A bone to pick on the subject of scale

Started by n2s-astronomy, 10/21/2007 02:46PM
Posted 10/21/2007 02:46PM Opening Post
We know the derision that most of us feel when we see a tiny department store scope packed in a box proclaming some absurd amount of power and decorated with Hubble quality images. But, there is alot more of that sort of thing happening all throughtout the hobby of astronomy. We need only google a Messier object, or enjoy one of "Pictures of the Day" on our home page, to be awed by brilliantly detailed photos of strikingly beautifull objects. Unfortunately, almost all of those images are completely alien to what we can expect to see by simply turning our scopes skyward and taking a peek.

What seems to be missing is an element of scale; the lengend on our mental map of the sky which proclaims a measure equivalent to so many miles to the inch. But, not only along the one dimension, since so many other factors determine what can actually be seen and how it should be expected to appear.

Let's look at some of the major points of confusion:

1) The apparent size of the target: It is perfectly natural to want to illustrate each subject with the best available image. But, in doing so we usually add to the confusion by readilly mixing up the apparent size of objects. The same textbook, or article will contain a vast array of scales with no attempt to juxposition the one against the other. The moon may generously fill your field of view at 100x; but, what about the other planets, or beyond them some of the endless other objects? Should we be looking for an elephant that generously fills FOV, perhaps a tiny version of the same, or merely a trunk, a tail, a leg, or an ear?

2) Just what are we looking for: There is a vast difference between imaging and visual astronomy. Just becuase we can capture vast amount of detail on an image does not mean that the details are useful to visual astronomers. Sometimes the excess details merely obscures the relevant landmarks. I was looking at an old series of plates reproduced in David Bergamini's The Universe (Time Life, 1962, pg 57) which shows four photos of a star cluster taken through the 100 inch at mt. Wilson. The images show the same object after a 6 minute exposure, a 15 minute exposure, a 37 minute exposure, and a 94 minute exposure. The appearance of the object changes so drastically as to be barely recognizable. Just how is an image processed through hundreds of stacked plates, and representing many hours of exposure, helpful in identifying an object when merely viewed visually?

3) What about aperture: Aperture is king, the bigger our scopes the more light that they can collect, and the more likely that our eyes will be able to resolve the finer details of the object. But just how should an object appear in a six inch scope, and how would that image appear differently in a 20 inch, or a mere two inch? Similar to exposure length, we can expect the object to look vastly different; what may seem little more then a dim fuzzy can resolve itself into something sharp, colorful, and awe inspiring. Once again we are often led astray, our popular image does not align with what we can expect to see through the eyepiece.

4) Which way is up?: There may be no up or down in space; but, there certainly is an up or down to the Earth bound observer. How many times have we seen images rotated out of true, with no indication as to which side of the image should be aligned with the celestial pole?

Perhaps what we lack is a common language. We know that what we will each see when we look up at the sky will vary depending on many factors, certainly not the least of which is ourselves. But, we seem to lack the means to communicate; how do we describe a landscape where the landmarks change in apparent size and shape? Do we have a program out there where you can define your equipment,eyepiece, location, seeing, and exposure and have it return a photo-realistic recreation of how the sky should appear through your eyepiece? Experience and computer aided navigation can cure much of this; they can point our scopes in the desired direction. But, there is still a gap between what we can actually see and how we describe it.