University Lowbrow Astronomers

Ya Know What I Mean?

by Doug Scobel
Printed in Reflections: June, 2006.

What does this picture and astronomy have to do with each other?
Doug Scobel can tell you, read his article and you’ll know what he means.

Most hobbies have their own lingo and buzzwords, and amateur astronomy is no different. But sometimes the terminology can get quite complicated, and words can sometimes be confused and interchanged, sometimes to the point that you need an interpreter to understand what the other person is talking about. OK, maybe it’s not quite that bad, but it does help when everyone is on the same page so to speak. So, at the risk of being a little over the top on this (OK, I’ll admit I can be sometimes), here’s my attempt at clarifying a few commonly interchanged terms so that we can all understand each other better. I’m not mentioning the names of the people from whom I’ve heard these things, so there’s no need to take offense. But if the shoe fits....


These are two different words that mean two different things.

Collimation (coll-i-MAY-shun) is the act of aligning the optical elements in a telescope, for the purpose of minimizing off-axis aberrations and realizing the best images the scope is capable of producing. This is most often done in reflectors with so-called Cheshire eyepieces and/or laser collimators. Owners of fast Newtonian reflectors are always fussing over their telescope’s collimation, or at least they should be.

Culmination (cull-mi-NAY-shun), on the other hand, means the highest point in the sky that a celestial body reaches. For non-circumpolar objects, it occurs when the object reaches the north-south meridian, or in other words when it transits. Circumpolar objects, which transit twice in a 24-hour period, have two culminations, upper and lower. But generally one would mean the upper culmination, or its highest point.

Now at the 2005 Black Forest Star Party, at the door prize drawing someone won a laser culminator. Really. That’s how they announced it, and more than once! I’m not sure how it would work, but I would like to get one myself. It would be really handy for observing those objects that are just rising and I don’t want to wait for hours for them to get higher in the sky and out of the soup. Just point that baby at Mars and boom!—it’s culminated!


Here are a couple terms that sound similar but really mean two different things.

Offset is often used when describing an offset mask, as is sometimes done with large Dobsonian reflectors when atmospheric steadiness is really bad. The mask covers the upper tube opening, but has a circular aperture that lines up in the opening between the secondary mirror and the edge of the primary mirror, and between two spider vanes. It results in an unobstructed light path, and can aesthetically clean up the image, although the aperture is greatly reduced. Some folks call these masks off-axis, but they are offset, not off-axis. The light path (as long as the scope is collimated) can still be on-axis; it’s just that the aperture is offset from the optical centerline.

Off-axis refers to incident light rays that are not parallel to the optical axis of the telescope. If the scope is properly collimated, then the light from any object that is centered in the field of view is on-axis, and the light from any object not in the center of the field of view is off-axis. Off-axis light rays can cause all kinds of aberrations, such as astigmatism, distortion, and coma. And the farther the light rays are from the center of the field the worse the aberrations. This is particularly true in telescopes with fast focal ratios, where off-axis light rays from near the edge of the field of view arrive at the focal plane at steeper angles compared to slower focal ratios. They also cause many an eyepiece designer sleepless nights, since the correction of any one aberration will usually increase another. Which helps explain why eyepieces that are well corrected to the edge of the field in fast scopes also tend to be quite pricey.

Magnitude/Order of magnitude

Here’s a case where the same word is used singly or in phrases, but means two different things depending on the context.

When used alone, magnitude means the total amount of light being received from a celestial object. In other words, it’s a measure of how bright something is. In the magnitude scale we use for indicating star brightness, each number represents a gain in brightness about 2.5 times over the previous number.

Order of magnitude is a phrase that means a change by a factor of ten. Each “order of magnitude” can be calculated by taking the number ten to the order’th power.

For example, when comparing a 1st magnitude star against one of 4th magnitude, a gain of three magnitudes, the brightness increase is about 15.6 times. But three orders of magnitude means a change by a factor of ten to the third power, or 1000. Not exactly the same thing, huh?

Minutes of Arc/Minutes of Right Ascension

Here’s another case where the same words are used singly or in phrases, but mean two different things.

A minute of arc, or arc minute, is a constant angular measure, and is always 1/60th of a degree. It doesn’t matter where in the sky you measure it, it’s always the same angle.

A minute of right ascension is also an angular measure, but the actual angle depends on the declination where it is measured. Right ascension is the way astronomers measure the angle around the earth’s rotational axis, with a full circle being split into 24 hours. Each hour of RA is further split into 60 minutes. Because the hour circles get smaller and smaller, each minute of RA subtends a smaller and smaller angle as you move from the equator to the poles. For example, at the equator each minute of RA is 15 arc minutes. But at plus or minus 60 degrees declination, where each hour circle has half the diameter as that at the equator, each minute of RA is only 7.5 arc minutes.

