Glass v mirror.

[rory]

Does the same aperture of a refractor and reflector equal same brightness of image ? Or is there some difference ?

So would a 90mm refractor for example be a the same as a 114mm reflector.

I was asked this today. , and I've no idea. , I'd not thought about the characteristics before now.

Is there perhaps a rule of thumb . Or are they indeed equal .

Thanks .

[nmoushon]

The brightness would be the same, I would think. What the difference would be is that glass (or a lens) would refract the light when it passes through and would create false color around bright objects if its not corrected with other lenses. This varies on how fast the f/ratio is. The faster it is the more visible this becomes and the harder it is to correct. Thats why their are Achro and Apo refractor telescopes. Apos correct this better than Apos (also cost a lot more) but most the time Achro scopes are a lot slower and thus its not as visible. Some people don't mind the slight false color and other can't stand even the slightest bit. Reflectors don't have this problem as they just reflect the light off mirrors and into your eye. But no matter how they collect the light the collect the same amount if the apertures are the same. At least this is how I understand it.

[Stu]

Correct apart from the impact of the secondary mirror and supports which do block some light and reduce contrast a little.

You can compare the two if you work out the area. The calculation is PI x the diameter (Pi is roughly 3.14). For the Newtonian you subtract the area of the secondary from the area of the primary and this will show you the light gathering power vs the refractor.

Many other factors come in to play too such as the reflectivity of mirrors and transmissiveness of the glass and coatings on a refractor, plus any diagonals used etc etc etc

Stu

[nmoushon]

Correct apart from the impact of the secondary mirror and supports which do block some light and reduce contrast a little.

You can compare the two if you work out the area. The calculation is PI x the diameter (Pi is roughly 3.14). For the Newtonian you subtract the area of the secondary from the area of the primary and this will show you the light gathering power vs the refractor.

Many other factors come in to play too such as the reflectivity of mirrors and transmissiveness of the glass and coatings on a refractor, plus any diagonals used etc etc etc

Stu

GAH! Can't believe I forgot the secondary.

[Stu]

:-) Easily done

Stu

[ronin]

The same diameter collect the same amount of light. However the eye does not respond to brightness alone, the contrast is a significant factor in how you see things.

The spiders and secondary tend to lower the contrast so the eye can "see" more even if dimmer but the contrast is higher. The eye is more complex.

Another factor is if you can collect enough light through a 100mm refractor to have a good bright image then a brighter one through a 150 mirror doesn't really add much, just it's brighter. Do you stop driving a car on an overcast day because the sun is not shining? What is the purpose of a moon filter if not to make the image dimmer.

[ollypenrice]

If you want a rule of thumb I'd make that, 'premium refractors punch above their aperture,' both in imaging and in visual use. By how much is a moot point. Even though I'm a refractor buff I'd say it was by a fairly small amount these days, when mirrors are so reflective. But another good rule of thumb would be, 'Newtonians punch above their price tag.' Yet a third rule of thumb might be, 'A bad reflector beats a bad refractor' but that is really just rephrasing the point about the bargain price of Newtions.

Olly

[rory]

Cheers guys. Generally speaking then , there's not much in it .

[Stu]

Brightness wise, there is not so much in it, provided the secondary is not too big. Even if the percentage looks significant vs the diameter, once you calculate the areas it is normally a relatively small percentage reduction by area.

The main differences are qualitative rather than quantative. By this I mean you get diffraction spikes on Newtonians, but don't on refractors, whilst you get false colour on achro refractors, but not on Newtonians.

Stu

[Astro Imp]

Interesting thread, it seems that even answering an original simple looking question is more complex than it seems.

[John]

Interesting thread, it seems that even answering an original simple looking question is more complex than it seems.

Indeed. This one would keep them busy for weeks on some forums 

[rory]

Interesting thread, it seems that even answering an original simple looking question is more complex than it seems.

I was worried I was asking a really daft question. When I was asked the question originally my immediate thought was aperture is aperture . Then I got to thinking about mirrors and how much they reflect back , spider veins and another mirror. Then there's various amounts of glass with different coatings and so on.

So although I answered " they are the same " I was left with doubt . Where else to get an answer than good old sgl ?

[Astro Imp]

 Where else to get an answer than good old sgl ?

First port of call for everything astronomical.

[John]

It could lead on to a discussion about the total optical performance of systems. We see published data for the primary mirrors and occasionally objective lenses but what we see at the eyepiece depends on the combined performance of secondary mirror, diagonal (refractor / mak-cass / SCT) and the corrector (SCT's / mak-cass) and the coatings on those as well. 

I wonder what the variance in overall light transmission between these designs is ?

There are light transmission figures published for eyepieces now and then but I can't recall seeing much on the full optical system of scopes.

[Chris]

qualitively I prefer the views of jupiter through my ED120 than my old 200p or C8, then vise versa so far when it comes to DSO's. Obviously the 200mm Newt and C8 has quite a bit more aperture than the 120mm ED frac so I'm assuming planets look better with the 120 because the contrast is simply a bit better. The overal light grasp isn't quite as good as the 8" scopes for DSO's mind. I'm sure there are even more factors between fracs and newts other than secondary mirror and vein obstuction and refrac vs reflective index. e.g. Does more light get scattered in newts and SCT's from being folded over on itself?

Chris