Reflector vs Refractor

[Mehraj Anik]

Now that I know how refractors and reflectors work, thanks to you people, can anyone please tell me what differences these two make in both viewing and imaging?

Also, most of the times, I have found refractors have smaller aperture (higher ƒ value) at a focal length comparing to that of reflectors. Why so? Do they get more light than reflectors, may be because light travels twice in the scope in a reflector?

[Uranium235]

The F value is directly affected by apeture and/or focal length.

Newtonians are typically faster than refractors becuase they have the kind of apeture and light gathering ability that isnt available with refractors (well, to most mortals anyway)

Examples are as follows:

Refractors:

600mm (Focal length) / 80mm (apeture) = F7.5 (slooow... like a snail)

Newtonian reflector:

1000mm (Focal length) / 200mm (apeture) = F5 (speedy... 2.25 times faster than f7.5)

800mm (Focal length) / 200mm (apeture) = F4 (warp speed scotty!.. 3.51x faster than f7.5)

Speed only really comes into it when youre imaging, where speed is king.

For example (get a scientific calc for this):

F7.5 / F4 X^2 = 3.51

Basically that means that for every hour imaging at F7.5, you would get 3.51 hours at F4. A massive timesaver. However, speed isnt cheap. The faster you go, the more expensive it gets.... especially in the case of APO telescopes or telephoto lenses.

Be happy at F5, its far better than a poke in the eye

[nicnac]

price, there is a big difference, mirrors are cheaper than lenses

[rowan46]

for viewing you need apparture. the bigger the better. imaging requires a different parameter , it needs accurate tracking. refractors are chosen by imagers because of their size. imaging doesn't require a large apparture because the camera can be left on a long exposure, therefore having a smaller scope means that the mount doesn't need to be as large and expensive. again fast scopes are chosen for the same reason it puts less strain on the mount. in theory you can image with a slow scope and large apparture, in practice this means a hyper expensive and accurate mount. which is why most imagers use small fast apo's it's still expensive . but it's the difference between nearly a grand for an eq6 and £20 000 for a paramount me. visual only systems don't require the same sort of accuracy of mount. So although an eq6 would take a 300p easily for visual it would not make a suitable imageing set up. the general rule is for imaging that you should only load a mount to %50 max of its available payload ( this includes cameras, guidescope, filters etc. The Heq5 is considered the smallest serious imaging mount. To get back on topic essentially the reason why reflectors are chosen for viewing is because they are the most cost effective route to a large apparture, which is the most desirable attribute of a visual scope. (Providing the optics all work properly) Refractors are chosen for imaging because of their clear optics in a small package. A fast 4" apo refractor won't be as useful for visual work as an 8" reflector. but it will make the better imaging instrument

[cs1cjc]

A fast 4" apo refractor won't be as useful for visual work as an 8" reflector...

but a 4" refractor is easier to maintain, store and transport. It will give wonderful wide views (with a suitable eyepiece), can be used for solar (with the right accessories) or terrestrial viewing, but will not show as much detail on planets nor as many faint DSO's.

[Ad Astra]

Hi Mehraj!

Reflectors and refractors handle light in different ways. Allow me to explain!

A refractor lens is like a prism in a way - although white light enters, the lens tends to separate light out into separate wavelengths (like a rainbow!) This is called chromatic abberation. Basically, the colors will each focus at a slightly different focal length with green in the middle and red and blue focusing on either side. The result at the eyepiece is that everything will have 'color error' or colored halos around every star. Very annoying!

There are two ways to defeat this problem. 1) You can have a long focal length. People knew this by Newton's time, and some of the 17th century refractors were extremely long to help get rid of color error. 2) You can also have multiple lens elements, each with a different index of refraction. This is essentially multiple lenses that correct for each other's errors. Simple 2-element lenses with a thin layer of air between them are called Air-spaced Achromats. This lens was a huge jump in technology and occured after Newton's death. Modern (20th century) glass technology and computer aided design allowed astronomers to develop 3 and even 4 element lenses, sometimes air-spaced, other times oil-spaced which make correction even better - and at shorter focal lengths. These lenses are usually called Apochromats or just Apo's for short.

