The focal ratio is the ratio of the focal length to the aperture. So, a telescope with a 2500 mm focal length, and 250 mm aperture is a f/10 scope. A scope with a smaller focal ratio always benefits the users in imaging as it shortens the exposure time. Some people will call it photographic speed.
Why it takes more time to have the same result using the same imager but a slower telescope? Let’s say I have a f/5 and f/10 telescopes and both of them have the same aperture. Let’s make them 8″. Now, I move from f/5 to f/10. Since the aperture is fixed, focal length is the only increasing factor. In this case, it is doubled. As the image you see is in 2 dimensions, it needs a least 2 rays of lights to form an area. Therefore, the lights are spreaded into a factor of 4. In conclusion, you take 4X more the exposure time to get the same S/N.
*S/N refers to Signal/Noise ratio
Below is the formula to calculate the exposure factor,
Exposure Factor = (Focal ratio 1)^2 / (Focal ratio 2)^2
Once you figured out the exposure factor, you know how much is A scope faster than B scope or vice versa.
