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THE
ROLE OF MAGNIFICATION
A faint extended object (e.g. a galaxy, a bright spot within a
galaxy or nebula) should be viewed with enough magnification so it appears
several degrees across to the sky. To be detected, it must be surrounded by a
darker or lighter background, so the eye can distinguish contrast. Various
magnifications should be tried to bring details into the range of best
detection. At each magnification, considerable time must be spent examining for
detail. Higher magnification should be tried until the object is totally lost
from view. The eye should be dark adapted for at least 30 minutes so the
photochemical visual purple is at full abundance. Bright stars and extraneous
lights will tend to destroy dark adaptation." (Clarke 1990:18)
One piece of conventional wisdom is that low magnification
should be used for deepsky observing, so that light from a faint object is
concentrated on a small area of the eye's retina. Clarke argues: "This
would be true if the retina worked passively, like photographic film. But it
doesn't. The visual system has a great deal of active computing power and
combined the signals from many receptors to detect a faint extended object.
Increasing the magnification spreads the light over more receptors, and the
brain's processing power can then bring into view fainter objects having lower
contrast.
When you switch from a low to a high power, you could gain a
magnitude or more in faint stars. This works because high power reduces the
surface brightness of the entire field by spreading out the light. Doing this
dims the sky background without affecting the total amount of light arriving
from small, discrete objects. Stars appear so tiny that their surface brightness
hardly looks changed at high power. But even an already dim, diffuse galaxy
won't be rendered any less visible when its surface brightness is lowered (at
least within limits), because the galaxy's contrast with the sky remains the
same. You're actually likely to see it better, because your eye perceives
low-contrast objects better when they are large. The neural network in your
retina is smart enough to gather and correlate the galaxy's light from a wide
area. Deepsky vision is quite different in this regard from the behaviour of
'dumb' photographic film, which responds to surface brightness only."
Clarke also notes: "It is normally accepted that the
highest power [usefully employed on a telescope] is about 50 to 60 times the
objective in inches. This limit is correct only for bright objects ... for
fainter objects the eye has less resolution and needs to see things larger, so
higher powers are called for. At the limit of the eye's detection ability, the
highest useful magnification is on the order of 330 per inch of objective!
High magnification demands a lot from your telescope. If the
mounting is unstable, or the slow-motions not smooth, then each vibration or
bump will cause the image to dance about wildly. Further, the field of view
becomes narrower as you magnify, making objects a bit more difficult to keep
track of. The effects of an unstable atmosphere are exaggerated at high
magnification. Clark notes that "magnification also reduces the surface
brightness of everything in view. It must not reduce an object's surface
brightness below the eye's detection limit, of course, or the object will
disappear.
So while higher magnification does decrease surface brightness,
MacRobert notes, the total number of photos of light entering the eye remains
the same. It doesn't really matter that these photons are spread out over a
wider area; the retinal image-processing system will cope with them. At least
within certain limits. A trade-off is needed to reach the optimum power for
low-light perception: enough angular size but not too drastic a reduction in
surface brightness.
Mel Bartels on his Visual Astronomy page concludes
that there are a number of ways to make an object detectable. Regarding
magnification, he says: Use sufficient magnification to make the
background invisible and the object about one degree in apparent size. Most
amateurs today use too low of power because their scopes don't track, and
because 'that's what everyone else does'. John Dobson was the first large
aperture observer to point out the advantages of high magnifications. Al Nagler,
and Brian Skiff, among others, have recommended high magnifications. By
increasing magnification, you are decreasing sky background brightness, and
making the object larger in apparent size, both crucial to detectability. For
small extended objects, you may exceed the old double star observers' rule of
50x per inch of aperture.
Nils Olof Carlin summarises: "To detect a faint object,
you can increase magnification till the sky is so dark that you have difficulty
seeing the field stop, or till the object has an apparent size of 1 degree,
whichever comes first." |