Astronomy Principles and Practice Fourth Edition.pdf

Image photometry 319
telescope aperture of diameter D at any given instant is given by
( ( ) )
D
2
5·6exp −0·1557 . (19.12)
r 0
This essentially defines the probability of obtaining a lucky snapshot with a non-distorted or sharp
image. As might be expected, such a probability depends on the ratio of D/r 0 . According to the
probability formula with a telescope-seeing combination with D/r 0 ≈ 5, there is of the order of a
1 in 10 chance of obtaining a sharp picture. It is for this reason that amateurs with small telescopes
using fast CCD cameras are able to obtain very respectable pictures of planetary disc features fulfilling
the potential resolution of their telescopes. For a telescope of medium size, such as the Nordic 2·5 m
instrument on the island of La Palma, the D/r 0 ratio is ∼7 giving the chance of about one good
exposure out of 350 and such odds have been used to advantage in recording the close separations
of some bright stars. For reference, it may be mentioned that the Hubble Space Telescope also has a
diameter of 2·5 m but, as it is in orbit above the Earth’s atmosphere, it is not subject to the problems
of seeing. For very large telescopes with D/r 0 ≈ 10, the probability of getting a sharp picture falls
dramatically to ∼1in10 6 .
Following the simple calculation of the atmospheric height at which ‘seeing’ is generated, it is
obvious that observations made above this zone should be free from any major disturbance. This is
confirmed by the brilliantly sharp pictures obtained by balloon-borne imaging. It goes without saying
that all seeing problems are completely eliminated by observations from orbiting satellite platforms.
The effect of seeing does not always cause significant deterioration in measurements and, in the
cases where it does, it is sometime possible to design the recording equipment so that the effects can
be overcome or compensated. The fact that all recorded star images are in the form of a seeing disc
does not prevent positional measurements being made with an uncertainty which may only be a small
fraction of the size of the disc. It is perhaps in connection with analyses which involve the measurement
of one star which is apparently close to another, or those where the analysing equipment has a very
small acceptance angle, that the size of the seeing disc is inconvenient. However, this can also be
addressed by using adaptive optics (see section 20.2) in the telescope design.
Scintillation may also not always be important since, for example, a long exposure of a
photographic plate or CCD chip smooths out the rapid intensity fluctuations. It is only when an
‘instantaneous’ stellar intensity measurement is obtained, say, from a photoelectric cell that scintillation
noise appears in the output. In photoelectric scanning spectrophotometry, the departures from a mean
intensity level may be on the same order as the depths of spectral lines and, in this case, it would be
impossible to detect them against the scintillation. The remedy for this particular problem is to have a
second detector which accepts a fixed band of the spectrum close to the section which is to be scanned
and allow this to monitor the scintillation noise. By using the second channel to divide out the noise
in the scanning channel, the recorded spectrum is, to all intents and purposes, freed from scintillation
noise.
19.8 Image photometry
19.8.1 Photographic photometry
One of the purposes of recording a star field on a photographic plate is to allow determination of stellar
brightness. The usual procedure is to place the photographic plate in the focal plane of the telescope
so that in-focus images are produced. The spectral range may be limited by placing colour filters prior
to the emulsion. Exposures may last from a few minutes to a few hours.
An inspection of any star field plate immediately reveals that the images vary in size and are in
some way related to the brightnesses of the stars. It should be noted that the distribution of image