Managing Expectations

I love visual astronomy. It's probably the most fulfilling hobby I've ever had. Finding tough objects, appreciating the greats, and sharing those observations either in person through sidewalk astronomy and observatory viewing sessions or through recording them in my log.

But sometimes I do think, "I'm just looking at fuzzy gray blobs and white and brown dots."

And it doesn't really phase me. The result may not be the equivalent of an expensive astrophoto or hubble space telescope picture, but I'm using my own eyes, augmented through nothing but mirrors and glass, to view and record actual astronomical objects at great distances. I live to see as much detail and structure in the blobs and dots as I can, but I know the limitations of visual astronomy.

But with laypeople on the sidewalk, or beginners with their new telescope, their expectations might not match those limitations, and it ends up with disappointment. However, if you know what to expect to begin with, the experience will be much better.

Let's imagine, for a moment, that we flew up to the Hubble Space Telescope with an adapter that would let us put an eyepiece in it, and we could look through it. You might think that surely, with a 2.4 meter (8 ft) aperture, that you ought to see bright and brilliant colorful detail just like in the photos. However, there is a limitation to the maximum surface brightness any image seen through a telescope can have. Telescopes can't actually show an image with a larger surface brightness than if you just used your unaided eye. All the telescope does is take that image and make it larger, so you can see more detail in it, and it can increase the apparent contrast against the background too.

What this means is that a normal 6" amateur telescope will show objects like galaxies and nebula at no better than the same surface brightness as the visual-modified imaginary Hubble. The difference is it can do that at a much higher magnification, allowing your eyes (which aren't great at seeing sharp detail in faint objects) to see sharp details it never could have before. It's the difference between a Globular Cluster being a blurry ball of dull light and it being a brilliant sphere of individual stars filled with haze. It's the difference between the Orion Nebula being a fuzzy winged blob, and it being a world of bands of light and dust. But it ISN'T the difference between a dim ghostly image and one exploding with light and color. The maximum possible surface brightness stays the same, even for our hypothetical visual Hubble.

This is, I think, somewhat consoling. It means that a small telescope can really do pretty well for aesthetic viewing of certain especially bright deep sky objects, and so I tend to get a fair bit of use out of my smaller telescopes. Don't get me wrong, the image through a big telescope will almost always be far better, but if you understand the limitations of your instrument, a small one can suffice. Some astronomers have even taken to hunting Deep Sky Objects with just their unaided eye.

Another example of telescopic observing which suffers from unmatched expectations is planetary observing. People expect to see hubble-scale detail in the planets, and are sometimes disappointed when they can't.

A large telescope at maximum power can show planets at theoretically very high resolution indeed. But there are a few optical reasons why you can't get to very high powers. For one, a telescope can only magnify to about 2x per millimeter or 50x per inch of aperture. Beyond that you start wasting magnification and you just get a dimmer, blurrier image. This is a limit imposed by the physics of light, and would apply to Hubble just as much as your average beginner telescope. A larger aperture will let in more light to fill in the picture so to speak, and the resolution would increase. If you did use the imaginary visual Hubble, you could probably get some pretty amazing views which approach hubble photos. But in practice, even massive observatory telescopes still won't show hubble-ish views.

The problem is atmospheric seeing. We live on a planet with a fairly dense atmosphere, and this has important effects on our ability to do astronomy. Imagine if you were trying to look out from inside a shallow lake. If the lake were very calm and still, you could probably make out certain details in your surroundings, but if it were windy and the water was turbulent, the image would be wavy and distorted to the point you couldn't really see anything besides a blur. The same thing happens with atmospheric seeing, and so on nights when the air is turbulent, stars will twinkle more, and the resolution is limited. Some nights the seeing gets down so much you can see right up to the maximum magnification of your telescope. Others it might be limited to medium or low powers only. Generally, average seeing limits your maximum useful power to about 200x, poor nights limit it to less than 100x, and excellent nights probably can't do much better than 400x-500x. (Astrophotographers get around these limitations by taking videos of the planets and using stacking software to put together only the frames where the seeing distortions happened to be particularly low, which is how even amateur astronomers can get some seriously amazing shots of planets without going to space.)

Consider the Moon. When you look at the Moon in the sky, you can see only albedo features: the "seas" or "maria" of the Moon. You can't see any craters (unless you count the maria as craters), you can't see roughness or texture, it looks like a spotted but perfectly round ball. This is because the Moon takes up a vanishingly small part of your actual field of view, equivalent to the fingernail on your small finger held at arm's length--or about 0.5 degrees. (That is, the angle from the north pole of the moon, to your eyeball, to the south pole of the moon, is about 0.5 degrees.)

Mars, at its closest approach, takes up an angle of 0.006972 degrees. If we magnify it with a beginner telescope to 100x, the apparent size would increase to 0.7 degrees, which is not much bigger than the Moon. Viewing the Moon like that would still not reveal individual craters, it would still not reveal sharp and crisp detail. Even at 200x or 300x we're still much closer to "naked eye moon observing" than "individual surface features."

But just as viewing the Moon with your own eye can be pleasant and rewarding, so too can observing planets at a medium power through a telescope using your own eye.

However, there is something important to note: if you don't see detail at first, keep looking. As your eye adjusts to what it's seeing, it'll start to pick out more subtle details through the seeing and through eye floater blurring, until you can see much more than before.

There are other ways to improve your viewing of planets--waiting for good weather, making sure your telescope is acclimated to the outside air temperature, but that is a little outside the scope of this article.

The good news is you don't need to manage expectations with the Moon, because the Moon through a modest telescope's eyepiece is better than any picture, which is why it's such a good sidewalk astronomy target.