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Why aperture matters

August 15, 2012

Most people who have gotten as far as even doing research on buying a telescope can probably rattle off the two benefits of aperture: greater diameter allows increased angular resolution–the ability to resolve fine details–and greater collecting area simply gathers more light. But what do these advantages really mean? Just in the past month I’ve read a couple of explanations that gave me new ways to think about this, and that I think need to be more widely read.

The first comes from Richard Panek’s fine little book, Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (page 111):

Double the diameter of the aperture, and its light-gathering capacity increases fourfold; triple it, and the capacity goes up ninefold. At the same time the brightness of the object under observation depends on the square of the distance. Double the distance of a light source, and its brightness decreases fourfold; triple it, and the brightness drops off ninefold. The implication was clear: Double the diameter of the aperture, and you double the distance the mirror can see. Herschel understood that for the purpose of investigating the starry depths the major advantage of a reflecting telescope wasn’t in greater magnification, but in gathering more light–wasn’t in seeing more detail, but in seeing farther.

Okay, cool stuff. I knew that light grasp increases proportional to the square of the diameter, and that brightness drops off proportional to the square of the distance, but I’d never thought to put those two thoughts together.

Still, deep sky observers usually start with the bright stuff, like the Messier objects, and with sufficiently dark skies even the Herschel 400 are within reach of a 2-inch scope. Given that you’re already looking out more than 60 million light years to see the most distant Messiers, does seeing father really help?

There’s another piece to this, and it’s one I’d never considered until I read a very clear explanation of it by Ed Moreno (CN username Eddgie) on Cloudy Nights. Here’s the link to the CN post, but it’s so good I’m just going to copy and paste the whole thing here:

Make no mistake here. If you want to see more structure or detail in any kind of extended target such as nebula, galaxies, or planets, there is absolutely no substitute for clear aperture.

Almost all of the design discussions totally omit the function of image brightness/image scale that is offered by a larger aperture. We tend to discuss equipment like we don’t have eyeballs.

But we do have eyeballs… Or Cameras.

The image brightness and the ability to trade it for image scale is a huge part of the value of a larger aperture.

Keep in mind that a 16″ telescope and a 4″ telescope can both show the Orion Nebula with the exact same brigntness. Is all one has to do is use an eyepeice in each scope that has the same exit pupil.

For example, if I use a 16″ telescope at 100x, this will give about a 4mm exit pupil for the observer. In a 4″ scope, to get the same exit pupil (same image brightness), you will have to use 25x.

Now in the 4″ scope, the image is exactly as bright as in the 16″ scope, but the scale is only 1/4th as large.

Now is where it gets interesting. The structure inside the nebula is often very low contrast and very fine.

It is often stated on these forums that the human eye can resolve down to one arc minuted. This actually greatly overstates the eye performance of the scotopic eye for low contrast detail. This figure (one arc minute) is only achievable when the target is well illuminated and the contrast is 100%.

If the eye is in scotopic mode, and the contrast is very low, the eye may struggle to resolve detail that is smaller than about 3 or 4 arc minutes of apparent field unless the contrast is very high or the target is well illuminated.

There are two ways to increase the likelihood of the human eye (or camera) being able to resolve this very low contrast detail. They are to A: Make it bigger by magnifying it more, or B: make it brighter. Of course if you can make it slightly bigger and slightly brighter, this greatly enhances your ability to resolve fine, low contrast structure present in the extended target.

Now, lets take our 16″ scope and our 4″ scope and use the 16″ scope with our 4mm exit pupil. Remember, the illumination (determined by the exit pupil) is the same, but the angular size of any detail in the target will be four times as large. Detail within the target that does not have the angular magnification to be resolved in the smaller scope is now presented as four times as large. This means that there is now a lot more detail that has been magnified enough to be seen in the much bigger scope. Of course I can make the detail larger in the smaller telescope but this comes at the expense of loosing illumination (smaller exit pupil) which makes it harder for the eye to see the detail.

The other option is to make the image bigger and brighter in the bigger scope. Suppose I use the bigger scope with a 6mm exit pupil. Now, the exit pupil being larger gives me a brighter image. Once again, the eye likes illumination as one of the two ways to improve the ability to detect detail. When I make the image brighter in the larger scope, I see the extended target as extending over more area. The added brightness expands the extent of a nebula or galaxy. And guess what…. With the 6mm exit pupil in the 400mm scope, the magnification is 66x, so every detail present is still presented at almost 2.5 times the image scale as it would be in the 4″ aperture!!!!!! So now I have an image that is both bigger AND brighter in the larger aperture. I don’t care what kind of scope this is… It can be a refractor, a reflector, or a compound scope. If I make the image bigger and brighter, even if the contrast of the detail is exactly the same, my eye will have a better chance of seeing that detail if I make it bigger or brighter, or both at the same time.

Again, make no mistake about it… For seeing the most detail in extended targets of all types, clear aperture is king. You cannot isolate the telescope from the detector, and when the human eye is the detector, with a larger aperture you usually get both increased scale AND better illumination of the target and all of the structure present in the target.

