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Aperture vs. Light Pollution

Posted by Ed Moreno   09/23/2004 12:00AM

I am going to start with a direct quote from a leading telescope company’s product catalog: “A refractor is the scope of choice if you will be doing most of your stargazing from city or suburbs, where the night skies are moderately light-polluted. Here, more aperture doesn’t gain you much, since viewing is restricted mostly to the Moon and planets. In fact, a big scope would only amplify the sky glow, yielding poor washed out images.” I see similar statements repeated even in books geared to astronomy novices.”

There are two problems to this statement. The first is that it badly misleading. The second is that someone that takes this advice and buys a small refractor as their first telescope may have a bad experience with astronomy! Let me explain.

First, let me tell you about my direct experience from my moderately to highly light polluted sky. I live in central Austin, Texas, which any local will tell you, is struggling to stay weird. It has already lost the struggle to stay dark. Anyway, I live under Magnitude 3 to 4 skies much of the time, though sometimes when transparency is superb, I can get to magnitude 4 to 5 stars with the naked eye.

I have recently owned several telescopes: an 80mm Stellarvue refractor, a 102mm Vixen refractor, a 140mm Vixen refractor, a 150mm APM refractor (achromat), a Meade 152 ED refractor, and a NexStar 11 SCT. While these are just the latest in my inventory, I have owned maybe 20 telescopes in the last 10 years. Still, I will use these most recent scopes in inventory and some common objects to explain why I feel that the above statement is patently false.

On a recent night, I set up my 80mm, 140mm, 152mm refractors, and 278mm SCT telescope in my back yard for some direct observations and comparisons. Here is how I thought these scopes compared on 4 typical observing targets: Globular clusters, open clusters, nebula, and double stars.

Let’s start with globular clusters M92 and M13. These are actually both quite splendid objects. M92 is nearer to the globular cluster mainstream in its brightness than M13, so I think that it provides representative performance of how a telescope similar in size to mine might perform on most globular clusters. Here goes from the bottom.

Under my Magnitude 3 skies last night, M92 was visible in the 80mm refractor as a hazy spot. It is recognizable as a globular, but only just so. In fact, on an average night under my local seeing conditions, it really doesn’t show any kind of features at this aperture. It is just a small nebulas glow.

In the 140mm refractor, it starts to take on character. I see it as being very much more graduated from the edge to the center, and using averted vision, I can see some stars across the nucleus.

M92 starts to get really interesting in the 6” APO, because many of the stars across the face start to show with direct vision. In addition, some of the extended glow starts to resolve as stars with direct vision, but it is only partially resolved at the core. Still, under my sky conditions, this is the aperture where this object starts to get exciting to look at.

The 11” SCT shows numerous strong graduations with stars resolved quite close to the nucleus. Under my light polluted sky, I find that the 11” aperture of the NexStar 11 is the only scope that I own that does this object any kind of justice. My C9.25 of a few years ago also did really well with this target.

M13 presents a similar picture, though here, even the 80mm scope clearly presents M13 as a globular. Still, in the 80mm scope individual stars are difficult to discern. The 140mm shows a great number of stars with direct vision, and the 152mm refractor starts to resolve the core. Things change dramatically with the 11” telescope. The core is almost totally resolved. There are stars EVERYWHERE. With a 12mm Nagler eyepiece in the N11 (233x), the field of view is literally covered with stars. Here again though, the 152mm refractor was the smallest aperture that made M13 look really dramatic under my light polluted sky.

