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Refractor vs reflector?


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Hi guys

I'm just wondering about something ... I'm sure I can recall reading, many years ago, that a 3" refractor (a good one presumably) will perform at least equal if not better than a 6" reflector ... In terms of what can be seen/resolution I think ... Is this true? Can a good lense of smaller diameter equal a larger mirror, or even offer some sort of viewing advantage? I certainly do recall the late great Patrick Moore recommending a minimum 'useful' aperture of 3" for a refractor and 6" for a reflector ....

Very interested in opinions experiences and plain old superior knowledge!! :D

Donaldo

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I think that old comparison is well past its sell-by date. Reflectors now have far higher reflectivity and mass produced mirrors can be of an almost absurdly high standard. You will go far deeper with a modern 6 inch Newt than with a three inch refractor.

There is still a debate regarding the charms of very high end apochromatic refractors and whether, with their tiny stars and high contrast, they punch above their weight. I think that they do, but not so much in terms of going deeper as simply in terms of the aesthetic quality of the view. While I do subscribe to this opinion I'd be the first to say that it was highly subjective and that, in visual use, aperture is king in terms of going deep. In terms of 'going pretty' I'd still back our TEC140 against all comers... but that can be dismissed as a fairly trivial point!

:grin: lly

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I have just looked at the old Observers book of Astronomy  and the Amateur Astronomer both by Patrick Moore.

Both the  Observers and The Amateur Astronomer state a 3'' refractor would be a minimum size for any usefull work and a 4'' reflector the minimum usefull size but recommends a 6'' reflector as best. He also mentions prices like the 3'' being £20 and you could tool up and make a 6'' mirror for £10.

I think that with the advances made nowadays in figuring mirrors and better glass quality the views with a modern 3'' refractor or a 6'' reflector will be superior.

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I have just looked at the old Observers book of Astronomy...

Showing your age? lol Though I fear I might just be able to quote sections verbatim... :p

I suppose, in those days, the much fabled "3-inch refractor" was an 1-dioptre achromat?

Reflectors were quoted as "one quarter wave astro"... as a point of significant merit etc. 

(Edit: Still can't spell achromat) :D

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I think the main advantages of a small refractor is they are less susceptible to seeing conditions. Both the small unobstructed aperture and closed tube design go to providing sharp contrasted views far more often than not. While there will never be beating bigger apertures for their larger and brighter views, seeing conditions and transparency play a larger part in their success for giving the best views they have to offer.

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The 2 options procduce a slightly "different" image.

The bigger reflector will collect more light.

The "drawback" being that with the secondary and supports the resultant image tends to be a bit "softer".

So you have collected more light and a brighter image but a little "soft".

The refractor will collect less light, a dimmer image but the image tends to be "sharper".

So the image has a better contrast.

This is where the operation of the eye comes into it. The eye "sees" contrast more then we assume. So you now have a dimmer image that the eye works better with.

I suppose the other aspect is that years back many mirrors were spherical so a lower quality image whereas now they are mainly parabolic and a better image.

So the choice is not as simple and the comparison is not sort of 1:1, and the actual equipment has moved on. They didn't have FPL-53 years ago and they didn't have the computer controlled grinding equipment either. Go back 20 years with a modern FPL-53 Triplet and noone will believe the performance.

To me the answer is you do not see more, you see different.

If that different you find better or worse is relevant to you.

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this is a very old "can of worms" and debate is still ongoing,none has been able to dig to the bottom of the can as of yet.

I agree with Olly`s post and a few others above.Reflector or DOB will go deaper on DSO`s and will be cheaper if you are after bigger apperture,but refractor will produce "nicer" image on planetary and stars as refractor does not have any central obstruction.Obivoulsy there is a cost of each involved.

At the end,i think it is down to individuals preference really.Do you like the dob or do you like refractor and this debate can go on endless.If i was living in rural area with no light pollution and money no object,i would love to have both:big 20+ inch dobsonian for DSO`s and nice 6-8" APO or ED for planetary.

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I think that the poor old reflector has been dealt an unfair blow by comments like those of Patrick's. You have to remember the era that comments like that were written in. Back then speculum was still in some older Newtonian, this tarnished rapidly and needed regular refiguring. Many amateur's would grind, polish and figure their own glass mirror blanks which were then silver coated. These mirrors were not always figured properly, leading to under correction, over correction, zonal error's, turned edges, spherical aberration, astigmatism and wedge. In reality its a wonder many of these scopes worked at all. Comparing such home grown workmanship with professionaly made refractors, its little wonder that the Newtonian reflector got such bad press.

