Does 75% More Light Gathering Mean....

[sabana]

The Difference between an 8 inch scope compred to a 6 inch scope is roughly 75% more light gathering capability? I think this is correct.

Does this mean you can see 75% further into Deep Space? (With the same eyepieces eg. 10mm). Or just you can see the same distance but it will be 75% brighter in the viewfinder.?

The scopes i had in mind were the Nexstar 8se and 6se.

Clear skies

[Bizibilder]

It means that the image will be brighter - therefore you can see more objects as some of those too faint to see with a 150mm scope will now be bright enough, in the 200mm, for your eye to "see" them. Distance is not really relevant as there are close dim objects and distant bright ones all jumbled up together!

"75% more" is a salesmans term and is really meaningless!

[Omar]

From photography point if view, I guess it means less exposure time, for deep sky long exposure photography. I am not sure about that, it is only a guess...

I'm still reading about astronomy and astrophotography..

I hope I have been of help

[E621Keith]

It means an object will be brighter.

An object's brightness is affected by many variables, such as its distance and how much light it emits/reflects. Also an object's brightness is inversely proportional to the square of the distance, so an object twice as far will appears four times dimmer.

[assasincz]

I have to admit I am rather confused about it myself and I beg for explanation from someone competent.

I have recently done a bit of math and compared the surface area of my current scope's primary with that of a scope I would like to buy.

So that is 4.5" vs. 12"

Now excuse me for doing it in metric, but:

The 114mm mirror has the surface of 102 cm2

The 300mm mirror has the surface of 730 cm2

That is 615% more than the surface area of the smaller mirror

Scope calculater spews 13.0 and 15.0 limiting magnitudes.

Now I have read that 200mm scope has 80% more light gathering than a 150mm one and 140% more than a 130mm one.

The surface area of the 200mm (314cm2) is 78% larger than that of the 150mm (176cm2) and 137% larger than that of the 130mm (132cm2).

Is my inference that the "light gathering" = "mirror surface area" correct or am I missing something out? Is saying that 12" scope gathers 615% more light than 4.5" one correct? It seems like a whopper of a difference to me

[Bizibilder]

Comparing mirror surface areas is how these companies get the "percentage bigger" figures. However this is not the full story as factors such as secondary size and position also play a part as well as the reflectivity of the mirror's surface.

Limiting magnitude is another measure but most people are actually limited by light pollution and seeing rather than any theoretical values.

Mirror size also determines resolving power (the ability to seperate two very close light sources) but again, in the real world, atmospheric seeing overpowers any theoretical values.

[ambrishnat]

Also a doubling of brightness, in terms of the amount of light that's there, doesn't translate into a doubling of perceived brightness.

This is an interesting link. A guy has created a scale of perceived brightness vs. real brightness, for the measuring of the brightness of flashlights: http://www.candlepowerforums.com/vb/showthread.php?271967-Perceived-Brightness-Index

[ronin]

It simply means that the 8" collects 1.77 times as much light.

If the visual magnification is the same in both scopes then the 8" will be brighter, if the magnification is not the same then the brightness depends on the level of magnification.

For astrophotography the f number is relevant, same f number same brightness of the primary image.

Once the light is collected, 6" worth or 8" worth, it depends on what you do with it that determines the final brightness.

[cantab]

A larger scope will collect more light. For point sources like stars, that means you can see fainter. One magnitude is a 2.5 times brightness change, so the difference between a 6 inch and 8 inch scope is about half a magnitude.

For extended sources like DSOs, the greater aperture means that you'll get greater surface brightness at the same magnification, or for the same surface brightness you can use higher magnification that will show more detail. (However, effects like diffraction, CA, and other optical errors may impair detail).

[Charon]

Excellent question Sabana, great thread.

[Syntarsus]

Excellent question Sabana, great thread.

Indeed it is.

[acey]

Difference in light-grasp between an 8" and a 6" is indeed given by the ratio of primary mirror's surface area (assuming central obstructions are in proportion): 8^2/6^2 = 1.78, i.e. the 8" gathers 78% more light. Comparing a 12" with a 4.5" we get 12^2/4.5^2 = 7.11, i.e. the 12" gathers 611% more light.

But how much brighter does the image look? The problem is that "brightness" is not a precisely defined concept. You could define it as the number of photons being received per second from a star, but the eye is not equally sensitive to every kind of photon: we can't see infra-red, for example, and some colours look brighter than others. So you could modify the definition to include the different way we perceive different colours (there is an international standard for this), but again, the standard is based on photopic vision, i.e. normal daytime vision, not scotopic vision, which is what we use in low light levels.

In the 19th century it was thought that brightness perception is logarithmic: if you multiply the intensity by some factor, then the perceived intensity will increase by an additive amount. This is called Weber's Law (or Weber-Fechner Law) and was the basis for the logarithmic magnitude scale: multiply the intensity by 100 and the magnitude increases by 5 (so an increase of 1 magnitude corresponds to increasing intensity by a factor of 2.512).

But it turns out that brightness perception isn't logarithmic, in fact it follows what's called a power law (this was discovered by Stevens, hence is called Stevens power law). And the added complication is that the law depends on the size of the object being viewed.

Upshot is that there is no simple, absolute, clear-cut way of saying what difference it makes if you go from a 6" to an 8" scope. But as a rough estimate we can think of "magnitude gain", meaning how much fainter we would expect to see, leaving aside all the technical quibbles.

Going back to the definition of the magnitude scale, we would expect an 8" to beat a 6" by 5log(8/6) = 0.62 magnitudes. A 12" should beat a 4.5" by 5log(12/4.5) = 2.1 magnitudes. That maybe doesn't sound a lot but it's actually a huge difference in terms of what you can see: 2 magnitudes is the difference between the faintest stars you can see from a brightly-lit town and the faintest you can see from remote wilderness. As for the difference between the 6" and 8", it's nowhere near as big but still appreciable. On the other hand, going from a 10" to a 12" you'd expect a gain of 5log(12/10) = 0.4 magnitudes. Many users would hardly notice that sort of difference and might feel disappointed, even though the increase in light grasp is a sizeable sounding 44% (12^2/10^2 = 1.44).

Percentage increase sounds far more dramatic: that's why you see it in telescope adverts trying to persuade you to go that extra inch or two.

Finally note that the foregoing refers to point-sources, i.e. stars. With extended objects (galaxies, nebulae) we are more concerned with "surface brightness" (luminance), which is another story.

http://en.wikipedia.org/wiki/Apparent_magnitude

http://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law

http://en.wikipedia.org/wiki/Stevens'_power_law

http://en.wikipedia.org/wiki/Photometry_(optics)

http://en.wikipedia.org/wiki/Brightness

http://en.wikipedia.org/wiki/Limiting_magnitude

http://en.wikipedia.org/wiki/Luminance

[todd8137]

if you had 50 red aples on a table ,and added 75 more red apples giving you more in the same as light