maximum magnification, reflector vs refractor

[bomberbaz]

I am a little curious as what the reason is that refractors with smaller aperture seemingly seem to offer higher mag than its cousin the reflector.

I have read about the light loss, only 85% being collected by a reflector (correct me if I am wrong there with that %), but that doesn't add up because from what I can gather, a decent 90 refractor gathers as much as a 130 reflector or sometimes more.

Is this right or am I missing something?

Steve

[Ags]

My Nexstar 4SE transmits 83 percent of the light it gathers, while my ST80 probably transmits in the high nineties. Even so, my mak has an aperture of 102mm and this is still comparable to a refractor of slightly over 90mm - not much difference.

Magnification is a funny thing and you will see all sorts of odd rules. The 2x aperture rule is often quoted, but this only applies to very small apertures. For example, its hard to get more than 160x out of an 80mm scope as the view gets too faint. But as you go up in aperture the 2x rule falls away. The 130mm reflector is theoretically capable of 260x magnification, but in practice this is usually limited by seeing and the target to a lower number. A 250mm dob is theoretically capable of 500x but this is truly theoretical, especially if the dob requires manual nudging! In all three scopes mentioned, the max magnification for a typical night observing Jupiter would probably be the same, around 150x regardless of aperture.

[Moonshane]

I don't know much about the mechanics or rules of optics but in my practical experience, I find focal ratio more important than aperture when it comes to magnification. aperture not only affects light gathering but also resolution so in theory more aperture = more detail but seeing is the limiting factor in most cases when viewing at high power (magnification).

I find that with my 6" f11 I can regularly get to 250x. with my 16" f4 masked to 170mm f11 I can sometimes get to 300-500x when needed but this is rarer. I see more contrast and to me more detail with the big scope masked down (for planets/lunar) than with full aperture apart from on the best nights when the full aperture is amazing.

more focal ratio (i.e. more slowness / a higher f number) also appears to provide better double star splitting to me also.

another factor is tube currents. with refractors you have a smaller 'sealed' unit which creates more stable air and hence potentially less disturbance than an open system like a newt.

there's always lots to consider, especially on rainy nights like tonight.

[ronin]

A 90mm refractor collects less light then a 130mm reflector - that bit is simple.

The difference is that a refractor tends to have a higher contrast image and your eye works as much on contrast as it does on brightness possibly more.

A consequence of the above is that a reflector is described as having a softer image, 2 possible reasons for that. So taking Jupiter if the dark banding tends to blend a bit with the paler bands you do not have such a distinct separation.

I suspect that it is this sharpness that allows a smaller refractor to out perform a slightly bigger reflector and give the impression of a brighter image.

As to the magnification the high magnifications come with the apo (expensive) refractors. In the refractor line you have these days 3 types - achro, ED, apo. You will not get 300x on an achro, you might on an apo.

In reflectors there is basically just a standard parabolic reflector. There are imaging newtonians and these may enable higher visual magnifications, these are expensive as per apo refractors. A normal parabolic give rise to coma that upsets the image and will pervent high magnifications much the same as an achro does in a refractor.

Parabolic is not the ideal shape, it is good but not the final answer, so inherent aberations that are doubled by a secondary mirror (laws of reflection). Want a better mirro then look for a hyperboloid one.

[Peter Drew]

A "normal" parabola does not limit the magnification of a Newtonian, normal being F5-F8, Coma becomes noticeable at faster F ratios and limits the field of good definition. Higher magnifications reduce the field and hence the effect of coma. A first class normal parabola will be limited mostly by collimation and seeing conditions. You couldn't use a hyperbolic mirror in a standard Newtonian, the correction would be way out.

[John]

As well as the all important seeing conditions, the ability of a scope to handle high magnification well is linked to the quality of the optics. A mirror or objective lens with a smooth, accurate figure seems to allow higher magnifications to be used than a poorer quality one.

I used to have a 150mm F/8 achromat refractor. In it's normal state magnification above 200x saw little improvement in the quality of the views even in the seeing conditions were good. I added an optical corrector which was designed to significantly reduce chromatic aberration (CA) and another aberration commonly found in chinese refractors, spherical aberration (SA). With the corrector in place the optical system was performing like a decent quality ED doublet refractor and, as well as the major reduction in CA the scope was able to take higher magnification much better. I found that 300x and even 350x became quite usable on the moon, Saturn, Mars and binary stars while at 200x the scope seemed to be just "ticking over". I reckon that was more to do with the removal of the SA than the reduction in CA.

I'm sure that the above would apply to reflecting systems too, assuming that they are in good collimation. Mirrors with a smooth polish and accurate figure will handle high magnifications better. I know that Shanes scopes fall into this category

[bomberbaz]

Great replies guys. Can't say I fully grasp everything you say because there are some things mentioned I am still figuring out but I get the gist of it.

Its the same whenever you try to expand your knowledge into something that is of interest, you want to explore as much as possible in as wide a range as you can before you start to settle in the area you feel your own personal interest lies. So thanks for your replies as always gents and ladies, it is always gratefully appreciated.

Steve

[Moonshane]

good points all round John.

