Objective relationships between telescope types

[JulianFR]

Hi... Can anyone explain for me the light-gathering power relationship between different telescope types? For instance, Schmidt-Cassegrain / Maksutov type telescopes generally seem to have smaller diameter objectives than Newtonian reflectors, so does that mean they are less able to gather light and so are less 'powerful' as light buckets than reflectors?

To put it another way, I have an 8" Dob, so what size would the equivalent Schmidt / Mak be and what size would constitute an upgrade observing-wise?

I'm not intending to change scope any time soon; I'm just curious.

Many thanks, Julian.

[cantab]

Aperture is aperture.

There's some effect due to the central obstruction, SCTs and Maks tend to have bigger obstructions than Newts, while refractors have none. But in terms of area, and thus effect on light-gathering, it's small.

So an 8 inch Dob, an 8 inch SCT, and an 8 inch refractor will give comparable deep-sky views.

The general rule of thumb is that going up "one step" in scope size won't bring much of an improvement, but "two steps" is noticable. So you'd want a 12 inch scope to really notice an improvement over the 8 inch.

[ronin]

1:1

A 6" in one type collects the same as a 6" in the others.

[nightfisher]

Aperture is aperture.

There's some effect due to the central obstruction, SCTs and Maks tend to have bigger obstructions than Newts, while refractors have none. But in terms of area, and thus effect on light-gathering, it's small.

So an 8 inch Dob, an 8 inch SCT, and an 8 inch refractor will give comparable deep-sky views.

The general rule of thumb is that going up "one step" in scope size won't bring much of an improvement, but "two steps" is noticable. So you'd want a 12 inch scope to really notice an improvement over the 8 inch.

thats mainly correct, but another thing to consider is contrast....a Mak or frac will have better contrast than a same size newt, also large Maks and SCT`s have very long focal lengths, making them not quite so good at deep sky work compared to a newt

[Moonshane]

I don't think there are any major differences between different scopes of the same aperture and all will show the same sort of detail. focal length is what makes a large difference. for scopes of the same aperture it either makes a scope more compact or harder to mount, increases or decreses available field of view, increases or decreases the required secondary size and exit pupil. in my experience, contrast is enhanced (at least aesthetically) with a slower scope although there is more detail with more aperture. stars are also tighter points with a slower or unobstructed scope.

so these things are what determine my scope choices rather than aperture or telescope type although for me, viewing comfort is also essential and I find newtonians on a dobsonian mount the best of all worlds for visual (had to get that in somewhere eh?).

[ollypenrice]

Yes, aperture is aperture, related to central obstruction. I think it's fairly uncontrovertial (though don't count on it!!) to say that premium apochromatic refractors punch above their weight but (honest Olly) also punch miles below their price weight!!! The fact that I have a large Newt, a medium SCT and assorted refractors does nothing for my objectivity. I'm as biased and unreasobnable as the rest of them!

Runs for cover...

Olly

[ScottS]

1:1

A 6" in one type collects the same as a 6" in the others.

Yes, this I would say. It's how the different types of scope manage that light that matters.

[JulianFR]

Thanks for these explanations ... Very interesting and not what I was expecting. I had assumed in my ignorance that Maks etc have some inherent superior resolving power, which enables them to have smaller diameter objectives for the same effect as a larger Newtonian.

I think this shows that Dobs are remarkably good value for money even if Mak-type telescopes and refractors have important strong points. Maybe if I win the National Lottery I'll buy an 11" Mak to accompany the Dob. Presumably the price of an 8" refractor would be (no pun intended) astronomical!