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Refractor vs Reflector


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Could a 4" f/9 ED refractor compete with a 10" f/5 Newtonian under any circumstances?

A good refractor gives more 'pinpoint stars' than most reflectors.

A 4" refractor will also be less affected by poor seeing (turbulent

atmosphere) and may split double stars at a lower magnification.

But under decent conditions a 10" Newtonian will resolve more

planetary detail, show more 'faint fuzzies', objects like globular

clusters will come alive, and open clusters will look far better with

the magnitude penitration of the larger aperture.

HTH, Ed.

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I have a 4" Vixen F/6.5 ED refractor and an Orion Optics 10" F/4.8 newtonian with Hilux coatings and 1/6th wave PV optics (according to the test result supplied with the scope).

I enjoy both scopes for different reasons but, assuming reasonable observing conditions and that the newt is in collimation, the 10" will always out perform

the 4", the margin depending on what's being viewed.

Despite this the refractor gets used more because it's just so quick and easy to set up.

Edited by John
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I agree with the above posts. I have an 80mm F/6 refractor, but the 200mm SCT beats it on planets (except when using low magnification), and more so on DSOs. The only things the 80mm F/6 does considerably better are wide-field objects such as the Rosette Nebula, Pleiades, Beehive, etc. It is also easier as travel scope as it can travel as hand luggage (I am taking it to a conference in Italy in July).

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The answer has to be no.

One scope is collecting 6 times as much light as the other and with thst differencethe bigger 10" has to win out.

Rather like a drag race, one car with a 1 litre engine the other with a 6 litre and otherwise similar bodies and weights.

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The answer has to be no.

One scope is collecting 6 times as much light as the other and with thst differencethe bigger 10" has to win out.

Rather like a drag race, one car with a 1 litre engine the other with a 6 litre and otherwise similar bodies and weights.

Unless you use a modern 1 litre turbo-charged engine against a very early 6 litre :). But assuming optical quality is similar, you are right

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You spot a gap in the cloud and want a quick peep at Saturn. The refractor will perform much better because you will have the scope pointed at Saturn and you will be looking through the eye piece. Views through the newt will be awful because you will still be balancing the mount/rotating the ota or collimating and by the time you've done that it will be cloudy again.

Apart from that silly example, it wont be unusual to find that the refractor is able to split double stars which the larger newt can't.

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My 10" SW reflector beats my 4" ED100 refractor on pretty much all targets. But, that doesn't means the views are more pleasant. Open Clusters are far more enjoyable using the 4" refractor, because of the pin-point stars.

On the other hand, my 5" Meade refactor beats the 10" reflector (which is regularly collimated) hands down on most targets. Surprisingly, even on faint DSOs. For example, M97, which is quite dim, is far easier to find in the refractor, probably because of the vastly improved contrast. The exceptions are globular clusters, which seem to appreciate aperture.

My reflector rarely gets used these days, because it's not as good as my refractor and less comfortable and practical to use.

That's just my experience. Truth is, there is no right answer. It depends on many things, including sky conditions (I live under moderately light polluted suburban skies).

Edited by 26Left
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A 10inch Newt can almost always show more than a 4 inch refractor, but as others have alluded to here, there's a reason many people have a 3 or 4 inch refractor.

In my own personal experience with a 10inch dob and a 80mm refractor, I have seen much more with the smaller scope than the larger. So much so, that I sold the dob. Sure there were a couple of nights when I managed to get the dob out, collimated, cooled and pointed at something which really showed the benefits of the larger aperture, but they were few and far between.

However, I guess I'm a bit lazy, I observe more with the refractor because it's easier, it shows just enough to be interesting, and it's easy to have a list of targets that will be 'just on the limit' if you want to challenge yourself.

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You spot a gap in the cloud and want a quick peep at Saturn. The refractor will perform much better because you will have the scope pointed at Saturn and you will be looking through the eye piece. Views through the newt will be awful because you will still be balancing the mount/rotating the ota or collimating and by the time you've done that it will be cloudy again.

