte]

Hi Eduard,

In your setup,  How many spectral channels in Ifavg are you using?

Thanks,

Mike

[Anthony]

Hi Eduard,

Alex P. asked me about this some time ago, which took me time to figure out how to do this since I am not using SDR#, but BAA Seminar. 

I’ve done testing and got two independent estimates of my system temperature:

---

1. Cygnus A–based calibration

• Cyg A flux: ~1 500 Jy at 1 423 MHz

• Measured peak correlation: ≃4 × 10⁻⁶ (from BAA Seminar’s Correlator)

• Dish effective area: Aₑ ≃ 0.6·π·1.5² ≃ 4.2 m²

• Radiometer relation:

  SEFD=ρpeakSCygA⟶Tsys=2kSEFDAe≈50K (ideal)

• Real-world additions (spillover, foliage, supports): +30 – 50 K
  → total 80 – 100 K

---

2. Informal Y-factor check in BAA Seminar

• Hot load (foam over feed): ⟨Sum⟩ₕₒₜ ≃ 0.40 (arb. units)

• Cold sky (blank zenith): ⟨Sum⟩𝚌ₒₗ𝑑 ≃ 0.10

• Y = 0.40/0.10 = 4

• Compute:

  Tsys=4−1300−4×10≈87K

---

Both methods converge near 90 K, so that’s what I’ll use for planning RRL observations. The Cyg A calibration is my primary reference, with the Y-factor run as a useful sanity check.

—Anthony

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[b alex pettit jr]

Hello Anthony,

I would suggest you verify the Calibration of the spectral amplitude scale by using a 'stable' noise source ( 50 Ohm Load )

and imposing a few attens in the SDR input and confirm an equal amplitude shift.

Alex P

HLine3D

[Eduard Mol]

Hi Mike, 

I’m using 512 channels, which gives me a resolution of 11.71 KHz. 

The RRL is much wider than that, so I can later reduce the resolution by averaging spectral channels together to improve the SNR. 

Op vr 27 jun 2025 om 14:32 schreef Mike Otte <mike....@gmail.com>

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[Anthony]

  Thanks again, Alex—I'll set this up this weekend and post the results!  

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[Anthony]

Hi Alex,

I have a plan to verify the linearity of BAA Seminar’s Total Power scale using my existing 60 ft of LMR-400 plus a short 1 ft  LMR jumper:

1. Baseline (60 ft cable only):

   • The dish feed is always attached through 60 ft of LMR-400 (≈3.6 dB loss).

   • I terminate the LNA input with a 50 Ω load and record the Sum channel for 60 s.

   • Then Call the average reading P₀.

2. Insert 1 ft jumper:

   • I add a 1 ft LMR400 jumper in series (now 61 ft total, ≈3.66 dB).

   • I'll record the Sum channel again for 60 s, that will be called → P₁.

3. Compute the incremental loss:

   ΔdB=10log10(P1/P0).

   A 1 ft LMR-400 section, I think, should introduce about 0.06 dB of additional loss. If my measured ΔdB matches ≃–0.06 dB, that confirms the Sum channel is linear in dB.

4. Optional double-check:

   • Loop the 1 ft jumper twice (62 ft total → ≃3.72 dB) and repeat. I am expecting to see ≃–0.12 dB, which is easier to resolve above noise.

This should prove to be a good test that 0.06 dB insertion produces a 0.06 dB drop in Sum, I should be able to convert BAA Seminars arbitrary‐unit readings into true decibels (and then kelvins) for both calibration and RRL sensitivity estimates.

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[b alex pettit jr]

I think attempting to discern a 0.06 dB change using  a random noise source is a 'bit'  optimistic.

This is a nice calibration set

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

[b alex pettit jr]

That photo was incomplete the set includes a -20 dB atten also

• A convenient, complete package of 6 different 50Ω SMA in-line attenuators
• 
  
• Includes values of 1dB, 2dB, 3dB, 6dB, 10dB and 20dB,
• 
  
•  accuracy of +- 0.1dB through to 3GHz

[Anthony]

Yes, I have those already...

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[Anthony]

We'll have to see,Alex. 😊

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[b alex pettit jr]

It would be more valid  to use 6 10 20 dB etc

[Anthony]

Ok, thought about it, I am convinced. I'll do it yourway, Alex. 😊

If each measured drop matches the pad value I’ll know BAA Seminar’s Sum scale is truly linear in dB. From there I can convert arb-units into kelvins confidently.

Thanks, this will simplify the calibration a lot!

On Fri, Jun 27, 2025 at 1:30 PM 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

It would be more valid  to use 6 10 20 dB etc

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[b alex pettit jr]

Insertion Loss of the two cable end N connectors  +  more for the adapter

The Type N connector is a threaded, weatherproof, medium-sized connector for durable applications that can easily handle frequencies up to 11 GHz. 