Now consider a 120mm Apo for a minute. Three Elements, each with a different type of glass. This means Six Optical Surfaces, each must be perfect to within 1/10th a lightwave. Each space must be exactly correct, and the lens elements must all be aligned perfectly with each other, and with the tube. Oh, one more thing - a refractor's lens must be perfect throughout its volume. No flaws, bubbles, or ripples in the interior of the lens.

No wonder these scopes are expensive!!!

Now let's look at the Reflector telescope or Newtonian. This scope was developed in response to the problems with refractors. As you know, a mirror reflects light of all colors virtually equally. There is no color error in a reflector because the light never goes through any glass - so there can be no 'prismatic effect'. Newton himself considered the color problem to be insolvable - and said so! His pronouncement stopped research on 'color-free' lenses like Apo's and apochromats for more than 50 years!

Clearly, color error isn't the Newtonian's weakness - instead it is the obstruction in the optical path. The spider and secondary mirror cause diffraction spikes, lower contrast, and readily introduce collimation errors. (A refractor is almost never out of collimation unless it has been severely damaged.) Still, when you consider the optical construction of the Newtonian, the difference in price becomes easy to understand. A Newtonian has Two mirrors, and only one of those has a single curved surface (the secondary mirror is perfectly flat). Because it is a first surface mirror, the reflecting part is on top (unlike your bathroom mirror at home). This is why you must be so gentle cleaning a telescope mirror compared to the way you may scrub a household mirror clean. Also, first surface mirrors don't have to be perfect inside as a lens does. If there is a bubble or a ripple under the reflective coating - no one cares. Quality of glass? No one cares. Index of refraction? No one cares. The glass here is only a physical shape to hold the reflective coating. You could make a telescope mirror out of anything that would hold a precise, polished shape and take a good reflective coating. All mirrors in Newton's day were, in fact made of metal and polished each night before observing. Glass mirrors with reflective coatings (first silver, then later aluminum, and finally aluminum with enhanced coatings against tarnish) are a much newer innovation.

So with a Newtonian (almost any reflector) you virtually eliminate color error, but you get diffraction spikes (annoying in photos) and lose contrast and resolution (detail) due to the obstructed nature of the design. But the HUGE difference in the difficulty of making a refractor (compared to a reflector) means that you can compensate for diffraction, resolution and contrast problems by being able to afford more aperture.

The stuff in the posts above about focal length and exposure times are true, but with today's stacking technology (stacking many shorter photos on a computer) - the exposure time thing is losing its sting. I think refractors will always give a better image when the optical quality and aperture is similar. Still, I know many people with 8-12 inch Newtonian scopes - I've seldom seen a refractor larger than 150mm (6-inch) aperture - and those would have cost you $5-8,000 dollars just a few years ago. Now Celestron, Skywatcher and others offer 150mm refractors for around $1,000 to $2000 - a real bargain! Needless to say, these are achromats, not Apo's. A 150mm Apo would still set you back $10,000 or so.

I hope that helps,

Dan

For what it is worth to you, my personal scope is an 133mm f/12 Apo (refractor), but I use reflectors to teach with. My 5.25" scope is 7-foot long, and almost 24 kg, so it takes a very large and expensive mount to use it properly. You can see the scope and mount HERE, HERE, and HERE.

Cheers!

[rowan46]

but a 4" refractor is easier to maintain, store and transport. It will give wonderful wide views (with a suitable eyepiece), can be used for solar (with the right accessories) or terrestrial viewing, but will not show as much detail on planets nor as many faint DSO's.[/QUOTWhy is it whenever someone quotes what is essentially a law of physics to refractor owners they get on their high horse. The guy asked why visual users use reflectors and imagers use refractors. FACT a 12" dob is going to see a damn sight more than a 5" apo you can mention all you like about the quality of the views which is pretty subjective. But if I got a 12" skywatcher dob I would still see more than any 4"or 5" apo no matter how much you paid for it. So the simple fact of why the majority go for reflectors for visual is cost per apparture. It's annoying that almost everytime somebody says something is better than their scope for something a defence has to be mounted. Refractors are great. and if i could afford a 12inch fast apo and the mount necessary. I would love one. but I guess I'm like 90% of amateur astronomers who can't afford that sort of kit Its no slur on a scope to say the reason most people don't use it is because they can't afford it.