I have owned perhaps 40 telescopes in the last 10 years, and for all classes of extended targets I would rank them in terms of performance in almost 100% agreement with the amount of clear aperture the scopes provided. I personally have never compared two telescope (even of different types) that the scope with the most clear aperture didn’t equal or exceed the performance of the smaller clear aperture.

There is no substitute for aperture. That is why the Hubble is a 2.4 meter instrument and not a 4″ refractor.

Aperture gives you image scale and illumination. It doesn’t even matter how you get that aperture (design). It is only important to have a lot of it.

Big telescopes simply show more. At the end of the day, if you want to see more structure in all class of extended targets, or if you want to see these extended targets at their maximum size, the best way to do this is to use as much aperture as you can manage.

The problem with refractors is that to get any meaningful amount of clear aperure (I recommend 8″ to 10″ for seeing a lot of structure in most deep sky extended targets) the expense becomes impractical as does the physical size of the instrument.

Everyone gets to use what they like, but the physics of image formation give clear aperture the best image formation, and lots of aperture gives the human eye the best chance of seeing any existing detail that might be present in the target.

There really isn’t any substitute for clear aperture. The more you apply to almost any target in the sky, the more structure or detail you will be able to see.

Now, I had experienced this, but I hadn’t thought about it–in fact, hadn’t known about it–in terms of brightness and image scale. And now that I have,  I agree with Ed completely.

I like small scopes. A lot. Maybe more than is healthy. But I will readily admit that my XT10 blows away all of my smaller scopes, on almost every target. Maks are often described as being fine planetary scopes, and they are. My 5″ Mak rocks on planets. But the XT10 just annihilates it on max detail. Even on the moon–I will put one of the smaller scopes on the moon and think I’m getting a good view, and then I put the XT10 on the moon and BAM! It’s the same field of view, but not the same view–suddenly everything is just bursting with details that I couldn’t see before. And now I know why–it’s not just angular resolution and not just light grasp, but rather the ability to get an image that is bigger, brighter, and more detailed, all at the same time.

Fortunately, I didn’t buy the Mak under the illusion that it would outperform the dob–I bought it because it fits in a space only a little bigger than a gallon milk carton, for those times when I don’t have room for anything bigger. And there is in fact one class of targets where I strongly prefer the Maks, and that is double and multiple stars. Oh, there’s no doubt that the XT10 will split closer doubles, period, and split wider doubles more easily, but I have not (so far) been going after doubles that are at the limit of resolution of a 10″ scope. And for things the Maks can split, they provide a more pleasing image than the dob, with no diffraction spikes. There is something entrancing about seeing a just-barely-split star as two clean little balls of light separated by the thinnest of black lines, surrounded by nice concentric diffraction rings but with no other visual noise to clutter the view.

So, dobs are cheap, but people end up buying other scopes for other reasons, and probably the two most important of those reasons are portability and aesthetic “cleanness” of view. Those are, in fact, the very reason that I own scopes other than dobs. BUT if you are interested in the deep sky–or pretty much anything else up there–and you’re on a budget, then clearly it is in your best interest to get as much aperture as you can afford. So this post is a deliberate follow-up to the last one; that one showed which scopes provide the most bang for the buck, and this one explains why you want as much bang as possible.

All of this is just part of an extended run-up to the Suburban Messier Project. I’m not ready to get started on that, not yet. I’m too busy with teaching, and my skies are lousy right now anyway. But I am thinking about the SMP, and when and how to best get started, and what information you may want or need if you’d like to tackle the project with me.

Until next time, clear skies!

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4 comments

  1. [...] think they don't have time for stargazing. « What’s on your bucket list? Why aperture matters » What aperture costs August 14, [...]


  2. [...] more than anything else. It’s one thing to hear that aperture rules, another thing to really understand why, and still another thing to discover that for yourself, out of your own experience. Like [...]


  3. “There is no substitute for aperture. That is why the Hubble is a 2.4 meter instrument and not a 4″ refractor.”

    Yeah, but for slightly different reasons than for the eye: With Hubble, you can use a longer integration time for the camera(s) (unlike the eye) and go make it a diffraction-limited system.

    Of course larger aperture => shorter integration time => more objects to image per time, which is nice if you put a multi-million (billion?) dollar telescope in LEO.

    And if you do astrophotography here at Earth, than the diffraction limit probably matters more than brightness – that is until you want to see what you photograph.

    (Which reminds me of the time I took a photo of a unlit industrial building, when it was to dark to see *anything* in the DSLR-viewfinder with the naked eye, and I had to take 30sec/1600ISO open aperture images and view them on the TFT, just to compose and focus the final 4min/100ISO aperture-stopped-down photograph…)


  4. […] A lot of people, myself included, find that eyepieces become a lot less comfortable to use when the exit pupil gets under 1mm. In contrast, large exit pupils are very comfortable because you can move your eye around a bit without losing the light beam. Most veteran deep-sky observers recommend an exit pupil of about 2mm as the optimum for picking out faint details. This is explicitly a trade-off between brightness and image scale: lower magnifications offer a brighter image but smaller image scale; higher magnifications give a larger image scale but spread out the light so the image is dimmer. The only way to beat that trade-off is to move up to a bigger scope, which will let you have a brighter image at a larger image scale. That’s why aperture matters. […]



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