Now let’s look at a nice example of open clusters. NGC 6633 is a large, bright (magnitude 4.6) cluster in the northeast part of Ophiuchus that should be, and is, a quite beautiful object for small telescopes. In fact, because of its proximity to the Milky Way, the 80mm scope shows is as a bright cluster in a beautiful field of stars. But then again, so did the 140mm scope (thought the Vixen 140 is a VERY fast refractor at f/5.7 and only 800mm focal length). The 140 showed even MORE field stars, and the cluster really started to become more interesting at this aperture, with maybe double the number of member stars. My Meade 152ED is a superb refractor, and it made this cluster really sparkle. The cluster itself started to show many more members, but the BACKGROUND suddenly came more alive with the 6” scope. NGC 6633 sits on the edge of the Milky Way so the background SHOULD be rich in field stars, and suddenly at 6 inches of aperture, this became apparent as many of these faint stars started to show against the sky glow. The smaller scopes didn’t really give me nearly the same perspective of showing the cluster embedded in the Milky Way, so the cluster really looked magnificent here. The 11” showed even MORE stars in the cluster, however here, the shorter focal length of the other scopes helped show the cluster in a beautiful field, while the 2800mm focal length SCT was limited to showing mostly the cluster and only the immediate half-degree field around it, so in effect, this target was BEST with the 6” scope (which is why many astronomers have more than one scope!). I think an 8” or 10 dob would have provided superior views to the 152mm refractor though, because they would have shown perhaps even more background stars and because of their similar or even shorter focal lengths to my 6” refractor, they would still allow framing the cluster in a beautiful background field.

The Wild Duck is FANTASTIC in the 11” Scope, and while it is quite visible in all of the other apertures, the stars just didn’t quite pop out against the light polluted background. Only the 11” telescope really dazzled me on this object. The Wild Duck cluster itself is actually quite bright, but with the exception of a single relatively bright star near the center of the cluster, the member stars are all rather dim, and in the smallest scope, the cluster looked more like a grainy nebulosity. Still, as with most of the other objects, the 6” aperture scope really made the breakthrough to providing a thrilling view.

How about some nebula now? The Dumbbell Nebula (M27) is a magnitude 8.1 nebula in Vulpecula. It is an easy nebula to see in even a small scope, and one that is easy to recognize. The small scope showed it (barely), and actually showed enough of the shape to positively identify it. The bigger scopes did make the shape far more recognizable though. With the 80mm, the Dumbbell was more rectangular, while with the larger scopes, it showed the more classic pinched middle. The 11” scope, as you can imagine, did the best here, but the difference wasn’t as great. The nebula did extend a bit further in the 11” than the 6”. I see this on these kinds of nebula that have more distinct edges. In comparison, nebulas like M42, the Great Orion Nebula, just get expand out bigger and bigger with greater aperture.

M57 was similar. The Ring Nebula barely stands out from the background in the 80mm refractor, and it only faintly shows the ring structure. As aperture increases, the doughnut shape starts to show a pronounced graduation toward the center, and on the longer “sides” extending outward. The 152ED showed this the best of the refractors, but the 11” started to show the same effect at the ends of the oval. In addition, the very dim 13th magnitude field star near one side of the nebula came into view with direct vision with 11” scope. This star was invisible in all of the smaller apertures, though the 6” refractor hinted at it with averted vision.

Good color contrast double-stars are beautiful through any aperture, and here is one place where the smaller refractors hold their own. On nights of average seeing, brighter doubles always look best in smaller refractors. Last night, I observed Alberio, Mizar, and the Double-Double in Lyra as a nice sampling. In every case, the small refractors showed these objects beautifully. Colors in Alberio were pronounced, even in the 80mm scope. On most nights refractors also present lovely airy disk patterns on many nights where larger reflectors just show muddy blobs, though I can split a GREAT many of those blobs with the 11” scope. As an example, in the Double-Double, the component stars of each pair were in contact with the 80mm scope. The 140mm scope moved them far enough apart that the first diffraction rings were touching, forming a nice figure 8.
The 6” 152ED, by virtue of having VERY highly correct optics and a little greater aperture, moved the components apart enough so that the rings were not touching, and the first diffraction rings were so faint that they could hardly be seen. The 11” scope moved the components of each pair so far apart that there seemed to be enough space to squeeze a third star in-between. Because the seeing was almost PERFECT last night, the first diffraction ring was actually quite evident, but it was never steady, and this was perhaps the best I have ever seen them in the larger scope. From a purely aesthetic viewpoint of being able to show a nice stable airy disk, the mid-sized refractor and large refractor almost always do better than anything else.