[John]

..Why is it whenever someone quotes what is essentially a law of physics to refractor owners they get on their high horse. The guy asked why visual users use reflectors and imagers use refractors. FACT a 12" dob is going to see a damn sight more than a 5" apo you can mention all you like about the quality of the views which is pretty subjective. But if I got a 12" skywatcher dob I would still see more than any 4"or 5" apo no matter how much you paid for it. So the simple fact of why the majority go for reflectors for visual is cost per apparture. It's annoying that almost everytime somebody says something is better than their scope for something a defence has to be mounted. Refractors are great. and if i could afford a 12inch fast apo and the mount necessary. I would love one. but I guess I'm like 90% of amateur astronomers who can't afford that sort of kit Its no slur on a scope to say the reason most people don't use it is because they can't afford it.

Talking of horses, this reads a bit like you have just mounted yours

[rowan46]

Talking of horses, this reads a bit like you have just mounted yours

SORRY my first and only time, but everytime anybody dares mention there is a problem with apo's and that problem is the price someone comes on to suggest the view is so much better. I have heard claims of between 2" to double the apparture as being the difference. It is against the laws of physics to suggest that a 4" apo can see more than an 8"reflector but some apo owners still make the claim. I agree that per inch the apo has better quality in just about every objective and subjective criteria. but until they can make an affordable 8" apo it is never going to be the best instrument out there for the majority of visual astronomers

[Ad Astra]

SORRY my first and only time, but everytime anybody dares mention there is a problem with apo's and that problem is the price someone comes on to suggest the view is so much better. I have heard claims of between 2" to double the apparture as being the difference. It is against the laws of physics to suggest that a 4" apo can see more than an 8"reflector but some apo owners still make the claim. I agree that per inch the apo has better quality in just about every objective and subjective criteria. but until they can make an affordable 8" apo it is never going to be the best instrument out there for the majority of visual astronomers

The laws of physics you are quoting there are for the ideal case with perfect optics and no allowance for central obstruction resolution loss.

There are very few commercial reflectors that are made with the optical precision of a premium apo - and optical accuracy (surface P/V error), is much harder to accomplish on a large mirror. More aperture compensates for this, but not totally. These scopes are very individual - each has its own characteristics.

I observe with a lot of equipment in my teaching - and I can tell you that on the Moon, Saturn, Mars, Jupiter, and almost any double star - my Apomax blows the doors off our C-11 in detail and image quality. The C-11 is certainly brighter, but doesn't reveal more detail unless these details are very faint. In DSO's and other faint stuff - the C-11 is the very clear winner, but for Lunar and planetary work, my refractor is the King 'o Views.

I will easily admit to you that the big Apo is a specialty scope. It is not a general purpose instrument (like an SUV), rather it is like a sports car that does what it does superbly, but it isn't a 'general purpose' instrument.

Hey, if you're out my way sometime, we'll set up our scopes together and try them out against each other. The guy with the best Lunar views buys the Guinness, the guy with the best DSO views buys the lunch. That way we'll both go home happy!

Dan

[Mehraj Anik]

Thanks Dan for you excellent explanation!

And Rowan, :hello:buddy, learning is fun this way!

[rowan46]

[quote=Ad Astra;

"Hey, if you're out my way sometime, we'll set up our scopes together and try them out against each other. The guy with the best Lunar views buys the Guinness, the guy with the best DSO views buys the lunch. That way we'll both go home happy"

Dan I have a 5" sct I am afraid it would be no contest. What with the price of Guiness, food and the airfare over. And the fact that yours is significantly nicer than mine. I know who would be the happier. Even so I'm tempted just to get a look at that wonderful scope of yours

[Ad Astra]

Dan I have a 5" sct I am afraid it would be no contest. What with the price of Guiness, food and the airfare over. And the fact that yours is significantly nicer than mine. I know who would be the happier. Even so I'm tempted just to get a look at that wonderful scope of yours

Ok, ok... the Guinness and the lunch are on me. Heck, you can even sack out at our place! Just let me know when the plane lands so I can have the Guinness properly cool. Here in the desert that can take a while - especially in the summer! We'll put the CGEM-1100 and the Apomax both out on the Observing deck and have a contest hunting down Saturn's moons and we'll both be happy!