This type of connector follows MIL-STD-348 and is widely used in lower frequency microwave systems where ruggedness and low cost are needed. 

Higher quality antennas and Hotspot outdoor enclosures typically use the standard polarity N connector, 

giving the needed robustness to the link between antenna and gateway.

These connectors have the lowest insertion loss from the 3 types in this article.

 A good N-type connector in general will have less than 0.3 dB insertion loss. Some lesser brands could go up to 0.5dB.

As a rule of thumb when installing add 1dB loss to the link budget for every 2 connectors, for example on both ends of a cable.

[Marcus D. Leech]

On 2025-06-27 13:42, Anthony wrote:

Ok, thought about it, I am convinced. I'll do it yourway, Alex. 😊

If each measured drop matches the pad value I’ll know BAA Seminar’s Sum scale is truly linear in dB. From there I can convert arb-units into kelvins confidently.

Thanks, this will simplify the calibration a lot!

Excursions from linear aren't going to come from the application, per se, but the radio hardware.

The total-power calculation in baa_seminar use the standard way of extracting a power estimate from a complex base-band:

AVG(I*I + Q*Q)

There will inevitably be *slight* deviations from "perfect" because the band-edges don't have infinitely-steep roll-off, so the contributions
  from the band edges are less than in the center.  This should still produce a linear result, though, because that roll-off is the same
  regardless of power level.

Power-level changes near the very bottom of the range will not appear to be linear.  Because remember, you're measuring:

G(cal-noise+Tsys)

Where G is the gain.  But since it will be the same in both cases, it can safely be removed, leaving:

cal-noise + Tsys

Since you're effectively measuring two different values for "cal-noise", you're doing

(cal-noise1 + Tsys) / (cal-noise2 + Tsys)

You only get good values when the noise levels are considerably larger than Tsys.

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[Eduard Mol]

Hi all,

Here are my preliminary results from the Cygnus X H167a survey. So far this has been a very challenging project but I think I finally got some promising results. 

Below is the stacked spectrum of 8 driftscans- this is 30 minutes per driftscan so total accumulated integration time is 4 hours. The sharp dip at -100km/s is from an RFI spike at 1400MHz. 

Here is a version with the background curve removed with a second order fit and spectral resolution reduced from 12 kHz to 58 kHz. The conversion to Kelvin on the vertical scale is only to check whether the line intensity is in the right ballpark (it’s assuming the same conversion factor as at 1420 MHz but I have not independently verified if this is still accurate at 1399 MHz!)

In order to check whether this is not just some RFI I did the same analysis on another part of the sky well away from Cygnus X, centered at RA 18:50. As expected, no RRL-like signal is detected there. Of course this does not completely rule out RFI but atleast it seems less likely. 

Best regards, 

Eduard

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[Dimitry UA3AVR]

Well done, Eduard ... it seems, the strength of H167a line is close to H166a. Is it correct for other close lines (H168a, H165a, H164a ...)?

воскресенье, 29 июня 2025 г. в 20:20:51 UTC+3, Eduard Mol:

[Eduard Mol]

From my limited reading in the literature it seems indeed true that close RRL lines are indeed of similar line strength. You can also see this in the measurements from the Astropeiler survey I linked in my first post. I also encountered this study covering RRLs in the Orion nebula (https://www.aanda.org/articles/aa/abs/2019/06/aa34532-18/aa34532-18.html) There atleast the Hna RRL lines become stronger with lower n, so at higher frequencies. 

Best regards,

Eduard

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[Dimitry UA3AVR]

OK, Eduard, thanks ... GhatGPT tells the same, - temperatures or intensities of close lines are close.

понедельник, 30 июня 2025 г. в 22:55:49 UTC+3, Eduard Mol:

[Dimitry UA3AVR]

Here is H166a line from the brightest part of Cygnus X (DR21 star forming region). Observed with Green Bank 20 m dish (Skynet) today, integration time 200 s, average of 2 linear polarizations. May be the system temperature Tsys is somewhat overestimated, so, the brightness temperature Tb on the plot is higher than expected. 

The cost of the observation - 200 credits.

вторник, 1 июля 2025 г. в 11:28:44 UTC+3, Dimitry UA3AVR:

[Eduard Mol]

Great work! 

At first glance the line width and velocity seem similar to what I found. 

Op vr 4 jul 2025 om 23:37 schreef Dimitry UA3AVR <ua3avr...@gmail.com>

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[Adrian]

What a superb effort in detection of the RRL emissions with such a modest aperture. Congratulations, and well done.
Adrian