Apart from that silly example, it wont be unusual to find that the refractor is able to split double stars which the larger newt can't.

Binoculars are better still :). If a 10" Newtonian cannot split a double a 4" frac can, the newt is out of collimation. I could set up my old 6" newt in no time. With careful handling I rarely need to recollimate (F/8 so a real "apo-killer" :hello2:).

In my case I have seen WAY more with the C8 than the 80mm (OK, spring-time galaxies need APERTURE :)), and about the same number of objects with bins.

Edited by michael.h.f.wilkinson
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Open Clusters are far more enjoyable using the 4" refractor, because of the pin-point stars.

Open clusters are just like that aint they?:)

I always prefer them through my 10"rather than my 16" they just don't appreciate light buckets.

Through a frac they're just lovely. The double cluster through a 4"-5" frac is just wow!

Through my 16" it's just a rich star field. loses all it's splendor IMO.

Doubles are just great fun with a frac too. Although the Mak/Sct brigade will probably have some say on that one.:)

Diffuse Nebula and Galaxies. Give me a big reflector every time.

IMO even a 5" frac simply cannot compete with the sheer light grasp of a 10" scope.

Between a 5" frac and an 8"Newt it's a lot closer.

Regards Steve

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If a 10" Newtonian cannot split a double a 4" frac can, the newt is out of collimation.

I'm not sure that's always true Michael. May be other optical aberrations but I use a hotech collimator every session and star testing confirms good collimation. I haven't done any side by side comparisons with my ED120 and 12" Skyliner but my feeling is that the ED120 is generally a better splitter than the newt. Must be something to do with the airy disc rather than resolution.

An opticophile might happen along soon and put me straight!

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The airy disc is the diffraction limited resolution, and decreases linearly with aperture. I think seeing limiting kicks in first most of the time though.

some people might find the point like stars of a refractor easier to split, but i actually find the diffraction spokes of a reflector useful sometimes.

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I've never seen a reflector of 11" or less beat my 133mm f/12 for planetary or lunar detail, or for splitting doubles. My big 'frac isn't your average 'scope, however - far from it. It is a hand built planetary scope custom designed from the ground up for that purpose. I regularly take the CGEM 1100 out with the Apomax, and the only thing the C-11 does better is finding the faintest moons of Saturn - and if the seeing is at all turbulent, it doesn't even do that as well as my 'frac.

With that caveat :), I'd have to say that I agree with others who've posted here. A commercial grade 4-5" refractor will not compete with a comparable commercial grade (well collimated) Newtonian. Aperture controls resolution (angular resolution is a mathematical function of aperture) and more obviously, brightness. These factors translate directly into more detail on every target, and more targets within the grasp of your kit.

I'm a very serious lunar/planetary observer, not so much on DSO's, so for me, the 'frac was an obvious choice. But as many here have probably noted, when someone asks for my recommendation on a good "All 'round" scope, the classic Dob tops my list for value and functionality.

Dan

Edited by Ad Astra
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...

Doubles are just great fun with a frac too. Although the Mak/Sct brigade will probably have some say on that one.:)

...

Doubles are just great fun... fullstop!

Got both a Mak and a 'frac and they're stunning in both :)

Not got nearly enough experience to make an "authoritative" judgement, but to me doubles just have a better "eye-feel" in the 'frac too.

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Doubles are just great fun... fullstop!

Not got nearly enough experience to make an "authoritative" judgement, but to me doubles just have a better "eye-feel" in the 'frac too.

The unobstructed optical path of the 'frac generally gives better resolution and a cleaner separation (darker space between closely spaced stars) than a Newt will. This assumes, of course, that the refractor is well corrected for chromatic abberation. A fast 'frac will sometimes have color issues that will cancel out the unobstructed OTA advantage.

