Aperture

[OzStarGazer]

Hello,

I saw an ad of the Skywatcher Heritage 130cm that said it collects 3 times more light than a 114cm telescope. This is a difference of about an inch. So I was curious to know if an additional inch always means 3 times more light or if it varies? Is there a rule?

Thanks!

[glend (Glen)]

Simply calculate the area inside both circles (mirrors) using the formula A=3.14 x the radius squared, and then subtract the smaller one from the larger one. Take that difference and divide by the smaller number to get the percentage increase in the larger mirror - hope that make sense. A quick calculation shows the area difference is 30%, so the larger mirror would have 30% more light gathering area.

[OzStarGazer]

Thanks! That's exactly what I needed to know.

[Robh (Rob)]

Another way to do it (^2 means squared) is

(130/114)^2 = 1.30 or a 30% increase.

Regards, Rob

[OzStarGazer]

Thank you, Rob!

[raymo]

If you work in inches, the difference in light gathering ability is immediately obvious. Just square the objective diam. For example
4" x4" =16 5"x5"=25 6"x6"=36 8"x8"=64 !0"x !0" =100.
Being smaller numbers than millimetres, it is easier to see the relationship between scope sizes. From these figures it can instantly be seen that a 10" [100] has 4x a5"[25], or an 8" [64] has 4x a 4"[16].
For less straightforward numbers it can be seen for example that an
8" has 64% of a 10", or a 6" has 36% of the 10"
just another way of looking at it.
raymo

[OzStarGazer]

Thanks - yes, that is possible too.