Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Confused, F ration and focal length


Recommended Posts

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?

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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!

Edited by brianb
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.