Telescope Reviews: "Aperture is king"

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 absolutly no substitute for clear aperture.

Almost all of the design discussions totally omit the function of image brightnss/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 brigntness 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 Oroin 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 performnace 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 likelyhood 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 magnificaiton 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 brigter 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 almsot 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 class of extended target and 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.