How do you work out the extra exposure time between f numbers?

[swag72]

I have seen it said on here numerous times that if you have an f/5 scope and an f/7 scope, then you need x amount of extra exposure time in order to catch the same amount of detail. How do you calculate that?

Amazed by a Pacman in DSO section, where the data collected for OIII and SII was insignificant compared to mine. Looks like he's running an f/4.8 scope and mine is f/6.5 - So how does that wok with regards to exposure times? And can that be TOTAL exposure time or does it only refer to each sub?

[Mike Hawtin]

Hello Sara,

to the best of my knowledge you simply divide the focal ratio of the slower scope by the ratio of the faster one and square the result. I the case of F/4.8 and F/6.5 your F/6.5 scope will require 1.8 times the total exposure.

[GlassWalker]

Every 1.4x difference between two f numbers is a factor of two change in exposure time. So in your first example the f/7 scope would need double the exposure time of a f/5 scope to get the same output assuming all else is equal. To work it out for any arbitrary values, I've squared before dividing but Mike's way is equivalent and saves an operation... why didn't I think of that earlier?

As for exposure times, roughly speaking the exposure of a sub determines how faint stuff you can detect is. The number of subs reduces the noise.

[dph1nm]

If the two scopes have different cameras, then you need to look at the combination of camera pixel size and f-ratio, not just f-ratio. So an f5 scope with 3um pixels is the same as an f10 scope with 6um pixels.

NigelM

[swag72]

Excellent - Thanks folks - Will save this one somewhere special!