Telescope Reviews: 8 vs 10 vs 12 Real Comparison Real Skies Real DSOs

There are always and frequently posts by folks wanting to know about buying a scope and aperture/size. Most often it's 8" vs" 10" vs 12.5"

Over last few months and years I have had opportunity to compare views of various DSO's in several scopes. The scopes were all truss dobs:

8" with excellent Meade mirror f/6
10" with Meade research grade mirror f/4.5
12.5" with Zambuto mirror f/5

All scopes were well collimated when tested. Same ep's were used, too. All used Baader Hyperions 8, 13, 21, and 31.

Aperture won on many targets, but not all. In all, the images are larger with the larger scopes and you get more magnification with larger scopes, but on many targets the 8" is quite satisfactory.

Aperture was the dominant factor on all globulars. If globulars are your thing, buy the biggeest scope you can buy. You'll be disappointed on globulars in an 8". The larger globulars look like a dirty snowball (and small) in the 8". In the 10" you get some resolution and patterns of stars on M22, M28, M92, and M13. In the 12.5 you get lots and lots of resolved stars. The differences on globulars between scopes of different sizes is a large difference. Aperture absolutely rules on globulars. But - the 10" does quite well. The difference between the 10 & the 12.5 was not as large as difference between 10 & 8

Open clusters were the best in the 10" f/4.5. Some of best objects I viewed were in the 10 looking at M11 Wild Duck Cluster with a 13mm Baader Hyperion ep. Same for M23 M24 & M25, and the Double Cluster in Perseus. An f/4.5 mirror and an ep with a large fov blew away the other two scopes. If open clusters are your thing, a fast 10" scope with good optics and wide field ep's will keep you coming back for more night after night.

How about nebulae? On M8 Lagoon & M17 Swan the little 8" does quite well. It yielded a very bright albeit small image. You can see the Swan as a swan (which btw you can't on 4" - 6" scopes. In an expensive 4" APO refractor M17 looks like a diffuse rectangular shape). The 8", though, does have trouble yielding the dark "lagoon" area in M8. On M20 the 12.5 won easily - in both the 10 & the 8 it was a very dull object with no wow factor.

On M16 Eagle Nebulae the 8" could not find it. The 10" and 12.5 easily found it, and there was not much difference between the two scopes.

How about M57 Ring Nebulae? You can see it in the 8; it's a tiny specs with a slightly darker center. But you can't "observe" and study it. On Ring aperture difference between the 8 and the others was very evident. Even on the 10 the difference between it and the 12.5 was very clear. The 12.5 yielded very very large and detailed image of The Ring and the low magnitute star just outside the ring was brighter in the 12.5 on a night of marginal seeing. Still, on a better night of seeing the difference was a lot smaller. I'd still have no hope of seeing the 15th mag star in middle in the 10, nor the other two faint stars in the smokey ring, but with the 12.5 you can spend an evening looking for them.

On M27 Dumbbell I saw a big difference between the 8 and the others. Dumbbell, IMHO, always takes magnification well. But with the 8" the image is still small. I got a huge image in both the 10 & the 12.5 and they were very similar. I could not pick a winner between the 10 & the 12.5 on M27 Dumbbell.

NGC 6960/6992/6995 Veil is best viewed in the 12.5. It will take your breath away on a dark nite with good seeing. The 10 does well, but not as well as the bigger scope. The 8 did about as well as the 10.

M31 Andromeda - I prefer the 10" f/4.5. On a good night the core was bright and you could see dust lanes - image was very comparable to the 12.5. The 10 gave a great image even with a higher mag 21mm Hyperion ep: I could frame M31 M32 & M110 all in same fov in that ep, get a lot of magnification on M31, and still get a wide fov. It was one of best objects I observed, and it was best in the 10.

Conclusions: the 12.5 may be be the best overall "small" size scope in terms of portability vs aperture and resolution. If size is a concern, the the 10 is a remarkable scope and will yield a lifetime of pleasing evenings under dark skies. You WILL NOT be disappointed with a 10" fast reflector, but you may be disappointed with an 8" and get aperture fever quickly. From my comparisons, there is not much difference in size, easy of transport, and cost between an 8 & a 10 but there is a huge difference in performance. The 8 I used was nice with excellent optics, but it comes up lacking in terms of performance on DSO's.

I will qualify my conclusions by stating that one variable - focal ratio - was not the same in the 3 scopes. The 8" was f/6 and 10 was f/4.5, while 12.5 was f/5. This is a factor on some objects, yielding a larger fov on such objects as open clusters and M31.

Hope this helps all y'all out there contemplating a scope purchase and you're looking at these sizes.