Curious question about light gathering capability

[M4lcs67]

Hey peeps,

Just another question from yours truly. I know that the scope manufacturers always go on about this scope has 50% more light gathering capability than that scope etc...... What is the calculation to work this out???? Reason I am asking is that I have a 127 SLT at the moment (5"). But i am expecting a C9.25 soon and was wondering what the light gathering capability of the new scope would be compared to my 127 SLT?

Just curious really, but I guess it would be handy to know what the calculation is for future reference.

Thanks muchly

[Capricorn]

It goes by the area which as a ratio is the square of the diameters.

5 squared is 25,

9.25 squared is 85 (close)

So the ratio is 85/25 = 3.4

The 9.25" will collect 3.4x the light of the 5"

[Merlin66]

Geee, I love the occasional easy question!

The LGP is just the area of the lens or mirror. This can be compared to the human eye (6mm diameter) or any other lens/ mirror.

The area is Pi *d^2/4 where Pi= 3.14157 and d is the clear diameter of the lens.

When comparing lens the Pi/4 is a constant, so it boils down to the ratio of d^2.

The eye = 6*6 =36

Your 125mm = 125*125 = 15625

The C9.25= 235*235= 55225

The 5" therefore collects some (15625/36)= 434 times more light than the human eye

and the C9.25 (55225/15625) = 3.53 times more than the 125mm

Hope this helps

(An interesting question for you to consider...what does the x3.53 more light mean in terms of fainter magnitudes??....)

[Merlin66]

Hmmm

talk to much - type too slow!

[steppenwolf]

This is all to do with the area - for example, a 200mm aperture instrument has 77% more light gathering power than a 150mm telescope = simply apply pi*r² to get the area of each primary and then divide the larger area by the smaller area, multiply by 100 then subtract 100 to get the percentage.

[steppenwolf]

I'm too slow too!!! At least you got your answer ......

[M4lcs67]

Thanks to you all. Wish I hadn't asked now, lol.....