Aperture

11.19 - Understand that the ‘light grasp’ of a telescope is directly proportional to the area of the objective element and thus the square of the diameter of the objective element

11.20 - Know that the aperture of a telescope is related to the diameter of the objective element

Aperture
One of the key components of using a telescope is collecting light. Aperture is the amount of light that a telescope receives. The larger the diameter of the lens or mirror, the higher the aperture and how much can be seen. Aperture determines how well we are able to view fainter objects.

Light grasp
This is also known as light-gathering power and is a measure of a telescope's ability to capture light and is directly related to the size of the instrument's main lens or mirror. The larger the objective lens or mirror the greater the instrument's light grasp.

Understand that the ‘light grasp’ of a telescope is directly proportional to the area of the objective element and thus the square of the diameter of the objective element

Light grasp can be used to compare one telescope to another using the inverse square law. A 160mm telescope collects 4 times as much as a 80mm telescope.
1602 = 25600
802= 6400
25600 / 6400 = 4

 

Question

Telescope A has an objective of 250mm. Telescope B has an objective of 160mm. How much more light does Telescope A receive than Telescope B?

2502 = 62500
1602= 25600
62500/ 25600 = 2.44

 

 

Links

Starizona Observing Theory

Highlands Astronomical Society Telescope Basics

Astronomy Tools Light Grasp Ratio Calculator