maximum magnification, reflector vs refractor

[bomberbaz]

2794mm to be exact, focal length that is. Which is of course nearly 3 meters, and remember, this isn't a compact system.

give over, on the roof rack at up to the moors in 20 minutes

[John]

Thanks for that John, that pic is a little mind blowing, although I wonder why its open to the elements. Amazing none the less. This hobby can get expensive if you let it judging by that picture, lol

The guy standing next to the scope is John Pons who also made it. The objective lens is a Zeiss though so about as good as it gets. John is based in Florida I think so the scope is at less risk from the elements than it would be here but the scope does just about fit in his garage I believe.

That is the issue with refractors - they get very expensive as well as somewhat unwieldy, when you get over 6 inches in aperture.

[Damo636]

I love that pic, I would imagine the views through that baby would be jaw dropping!

[Naemeth]

I love that pic, I would imagine the views through that baby would be jaw dropping!

Lets hope the mount is up to the job, otherwise it would be telescope dropping :Envy: .

[Naemeth]

I'd just like to add, that according to one of my lecturers, refracting actually results in more light loss than reflecting, the secondary mirror in a reflector of course does decrease the contrast, but refractors probably transmit about the same amount of light as a reflector.

[John]

I'd just like to add, that according to one of my lecturers, refracting actually results in more light loss than reflecting, the secondary mirror in a reflector of course does decrease the contrast, but refractors probably transmit about the same amount of light as a reflector.

I'm not sure I agree with your lecturer on that one. It varies depending on the quality and age of the coatings on the mirror of course but I believe there is a difference in the overall light transmission (even allowing for a mirror diagonal in a refractor) of between 5% and 10%.

[Naemeth]

I'm not sure I agree with your lecturer on that one. It varies depending on the quality and age of the coatings on the mirror of course but I believe there is a difference in the overall light transmission (even allowing for a mirror diagonal in a refractor) of between 5% and 10%.

I should add I didn't add that this was for telescopes. Is there any way to actually quantify this without resorting to conjecture?

[John]

I should add I didn't add that this was for telescopes. Is there any way to actually quantify this without resorting to conjecture?

Well I've directly compared reflecting scopes with refracting scopes of the similar aperture at the same magnification on the same object on the same night, if thats what you mean.

Try it and see what you think

Edit: Don't want this to turn into a refractor v's relector thread though, other than the issue that the OP raised. They are both great scope designs and the relfector easily out runs a refractor in the "bang for your buck" stakes

[Moonshane]

good steadying of the ship John

I think ultimately the optics are broadly the same no matter what design you choose and it's other factors like aperture/portability, observing position, preferred objects and budget that tend to determine what people buy. no matter what, the average decent telescope of any type provides great views at not that much cost.

[acey]

If a telescope has perfect optics then there is no inherent limit on magnification: it's just a case of choosing the objective/ primary mirror focal length to be as great as possible and the eyepiece focal length as short as possible. This is regardless of whether it's a reflector or refractor. In practice there are three limitations. Apart from the fact that optics are never perfect, these are the atmosphere and the human eye. It is the latter condition that leads to apparently higher recommendations for refractor as opposed to reflector.

The eye imposes a limit because if you look through a tiny hole then the light will be diffracted; a point of light will look like a spot. Make the hole smaller and it will be diffracted even more: the spot will look bigger. The exit pupil of a telescope (the disc of light on the eye produced by the eyepiece) is effectively a small hole through which the observer is looking. As magnification increases, exit pupil decreases. If the exit pupil is too small then diffraction will dominate and there will be no further visual gain from increasing magnification. This limit varies from person to person, but as a rule of thumb can be taken as about 0.5mm or 1/50 inch.

Exit pupil = aperture/magnification, hence Maximum magnification = aperture/(minimum exit pupil).

If we take the minimum exit pupil as 1/50 inch then the maximum magnification is 50 times the aperture in inches. If we take it as 0.5mm then maximum magnification is 20 times aperture in centimetres.

Strictly speaking, however, we should not be using aperture, but "entrance pupil", meaning the actual light gathering area. In a refractor there is no difference because the objective lens is unobstructed: the entrance pupil diameter is the same as the clear aperture. But a reflector has a central obstruction that blocks part of the primary mirror, hence the entrance pupil diameter of a reflector is slightly less than the aperture.

As an illustration, compare a refractor and a Newtonian reflector which both have an aperture of 150mm, and suppose that the Newtonian has a 50mm diameter secondary mirror. The refractor has entrance pupil diameter 150mm, the Newtonian has entrance pupil diameter 141.4mm (= sqrt(150^2 - 50^2)). If we assume the optics to be perfect and assume that the atmosphere places no limit on magnification, and if we assume that the minimum useable exit pupil is 0.5mm, then the refractor will have a maximum useable magnification of 300 (=150/0.5) and for the reflector it will be 283 (=141.4/0.5).

