Exposed!

[AndyBooth]

OK Eggheads,

Here's a question.

I've been loving looking at all the DSO photos by all the great masters here on Stargazers.

My question is - how does exposure, Fratio, and aperture interrelate?

e.g If some one takes a shot with an F8 100mm refractor, what would be the exposure for a 300mm F5 be?

to give the same result? (FOV different of course).

I guess there must be a formulae to relate dia/f number and time?

Thanks for any info..

Regards,

Andy

[jgs001]

Andy, telescopes and lenses work in the same way, except the focal ratio on a scope is fixed and on a lens it's not. For each stop of Focal ratio, half the light gets to the sensor, therefore you need to double the exposure time. In the case of your example.... f5 to f8 is approximately one stop, therefore, you would need to double the exposure time to achieve a comparable exposure.

The diameter is only used, for exposure time, in the calculation of the Focal Ratio.

[Whippy]

This could be interesting... I think there's many takes on this, I'm sure I've read somewhere that ratio/aperture is almost irrelevant and it's the signal to noise ratio that's the important factor.

The one that makes sense to me (bear with me here, I probably won't explain this particularly well) is the 'ratio squared' explanation. For example a scope that has a focal ratio of f7 takes 49 minutes (7x7) to get x amount of data whilst a scope with a ratio of f5 get the same amount in 25 minutes (5x5), so essentially the f5 scope is twice as fast as the f7 model.

As John has already said, aperture is only used in relation to working out the ratio.

That's what I go on anyway, it's probably wrong but I haven't seen an explanation to prove any of the explanations right! What I do know is that faster is better .

Tony..