Night sky emission

We first consider emission from the sky, which comes from several sources:

• air glow

• zodiacal light

• aurorae

• sunlight

• moonlight

• light pollution

• telescope and dome!

• unresolved stars and galaxies

Most of these source are emission line sources, not contiunuum sources, as shown in this plot How bright are these sources?

• air glow : strong in lines

• zodiacal light : $m_V \sim 22.2 - 23.5$ depending on ecliptic latitude (and to a lesser degree on ecliptic longitude)

• sunlight : small away from twilight

• moonlight : variable, can be very bright! $\sim$10 times brighter at full moon.

  lunar age
  (days)	U	B	V	R	I (mag/square arcsec)
  0	22.0	22.7	21.8	20.9	19.9
  3	21.5	22.4	21.7	20.8	19.9
  7	19.9	21.6	21.4	20.6	19.7
  10	18.5	20.7	20.7	20.3	19.5
  14	17.0	19.5	20.0	19.9	19.2

• light pollution : strong in distinct lines

Optical:

For broadband work, for example in the V band, $m_{sky} \sim 22$ mag/arcsec at good site so we switch over to background limited around m=22 for good image quality, switch over around m=20 for poorer image quality. Consequently, image quality matters for faint objects!!

One can choose narrower filters to improve the situation, but still have to then pay the price of longer exposure times.

Optical spectroscopy: sky emission generally not much of a problem except around lines.

IR: For broadband work in H band, $m \sim 13.5$ mag/arcsec; in K band, $m\sim  12.5$ mag/arcsec. So for all except bright objects, we're background limited. For infrared spectra, it's hard to estimate S/N : depends on where your feature is located.

Sky brightness from most sources varies with time and position in the sky in an irregular fashion. Consqeuently, it's essentially impossible to estimate the sky a priori: sky must be determined from your observations, and if your observations don't distinguish object from sky, you'd better measure sky close by in location and in time: especially critical in the IR.