The refractor has a higher maximum magnification simply because it has a larger light gathering area than the reflector of equal aperture.

[bomberbaz]

If a telescope has perfect optics then there is no inherent limit on magnification: it's just a case of choosing the objective/ primary mirror focal length to be as great as possible and the eyepiece focal length as short as possible. This is regardless of whether it's a reflector or refractor. In practice there are three limitations. Apart from the fact that optics are never perfect, these are the atmosphere and the human eye. It is the latter condition that leads to apparently higher recommendations for refractor as opposed to reflector.

The eye imposes a limit because if you look through a tiny hole then the light will be diffracted; a point of light will look like a spot. Make the hole smaller and it will be diffracted even more: the spot will look bigger. The exit pupil of a telescope (the disc of light on the eye produced by the eyepiece) is effectively a small hole through which the observer is looking. As magnification increases, exit pupil decreases. If the exit pupil is too small then diffraction will dominate and there will be no further visual gain from increasing magnification. This limit varies from person to person, but as a rule of thumb can be taken as about 0.5mm or 1/50 inch.

Exit pupil = aperture/magnification, hence Maximum magnification = aperture/(minimum exit pupil).

If we take the minimum exit pupil as 1/50 inch then the maximum magnification is 50 times the aperture in inches. If we take it as 0.5mm then maximum magnification is 20 times aperture in centimetres.

Strictly speaking, however, we should not be using aperture, but "entrance pupil", meaning the actual light gathering area. In a refractor there is no difference because the objective lens is unobstructed: the entrance pupil diameter is the same as the clear aperture. But a reflector has a central obstruction that blocks part of the primary mirror, hence the entrance pupil diameter of a reflector is slightly less than the aperture.

As an illustration, compare a refractor and a Newtonian reflector which both have an aperture of 150mm, and suppose that the Newtonian has a 50mm diameter secondary mirror. The refractor has entrance pupil diameter 150mm, the Newtonian has entrance pupil diameter 141.4mm (= sqrt(150^2 - 50^2)). If we assume the optics to be perfect and assume that the atmosphere places no limit on magnification, and if we assume that the minimum useable exit pupil is 0.5mm, then the refractor will have a maximum useable magnification of 300 (=150/0.5) and for the reflector it will be 283 (=141.4/0.5).

The refractor has a higher maximum magnification simply because it has a larger light gathering area than the reflector of equal aperture.

there you go, easy peasy.

Seriusly, nice explanation, well put

[RikM]

there you go, easy peasy.

Seriusly, nice explanation, well put

It also supports John's '1" extra needed on the reflector to give equivalent views' experience.

[Naemeth]

Well I've directly compared reflecting scopes with refracting scopes of the similar aperture at the same magnification on the same object on the same night, if thats what you mean.

Try it and see what you think

Edit: Don't want this to turn into a refractor v's relector thread though, other than the issue that the OP raised. They are both great scope designs and the relfector easily out runs a refractor in the "bang for your buck" stakes

Sorry, I really didn't mean to start that. Your way was sort of what I meant, perhaps there is an accurate measure of how many photons are coming through the scope, no?

[Moonshane]

ha ha you can tell it's cloudy again!!

[John]

Sorry, I really didn't mean to start that. Your way was sort of what I meant, perhaps there is an accurate measure of how many photons are coming through the scope, no?

I guess it should be possible to pass a known amount of light though an optical system and then measure what % of it reaches the focal plane. It's probably been done but I've not been able to find any reference to it on the web, as yet.

If I find anything, I'll post it here

Edit: and it is cloudy here too

[Naemeth]

Yes... it is cloudy here.... again! Perhaps good viewing tomorrow..

[RickV]

My limited experience with my Orion 100mm ED 900mm and my Celestron C11 says:

a) My ED Refractor - can push the magnification to about 3X the aperature (3 x 100mm = 300X) or 3X the focal length (3 x 900 = 270X).

    e.g. a 3X barlow.  Anything more and the image is bigger but has lost detail and is blurry/fuzzy.

    With the refractor, I think I am limited by the optics.

My C11 SCT - can only push the magnification to about 1.5 x the aperture (3 x 279mm = 418X) or 1.5X the focal length (3 x 279 = 418X).

     e.g. a 1.6X barlow is marginallly over but OK.  A 2X Powemate is too much; image loses detail and goes blurry/fuzzy.

     With the SCT, I think I may have been/am passing the limits of seeing at 400X.

In both cases, the target was Jupiter recorded with a 1.3MP MallinCam Solar System Imager (webcam) with 3.63U pixels.

Those are my 'Rules of Thumb', 3X for quality refractor and 1.5X for an SCT.

RickV