Let’s talk about the physics and the physiology involved for a minute.
I’m an armature astronomer with my own observatory in the backyard so I’m pretty well nerved out on how optics work.
There are several things that help produce better, sharper, images.
1) Resolution, or the resolving power, of a spotting scope is an artifact of its objective lens diameter. The larger the objective lens the smaller the details it will be able to make out.
So the larger the objective lens the better.
2) Lens quality also matters as a high quality objective lens will produce sharper images than a low quality objective lens of the same diameter. Once a lens is producing the sharpest possible image it is referred to as “diffraction limited”, which just means physics says the image can’t get any sharper.
So the larger the objective lens the better, *as long as the quality is decent*
That creates some potential compromises as given equally good quality, larger objective lenses cost exponentially more to make. For example an 80 mm objective is only 20mm larger than a 60mm lens, and is only 33 percent larger. But it might cost twice as much to make the lens to the same level of quality.
3) There’s also the focal length. Longer focal lengths either require longer tubes or a series of mirrors or prisms to bend the light back and forth inside a shorter tube. Since spotting scopes need to be reasonably short, they have short focal lengths usually no longer than f/5 and that has an impact on depth of field and in turn how much distance can be in focus at the same time, and how precise the focus has to be to get a sharp image.
So, a short focal length spotting scope needs a focuser that is precise. That’s also a trade off as more turns to focus is less convenient, and a focuser that is too slow with lots of turns lets your eye adjust and makes it hard to precisely focus. Ideally it will have a larger knob for quick focus and then a smaller knob stacked on top for precision focus, not not many spotting scopes have that feature.
But even with a single speed focuser you want one that doesn’t have a lot of hysterisis or sloppiness in the adjustment.
4) More magnification isn’t better, and exit pupil is very important. Way too many people think that more magnification will make small things easier to see and that’s not the case. Exit pupil is the reason.
Exit pupil is the objective lens diameter divided by the magnification. In other words a 60mm spotting scope at 20x will have a 3mm exit pupil. In comparison a 80mm spotting scope at 20x will have a 4 mm exit pupil.
On a bright shimmy day when the pupil in your eyes is around 2.5 mm in diameter it won’t make much difference.
But on a heavy overcast day when your pupil is about 4mm the difference is significant. The 4 mm exit pupil of the 80 mm scope will fully illuminate the retina in your eye, while the 3mm exit pupil in the 60mm scope will not fully illuminate your retina. The end result will be an image that looks both grainy and dim.
With a 60 mm objective that over cast day, the magnification would need to be reduced to 15x to get that same 4mm exit pupil and a bright, non grainy image.
So…a larger objective not only produces a sharper image, it also allows you to use more magnification, and more magnification under lower light conditions.
5) Fixed versus variable magnification - two things are important.
First, looking at a telescope also keep on mind that the realistic maximum magnification is going to be what it can produce with a 4 mm exit pupil. 50mm = 12.5x, 60mm = 15x, 80mm = 20x. Most variables will advertise and offer magnifications twice that or more. But you’ll rarely be able to use that much magnification and get a sharp image.
Even when you can, it’s a lot easier to use the scope of its putting out a much larger exit pupil. For example an 80mm scope at 15x produces a 5.3 mm exit pupil and on a reasonably bright day when your pupil is 2-3mm wide, the extra room makes viewing through the scope easier.
A longer, wider eye relief also helps and that’s an artifact of the eye piece design, with more expensive multi lens designs costing more money, but giving an easier to work with eye relief.
Second, fixed magnification spotting scope designs make it much easier to keep all the lenses in alignment, and they cost less to produce to a given level of quality. You might for example be able to get a more expensive eye piece that offers better eye relief for the same money as a variable magnification scope.
But you need to be careful and make sure the fixed magnification is has isn’t exceeding the exit pupil and magnification limits discussed above.
6) Lens coatings have a big impact on image quality and brightness. Lens coatings reduce the amount of light that is reflected back by a lens. For example an un coated lens might reflect 10% of the light that passes through it (about what automotive glass does). With a single lens it’s not a big deal. But when you have an optical doublet in the objective lens plus a three more lenses in the eye piece, you’re now down around 59% of your original light.
