f/ ratio and photopgraphy

[Earl]

I few hewbie Questions.

What are the diffrences between (for example) a F4 scope and a F10 Scope in regards Prime Focus Astrophotography?

[themos]

To a first approximation, f4 would be good for nebulae and deep-sky, f10 would be good for planets.

[Demonperformer]

An F10 scope would require 6.25x the exposure time

[Earl]

Cheers.

So for DSO work a big Newt is where you want to be looking?

[LukeSkywatcher]

Cheers.

So for DSO work a big Newt is where you want to be looking?

Looking upwards would be better. LOL.Sorry. Yeah a newtonian for DSOs.

[TheThing]

A fast newt, as opposed to a big newt (although big is good!)

[themos]

The Newtonian (well, a parabolic reflector with a flat secondary) is certainly the cheapest option but has inherent distortions ("aberrations") that limit its usefulness for DSOs that require a large field (in terms of angular size).

[david o]

Looking upwards would be better ...

Paul, you owe me a new keyboard - the coffee went everywhere!

[dph1nm]

The real answer to this question is it depends what CCD you put on the end. If you chose your ccd+scope combination so that the scale in arcsec/pixel is the same on both scopes then there is no real difference between imaging at f4 or f10. People who decry imaging at long focal lengths tend to forget this.

NigelM

[hunterknox]

The exposure time generally varies according to the square of the focal ratios, so Demonperformer's answer is based on:

10^2 / 4^2 = 6.25.