Your milage may vary!

Dan

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.....no one has mentioned imaging.......just wondering if the 4" refractor would be considered by others here as superior for imaging to a reflector around 11" ......I'm mindful that the 'restricted' aperture of the 4" can be somewhat compensated for in imaging by taking more shorter exposures......Just another thought.......

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.......& what about pushing it to a 6" refractor - okay, its going to cost an order of magnitude greater, but when the central obstruction of the reflector is taken into account, plus the poorer efficiency of the light reflecting mirrors, a 6" refractor must be close to a 10" reflector?????

.......Oh, one other - the reflector is going to lose about 1 - 1.5% of its reflection performance year on year - the refractor, if cared for normally, will still be close to its peak performance decades from now.....

........oh, & another collimation.....but that might be considered in comments others have made about setup times already......

.......but I'm new, so take my comments with a pinch of salt.......I'm just repeating what I've picked up on my readings of what 'scopes to go for......

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I would think a 6" Apo (3-element, well color corrected) would cost you a mint. As you get above 100mm, Apo's and ED objectives get very expensive. And 2-element scopes tend to be offered in longer focal lengths to compensate for poor color correction, which works against you in terms of exposure times.

I think you'd be better off with an Apo of 90-100mm than a 150mm achromat (2-element lens). Not so much of an issue with visual work, but it will really hamper your imaging. Cameras are even more sensitive to color problems than your eye - exacerbating the problem greatly.

Dan

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A TEC140, not quite six inches, costs 5,500 dollars new, presumably pre tax. http://www.telescopengineering.com/

It is very good indeed though...

Imaging is an interesting question. In theory a Newt with coma corrector ought to be a dream because it has aperture, f ratio and freedom from false colour. In reality, though, most of the highly flat-fielded, colour corrected Newts like the AO series, Tak Epsilon etc end up being very expensive. Also in reality, there seems to be more work involved in getting them to work well every time.

For this reason, I refract!

Olly

Edited by ollypenrice
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.......& what about pushing it to a 6" refractor - okay, its going to cost an order of magnitude greater, but when the central obstruction of the reflector is taken into account, plus the poorer efficiency of the light reflecting mirrors, a 6" refractor must be close to a 10" reflector?????

.......Oh, one other - the reflector is going to lose about 1 - 1.5% of its reflection performance year on year - the refractor, if cared for normally, will still be close to its peak performance decades from now.....

........oh, & another collimation.....but that might be considered in comments others have made about setup times already......

.......but I'm new, so take my comments with a pinch of salt.......I'm just repeating what I've picked up on my readings of what 'scopes to go for......

Mirrors nowadays reach 97% reflection with a dielectric coating and hardly degrade because of that. Glass readily absorbs 5% of light at 5mm thickness. This does not take reflection losses into account. A thick triplet easily loses 8-10 % of light, which gets worse with aperture, because the thickness increases.

There is no way the "inefficiencies" of mirrors could undo the roughly 2.5x more light the 10" gathers with respect to the 6".

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Celestron state that an XLT coated catadiatropic has 83% transmission. From their wording I believe that does not include light loss from the CO and maybe does not include the diagonal either. Anyway, its a long way from 97%.

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Celestron state that an XLT coated catadiatropic has 83% transmission. From their wording I believe that does not include light loss from the CO and maybe does not include the diagonal either. Anyway, its a long way from 97%.

A hilux coating has 97% reflectance, and 99% coatings exist. The 32% central obstruction boils down to 10% light loss. This yields 0.97*0.97*0.90 or about 0.83 transmission, i.e., 83%. Diagonal is not included, because it is not part of the OTA. Thus, a reflector with a huge CO loses 17% of the light. A refractor with 10 mm thick glass objective will lose about 10%.

Thus the factor of 2.6x light gained by the 10" aperture compared to the 6", is offset by 7% extra light loss. No contest really.

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