Good lens coatings can reduce that reflection to around 1-1.5 percent per lens. With 1.5 percent and 5 lenses, the light through put is around 93% compared to just 59%. At 1% the through put with 5 lenses is 95%, so it’s better but there’s obviously a point of diminishing returns.
Advertising is also important. “Coated” means the objective lens is coating, perhaps only on one side. “Fully coated” means all the lens surfaces are coated. “Fully Multi-coated” means all the lens surfaces are coated with two or more coatings.
Each lens coating works best at a certain wavelength and multiple coatings ensure there is minimal internal reflections at wavelengths for red, blue, and green light.
So you want fully multi coated lenses. Beyond that lens coatings are very proprietary and good coatings can produce very nice color fidelity and brightness.
7) there’s also different optical systems.
The rainbow produced by a prism happens because the light refracts and separates in the prism. The same thing happens in an objective lens. It bends the light and the different colors then come to focus at different focal lengths. That’s where a long focal length telescope (f/9 or higher) is better as it will have very little chromatic aberration. But a spotting scope at f/5 or so will have a fair amount of chromatic aberration.
It creates both a loss of focus (as the different wavelengths focus at different points) as well as false color which will be see as a purple fringe around a bright white object. Some folks don’t notice it much, others notice it a lot.
Achromatic lens systems reduce the chromatic aberration by using a double of flint and crown glass in the objective lens to bring two of the wavelengths of light to the same focal point with sharper focus and less false color fringing.
Apochromatic lens systems use a three piece triplet in the objective lens to bring three wavelengths (red, blue and green) of light to the same focal point with an even sharper image and no false color fringing.
There’s also ED glass which is “extremely low dispersion” glass. That just means the objective lens doesn’t spread the focal lengths of the different wavelengths as far apart. ED glass costs more than regular flint and crown glasses, but it means an achromatic lens system using ED glass can produce excellent images that rival a much more expensive Apochromatic system.
8) So far we’ve been talking about refractor telescopes that uses a lens to bend the light. But there are also reflector telescopes that use a curved mirror.
Again, objective size matters. Bigger mirrors collect more light and create sharper images.
The same focal length issues apply as well but since they reflect the light back in a single mirror Newtonian telescope and back and forth in a Schmidt Cassegrain or Maksutov Cassegrain telescope the focal length can be 2-4 times as long in the same length tube.
Most Cassegrain design scopes will have focal ratios of f/8, f/9, f/10, f/12 or even f/14 or f/15. Those longer ratios make focusing easier by increasing depth of field. Since they also use mirrors rather thanon objective lens they don’t have any chromatic aberration.
But the downside is that they are less robust and may need periodic collimation (alignment) of the primary mirror.
Another downside is that Cassegrain scopes have a central obtruction in the corrector lens at the front to support the secondary mirror. That obstruction reduces the image sharpness/ resolution of the scope.
So, no false color or chromatic abberation, but less image sharpness. But…if it’s got a big enough mirror, it makes up for it. The Maksutov Cassegrain design also uses a smaller secondary mirror and will give a charger image than a Schmidt Cassegrain of the same size.
In short, a 5” / 127mm Maksutov Cassegrain can make a very impressive spotting scope and can be had for around $500-$600. The Mak-Cass style scopes also travel better than the Schmidt-Cass scopes with less risk of losing colimation.
A 90mm Mak-Cass is also a good option.
These scopes all come with separate eyepieces and you can use different eye pieces to change the magnification. Plus you can get a good quality eye piece with excellent eye relief and viewing comfort.
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What’s that mean in the real world?
For one it explains why some very old scopes keep getting recommended.
For example the Bushnell Spacemaster II is a 50 year old scope, but the optical and mechanical quality was excellent and with the 60mm objective and a 15x eye piece it offered good resolving power and exit pupil in a compact and durable scope. (You could also screw different eyepieces in the back, but anything higher than 15x came with compromises.)
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I have a Vortex Diamond-back 20-60x85 spotting scope and still have it. It’s not a bad scope.
However, I prefer my Leupold 20-60x80 SX-2 Alpine. The lens coatings are better, it gives a brighter image with better color, and the eyepiece is better with more comfortable viewing (even though the eye relief itself is about the same). The focuser is also better on the Leupold. It offers more bang for the buck in my experience.
I use it mostly at 20x where it has a 4mm exit pupil and if it were a fixed power scope I’d be just as happy with it.