The moon and planets aren’t affected by light pollution to the extent that these other objects are. Once again, though, my experience with these kinds of objects is that generally, the amount of detail seen seems to make noticeable steps with the scope mentioned in this article. As theory predicts, the jump form the 152 to the 11” SCT actually shows the SMALLEST improvement here. On every other class of objects, the jump between successive apertures was quite pronounced. Here, it was pronounced between the refractors almost exactly in proportion to the relative aperture. The difference between the 152ED and the 11” is much more subtle. In fact, the 152ED is the first scope I have owned that actually challenges the 11” on planetary performance. My calculations suggest that the Modulation Transfer Frequency curves on the 6” APO and the 11” SCT are going to be VERY close, so contrast on planetary performance should be similar. I guess we will all have to wait until next year when the gas giants get high again to find out, but to date, nothing smaller I have owned has surpassed my N11 on the planets WHEN THE SEEING IS GOOD.

To sum up everything to this point, I find that under my probably typical light polluted sky, the 6” refractor offered the magic aperture that started to really showcase the better night sky objects. I would say that a reasonably good 8” reflector would offer similar performance on the clusters and deep sky objects as my 6” refractor does, and in general, would be MUCH more affordable and when dob mounted, far easier to set up and use. In my estimation, the 8” dob starts to look like what I would consider to be the best all around minimum aperture that I could recommend as a really useful city/suburban use scope. It is my STRONG belief that to start with a 4” of 5” refractor will only serve to LIMIT the beginning astronomer to the brighter solar system objects and double star targets.

Part two of my article deals with why I think that the marketing materials that tout the merits of small refractors as being perhaps suitable telescopes for urban or suburban dwellers is damaging, and it is directly related to my observations above. The reason why this marketing miss-information is damaging to astronomy is because many people new to astronomy might run out of INTERESTING targets for their small scopes quickly. As my example of M92 above Illustrates, there are great numbers of globular clusters that start to resolve at around 8” of aperture under even typical light polluted skies. Someone that takes these marketing statements as fact may just assume that there really is no point in buying a larger telescope, and maybe will even question the decision to purchase a telescope at all! And that, my friends, would be sad, because there are a great number of incredibly beautiful objects available to an 8” aperture under many suburban skies.

Now, I love refractors, and I own plenty of them. The small refractor is still a great scope for wide-field viewing, and the size of a 4” refractor on an equatorial mount is about as manageable as you can get. Also, I would be the first to admit, that there IS a nice aesthetic appeal to the view through a good quality 4” refracting telescope. I actually think that they are GREAT scopes for EXPERIENCED observers, because experienced observers already KNOW that the objects they are looking at are magnificent. But if your first view of some of these objects is with a small scope under light polluted skies, they may not be particularly impressive.

Don’t let incorrect marketing statements put you in a small aperture box which disappoints you by not providing access to a far larger number of potential observing targets. I also want to make this a call to all companies that market telescopes and all authors that write books focused on beginners to abandon this message that suggests that there is no practical gain over a small to medium refractor if the user lives under moderately light polluted sky. It is simple untrue, and if it causes even a fraction of beginners to abandon astronomy because they started too small and came away disappointed, then it is a serious disservice to the pursuit further popularizing amateur astronomy. I personally think that anyone except perhaps a child should make their first scope 8” or larger. After all, one of the same companies that says that a small refractor is a better choice for light polluted environments, sells a great 8” dob for $400! My experience suggest that most people (because most people today live under light polluted skies), would do far better starting with the larger scope so that their first visit with their OWN telescope to wonderful objects like M92 will be a MEMORABLE and MOVING experience, rather than leaving them to utter the dreaded words that I have heard more than once: “Is that all there is to it????” That one, my friends, is likely to get away, because the hook was not set properly.

I wish you a good day, and a better night.

My regards,
Ed Moreno