f ratio 'fast-slow'

[omegacrux]

Why does a longer tube of the same diameter become slow can someone please explain I don't get it ? thanks

[Lester]

Hi David, a longer tube = more magnification. As the object is magnified, or spread it becomes dimmer. Hope that helps.

[Stardrifter_WA]

Yes, David, in addition to the above, conversely, if you maintain the same length of tube and dramatically increase the size of the mirror, you get a faster scope with much brighter objects, as it can collect more light. This leads to what is colloquially know "aperture fever". :)

[omegacrux]

But what if a 150x750mm and a 150x1000mm were both at 100mag wouldn't one be fast and the other be slow ? or both be the same . my head is about to go bang

[Stardrifter_WA]

Good question? If both were at 100mag, then the eyepiece in the 750mm would be a shorter focal length, a 7.5mm and in the 1000mm a longer focal length of 10mm. Not sure what the effect it would have. I should imagine the 1000mm would appear dimmer.

[barx1963]

I'll probably get corrected on this but as I understand it being fast or slow is not really relevant for visual use. They are really photographic terms. A faster scope will produce an image quicker than a slower scope on a sensor or film.
As your eye doesn't work but accumulating signal it is not really relevant to what is seen.

Malcolm

[omegacrux]

Thanks for the answers fellas I think I am more wiser

[The_bluester]

I actually asked a similar sort of question a while back (Actually, how changing the focal length of my scope and therefore the focal ratio which can be done with my SCT would produce images faster) I got a pretty good answer in there. I will dig it up and link to it.

Edit: found it.

http://www.iceinspace.com.au/forum/showthread.php?t=86248

[Satchmo]

A 6 inch telescope at the same magnification will give the same brightness view regardless of the focal ratio of the mirror. It's only the eyepiece that must be different to produce the same magnification in each scope.

[The_bluester]

I suppose that is where it gets more complicated, the thread that I put up is discussing the aspects from a prime focus point of view.

[Poita]

It is a bit relevant in that you get a more contrasty view through a slower scope, so the sky will look blacker through an F15 scope than it will through an F4, but most people care about if for imaging. For instance Hyperstar turns your F10 cope into an F1.9 light bucket, which makes a huge difference to exposure times.