Reflector vs Refractor

[DemonicOmelette]

Right, is there a simple equation to figure out the relative performance between apertures of refractors and reflectors? Ignoring such things as sharpness (which I understand refractors win on..) and price (same, reflectors). What I mean is, I am reading Turn Left at Orion, and when it says "This should be visible through a 4inch refractor, but not a 3inch", how would that translate to a reflector? I understand a larger-aperture reflector is (approx.) equal to a smaller refractor, but what's the ratio?

-eli

[Beyond_Vision]

It will be down to the area of the aperture. With a reflector or a SCT the area will be it's overall aperture minus the area of the central obstruction (secondary mirror). The refractor will just be it objective aperture. The only other factors although minor will be the Refractors transmissibility of light (percentage of light that makes it through the lens) and the reflectiveness of the primary and secondary mirrors on a Reflecting telescope.

Regards

Kevin

[philsail1]

Hello "eli"

Your question could elicit a huge response here, as there are "devotees" that favour their "apo" refractors, and an equal number who favour a good Reflector, or Dobsonian - and their others who love their SCT's!

In my opinion, one can spend one's life, and entire savings on the pursuit of the search for a telescope that will give the "perfect" image of what is being observed.

Whilst this pursuit is perfectly OK (if one has the money!), it is sometimes very easy to get caught up a frenetic and fruitless chase in pursuit of perfection.

The essence (again in my opinion only) of amateur astronomy as a hobby, is to enjoy it - whatever scope one has!

So, whatever scope you have - or want to obtain, just try and buy something that suits your particular needs, and then go out an enjoy the night sky!

No doubt you will receive many more opinions soon!

Best wishes,

philsail1

[DemonicOmelette]

I am perfectly happy with my scope, it is more out of interest. I have read that a refractor of equal aperture to a reflector is generally a much better scope, that you can see far more with it. Is that just not true? I am aware that it is like comparing apples and oranges, but I'm just trying to get an idea of how if a book says "visible through a 3inch refractor", how big a reflector you'd need!

And forgive me if I'm wrong, but isn't a Dob simply a reflector on a different mount?

-eli

[John]

Right, is there a simple equation to figure out the relative performance between apertures of refractors and reflectors?

I think the truthful answer is "no, there isn't".

On paper a refractor should marginally out perform an obstructed system (eg: newtonian, cassegrain etc) but in reality there are a whole load of other factors that will affect the actual performance achieved.

For example it's possible that some users of newtonians and cassegrains never experience the full potential of their scopes because of slight collimation issues, the scopes sensitivity to observing conditions etc.

I find the guidelines to what a particular aperture will and won't see have to be taken with a large "pinch of salt". A number of times I've been trying a split a double star or see a faint galaxy that should easily be in the range of my scopes only to find that the author of that advice had been using a scope on top of a 10,000 ft high mountain in Hawaii rather than from a semi-suburban back garden 15 miles SW of Bristol !.

John

[kniclander]

well, a 3 inch refractor has a bigger "effective" aperture than a 3 inch reflector because, as has been pointed out, some of the reflector's aperture will be lost because of the central obstruction. Also, a good 3 inch frac may give sharper more contrasty images and you might be able to use it effectively at higher powers. having said that, I bought a 3.5 inch reflector for £9.99 form David Hinds a year or two ago (ok it was on clearance, down form £49 ) and it gives lovely views of the moon and starflieds and you aint gonna get a 3 inch frac that's worth anything for that sort of money.

basically it's much easier to make a half decent reflector than a half decent refractor.

if you want to see people going mental over what's better, go to cloudynights.com for a laugh:)

[johninderby]

There is no real "right" answer as there's just too many factors involved. Theoretically it would be a refractor if you had no limit on your budget, but in the real world a reflector that you can afford to buy may outperform a refractor of the same price. While the reflector may not perform quite as well as an APO refractor per inch of aperture, the reflector is usually so much cheaper per inch of aperture that you can afford to go much bigger and therefore overcome many of the advantages of the refractor. Then there's the argument about an APO being sharper and more contrasty, but that depends on just what your looking at. The best scope is the one that you can afford and actually use.

I'd quite like a TMB 20" APO refractor, but as it costs twice the price of the average house, oh and then add the price of the mount and obs to put it in, it would have to be a BIG lottery win to pay for it.

John

[moondog]

Recently, at a dark site in New Zealand, I looked through a 66mm Apo refractor followed by a 110mm reflector and the views seemed pretty similar. If anything the reflector was fractionally better.

I believe one of Patrick Moore's early astronomy books suggested that a 6ins reflector has roughly a similar performance to a 3ins refractor.

MD

[John]

I believe one of Patrick Moore's early astronomy books suggested that a 6ins reflector has roughly a similar performance to a 3ins refractor.

That may have been the case in the 1950's but it's certainly not today. I reckon my 102mm ED refractor generally performs as well as a 127mm Mak and a 150mm SCT or Newtonian, except for DSO's where the extra aperture counts. That assumes that the non-refractors are in decent collimation.

On nights of indifferent seeing the refractor does a bit better than the scopes above wheres on really good nights the opposite might apply.

The 8" F/6 newtonian that I used to own outperformed my refractor on all counts in all but the worst seeing conditions.

John

[johninderby]

I think that the common view is that a 4" APO refractor is roughly equivalent to a good 6" reflector, which judging from my experience seems about right. Perhaps a better way to compare refractors and reflectors is by price and by what you get per £100.00 spent.

I've got a small APO refractor, Mak and a Dob and each has it's own strengths and weaknesses.

John

[acey]

Traditionally the comparison that was made was between a 4" refractor and 6" reflector, which were seen as equivalent. But there is no formula and never can be, because as others have said, there are too many factors that potentially compromise the performance of either instrument.

The visibility of faint objects depends on the darkness of the sky - you can see all the Messiers through binoculars if you're far enough away from light pollution. When an object is said to be visible in a given aperture you should assume this means at a dark site. If your sky has moderate light pollution then the required aperture will be possibly twice as much, if the object can be seen at all.

I'm a deep-sky observer so a 12" newtonian taken to a dark site suits me fine. If my interest were mainly in planets and double stars then I'd get myself a top-notch 4" refractor and use it in my back garden.

A commonly made argument is that atmospheric turbulence limits the resolution that can be achieved visually, so a 12" scope is rarely going to be able to show more detail than could be seen in a really good 4". The advantage of larger aperture is greater light grasp, making fainter objects visible.

Andrew