University Lowbrow Astronomers

What Do Telescopes Do? Part II.

by Doug Scobel
Printed in Reflections: September, 2003.

In my previous article I described some of the basic functions of telescopes.  Now it’s time to test your knowledge.  You may be surprised to learn that there is a lot of “common wisdom” out there regarding telescope usage that is simply not true.

True or false?  - Large telescopes are useless in light polluted areas because they make the sky background too bright.

False.  The truth is all telescopes of the same focal ratio will show about the same sky brightness through the same eyepiece.  It has nothing to do with the aperture.  To understand why, consider two approximately f/5 telescopes, one an 8 inch, the other a 16 inch.  The focal length of the 8 inch will be around 1000 mm, and that of the 16 inch around 2000 mm.  With a 25 mm eyepiece, the two scopes will give magnifications of around 40x and 80x, respectively.  The 16”, being double the diameter of the 8”, will collect four times more light.  Moreover, both combinations will produce about a 5 mm exit pupil.

Now you’re thinking that since the 16” gathers four times more light than the 8”, then the sky background should be four times brighter.  What you’re forgetting is that you’re also at double the magnification, so you’re only seeing one fourth the sky area as you’re seeing through the 8”.  Four times more light, but you’re only looking at one fourth the area, so the brightness per unit area is the same.  If you could lower the power in the 16” to 40, by using a 50 mm eyepiece, then you would see a four fold increase in the background brightness.  Unfortunately, such a combination will produce a too large (10 mm) exit pupil, and you’re no longer using all 16 inches of the primary mirror, making the background dimmer.

So, the truth of the matter is that in a light polluted sky, all telescopes are more or less equally hindered, and it just might be that it seems worse in a larger scope because you expect more out of it.

True or False?  - More aperture makes extended objects look brighter.

Well, this is kind of true, but a more accurate statement is that more aperture lets you magnify faint, extended objects more, making them easier to see.  But with more aperture, objects are not necessarily always brighter to your eye.  The reason is similar to what I explained in the previous discussion.  Suppose you are looking at M81 (a galaxy in Ursa Major) in the 8” f/5 scope with a 25 mm eyepiece.  You are looking at the galaxy at 40x.  Now switch to the 16” f/5 scope with the same eyepiece.  You’re now looking at it at 80x.  You’ve got four times the light, but the galaxy is now covering four times the area in your eyepiece’s field of view.  So, to your eye, its brightness per unit area is the same in both scopes.  Moreover, the sky background is the same brightness too, so contrast of the galaxy against the background is the same.  So why does it look so much better?  It’s because the galaxy appears twice as large in the 16”, covering four times the area, and you see much, much more detail.

Now if you switch to a 12.5 mm eyepiece in the 8”, then you will be looking at it at 80x and it will appear as large as it does in the 16” with the 25 mm.  But, you’ll notice that it is much dimmer and less detailed, because it is only one fourth as bright.  So, at equivalent magnifications, more aperture does indeed make extended objects appear brighter.

The bottom line is that the extra light provided by more aperture allows the observer to increase the magnification of an extended object to make it, and any details in it, easier to see.  But its actual brightness per apparent unit area in the eyepiece remains the same at equivalent effective focal ratios.

Note that this applies to extended objects only.  With stars, whose images are essentially points (not really, but close enough - see the next question below), more aperture does indeed make them look brighter in the eyepiece.

True or False?  - Ability to resolve close double stars is a good indication of the quality of a telescope’s optics.

Mostly true, but sometimes false.  Much of the time, slightly less than perfect optics can actually make it easier to split really close doubles, rather than make it more difficult.  To understand why, you first need to understand what a star’s image looks like in a telescope.

With reasonably good optics, the image of a star is not a point, but rather what is referred to as a diffraction pattern.  The diffraction pattern consists of a central disk, called the Airy disk (named after Sir George Airy, a nineteenth century English scientist and astronomer, not because it is “airy”), and a series of surrounding rings.  With perfect optics, 84% of the light goes into the Airy disk, and the rest is distributed into the surrounding rings.  And most of that remaining 16% goes into the first ring.  This pattern is the result of the wave nature of light, the rings being produced by alternating constructive and destructive interference of the light waves forming the image.

