23 IR Astronomy

Naseer Iqbal

Learning Outcomes 

After studying this module, you shall be able to

• Learn how infra-red radiations (part of electromagnetic spectrum) were first discovered by William Herschel.
• Learn the quantity ‘Optical throughout’ of the telescope and detector and its use in infra-red astronomy.
• Knowing the transmittivity at a certain wavelength of infra-red radiation with a given value of layer ‘L’ of perceptible water, we can obtain the transparency for other values of L of the infra-red radiation.

1. INTRODUCTION 

Infrared astronomy began in the 1830s, a few decades after the discovery of infrared light. The infrared light is the electromagnetic radiation with wavelength range from 0.75 to 300 ?m. The infrared part of the electromagnetic spectrum comes after the visible light as shown in figure 7.1. The discovery that some celestial objects emit most of their radiation in the infra- red region of the spectrum has stimulated an increasing growth of interest in infra-red astronomy, but this progress was limited. It was not until the early 20th century that conclusive detections of astronomical objects other than the Sun and Moon were detected in infrared light. After a number of discoveries were made in the 1950s and 1960s in radio astronomy, astronomers realized the information available outside of the visible wavelength range, and modern infrared astronomy was established. The air-born, balloon and rocket experiments have been performed to observe these sources outside the atmosphere or at a high altitude. The reason is that the Infrared light is absorbed at many wavelengths by water vapour’s in the atmosphere of earth, so most infrared telescopes are at high elevations in dry places, above as much of the atmosphere as possible. There are also infrared observatories in space, including the Spitzer Space Telescope and the Herschel Space Observatory.

Figure. 7.1: Percentage of Atmospheric absorption of EM Radiations from celestial bodies.

The extensive ground-based observations have also been carried out through the atmospheric windows in the near infrared, intermediate and very far infrared and in spite of the constraints introduced by the presence of a residual, non-negligible atmospherical emission; infra-red astronomy from ground observatories appears to be a valuable research tool. This is because of the flexibility of ground-based observations exceeds that of high- altitude observations. Now the realistic situation, to be discussed about the best ground based infra-red telescope depends upon the several parameters and features, such as the operating wavelength range, the matching problem between the detector and the telescope, the atmospheric contribution and the main aim of the observations (i.e.; a map of the sky, or a study of some well localized faint sources, or solar astronomy, etc.).

2. History of IR-Astronomy 

The infrared radiation was discovered by William Herschel, who performed an experiment where he  placed  a  thermometer  in  sunlight  of  different  colours  after  it  passed  through a prism. He noticed that the temperature increase induced by sunlight was highest outside the visible spectrum, just beyond the red colour, i.e., the temperature increase was highest at infrared wavelengths due to the spectral index of the prism rather than properties of the Sun. The fact that there was any temperature increase at all prompted Herschel to deduce that there was invisible radiation from the Sun. He went on to show that such radiation could be reflected, transmitted, and absorbed just like a visible light. Since such radiation has wavelength larger than visible light, so it was known as infra-red radiation. Then, the efforts were made starting in the 1830s and continuing through the 19th century to detect infrared radiation from other astronomical sources. The infra-red radiation from the Moon was first detected in 1873 by William Parson. So, the field of infrared astronomy continued to develop slowly in the early 20th century, as Seth Barnes Nicholson and Edison Pettit developed thermopile detectors capable of accurate infrared photometry and sensitive to a few hundreds of stars. The field of infra-red astronomy was mostly neglected until the 1960s, with most scientists who practiced infrared astronomy having actually been trained physicists. The success of radio astronomy during the 1950s and 1960s, combined with the improvement of infrared detector technology, prompted more astronomers to take notice, and infrared astronomy became well established as a subfield of astronomy.

The most common infrared detector arrays used at research telescopes is HgCdTe arrays, which operates well in between 0.6 and 5 micrometre wavelengths. For longer wavelength observations or higher sensitivity other detectors may be used, including other narrow gap semiconductor detectors, low temperature bolometer arrays. The special requirements for infrared astronomy include: very low dark currents to allow long integration times, associated low noise readout circuits and sometimes very high pixel counts. Low temperature is often achieved by a coolant, which can run out. Space missions have either ended or shifted to “warm” observations when the coolant supply used up. For example, WISE ran out of coolant in October 2010, about ten months after being launched.

3. Matching between Telescope and Detector 

A telescope is said to be ‘optically matched’, or in ‘optical contact’ with a detector, if

????=  ????… … … … … … … … … … … (1)

Where ??is detector area?

??is the solid angle of field of view of the detector,

and  ??, ??have the same meaning for the telescope.

The quantities ????and ????are frequently known as ‘Optical  throughout’  (OT) of  the detector and the telescope respectively. If the OT of the telescope is larger than the OT of the detector, the latter is excited by a fraction of the energy collected by the telescope; while in the opposite case, other rays, in addition to those coming from the telescope, reach the detector. In both the cases degradation is expected in the system performance, so that condition (1) is that of an ideally operating telescope-detector system. Moreover, as the OT of the telescope increases, we may observe more sources in a shorter time. More precisely, with increasing ??fainter sources can be detected, while with increasing ??the time required to perform a survey of the sky decreases. So in principle the OT must be kept as large as possible.

4. Sites for Observations 

The most important contribution to the atmospheric absorption is due to H20 and CO2 in the near and intermediate IR-region, and to H2O in the far infra-red. If we assume that the absorption coefficient at a given wavelength is ‘S’ for H2O and as the content of H20 in the atmosphere increases, the absorption also increases. We may introduce a layer ‘L’ of perceptible water, so that the absorption will depend upon ‘SL’. However, the Lambert law of exponential absorption is no longer valid for bands made of discrete lines and the effect of the layer on the transmittivity is shown in Fig. 2. If we know the transmittivity at a certain wavelength with a given value of L, we normalize the curve of Fig. 7. 2 at the same value of L and then we may obtain the transparency for other values of L. Practically, L ranges from 10 mm (sea level, high-humidity site) to 0.7 mm (high altitude over 3000 m).

Figure 7.2. Transmittivity of the atmosphere (in units of the absorption coefficient S) for various equivalent lengths L of precipetable water (taken from (S)).

4.  SUMMARY

In this module, you study

• The discovery of astronomical objects (other than the Sun and Moon) emitting the infrared radiations made the realization that the information available outside of the visible wavelength range, and so the infrared astronomy was established.
• In the observational infrared astronomy, the quantity ‘Optical throughout’ of the telescope must be kept maximum and is defined as the product of area of the telescope and solid angle of field of view of the telescope.
• The transmittivity of infra-red absorption through the atmosphere containing water molecules.

For More Details ( on this topic and other topics discussed in Text Module) See

1. Considerations About  elescopes for Infra – red Astronomy by F . Melchtorri
2. Glass, Ian S. (1999). Handbook of Infrared Astronomy . Cambridge, England: Cambridge University Press. ISBN0 – 521 – 63311 – 7.
3. Rieke, George H. (2009). “History  f infrared telescopes and astronomy”. Experimental Astronomy . 25 (1 – 3): 125 – 141.