Today amateur astronomers have been blessed as never before. We have refractors and reflectors as well as catadioptrics being slung at us from every direction. Advertisers want us to buy their ware's, but be aware! All telescopes have their advantages and disadvantages. Newtonians have the advantage of offering aperture and light grasp at usually affordable prices. They are in their element on the DEEP sky. They are also good all round performers, accommodating the astronomers desires without much difficulty. Most modern day Newtonians are quite short in focal length which can have a negative effect on the scopes contrast and definition as coma, a somewhat destructive abberation, increases as focal length decreases. Short F ratios also mean an increased secondary obstruction which also is damaging to the contrast. Keeping the Newtonian long will aid in its ability to perform as a near perfect planetary telescope.

The catadioptrics such as the Schmidt Cassegrain are a compromise and are often viewed as not being so good on the visual front. However that is also a misconception. The Schmidts do have a large central obstruction that has an impact on the diffraction pattern causing stars to appear somewhat bloated, but after being allowed to reach thermal equilibrium, their large aperture and long F ratio can deliver breathtaking views of most objects. They really shine however as imaging instruments, especially on the planets.

The refractor by comparison to the scopes mentioned above is usually limited in aperture. You will often hear of chromatic aberration being a problem, especially in shorter refractors, and ofcourse for many it is. Chromatic aberration like coma in a short reflector causes a loss on definition, so most short F ratio refractors, while giving breathtaking views of rich star fields that are hard to beat, give soft images of the planets that lose that laser etched view often seen in their longer focal length siblings.

The only way to have a good planetary refractor in a short managadvertisers with significant aperture, is to use classy glass or crystal lenses. The simplest being the ED refractor which is usually a doublet objective with one element made of an extra low dispersion glass such as FPL51 or FPL 53, a fluorite composite. Some higher end and consequently terrifyingly expensive doublets use Calcium Fluorite crystal as one of the elements. For many, myself included, these produce the most startlingly pure, contrasty and well defined visual views of any refractor. Then there are the triplet and quadruplet refractors that eliminate any trace of chromatic aberration. These are the preferred choice for many an imager.

All these ED and apochromatic refractors are superb specialist telescopes, but not all are equal, and price is no guide here. For example: A few years ago while observing Saturn with various scopes, a £3100.00 TeleVue NP101 was outstripped in no uncertain terms by a Vixen 102mm F6.5 ED doublet costing, I think, just over £1000. Another NP101 was completely thrashed by a Skywatcher 120 ED PRO in both lunar and planetary detail and purity of image. By comparison the NP101's gave a decidedly nicotine yellow view of the moon while the 120 ED gave a far cleaner fluorite view. The 120EDs use FPL53 in their construction. Does that mean the NP101's were not up to the task? No! It simply means they are different horses. They are rich field apos which are designed with the imager in mind. It also goes to show that you can't trust the advertisersadvertisers, which stated in TeleVue's adverts at the time "They are what you want them to be!" Well obviously they are not, at least not in my case.

Really all this just proves that there is no perfect telescope. Each one is a compromise and most do most things quite well, but none do everything perfectly. It really boils down to personal preference.

Mike

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The problem with this sort of comparison is that the answer is always "it depends".

It depends on things like:

  • Optical quality
  • Build quality
  • Collimation (of both)
  • Cooling state (of both)
  • Focal length and thus ratio
  • Observer experience and 'state of mind/tiredness'
  • Eyepiece quality
  • Seeing conditions and sky quality
  • Object chosen
  • etc

Assuming that all aspects are identical then I'd suggest you buy a lottery ticket that week  :grin:

Seriously though if all aspects are similar then a 6" newtonian would tend to 'beat' (whatever that means  :smiley:) a 3" refractor on almost all objects. If you pay substantially more for a 3" refractor than you would a 6" newtonian, it will start to fail the criteria above and are not comparing like with like.

Personally, I like the differences between different scopes and as you have a decent frac already would suggest that e.g. an 8" dobsonian would provide a worthy companion.

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A number of years ago I had the opportunity to compare a 4" Vixen fluorite refractor with a 4.5" F11 Newtonian. The former wowed everyone whenever it was aimed at a planet. This particular night it was looking at Saturn, as was the 4.5" reflector. To my utter dismay this child's toy of a telescope worth about £200, gave a more well defined view of Saturn and its ring system than did the £2000 fluorite. That little fluorite was renowned for its ability to give contrastier, more well defined views of the moon and planets than any other scope on the field, and there were some seriously big, high end contenders of various designs. The magic of that little Newtonian didn't lie in its aperture, but rather in its focal ratio. At F11 its optics would have been near perfect and that showed on that night in 2004.

I admittedly love high end refractors and have seen good apo's significantly outperform very much larger reflectors and catadioptrics time after time. And for my kind of observing, mainly lunar and planetary, a quality 4", 5" or 6" apo is preferable to any other off the shelf scope. Definition and contrast outweigh aperture every time, unless your thing is the truly deep sky.

