I used to have a 6" Dobsonian reflector, and the next telescope I buy, I want it to have the equivlant of about a 11/12" dobsonian reflector.

I'm looking at this Nexstar VX 9.25 , it says 235mm aperature, but is thats more than what a 235mm reflector would acheive right? How close is to a 11/12" reflector?

  • 1
    $\begingroup$ Can you clarify the question a bit more? The Nexstar VX9.25 is a reflector. It also isn't clear what you are asking regarding the aperture - 235mm is 235mm, one isn't 'more' than the other because one is a SCT and one is a dob. $\endgroup$
    – Jeremy
    Jul 15, 2014 at 11:39

1 Answer 1


Comparing optical instruments is very difficult. There are numerous variables involved, the performance is not a single number, and the theory is complex. Perfect scenario to baffle amateurs. Also, it sounds like you may have been listening to the wrong rumors.

The architecture of a telescope (refractor, reflector, catadioptric) is one of the least important factors in the performance analysis, regardless of how you define "performance".

Don't forget that how you maintain and where you use the scope are very important factors. Collimation will strongly impact resolving power - so make sure it's always perfect. Light pollution will impact you much more than light absorption within the instrument. Etc.

In terms of amount of light gathered, aperture is the dominant factor. 2x the aperture is 4x the area, so it's obviously very important. The central obstruction may factor in a little, but the effect is small. Going by diameter, a typical newtonian will have a 20% central obstruction; a typical catadioptric scope will have a 40% c.o. But if you translate that into areas, the newtonian is at 4%, whereas the catadioptric is at 16% - both are very small.

In terms of resolving power, again aperture is king. Bigger aperture = more resolving power, no matter what the architecture. But poor collimation will quickly destroy your gains, so keep that scope collimated.

In terms of internal losses, the newtonian will do a bit better - it's two reflections and that's it. The catadioptric will have two or more reflections (depending on design), plus absorption / reflection on the diopters. Again, with modern instruments, the difference is tiny.

Overall, at the same aperture, in theory, the newtonian pure catoptric scope delivers a bit more light to the sensor, compared to a catadioptric scope. In practice, the difference is far outweighed by other factors and you should completely ignore it.

You should make the purchase based on more important things. What's the size of the usable field? What kind of adapters can you use? Does it provide enough back-focus? What's the aberration in different points within the field? Is the telescope too heavy for the driven mount? Is it long focal ratio, or short? (each kind has different applications) Finally but not least important, price?

It's more common to see catadioptrics used as astrographs. That's not to say that a newtonian reflector cannot do the job - but if you're still new to the field, just do what most people do and get a catadioptric, with a good mount.

Final note:

When doing prime focus photography, the brightness of the image is given by both aperture and focal ratio - but not all objects behave the same.

For stars and other point-like objects (smaller than the resolving power), brightness is driven by aperture. Bigger aperture = brighter stars.

For extended objects (bigger than the resolving power), such as nebulae, many galaxies, the Moon, the planets, etc., the brightness of the object is given by the focal ratio. An f/4 scope will make brighter nebulae in prime focus, compared to an f/8 scope. Aperture doesn't matter. It's just that, at the same focal ratio, the bigger scope will also make a bigger image in prime focus.

This is true only for prime focus photography. If you're doing eyepiece projection, or something else, different rules apply.


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