DOB VS Refractor

[PSALM19.1]

I'm quite sure this has been argued about for decades....but here goes!
I was watching an epidsode of Sky At Night (an English show - by the way, if anyone knows how to get episodes of this on DVD I'd be really keen to have some!) and Sir Patrick Moore stated his opinion that to view planets and the moon was better with a refractor than a reflector? In fact, he said that a 3" refractor was equivalent to a 6" reflector in planet viewing and that the image was crisper in a refractor. When I bought my 8" Dob I was considering a Celestron Nexstar at the same time: one of the staff at the store said that the 8" Dob was better than a 150mm Nexstar by "a country mile"....anyway, is it generally true that a refractor is better for planet viewing than a Dob? I mean, the views through my 8" of the moon last week were nothing short of AMAZING! Thanks all....:question:

[Wavytone]

There is no easy answer to this, as there are too many variables and the quality of the optics is very variable - there are a few that have outstanding optics, many very "ordinary" ones, and a few pretty awful ones.

The first and foremost rule is buy the most aperture you can afford. Then there is a practical limitation - the limitations imposed by transport/storage (will it fit in your car, and can you lift it safely if this is to be portable) or the size of your observatory (permanent).

This must be tempered by the questions of focal length, overall physical length (catadioptrics) and weight, and the mount required to support it adequately. Dobsonians are pretty straightforward, the only real questions are the focal ratio and how good is the mirror. But a refractor optic opens up a minefield of possibilities:

- cheap achromatic doublet, vs apochromatic ED doublet or fluorite, vs air-spaced or oil-spaced triplet, vs Petzval design ?
- quality ?
- focal ratio ?

Then the question of the mount opens up a whole new minefield:

- is this just for visual use, or astrophotography ?
- do you want any means of finding things (eg setting circles) analog or digital, or are you going to "rough it" with a star atlas in hand and a torch in the old-school way ?
- do you want GOTO functionality ?
- do you want it integrated with a smartphone or tablet (eg SkyFi connected to an iPad running Sky Safari) ?
- computer interface (laptop) ?

Then comes the question of whether to go for a catadioptric telescope, e.g. Schmidt-cassegrain (the Nexstar is an example), Maksutovs (which I use for lunar and planetary) or Maksutov-Newtonians (excellent for astrophotography).

Lastly, if price is not an issue there is always a Questar.

To answer these properly you must first decide what you want to use the telescope for.

[PSALM19.1]

Thanks very much for that response...yes, seems as though there are many variables with refractors! I certainly (at this stage anyway) am not interested in photography (my daughter did snap a few very good photos of the moon through the eyepiece of my Dob however!); I just like studying planets and deep space objects...I guess my confusion is that if it's 8" of Dob apeture vs, say, 5" in a large refractor, how can the refractor give a better view (again; no photography here, just straight viewing) when the Dob is receiving so much more light? All a learning curve...but I am keen!

[glend]

Shaun, in terms of aperture value the DOb will win over the refractor. If your sticking with visual observation it's hard to go past the Dob, which is very easy to orient and understand. Where you will need to spend some money is in the area of the eye piece set as most lower end Dobs come with a pretty basic eye piece collection that just do the job but the light gathering ability of the DOB really comes into play when you add good eye pieces. Yes they can be more difficult to tranport than a refractor but is pretty easy to move a 12" dob around with a hand truck if your set up for that.

[PSALM19.1]

Thanks for the heads up (Glen) about eyepieces; recently upgraded one of mine to an Orion Edge-On 6mm and the difference kind of speaks for itself when compared to the Plossls that came with it. I simply can't afford TeleVue's at this stage, so I believe that the Orion Edge and Stratus range are alot better than standard Plossls? My Celestron 32mm does an excellent job as my "finder" eyepiece...I think I'll be looking for a 9mm or there-a-bouts by Orion shortly. Are there other fairly good EP's in that $120 to $180 dollar range?

[omegacrux]

There is a Ultima lx 8mm celestron on Icetrade
It is seriously good value ,a big lump of ep

David

[barx1963]

Shaun
Planets are bright objects so gathering light is not really the issue. Refractors have no central obstruction that Newtonians have, and that improves contrast, which can be helpful with planets. Also refractors can have fairly long focal lengths for their aperture and can handle reasonably high power. As planets are small, more power can be valuable.
Of course, refractors can be subject to colour fringing which was the specific problem the Newt was invented to solve.
From all this you can gather that there is no perfect scope, all involve some compromise and have strengths and weaknesses.
My main argument for getting a dob is that there are '000s of objects visible with even a small dob, while there are only 7 planets (and only 3 of those are really worth observing much!), and dobs still make reasonable planetary scopes.

Malcolm

[glend]

Shaun have a look at the GSO Superview eyepieces they are excellent value for $. Ihave both the 2" 30mm and the 15mm and I find I use those more than anything else, including my TMB 9mm.

[Robh]

Crisp views depend as much on seeing as on the telescope. Thus, the high magnification that in theory a larger aperture should allow is rarely attainable and often restricted to below 200x.

However, a larger aperture will gather more light and will give better resolving power. You probably don't need the extra light for the planets but you will for some of the moons e.g. Neptune's moon Triton, which is magnitude 13.5.

An 8" Dob will have twice the resolving power of a 4" refractor. You will pick up finer surface detail in any object that you look at. The extra resolution becomes quite noticeable when looking at the detail within the Homunculus (eta Carinae) with a 30" Dob. You just cannot get this detail in smaller apertures.

Regards, Rob

[PSALM19.1]

Wow, thanks all! I've owned a First Scope (which was my first scope..hahaha) (sorry) a 60mm refractor and now an 8" dob, and obviously, the dob is vastly superior. Actually, a first scope has a larger aperture than the 60mm refractor, but could not give the views that the refractor gave, so there's a point - smaller aperture but better size and detail! Will check out the EP's mentioned...thanks - and if I could, a plug for the Orion Edge-on 6mm ep, very very good on moon viewing!

[ZeroID]

+1 for the GSO Superviews. I have the 15 and 20 mm and bang for buck, they are excellent. New, you should be able to get both for about $100, for the pair that is, not each !
(they are a bit dearer over here in NZ but still worth the $$)