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Atmospheric seeing

23rd May 2005

Whilst taking an astronomical techniques course at Bayfordbury Observatory, in Hertfordshire, England, myself and two others were teamed up to observe and photograph three celestial objects through a 16-inch reflector. My assigned object was the Orion Nebula and I managed to get a reasonable, though unavoidably fuzzy, image via the attached CCD camera.

We each finished our respective task with some time to spare and decided to reposition the telescope on another object. The first of us tried to improve the focus of the image, using a hand-held control box, but succeeded only in making things worse. After a minute or two of attempted correction, he handed the control box over to the second student. His efforts produced a further significant decline in quality and he handed the box over to me. No matter which way I adjusted the focus, the image seemed to become more and more obliterated. Finally, we were left with a picture on the screen of what appeared to be a circular disc, with a couple of cross-hairs running through it, which we concluded was an image of the secondary mirror and its supporting arms!

Weary of the possibility of our professor coming in at any moment, I distanced myself from the instrument and went over to the door, at which point I was immediately motivated to call the other two over with the invitation, "Come and take a look at this!"

There, high above our heads, on a cold, clear night, was the most amazing display of stars. We had spent the best part of an hour inside the telescope dome, each struggling to get fuzzy images of our respective assignments printed off via the computer linked up to the CCD, before managing to put the telescope completely out of focus, while all the time we were missing an absolutely beautiful display, that was plainly visible to our unaided eyes. The latest, expensive technology had succeeded only in obscuring the reality of what we thought we were studying.

Gaining the technical competence to be able to control large telescopes is trivial compared to other problems that can not be so easily overcome. For instance, building really large telescopes such as the William Herschel Telescope (WHT), shown in Fig. 1, in no way means that we will obtain clearer and clearer images of fainter and fainter objects, as this quote from Dr. Ronald Parenti will demonstrate, "The imaging quality of [even 8- to 10-m telescopes] in the visible is seldom better than that obtainable from a 20-cm receiver." (A '20-cm receiver' is commonly called a 'back garden instrument'.)

Figure 1: The 4.2 m William Herschel Telescope on La Palma, Canary Islands.

Although the diffraction limit for the angular resolution of a telescope of circular entrance pupil is inversely proportional to the diameter of the aperture, and hence decreases as the size of the primary mirror increases, this limit can never even be remotely approached in reality, because of the distorting effects of the World's atmosphere, as commented upon by Isaac Newton. Constructing the telescope atop mountains 4- or 5-km high succeeds mainly in reducing the numbers that can operate them (due to altitude sickness), rather than reducing the effects of an atmosphere whose effective maximum altitude is 20-km.

Under atmospheric seeing, we obtain diffraction-limited speckles that accumulate randomly over time and whose composite image produces a blob, the approximate diameter of which, referred to as the seeing angle, is inversely proportional to the Fried parameter (named after Dr. David Fried, the greatest expert in this field). As we move from infrared observations, into the visible region, the Fried parameter decreases significantly (from ~ 50-cm to ~ 10-cm), thus producing a 2500% increase in the area of the point spread function (PSF), which determines how large a 'point' appears to be. Note that the seeing angle is irrespective of the primary mirror size.

Furthermore, the atmosphere is not static, but is constantly changing, which means that there is also a turbulence coherence time, over which a small region of the atmosphere (known as the isoplanatic patch) will be approximately invariant. In V-band observing, this time is of the order of 6.3 milliseconds for a zenith angle of zero, and 5.1 ms for a zenith angle of 45 degrees.

In an attempt to compensate for the distortion introduced by the World's atmosphere, either post-detection techniques, such as speckle imaging, or real-time, pre-detection techniques - adaptive optics - may be employed, with varying levels of success. Alternatively, one could try a satellite-based approach, as is claimed with the Hubble Space Telescope (HST). Such satellite telescopes would not work either, though, because they could not be locked onto a faint object for long enough, and are yet another example of science fiction masquerading as fact.

 

References

  1. Fried, D.L., 1966, "Limiting Resolution Looking Down Through the Atmosphere," J. Op. Soc. Am., 56(10), 1380-1384.
  2. Newton, I., 1730, "Opticks," fourth ed., Dover, New York (1979).
  3. Parenti, R.R., 1992, "Adaptive Optics for Astronomy," Lincoln Laboratory Journal, 5(1), 93-113.