Confused, F ration and focal length

[Theninjagecko]

Confused over the F ratio of a scope and its focal lenght.

I had a mak 127 f11 1500mm "or there abouts" mak.

I now have a 300p dob F4.9 1500mm.

views through both instruments with the 25mm yeild almost the exact same view, obviously the 300p shows much more. Example being the double cluster in Cassiopeia. on both scopes they just about fit in the view of the 25mm eyepiece. I understand this to be because of the focal length being the same, but what exactly does the f/ratio do?

[Capricorn]

Both are 1500mm focal length so with the same eyepiece they will give the same magnification and so look much the same.

Field of view should be smaller through the Mak and dimmer, smaller objective.

F number is the ratio between the focal length and the diameter, really a camera term as it defined the brightness of an image that is splatted on film/ccd direct.

For a telesascope the image size is dependant on the focal length, based on the same object, distance away and objective, then a bigger focal length give a bigger image. But as the light collected is the same then the bigger image is dimmer.

[Theninjagecko]

So the f ratio is pointless unless imaging? Or would two 1500mm scopes also yield the same image size on the ccd?

You say field of view will be smaller in the mak? So it would look more zoomed in a little?

[brianb]

So the f ratio is pointless unless imaging?

Sort of ... there are performance penalties to ultra short ratios (critical collimation, inability to work well with cheap EPs) but the tube length reduction balances this (to some extent).

Two scopes with the same focal length will get the same size image but the one with more aperture will give a brighter image / reveal fainter objects. Going from 127mm to 300mm gives you an increase in light grasp of (300/127)^2 = 5.58x which means a much shorter exposure (to beat seeing, tracking errors etc) or digging into objects which are 1.9 magnitudes fainter ... and half the objects you can see with any scope are within half a mag of its threshold!