# Do radio telescopes see other stars better at night?

As you know, we don’t see stars with the naked eye during the day because of the visible glare of our Sun.

As I understand our Sun emits radio frequency waves, too. Is there a radio ‘glare’ from the Sun such that radio telescopes can better focus on other stars during the night?

The Sun doesn't substantially impact radio observations during the day, because radio telescopes operate at long wavelengths. In general, light at longer wavelengths scatters less than light at shorter wavelengths, and so visible light from the Sun scatters much more than radio waves from the Sun.$$^{\dagger}$$ The former effectively fills the daytime sky, while the latter does not, leaving the radio sky dark even during the day. Couple this with the fact that the Sun - though bright in radio waves - is not extremely bright compared to other radio sources,$$^{\ddagger}$$ and you find that daytime radio observing isn't significantly different than nighttime observing.

Strictly speaking, you could argue that there may be indirect effects from the Sun. Water vapor is an important source of opacity, causing several extremely opaque bands, and atmospheric emission can add noise to the system temperature of your instrument, causing complications to observations. Humid summer days, for example, can be problematic for observing astronomical water vapor emission. As the Sun does impact weather and the climate, there are minute differences between day and night, but I don't believe those are significant when compared to how much the Sun affects optical observers.

As a final note: radio telescopes don't commonly observe stars, which aren't usually strong radio sources. There are occasionally highly-publicized stellar bursts or flares - BLC1 is an example, assuming it's natural - and star-exoplanet interactions can lead to radio emission, as in the case GJ 1151. But radio astronomers rarely look for radio emissions from stars, in comparison to other objects like radio galaxies, pulsars, and clouds of neutral hydrogen.

$$^{\dagger}$$The same is true for light traveling through interstellar space, which is why infrared and radio emission can travel through dust while visible and ultraviolet light can't; you need wavelengths much larger than the sizes of dust grains for minimal extinction.

$$^{\ddagger}$$A commonly quoted value is that around $$\sim1\text{ GHz}$$, the Sun has a flux density of $$S_{\nu}\sim10^6\text{ Jy}$$ when it's quiet, with an increase of 1-2 orders of magnitude during active periods. That's only a couple of orders of magnitude brighter than the next brightest sources in the sky, and the difference shrinks substantially in the $$\sim100\text{ MHz}$$ regime.

• The index of refraction and thickness of the ionosphere has a impaction on direction and phase of the incoming wavefront, especially at lower frequencies, and this is certainly modulated by exposure to the Sun, but as you discuss it doesn't count as "glare" per se. How large does refraction become in radioastronomy?
– uhoh
May 6, 2021 at 7:28