There is a saying in astronomy that aperture rules. Larger apertures collect more light and have higher resolution than smaller ones. This means they can detect fainter objects and resolve finer detail.
There is also a condition called Aperture Fever when astronomers just want bigger and bigger telescopes! So where is the balance?
Dobsonian telescopes are popular mainly because they provide the largest aperture for the price. 8- and 10-inch models are especially favored for their balance between price and performance, while still manageable for one person to handle.
The 12-inch Dobsonian, on the other hand, is the gateway to the truly large-aperture instruments.
Brightness increases quite a bit when you move up in aperture.
I used to own an 8-inch Dob: the Orion Skyquest XT8 Classic, which has been replaced by the SkyLine 8” Dobsonian (affiliate link). It was a wonderful telescope which provided me with over a decade of observations under the stars.
A few years ago, I upgraded to a 12-inch model (this is the SkyLine model), the Sky-watcher 300P Synscan. In this article, I’ll explain some differences in the views from each telescope and examine the pros and cons of upgrading to a larger aperture.
The actual differences between these scopes will depend on much more than just aperture.
We also need to account for viewing conditions, collimation, quality of optics, and the visual acuity of the user.
By which I mean, your mileage may vary.
However, this is what you can expect in reasonably good conditions.
Things to See With a 12 Inch Telescope
Below is a list of some of my favorite objects to view through my 12-inch Dob, with notes on the visual differences I noticed between these telescopes (comparing from memory):
Moon
Though large-aperture Dobsonians are viewed primarily as visual instruments, they are also wonderful for observing the moon.
Compared to a more typical lunar/planetary telescope, such as my old ED100 refractor, the 12” shows an abundance of fine detail.
I will never get tired of exploring the tiered rim of crater Copernicus, or the multitude of tiny craterlets scattered in the volcanic plains around it. There is an endless number of features to discover once you get that perfect night when you can crank up the magnification.
A rough comparison of a 12” Dob to a 4” refractor. Though we shouldn’t draw too many conclusions from these images, as one was shot with a cell phone and one with a DSLR, you can still see the difference in the level of detail around Copernicus.
Planets
Most of my best views of the planets have been through the 12” Dob.
Once again, the larger aperture reveals subtle details more easily than with either the 8” Dob or the 4” refractor, while the larger exit pupils it provides at high power make for brighter images and more comfortable viewing.
These pictures hardly do justice to the real-time view through the 12” Dob.
Jupiter reveals intricate details in its cloud bands, hints of those swirls and festoons usually only visible in photographs.
The Cassini Division on Saturn is easy, while the ring’s shadow on the planet and subtle shading of its cloud bands stands out nicely.
Stars
According to published specifications, a 12” Dobsonian has a limiting magnitude of 14.9, compared to 14.3 for an 8”.
This seems like a small difference, but in fact it means that the larger scope can see stars which are around twice as faint as what the smaller scope can detect.
Consequently, Milky Way starfields appear richer with background stars.
Star Clusters
Here is where the 12-inch Dob really shines. Even viewed from the suburbs, globular clusters are a magnificent sight.
In fact, they appear almost as they do in photographs, with stars resolved to the very center. The resulting view can seem three-dimensional, like a diamond-encrusted jewel or a firework frozen mid-explosion. M3, M5, and M13 leave me gazing in slack-jawed wonder.
Open clusters are also enriched, with fainter background stars joining the party.
Nebulae
In my experience, the main difference with viewing nebulae through the 12-inch compared to the 8 is brightness.
Objects like the Ring, Dumbbell, and Swan nebulas are very bright and distinct; M57 is a real treat. Compact planetary nebulae like the Blue Snowball and Cat’s Eye will benefit from the higher magnification capabilities of the larger aperture. Diffuse nebulae like the Lagoon and Veil will also show more readily in the 12”.
I should point out, though, that all of these objects showed very well in the 8” model.
Galaxies
Galaxy-hunting can be a tedious task for seemingly small reward, since most galaxies appear as little more than faint smudges through backyard telescopes. This is still the case with a 12” Dob, but you can pick out a greater variety of obscure NGC galaxies with the larger optic.
From a dark site, you may glimpse the spiral arms in the Whirlpool, or the majestic dust lanes of Andromeda.
Other Messier galaxies like M81 or the Leo Triplet may reveal some detail, but it still takes a trained eye, dark skies, and a little imagination.
For me, the thrill in galaxy hunting is simply in the detection of these incredibly remote objects, even if detail is minimal. A 12-inch scope opens up options not available in a smaller model.
Disadvantages of a 12-inch Dobsonian
Like everything else in this hobby, there are trade-offs to be made when choosing a 12-inch Dob over a smaller model:
Size and weight
A 12-inch Dob weighs roughly twice as much as an 8-inch and is around 30% taller.
This means they are more difficult to transport and set up. If space is a concern, I would recommend checking out a collapsible or truss tube design (affiliate link) such as the Orion XX series or Sky-watcher Flextube.
Cost
The price of Dobsonian telescopes rises rapidly as you get into larger apertures. Expect to pay around 3 times as much for a 12 as a comparable 8.
Fast focal ratio
To keep tube lengths manageable, most 12-inch Dobs are around f/5 or faster.
While this is not in itself a bad thing, it means your scope will be less tolerant of collimation errors and will also require eyepieces with better edge correction if you expect a sharp image right across the field.
Longer cool-down time
Your primary mirror needs time to reach thermal equilibrium after you put it outside. The larger the mirror, the longer this takes.