If you could look at a star through a hypothetically perfect telescope, with no central obstruction, one that is perfectly collimated, with an equally perfect eyepiece, with a perfectly steady atmosphere, at high power, then you would see a bright disk, a fainter first ring, and maybe a second ring if the star is bright enough.  But, all sorts of “defects” in real telescopes perturb the wavefront, both laterally and longitudinally, so that less light ends up in the Airy disk, and more light goes into the surrounding rings.  Atmospheric instability does it, the central obstruction in reflectors does it, and poor optical quality does it.  So with real telescopes, under real conditions, when light is removed from the Airy disk, it looks ever so slightly smaller to your eye.  In the case of a close double star, where the two diffraction patterns are nearly coincident, it can be easier to tell that there are two stars instead of one when the two Airy disks are smaller, rather than larger.  And slightly inferior optics can do just that.

Of course, there’s a limit.  Once the optical quality gets bad enough, then the entire diffraction pattern breaks down, and you might not see an Airy disk at all.  But for observing extremely close doubles under a steady atmosphere, 1/4 wave optics just might work better than 1/10 wave optics!

Also note that this applies to splitting close double stars only.  For viewing extended objects, especially those with low-contrast details like planets, good optics always out-perform poor optics, all other factors being equal (which of course never are!).

True or False?  - You should not use high magnification on faint, extended objects.

False.  This is a common sense argument that simply does not hold up in practice.  There are a great number of deep sky objects on which you actually need to use high magnification to be able to even see.

A good example is Stephan’s Quintet, a small cluster of five faint galaxies in Pegasus.  To find this little group, you need low power, but once you locate it, it’s hard to tell what you’re looking at.  At low power, (around 50x in my 13” scope) it looks more like a little amorphous nebula than a cluster of galaxies.  But increasing the magnification to around 200x makes a huge difference.  Now it’s easy to pick out all five galaxies, including their orientations.

Some think that it’s because increasing the magnification darkens the sky background making the object easier to see.  Not so, because the object itself is darkened by the same amount.  The real reason is that it now appears big enough to “pick out” from the background.  That’s why for faint objects that already appear large in a low power eyepiece, like faint comets, adding magnification usually makes things worse, which is probably where this bad advice originated.  But for small, faint objects, adding magnification often helps.

Adding magnification also lets you see details in the objects that you won’t see at lower power.  Many of the faint 11th and 12th magnitude galaxies in the so-called Herschel 400 list need to be viewed at relatively high power to see any structure in them, such as elongation, relative brightness of the core, etc.  And for bright objects, say the Ring Nebula in Lyra, high power can provide stunningly spectacular views.

True or False?  - Any red flashlight, regardless of its brightness, will preserve your night vision.

False.  Some think that as long as your flashlight has a red filter on it, that it will not harm your night vision.  But a too bright flashlight, regardless of the color, will indeed harm it.  Now red is probably the best color to use, although I don’t know for sure, and some even claim that a dim green flashlight is actually better than a red one.  But whatever its color, use as dim a light as possible.  I see so many folks using such bright flashlights while reading charts, and I’m sure that their eyes never become fully dark adapted.  Plus, be mindful of the background color of what you’re looking at.  If you can, use charts which have white stars on a black background (which will look more like the sky anyway), because the dark background will reflect little light back into your eyes.  If you need to read charts with a white background, then use as dim a light as will just allow you to read them.  If you are using a laptop computer, turn its monitor down as dim as it will go.

Personally, I use three flashlights while I’m observing.  I have one bright one, a two C-cell Mag-Lite with its lens painted with Testor’s red spray paint, that I use only for setting up and when putting everything away.  A “medium” one, a two AA-cell mini Mag-Lite with the lens painted red, for general chart reading and fumbling around, and a dim, single red LED with built-in magnifier for close-up reading of star charts, writing notes, and any time I have to look at a page which is printed black on a white background.  Even at that, I have noticed a slight, temporary loss of dark adaptation after I take a minute or two to jot down some notes on an object I’ve just observed.

So there, now you have something new to think about the next time you’re out with your scope.  And don’t believe everything you hear - or read.  And that’s the truth!

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Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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