>>>The two scopes are considerably different. One has a 1900mm focal length, the other 1200. One is around f/15, the other f/5. My question is this: What does this mean in practical terms to someone who doesn't yet understand all the physics?
Does one factor lend greater resolution than the other? What is the impact on eye relief (correct term?) or how far do you have to get to the eyepiece to see the whole view. (Some of the users wear glasses.) Does the lower f/number mean better contrast? What other things should I be asking, but don't even know about?
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Dennis:
You asked some good questions and I think I can answer them in practical terms. I have owned an ETX-125, I own a GSO 10 inch F5 DOB similar to the XT-10 and did own a XT-8...
In practical terms the most important factors to consider are the amount of light a scope captures, the scopes resolution and the scopes maximum field of view. The more light a scope captures, the brighter the image, the better the resolution, the better the detail, the larger the field of view, the easier it will be to find targets and the larger targets one can see.
In each case, the advantage goes to the Orion Dobsonian.
The size of the mirror determines the amount of light a scope gathers. In this case the ORion XT-10 will capture 4 times as much light and therefore at any given magnification will be 4 times as bright. (This corresponds to a factor an exit pupil that is twice as large). This is a huge advantage and means that you will be able to see about 1.5 magnitudes deeper. It is particularly important when looking for faint targets like galaxies and nebulas but even on bright targets like Saturn and Jupiter, it allows the use of higher magnification.
The difference between the ETX-125 and a 10 inch DOB on DSOs is big. Stuff one can barely see in the MAK can be bright and obvious is the larger scope.
Resolution is a direct function of aperture. A scope with a mirror that is twice as big will have twice the possible resolution. This means the XT-10, under ideal conditions is capable of resolving more detail on planets and splitting tighter double stars.
In practical terms, resolution is often limited by the "seeing" ie atmospheric condition so a large scope may not always be able to take advantage of the potential for better resolution. But, comparing the views on Jupiter, the XT-10 would be brighter and sharper in any event.
The field of view is related to two things: The focal length and the eyepiece format. A long focal length limits the possible field of view and 1.25 inch eyepieces are limited in comparision to the larger 2 inch format.
What this means is that the XT-10 with its 1200mm focal length and 2 inch focuser is capable of views of greater than 2 degrees, enough to see the Pleiades for example, while the ETX-125 is limited to about 0.84 degres, about 1.6 times the diameter of the moon.
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As an ex-owner of an ETX-125 and as someone who has helped first timers with ETX-125's I literally CRINGE every time I see someone like yourself considering the ETX-125.
The 1900mm focal length combined with that 5 inch aperture and a 1.25 inch focuser means that this scope is really only capable of relatively dim images at high magnifications, just the wrong thing for a first scope.
Yes, it is not bad for Jupiter or Saturn but looking for Deep Sky Objects, scanning the Milky Way, well it is disappointing. Also, this scope is dependent upon its GOTO because fo these limitations...
In comparison, the XT-10 is capable of bright images, widefield images and bright high magnification views of planets that are filled with detail. With a 2 inch eyepiece, finding targets is not difficult.
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In the past few years, in my wanderings as a solidary observer I have had the opportunity to help several first timers who bought ETX=125s. Most always they are having some trouble getting the GOTO working and frustrated. My normal travel scope is that 10 inch F5 DOB to a person, first time ETX-125 owners are impressed with the views through the 10 inch...
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There are some things to be aware of when considering any reflector, be it a MAK or Newtonian.
1. Thermal equilibrium. The scopes must cool down to give the sharpest views. Because of the open design of the XT-10, it will probably cool faster than the MAK.
2. Collimation. All scopes must be collimated. MAKs like the ETX-125 normally hold their collimation quite well but re-collimation requires a trip to the factory.
Newtonians normally require a check and may an adjustment of collimation before each use. It does require some skill but a Newtonian can be fully collimated by the user.
Eyepiece Astigmatism: Faster scopes are harder on eyepieces because the steep "light cone" must be brought to a point. In an F5 scope like the ORion XT-10 this generally means that the edge of the field of view may not be sharp.
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Bottom line here....
I have spent close to an hour writing this...
Why I have I taken this amount of time??
Because as an ex-owner of an ETX-125, I would hate to see you spend your hard earned money on this scope only to be disappointed by the views.
Though it is often sold as a beginners scope, it is really a single purpose scope, (high magnification) best used by someone who is experienced and is knows that is all they want.
Five inch scopes can be great first scopes, no doubt in my mind about that, but one needs to choose a scope with a more reasonable focal length and hopefully one with a 2 inch focuser.
Jon Isaacs