I mention this because I have a double star booklet where the author used minutes of arc instead of minutes of RA and misdirected the reader from star to star while star-hopping. I was having a devil of a time trying to star-hop to some stars using his directions until I discovered what was going on.


Here’s a case where the same word is used to mean two different things.

The most common usage is when describing how steady the atmosphere is when observing, also known as steadiness. Good atmospheric seeing, or probably more accurately good atmospheric steadiness, is important when using high magnifications to split close double stars or examine fine lunar and planetary detail.

But sometimes I’ve heard folks use the term seeing to describe a good, clear, night, in other words good transparency. Transparency is independent of how steady the air is, but more a measure of the amount of water vapor or other aerosols in the air, and the amount of light pollution present. In other words, transparency refers to how dark the sky background is.

More often than not, good transparency comes at the expense of steadiness, and vice-versa. If you want to be clear (pun not intended!) in what you’re describing, it’s probably best to use the word steadiness or transparency, rather than seeing.


Here’s another case where the same word is used to mean two different things. Some use it to describe the amount of scattered light in the eyepiece’s field of view. Others use it to describe the amount of detail that can be seen, for example in a planetary image. But they’re not the same thing, nor are they affected by the same factors. I would describe the former as field darkness, and the latter as sharpness.

Loss of field darkness is generally due to light being scattered more or less evenly throughout the entire field of view. The less scattered light, the more contrast there is between the sky background and the object being viewed. Factors that affect it are diffraction of light around the edges of things in the light path, like spider vanes or the secondary mirror in a Newtonian reflector. Dirty optical surfaces can scatter light. Internal reflections in the telescope and/or eyepiece also decrease field darkness. And of course, so does light pollution. But these do little to affect the amount of detail that you can see, say, in a planetary image. But if you’re trying to pick out a really faint galaxy on the edge of visibility then you need as dark a sky background as possible.

Conversely, it’s optical defects that adversely affect the diffraction pattern that reduce sharpness. Poorly figured optics do it by putting too much light in the outer rings, and not enough in the Airy disk. A too-large central obstruction can do the same thing. Poor atmospheric seeing affects it by blurring the pattern. Miscollimation affects it by introducing astigmatism or coma. All of these things will reduce the amount of detail you can see, but they will do little to brighten the sky background.

So when you’re describing a view in a telescope, do you mean contrast, or do you mean contrast? The terms field darkness and sharpness are probably more descriptive.


Here’s a case where the words look the same, but how they’re pronounced determines their meaning.

Vega (VEE-guh) is the brightest star in the constellation Lyra. In fact, it is one of the most brilliant stars in the entire sky.

The Vega (VAY-guh) was a small, lousy car built by Chevrolet a few decades ago. In fact, it was one of the dullest cars ever built. Please don’t insult the star by mispronouncing its name!


Here’s a case where both spelling and pronunciation come into play.

Betelgeuse (BET-el-joos) is the bright red giant in the constellation Orion.

Beetlejuice (BEET-ul-joos) is a creepy, fictional movie character.

Enough said.

Guard Rail/Guide Rail

Strictly speaking this has little to do with astronomy, but since it comes up during our annual pilgrimage to the Black Forest Star Party in Pennsylvania I thought I’d mention it.

In most states, where it would be hazardous to leave the roadway, roads have what they call guard rails along the edge of the road-bed. Their purpose is to prevent any would-be errant vehicle going somewhere it ought not go, like into a bridge abutment, over a bridge, or down a ravine. They guard you from such hazards. But not in Pennsylvania. There they don’t have guard rails, but instead have guide rails. Their purpose seems to be to guide you on your way. Fortunately, the Pennsylvania department of transportation tells you when they’re not present. In many places along the twisty roads around Cherry Springs State Park you’ll see signs that read “No Guide Rail.” My best guess is that you’re supposed to drive along with the side of your car scraping along the rail, so that it guides you along the way and keeps you on the road. But keep your eyes open for the “No Guide Rail” signs, lest you find yourself careening down the side of a mountain and into a tree!

Other Silliness

I’ve also seen lots of goofy spellings in my travels perusing the Astromart Forums. One of the most common ones is where folks spell aperture as aperature, which isn’t even a word. C’mon, people, it has a spell checker now so there’s no excuse! I’ve also seen people discuss the thickness of their spider veins (ugh!) instead of their spider’s vanes, and talk about armature astronomy (which is astronomy as it relates to electric motors I guess) rather than amateur astronomy.

Now that I’ve gotten all this off my chest, I feel much better now. And I hope you’ve taken a few “minutes” to read this and are now “seeing” the “contrast” between correct and incorrect terminology, at least in astronomical circles. And as far as me being a bit over the top on the subject, well as I said earlier, if the shoe fits....

Ya know what I mean?

Doug Scobel at Peach Mt.


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