Dan

[rowan46]

Ok, ok... the Guinness and the lunch are on me. Heck, you can even sack out at our place! Just let me know when the plane lands so I can have the Guinness properly cool. Here in the desert that can take a while - especially in the summer! We'll put the CGEM-1100 and the Apomax both out on the Observing deck and have a contest hunting down Saturn's moons and we'll both be happy!

Dan

Actually Dan being british I am quite used to warm beer

[Mehraj Anik]

~ erm, what is 5" SCT?

[ollypenrice]

SCT = Schmidt Cassegrain Telescope. Cassegrain invented the idea of having a curved secondary mirror and a hole in the back of the primary to let the folded lighpath out for the observer to see. Schmidt, much later, invented a glass corrector lens to reduce coma inherent in the reflectors of his day.

As you have seen, asking about refractors versus reflectors is likely to start a fire!

I love the quality of the view through very expensive refractors but you cannot see as much as in a larger reflector. Now I need to stand by with a fire extinguisher even for saying something like that!!!

[Uranium235]

Own both youl get the best of both worlds (he says, picking the splinters out from sitting on the fence!)

Or just dump both and go for a mak-newt, job done

[Mehraj Anik]

And mak-newt? sorry for asking too many stupid questions, I am a complete newbie

[Mehraj Anik]

mak-newt = maksutov newtonian?

[david o]

Yes, Maksutov-Newtonian.

Here are a few links that explain most of the basic telescope designs:

* Basic Telescope Optics

* Refracting Telescope

* Reflecting Telescope

* Catadioptric Telescope

HTH

[Davidv]

Be happy at F5, its far better than a poke in the eye

I was about to leave the Mak Newt question for someone else to answer anyway. My mate's grandad used to say it's not just a poke in the eye, it's a poke in the eye with a burnt stick.

[Ad Astra]

Actually Dan being british I am quite used to warm beer

Well spoken and true mate, but living for awhile in 45 Centigrade weather will change your mind about a lot of things. If you float an egg in 'room temp' beer on a summer afternoon here - it gets poached!

That said, Mrs. Astra keeps all the brew in the fridge and shakes her head when I take it out and let it sit on the counter for at least 30 mins before opening. (She's a Californian. ) I am as patriotic as anyone you are likely to meet - but "American" is not a universal epithet for "high quality"; "Fine American Beer" and "Fine American Cheese" being two obvious oxymorons that immediately come to mind. (We prefer aged white cheddar from Ireland or Wisconsin at our house.) As for American "beer", I imagine that it has to be so cold you can't taste it to be enjoyed (it that is at all the right word...), and such questionable products are never found here at the Ad Astra Observatory!!!

So the invitation still stands, Rowan.... Apomax and Guinness FTW!

Dan

[ollypenrice]

Yes, American cheese. Hmmm... Makes the elastomeric cheeses from the low countries seem quite exciting.

Olly

[John]

Own both youl get the best of both worlds (he says, picking the splinters out from sitting on the fence!)

Or just dump both and go for a mak-newt, job done

I'm hedging my bets / sitting on the fence as well - I've got 4" and 6" refractors, a 6" maksutov-newtonian and a 10" newtonian

All have varying strengths and weaknesses. The mak-newt is a pretty special instrument though - the one instrument that I've used that combines the tight star images and contrast of a refractor with the fast focal ratio / wide field capability of the newtonian. It's apochromatic as well of course

[MorningMajor]

(We prefer aged white cheddar from Ireland or Wisconsin at our house.)

AND Guinness - well, if Rowan ain't coming I'll let you know my flight details!! lol