I am a newbie as far as astronomy is concerned. I have read widely about scopes and the strengths and weakness of refractors v reflectors arising from this reading it seems that a 130mm refractor is roughly the equivalent of a 12" reflector in terms of image quality. Is this a correct interpretation of the rough/relative comparison between the image quality of refractors v reflectors. Or is it a result of my miss reading the relativised comparison between the two types?
Again from what I have read, I would much prefer to lug a 6" refractor around, rather than a 12" reflector.
I'll leave the details to those more experienced than I but it's not as simple as that. Scopes of all types can vary greatly in quality and the quality of the views they produce. Your choice of scope depends heavily on the type of observing or imaging you want to do.
I am a refractor kind of guy , see my sig , anyway there is a lot of plusses and minuses on both a refractor and a reflector ,
-'s on the frak , is CA ( unfocused colour on bright objects) , to get colour free you need an APO and here lies
-#2 , COST !!! an APO over 100mm or so gets very expensive and at the 6 inch size you are looking at 8-10k OTA alone , this brings us to
-#3 a Mount , to mount a 6 inch refractor you need a BIG ! mount , so in reality a 12 inch Dob is probably easier to lug around and set up true .
Ok Reflectors ? , they suffer from a thing called 'Coma' that is the outer 25% of the mirror does not come to the same focus point as the centre and stars look like comets , this like CA can be fixed using adaptors or very expensive TV type eyepieces ,, this brings us to
-#2 , high power viewing is frustrating using a push to Dob if not set up perfectly ,
-#3 image quality , because the reflector uses a secondary there is not the snap ( contrast) a good refractor shows , I could go on and on and on but I will let others contribute here . .
There is no one perfect telescope , I like refractors ( and my C9.25 CAT ) because I am a Luna/planetary kind of guy and that's where refractors shine , IMHO ,, gotta tread carefully here , as a good 12 inch Dob will show Jupiter just fine . .
And David , to IIS by the way .
I would say a 10-12 inch dob for deep space and say an ED80 on EQ5 mount for a quick look scope and dabbling in AP . .
Many thanks Brian & Pluto. It's difficult getting my head around the fact that a 6" (refractor) opening to the heavens can produce as useable an image as a 12" (reflector) it seems counter intuitive. The amount of light falling into a 12" opening is far greater than a 6" opening it seems as if the refractor Is working some l"magic".
Refractors are generally regarded as having better contrast so for a given aperture size you should be able to make out fainter stuff, also a good refractor can give very sharp views. Of course for a given aperture a quality refractor is also more expensive, plus it gets hard to compare apples to apples when looking at larger apertures as they just don't make refractors as big as reflectors - and that's ignoring all the quality issues like CA, coma, etc...
I have noticed that younger people normally like reflectors (high light gathering, sees faint galaxies etc), but older people like refractors (appreciate the sharpness of the views, whilst also being a bit over the heavier scopes with shorter-lasting mirrors (10 years? verses a lifetime) and CAN more easily afford them).
Nevertheless, serious observers often buy at least one of each over time to sample the best of both worlds.
Although I am a refractor guy (yes, 60 next month), I have found that non-astronomer, casual viewers are usually far more impressed with the brighter views given by a larger reflector, than the contrasty views given by a smaller (but often more expensive) refractor.
However, if space is a problem or portability is an issue, then the refractor may be a better buy.
People can (and do) discuss the pros and cons of this at length and there will be many personal exceptions to the rough guides that I have given.
Anyway, the thing to remember is that there are many types of scopes and all have strengths and weaknesses and deciding on the best scope you you is a matter of deciding what is important to you.
Personally, I love a big newtonian. Aperture simply is the best way to go deep on the really faint fuzzies which i love observing so it makes sense to get a big dob for me. But they have limitations. To get the best out of them you need to be a reasonable dab hand at collimation, the mirror takes time to cool, the big ones mean climbing a ladder to use and take a few minutes to setup with truss poles etc.
Having said that there is nothing like the view through a big dob, or even a 12" on a good night.
