There are several maser transitions accessible to amateur radio astronomers, although 22.235 GHz water (H₂O) is by far the most popular because the sources are strong and the equipment is relatively achievable.
The main amateur-accessible maser frequencies are:
| Maser molecule | Frequency | Wavelength | Comments |
|---|---|---|---|
| Water (H₂O) | 22.23508 GHz | 1.35 cm | The strongest and most commonly observed amateur masers. Star-forming regions, evolved stars, W49, W3, Orion KL, etc. |
| Methanol (CH₃OH) | 6.66852 GHz | 4.5 cm | Very strong Class II methanol masers. Excellent amateur target. Associated with high-mass star formation. |
| Methanol (CH₃OH) | 12.1786 GHz | 2.46 cm | Another strong Class II transition, but equipment becomes more challenging. |
| Hydroxyl (OH) | 1.612 GHz | 18.6 cm | Strong OH masers, especially evolved stars (OH/IR stars). Easy frequency for SDRs. |
| Hydroxyl (OH) | 1.665/1.667 GHz | 18 cm | The famous OH main lines. Strong in star-forming regions and late-type stars. |
| Silicon monoxide (SiO) | 43.122 GHz | 7 mm | Very strong in evolved stars (Mira variables), but much harder technically. |
| Silicon monoxide (SiO) | 86.243 GHz | 3.5 mm | Possible with specialist equipment, beyond most amateurs. |
Best targets for amateurs
1. 22 GHz Water
Probably the best first maser project:
- W49
- W3(OH)
- Orion KL
- Sgr B2
- W51
- Cepheus A
- NGC 7538
A 1–2 m dish can detect the strongest sources with a good LNA and SDR.
2. 6.7 GHz Methanol
This is arguably the second-best amateur maser band.
Advantages:
- Lower frequency than 22 GHz (easier feeds and lower losses)
- Many very strong sources
- Commercial LNAs and filters are available
Examples:
- G9.62+0.19
- W3(OH)
- G35.20-0.74
- Cepheus A
A small dish can work well because methanol masers can be very bright.
3. 1.6 GHz OH
This is attractive because:
- Your existing hydrogen-line experience is relevant
- SDRs cover this frequency easily
- Coax losses are much lower
- Filters and LNAs are inexpensive
Targets:
- OH/IR stars such as:
- OH 26.5+0.6
- NML Cyg
- VX Sgr
The challenge is that many OH masers are weaker than the famous water masers.
For your Lichfield Radio Observatory setup, I would rank them:
- 22.235 GHz H₂O – most exciting, but technically demanding
- 6.668 GHz CH₃OH – probably the easiest “next step” after hydrogen line
- 1.667 GHz OH – easiest hardware-wise and fits well with SDR techniques
- 43 GHz SiO – a future advanced project
A particularly interesting progression would be:
- keep your 1.5–2 m dish for 22 GHz water
- build a separate 6.7 GHz methanol feed/LNA chain
- use the same SDR backend and spectroscopy software
The observing techniques are very similar to your hydrogen-line work: stable frequency reference, narrowband FFT spectroscopy, calibration, and Doppler correction.
The required bandwidth depends on whether you want to observe one known maser source or any bright maser anywhere in the Galaxy. The numbers below are for a general-purpose amateur survey receiver, allowing for Galactic velocities plus the Earth’s orbital Doppler shift.
The conversion is:
Frequency shift = observing frequency × velocity / speed of light
so the higher the frequency, the more MHz of bandwidth are needed for the same velocity range.
Approximate bandwidth required for Galactic maser surveys
| Maser | Frequency | 1 km/s equals | Typical Galactic velocity coverage | Practical filter bandwidth |
|---|---|---|---|---|
| OH | 1.612 GHz | 5.4 kHz | ~400 km/s | 2-3 MHz |
| OH | 1.665/1.667 GHz | 5.6 kHz | ~400 km/s | 2-3 MHz |
| Methanol | 6.668 GHz | 22.2 kHz | ~400 km/s | 10-15 MHz |
| Methanol | 12.179 GHz | 40.6 kHz | ~400 km/s | 20-25 MHz |
| Water | 22.235 GHz | 74.2 kHz | ~400 km/s | 30-40 MHz |
| SiO | 43.122 GHz | 143.8 kHz | ~400 km/s | 60-70 MHz |
The velocity range includes roughly:
-200 km/s to +200 km/s LSR
+
Earth orbital correction of +/-30 km/s
For individual famous sources
If you are targeting a known source, the requirements are much smaller.
Examples:
W49 water maser
velocity range: about -20 to +30 km/s
bandwidth needed:
~3.7 MHz
plus Doppler allowance
≈ 8-10 MHz
W3 IRS5 water maser
velocity range: -70 to -20 km/s
≈ 3.7 MHz
plus Doppler allowance
≈ 8 MHz
6.7 GHz methanol sources Most maser features are within a few tens of km/s, so:
5 MHz can be enough for a known source
10-15 MHz is comfortable
Practical recommendations for amateur equipment
If buying cavity filters:
- 1.6 GHz OH: 5 MHz filter is ample
- 6.7 GHz methanol: 10 MHz filter is a good choice
- 12 GHz methanol: 20 MHz filter
- 22 GHz water: 30-40 MHz if you want a MaserDB-style survey capability
- 43 GHz SiO: usually no cavity filter; use a wideband front end and SDR filtering
For your planned 22 GHz water maser system, I would lean towards a 30 MHz filter rather than 20 MHz if you want to explore many MaserDB targets rather than just W49/W3/Orion. The SDR can then provide the fine spectral resolution (for example, kHz or sub-kHz channels).