Mike

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This whole subject seems to stir up an astronomers emotions like no other. You could insult their wives, their children even their mothers but not their telescopes.

The amateur astronomer that guided me into this hobby some 35 years ago, told me a tail about a similar debate. While he was attending an astronomical meeting back in the late 1960s, the same reflector versus refractor issue came up for discussion. As you might imagine the subject got quite heated, when my friend noticed an elderly man sitting against the wall, puffing on his pipe. My friend asked him what his thoughts were on the subject. He replied "Leave em to it lad, leave em to it!" His curiosity roused, my friend asked the elderly man what kind of telescope he had? He replied "A 6" Cooke refractor lad!" Then he added "Would you like to see it?" My friend jumped at the opportunity. On arriving at the mans home somewhere in Cheshire, he was guided down the garden path to a rather shabby looking domed observatory that looked like it had seen better days. When he entered the dome, there before his was a beautiful 6" F15 Cooke refractor in polished brass on an equally beautiful equatorial mount. The refractor virus had been caught, it was too late for my friend from that moment on, refractors were now in his blood. I caught that virus in 1980 and never looked back.

On reflection I think we should focus objectively and realise its OK to have a primary and a secondary scope if need be.

Mike

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There is no comparison between a  3" refractor and a 6" reflector... not even close.

Don't kid yourself.  Aperture still rules this sport.

so would a TV 76mm Refractor not provide better images than a 6" seben? it really depends on too many factors.

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...... For example: A few years ago while observing Saturn with various scopes, a £3100.00 TeleVue NP101 was outstripped in no uncertain terms by a Vixen 102mm F6.5 ED doublet costing, I think, just over £1000. Another NP101 was completely thrashed by a Skywatcher 120 ED PRO in both lunar and planetary detail and purity of image. By comparison the NP101's gave a decidedly nicotine yellow view of the moon while the 120 ED gave a far cleaner fluorite view. The 120EDs use FPL53 in their construction. ....

Thats interesting to read Mike - I happen to have both a Vixen ED102SS F/6.5 and a Skywatcher ED120 :smiley:

Both can be comprehensively outperformed by my 12" F/5.3 Orion Optics dobsonian of course although the ED120 gets surprisingly close to the 12" on planetary views given the massive aperture difference and I prefer the views of binary stars through refractors even though the 12" will "out split" both of them :smiley:

Peter Drew is right though - one scope is not enough :grin:

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Oh Myy Lordy,

Great thread, pandora's box opened. Currently have five scopes, SWMBO asked why five and only two eyes, asked her to get her jewellery box, I said so you only have five fingers on each hand....

Its preference, I hate EQ mounts, so I have az mounts. The best scope I had was an Lx200 10", the most used st120, 152mm frac , 150mm mak..It does not matter what you have so long as it gets used and does not collect dust in the garage.

If you then muddy waters with bang for buck its a dob :grin: ..

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I'm a refractor man myself, always will be to a large extent, there is just a purity to the views which I love.

That said, I am slowly being won round by the dob argument ;-). The planetary views in my 10" really are lovely, plenty of resolution and detail. I need to have a side by side with the Vixen to see which I prefer!

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I have dedicated my scopes for different targets. My 5" F/9.3  achromatic refractor is what I use for solar system objects. The views are razor-sharp and beautiful. My 200mm F/4 Newt is for wide-fields of space. My 12" SCT is for going after the deepest sky objects. And my ST80 is grab & go and solar observing.

Each excells for their designated purpose. And none is 'better' than any other. Rather like my viewpoint on people.

Clear Skies,

Dave

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Thanks guys, lots of interesting comments thoughts and information ... Admittedly, I think I rushed in to buying my ED80, I wish I'd bought a bit more aperture ... I had the cash at the time, I don't now. Hopefully I will one day though augment my ED80 with a decent Dob, then I'd have the best of both worlds ... But I'll still wish that I bought a 4" ED frac when I had the chance!

Never mind

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Donaldo, small refractors are superb little instruments and I personally think everyone should own a small, widefield frac, preferably with lovely glass like your own  :grin:

They make for excellent grab n go and trips away scopes. They can be set it up for white light and H-alpha, there are few things as beautiful as those stunning wide field views, a surprising number of multiple star system can be seen, spell-binding moon vistas, gorgeous Milky Way sweeps. Lovely framings of large nebulae, super tight, jewel box stars, and so on. No matter how many scopes one ends up owning, there's always a place for the small refractor.

In short, It's tough to beat a small refractor for convenience, ease of use, and super sharp views. The other good news is that they really complement a larger 8" to 12" dob which in terms of astro-gear can be picked up relatively cheap on the secondhand market. 

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