Star Spy: Size illusions
FREIDL HALE - TEKAPO STARLIGHT
How big is it really?
As a casual observer, can you tell how big celestial objects actually are? The answer is ‘no, you cannot’.
The obvious reason for this is the old, ‘Is it a bird? Is it a plane?’ question.
In the sky it is very difficult to tell the difference between large, distant objects and small near objects.
Behaviour can be a clue – if it suddenly and radically changes direction it probably isn’t a plane.
This is an even more difficult endeavour at night when your main clue, apparent size, is as useless as it is during the day, and your second clue, brightness, can be equally misleading.
Take two stars that are visible in the New Zealand sky: Alpha Centauri and Canopus. Alpha Centauri is the third brightest star in Earth’s sky. Canopus is the second brightest. But, as we have seen, Alpha Centauri is the nearest star to us.
You might therefore expect it to be brighter. In fact, Canopus is more than 70 times farther away! It is also thousands of times brighter than Alpha Centauri. It is a BIG star. It gets worse.
Lets take another look at the globular cluster, 47 Tucanae. We see it as a very dim, perhaps slightly fuzzy, star just next to the Small Magellanic Cloud. That is all that we can see of it using just our eyes, a dim “star”.
But what we see is just the brightest central core of it. If we could, for just a moment, brighten the whole of the cluster, we would see that, in its entirety, it is the size of the full moon in our sky.
The part we can see of it with our eyes is just the tiniest fraction of the cluster. However, when we look at a dim star, or a planet for example, the pinpoint of light that we see is the whole object.
It doesn’t turn out, on closer inspection, to actually be the size of the full moon in our sky.
This difference has a huge impact on astrophotography.
You might have thought that photographing planets would be simple compared to other types of astrophotography, the planets being so much closer than the star clusters, nebulae and distant galaxies.
However, the planets represent quite a challenge because of, as we saw above, their small size in the sky.
Of course they actually are tiny compared to the nebulae, star clusters, and galaxies, but it is their apparent relative sizes in the sky that becomes an issue when photographing them.
To photograph 47 Tucanae, the main requirement is a good steady tracking mount, a small telescope, and a long exposure to capture enough light.
A high-powered telescope is required to get much meaningful resolution of a planet and any detail on its surface.
You also need a dark, clear sky, and a very steady sky - what astronomers call ‘good seeing’.
It is the atmosphere that makes the stars and planets twinkle – poor seeing. And yes, planets do twinkle when the upper atmosphere is very turbulent or contains a lot of moisture.
Conversely, when it is settled, stars directly overhead do not twinkle at all. Twinkling is usually not a good thing for astrophotography.
Fraser makes his planetary images using the video feature of his Canon 7D SLR camera instead of still frames.
Video, simply a recorded sequence of usually between 15 and 60 images per second, avoids the small wobble caused by the camera shutter opening and closing to take still frames.
To make the images of Saturn and Jupiter he used his 9.25 inch Celestron HD Edge telescope as the lens of his camera, all attached to his mount as shown above.
He took a string of 750 images and combined the best 50 using Registax, free image-processing software available for download online.
The program evaluates all the images, chooses the best ones, and stacks them to produce the final photograph.
- © Fairfax NZ News
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