David i like refractors myself - but i reckon the approx image viewing is close to refractor size x 1.6 or thereabouts
eg 3" frac = 5" newt
5" frac = 8" newt
the 6" will be a bit behind a 12" newt
however cooldown is one thing in the favour of a refractor -they generally cool quicker -& i find they are a bit better in average seeing..when the atmosphere isnt great for viewing
under excellent conditions the 12" should be much brighter than a 6" frac
also the mounting of a 6" frac is no small feat [its a big heavy tube]-the dobsonian mount is much much cheaper & just as small
the viewing position of a frac can be close to the ground also
Many thanks Brian & Pluto. It's difficult getting my head around the fact that a 6" (refractor) opening to the heavens can produce as useable an image as a 12" (reflector) it seems counter intuitive. The amount of light falling into a 12" opening is far greater than a 6" opening it seems as if the refractor Is working some l"magic".
The reason you can't get your head around it is because it isn't true.
A 12" reflector has four times the light gathering power of the 6" refractor, and will see far more galaxies and other faint DSOs than will the refractor.
A 12" reflector has far more resolution than a 6" refractor, and will be able to split tiny little double stars that are invisible in a 6" refractor.
But reflectors have a central obstruction which affects resolution and contrast, compared to the same sized refractors. This means one needs bigger reflectors to get the same good view of a planet. But by getting bigger reflectors, one gets other problems like air currents in the tube and more susceptibility to poorer images from the atmosphere.
Dickenson and Dywer in the "Backyard Astronomers Guide" suggest a good 5" APO refractor is comparable to a good 8" Schmidt Cassegrain for doing stuff like looking at planets. But the 8" Schmidt Cassgrain will kill the refractor for stuff like looking at galaxies.
Refractors images are satisfying because one gets really nice views of stars/open clusters with lots of points instead of the lots of blobs that one may often see in reflectors, particularly early in the night. And the more contrasty view of planets compensates for the lesser resolution, relative to a reflector.
That said, the best planetary views I've ever seen bar none were through a club member's 10" reflector which he put together with the best parts, and which he had optimised for planetary viewing
Regards,
Renato
6" refractor is massive and heavy.
As a beginner one of the main advantages of refractors - IMO - is there is no need to collimate your instrument.
There are so many scopes out of collimation out there!
The best is if you could have a look through both and then you can make your mind. Compared to a cassegrain design, the refractor will be so much longer though, so you need to consider things like: storage and importantly whether you plan on observing from your backyard in a metro area or out in the sticks - will you then transport the whole setup with you? There is no point IMO in having a big scope in the city - i would rather take a smaller refractor or a mak and stick to planets in the metro area. So it kinda comes down to what you are looking for as well.
Last edited by Sylvain; 28-01-2015 at 11:02 PM.
Reason: typo
Newts often underperform for an instrument of a given aperture whereas fracs and cats are more likely to perform close to the theoretical limit. Commercial newts are not baffled, the 'black' paint is only reasonably dark, the secondaries are generally oversize for visual use and the short tubes let in stray light. Then the mirror coatings may be tarnished and are always at least a bit dusty. Even then I reckon outperform other design on a $ for $ basis. If you spend a bit of time and money you can improve the views: flocking the upper tube, a light extension baffle on the front of the tube, make sure around the primary is light tight, good overcoated aluminising etc. I've looked through a 6" f/8 built in a square tube with 3-4 baffles along the length and the tube extending about 6" past the secondary. It gave frac-like views for much less cost.
The weakness of big fracs IMO is the focusser. It simply is not possible to get a good focus with the amount of image shake that touching the focussing nob will induce. All scope benefit from upgraded focussers but on fracs it's basically mandatory. I started with an upgrade bolt-on motor drive and it was OK for a couple of years. Then the cheap cork guides on the drawtube wore to the point the image shift was a whole field of view wide in a medium power ep. So I upgraded to a motorised Moonlite and haven't looked back. I can see stars at least half a mag fainter, it's quicker to use and much less frustrating; it's lowered blood pressure and increased my endurance.
Regarding carrying and set-up: my 10" dobs is both easier to move and quicker to set up than the 6" frac. The 16" Lightbridge is heavier and harder to move but takes about the same time to set up as the frac.
Astral Traveller,
why is it that the opening to Newtonians are left open? Surely the cost of some non reflective glass over the tubes opening would add very little to the cost of the scope and thereby preventing dust etc: falling onto the primary mirror. Given that the mirrors coating is on the surface of the glass makes the argument in favour of a 'dust-cover" glass sheet even stronger. I have seen one or two Newts that look like Macs it just seems to make good sense to cover the opening is the a reason why the openings to Newts are left open? Is it just a cost thing?
