What is difference between receiver temperature, spillover temperature, ambient temperature, background temperature in radio telescopes?

In radio astronomy, several different “temperatures” are used to describe sources of noise. They are all expressed in Kelvin (K) because radio noise power is conveniently represented as an equivalent temperature.

The total system temperature is often written as:

T_{sys} = T_{rx} + T_{spill} + T_{sky}

where itself contains contributions from the cosmic background, atmosphere, Galactic emission, etc.

1. Receiver Temperature ()

This is the noise generated by the receiver electronics themselves.

Sources include:

• Low-noise amplifier (LNA)
• Feed losses
• Cables
• Mixers and filters

A perfect receiver would have K, but real systems add noise.

Typical values:

For your 1420 MHz hydrogen-line work with a good masthead LNA, might be around 30-50 K.

2. Spillover Temperature ()

This comes from parts of the antenna pattern that miss the sky and “see” warm objects such as:

• Ground
• Trees
• Buildings
• Feed support structures

The ground is roughly 290 K, so even a small amount of spillover can add significant noise.

For example:

• 95% of the beam sees cold sky (~10 K)
• 5% sees ground (~290 K)

Then spillover contributes roughly:

0.05 \times 290 \approx 15\ \mathrm{K}

Good radio telescope designs try to minimize spillover because it directly increases system temperature.

3. Ambient Temperature ()

This is the physical temperature of the local environment, usually around:

• 290 K (17°C)
• 300 K (27°C)

It is not normally a noise term by itself.

Instead it matters because:

• Warm cables generate thermal noise.
• Warm feed structures emit radio noise.
• Ground spillover sees objects at ambient temperature.

When performing calibration, an absorber at ambient temperature is often used as a “hot load.”

For example, a microwave absorber sitting in your observatory at 20°C represents approximately:

T_{amb} \approx 293\ \mathrm{K}

4. Background Temperature ()

This usually refers to radio noise arriving from the sky background.

The largest universal component is the:

Cosmic Microwave Background

which contributes:

T_{CMB} \approx 2.725\ \mathrm{K}

Additional background contributions may come from:

• Galactic synchrotron emission
• Atmospheric emission
• Unresolved radio sources

At 1420 MHz, looking at a quiet part of the sky, the background might be:

Putting it together for a small hydrogen-line telescope

A typical budget might be:

Thus the receiver itself often contributes more noise than the sky unless you use cryogenic cooling.

For a home-built 1420 MHz telescope such as those used at the Lichfield Radio Observatory, reducing spillover and using the lowest-noise possible LNA generally provides the greatest improvement in sensitivity.