Light gathering power of a telescope compared to another.

[Guest]

Probably discussed before. But I have an 80mm refractor. My question is how would you know the light gathering power of a larger telescope compared to it. Is a six inch telescope for example 3 or 4 times the light gathering capability of the 80 mm? Is there a simple equation?

[andrew s]

It the collecting area that counts which is dependent on the square of the diameter.

So you 6" = 150mm has (150x150)/(80x80) = 3.5 times  more light gathering power.

Regards Andrew 

PS is there is a central obstruction you have to subtract that from the area.

Edited July 20, 2020 by andrew s

[vlaiv]

It also depends on type of telescope and coatings applied. Mass produced newtonians in particular need to take this into account.

For example, 6" F/5 newtonian (28% CO) with regular "enhanced" coatings will have light gathering close to:

(75mm ^2 * pi - (0.28 * 75mm)^2 *pi) * 0.94 * 0.94 = ~14390.3mm2

which is equivalent to 135.36mm of clear aperture.

(mind you, refractors also need to take into account losses, but these are fairly small, something like 0.5% per Air/Glass surface. You have 4 of these for ED doublet, so total losses are about 2%).

 

[Guest]

Thanks for the info. As I thought. The 80mm ED refractor gives nice sharp images. While the six inch Newtonian resolves dimmer objects with it's greater light gathering power.

[John]

I think the reflective coatings get a bit less reflective each year as well. Maybe a few % loss per annum ?

[Nigeyboy]

Check out this useful tool - its the section at the bottom. Just put in both the telescope sizes and it will work out the difference.

Interesting if for the smaller scope you input 7mm (average human eye), to get an idea how much more light even a modest scope collects!!