I have seen flocking used in non-astronomical applications. I have a small Newt, where can I source flocking? The tube of my Celesstron just has a coating of Matt black paint, nothing more.
David
David, the reason is the newtonian has always been a low-cost light bucket.
To do what you suggest the cost of the glass costs more than rest of the entire Newtonian - this has to be optical glass, free of striae and strain, optically flat on both sides to 1/4 wave or better, and have no "wedge" error, and be anti reflection coated. Ordinary window glass is not adequate.
Pieces of glass like that cost more than the rest of a newtonian at 200mm diameter, hence the price of Meade and celestron SCT's.
As for larger apertures... Just take a look at refractor prices and you'll get the idea - at 300mm you could buy a car for what that glass costs, at 400mm it would cost as much as a modest apartment.
There is however another aspect - if you're going to the extent if doing that, make it a corrector for a maksutov-newtonian. The skywatcher and Orion ones around 190mm f/4.5 -5.6 are superb visually and photographically.
If you want to compare what you will see with a 130mm refractor v's a 12 inch reflector these images of Saturn are from a telescope simulator. Image 1 is with a Skywatcher ED120 with 10mm plossl and 2x barlow. Image 2 is 12 inch Meade Lightbridge collapsible dob, same EP 7 same barlow. The circle around the outside is the FOV. Remember, these are simulations only. There are a lot of variables that affect what you will actually see at any given time.
The type of scope you get will depend on what you want to do with it. Some will excel at certain tasks better than others. No single scope will do "everything" best. Decide what you want to do most and that will help narrow down the choice.
For instance will it be for visual use or photographic? Will it be for Moon and planets or DSO's (Deep Sky Objects)? If DSO's, what sort? Tiny galaxies or large nebula?
This is why some of us have a lot of different telescopes.
I'm a refractor type too.
Give me my 6" f12 frac anyday of the week over my 10" Newt. Don't know why but using the frac is very relaxing. It weighs a bit...14kgs, but that's OK to move and mount on my EQ mount. As for image shake as mentioned by David, I don't have trouble with that. It can be overcome with an electric focuser.
I'd say go with what you really like....get a reflector later
Some really good advice there from Matthew and Kevin.
I'm in the refractor camp myself but I also have experience with SCT and Newtonian. Again, it ultimately comes down to what you want to do with your scope, hence the reason why most folks own more than one. Personally, I find the refractor is great for long exposure deep-space imaging and performs quite well when imaging the larger planets like Saturn and Jupiter but lacks the power for the smaller planets. Don't expect to see the polar cap on Mars with using a consumer-level refractor.
I found the mirror wobble on the SCT to be problematic and the position of the focuser on the Newtonian to also be a bit of an inconvenience. These problems can be overcome with some modifications but I just found the refractor to be most convenient overall.
Image wobble is only going to be a problem on a long-tube refractor at higher focal lengths and even then, only when the tube is not properly secured to the mount. A wide dovetail plate and correctly spaced rings along with a motorized focuser puts this to rest.
The two images of Saturn tell a story that even a beginner can interpret. The contrast is better with the refractor, though the larger image produced via: the reflector makes the image more acceptable. I think this is heading towards the answer I was in the first instance seeking.
I really appreciate the time and effort you have applied to my question. Unfortunately I have a thousand more questions and I am beggining to think that I am going to wear down a few of the more experienced Astronomers at Green Point!
I have one more question...... as you head towards the larger SCT's there seems to be "nervous ructions" re: cool down times and mirror shake. What is the general set-up and user experience with a optical 12" SCT (excluding photography) are these larger SCT's difficult to use?
As Louie mentioned, the images shown are approximate simulations which only serve to demonstrate the size of a target in a "visual use" situation. It doesn't necessarily reflect the results that you're going to get photographically.
To give you an example, I've attached a photo of Saturn which I took last year using the very same 120mm refractor that was used in Louie's two image example. This was taken using a 4x barlow (for a focal length of 3600mm) and is a single 300ms exposure. It was my first attempt at planetary work and although not perfect, I feel that it does a reasonable job for the sake of this demonstration.
I am a refractor kind of guy , see my sig , anyway there is a lot of plusses and minuses on both a refractor and a reflector , 
true .
.
, as a good 12 inch Dob will show Jupiter just fine .
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