Lower f ratio

[Manoocher (Manoocher)]

Can someone explain the benefits of taking DSO images using lower f ration? More specific taking images with f/5 vs f/3.5.

[RyanJones]

Hi Manoocher,

The simple answer is that it takes less time to capture the same amount of data. There is far more to it and some physics to understand why but that’s the crux of it. Becareful though thinking that you should just buy the fastest scope you can afford because there are caveats. Fast scopes are much more sensitive to collimation issues, they are more sensitive to focus, they require much better curvature correction and depending on fast you go some filters may not be suitable. As the old saying goes..... nothing comes for free, there’s always a price to pay.

[raymo]

Lenses and telescopes range from photographically "slow" to "fast", literally.
The lower the f number the faster the scope/lens. Put simply, the lower the f number the more light that will get to the camera sensor in any given time.
In short, an f/3.5 scope/lens will provide the same result as an f/5 scope/lens
in about half the exposure time, allowing the user to get twice as many subs
for stacking from any given length of imaging session.
raymo

[Startrek (Martin)]

Fast scopes in theory do capture more data per same exposure time compare to slower scopes but that doesn’t mean slower scopes at f5 to say f7 are inferior
Fast scopes require a higher level of expertise to set up the image train and are less forgiving in relation to alignment of optics etc.. They are more suited for intermediate to advanced Astrophotographers.

Are you considering buying your first telescope for Astrophotography ?

If so, my recommendation would be to stick with focal ratios f5 to f7 to get started in the hobby

Cheers

[AdamJL]

a lot of modern glass also performs better "stopped down" to use a photographic terms.
Camera lenses in general are sharp (for terrestrial work) wide open, but when stopped down a good lens can rival telescopes.
Telescopes tend to sit at these "stopped down" f/ratios as they're aiming to hit that perfect combination between sharpness and light gathering capability. Plus, longer exposures negate the absolute requirement for fast telescopes.