[Naemeth]

In addition to all of the above, a 130mm refractor has no secondary mirror, thus all of that light that is gathered (or pretty much all of it, as lenses can't have 100% transmission) gets to your eye. With a 130mm Newtonian reflector, there is a secondary mirror in the way, an obstruction, maybe about 20%, and with Maksutovs this is higher, so not all of the light gathered reaches your eye, in addition the mirror reflects about 97% of the light (roughly) as a maximum.

[Moonshane]

but is it 20% by diameter or 20% by area? almost certainly the former so it's only 4% of the area of the aperture. the reduction in light gathering is almost invisible to the eye. the secondary affects contrast more than light gathering and does not visibly affect it when less than 20% by diameter.

[chiltonstar]

Maybe there are different answers depending on what you are observing.

In my experience with two refractors and looking through many others, you can sometimes exceed the 2x (or 5x per inch) rules with double stars, which will sometimes take very high mags, whereas planetary views do not generally improve above this mag.

Chris

[Naemeth]

but is it 20% by diameter or 20% by area? almost certainly the former so it's only 4% of the area of the aperture. the reduction in light gathering is almost invisible to the eye. the secondary affects contrast more than light gathering and does not visibly affect it when less than 20% by diameter.

Almost certainly diameter, but I'm not 100% sure either way. That 4% loss, combined with the 3% loss from each reflection (minimum, as in a Newtonian there are 2 reflective surfaces) does result in more light being lost that in a (high quality) refractor. Of course, light loss increases when you add more elements in an eyepiece, because light transmission cannot be 100%.

[Ags]

With my mak, I have sometimes pushed the mag to 330x (3 times aperture) on double stars, Mars and Saturn :-)

[Moonshane]

Almost certainly diameter, but I'm not 100% sure either way. That 4% loss, combined with the 3% loss from each reflection (minimum, as in a Newtonian there are 2 reflective surfaces) does result in more light being lost that in a (high quality) refractor. Of course, light loss increases when you add more elements in an eyepiece, because light transmission cannot be 100%.

I have no idea how much light loss a refractor has but I bet it's not close to 0%. Personally I doubt there's much difference in light transmission with good mirrors and good fracs (or at least none you can see) and I'd sooner have more resolution and light gathering in the aperture. One thing's for sure, a 130mm newt gathers more light and retains more light than a 100mm ED refractor even assuming 100% light transmission in the frac. I should caveat that though by acknowledging that people buy fracs for reasons other than light gathering.

[bomberbaz]

And so the actiual answer is????

[Naemeth]

I have no idea how much light loss a refractor has but I bet it's not close to 0%. Personally I doubt there's much difference in light transmission with good mirrors and good fracs (or at least none you can see) and I'd sooner have more resolution and light gathering in the aperture. One thing's for sure, a 130mm newt gathers more light and retains more light than a 100mm ED refractor even assuming 100% light transmission in the frac. I should caveat that though by acknowledging that people buy fracs for reasons other than light gathering.

Okay, I am wrong to say it's 100% certainly, it is probably quite close though.

I understand that a 130mm newt gathers more than a 100mm refractor, but, if both were 130mm, then the refractor would win in light transmission (but, it would be much more expensive).

Contrast is also improved by the absence of a secondary mirror.

Oh, and I'd sooner have more light gathering power too, the price of a good quality refractor is huge.

[John]

From my experiences comparing different scope designs in the 4-8 inch range I reckon, broadly speaking, that a decent refractor can perform, on resolution and contrast, as well as a decent reflector of around 1 inch larger aperture. It's rather rough and ready but it's how comparisons I've done seem to have panned out.

For observing deep sky objects though the additional aperture will "win" as it were.

I think the above would change with much larger or smaller apertures though. I just need to get hold of an 11 inch refractor to compare with my 12" newtonian so that I can find out !

[Naemeth]

From my experiences comparing different scope designs in the 4-8 inch range I reckon, broadly speaking, that a decent refractor can perform, on resolution and contrast, as well as a decent reflector of around 1 inch larger aperture. It's rather rough and ready but it's how comparisons I've done seem to have panned out.

For observing deep sky objects though the additional aperture will "win" as it were.

I think the above would change with much larger or smaller apertures though. I just need to get hold of an 11 inch refractor to compare with my 12" newtonian so that I can find out !

Couldn't you just build one?

[bomberbaz]

Couldn't you just build one?

hahaha, you tease

[John]

Couldn't you just build one?

Way beyond my DIY skills I'm afraid

Istar do sell a 10" F/7 or F/11 achromatic objective lens in a lens cell for around £3,400 though !

[Naemeth]

Way beyond my DIY skills I'm afraid

Istar do sell a 10" F/7 or F/11 achromatic objective lens in a lens cell for around £3,400 though !

10" F/11 Refractor, hmm... :Envy:

[bomberbaz]

10" F/11 Refractor, hmm... :Envy:

Sorry I am not sure but that works out at 2750, is that right?

[John]

Sorry I am not sure but that works out at 2750, is that right?

Yep - that's a 9 foot long tube !

Here is a 10" F/15 so you can see the sort of thing we are talking about here

[Naemeth]

Sorry I am not sure but that works out at 2750, is that right?

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

[bomberbaz]

Yep - that's a 9 foot long tube !

Here is a 10" F/15 so you can see the sort of thing we are talking about here

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