Re: Planetary imaging with fast Newtonian

>>>They aren't. If you make equal length exposures with the two scopes, operating both at the same focal length, the S/N will be better for the Newt because of its increased aperture. At no point in that analysis is it necessary to introduce the concept of focal ratio.
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Here is what you wrote:

""Why? Focal ratio tells you nothing about the exposure times. All you
care about is the S/N, which is independent of focal ratio. "

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Clearly in this case S/N is dependent on the focal ratio, not
independent of the focal ratio. That was why I took the time to lay
out this example.

One needs to know two of these three: Aperture, focal length, focal
ratio... Any two will do. I like all three.

Focal ratio is a measure of how the light will be concentrated, a fast
focal ratio means the same light is put into a smaller area thus making
a brighter image and therefore a shorter exposure.

I find this concept of some use, others seem to, you may not.

In the case of the Pronto and the Newt operating at 6000mm FL, one is
F85, the other F19. The exposure times will vary somewhat.

Best to all...

Jon Isaacs

.

Relevant Pages

  • Re: Planetary imaging with fast Newtonian
    ... Focal ratio tells you nothing about the exposure times. ... >care about is the S/N, which is independent of focal ratio. ... It is dependent on aperture alone. ...
    (sci.astro.amateur)
  • Re: Planetary imaging with fast Newtonian
    ... Focal ratio tells you nothing about the exposure times. ... what determines the S. An 12 inch scope operating at 6000mm will ... provide more S than an 8 incher operating at 6000mm.. ...
    (sci.astro.amateur)
  • Re: Aperture, F-Ratio, and Exposure Time
    ... Shame on you! ... > noise down, and that means the shortest possible exposure times, at the lowest ... More aperture means more photons, ... The effect of focal ratio is still secondary, ...
    (sci.astro.amateur)
  • Re: Stacking Question...
    ... > I don't find the concept of focal ratio at all useful in imaging. ... So focal ratio does NOT determine the exposure times, not for telescopes. ...
    (sci.astro.amateur)
  • Re: Planetary imaging with fast Newtonian
    ... >I agree that what we are talking about here is getting that 8 inch FL ... >1000mm scope up to 5000mm or 6000mm effective focal length. ... Focal ratio tells you nothing about the exposure times. ...
    (sci.astro.amateur)