Quote:
Originally Posted by Brycepj
Thanks Malcom
This does clarify my question.
Can you explain "f" ratio in telescopes
The lower the f ratio is better, which has been my understanding, if you have a low f ratio how does this relate to the clarity difference under various magnification levels.
Is this logical or do I have my wires crossed. Is a good scope a balance of these variables?
Is it dependant on what you wish to observe, which means different scopes for different applications? If this is true what is a good all round scope.? Is object luminosity, aparent / actual magnitude the guiding factor?
Sorry if this is getting deep it's just my inquisitive nature, that's why I love star gazing
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"f" ratio is simply the ratio of focal length divided by aperture. So your 80mm scope is 400/80 = f5.
It is not correct to say that lower f ratio is "better", it can have some advantages and some disadvantages.
For example in a newtonian scope, such as a dob, with a long focal length will have the eyepiece well off the ground, in a large scope, you need a ladder to observe most objects. On my 20" f5, I stand on the third step of my ladder to observe a lot of the time. If I bought a shorter scope, say an f4 20" I could start observing most objects from the ground and only need a small ladder, but the following would need to be taken into consideration:
- the image would display more coma, probably requiring a coma corrector
- the scope would cost much more, cost increases substantially as the f ratio gets smaller.
The lesson is that as in most things there is no absolute, every decision in scope design is to some extent a compromise.
Malcolm