Re: Planetary imaging with fast Newtonian

On 17 Oct 2005 16:44:51 -0700, "jonisaacs@xxxxxxx" <jonisaacs@xxxxxxx>
wrote:

>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. "

Yes, that's what I said. And that's what I meant. The focal ratio alone
tells you absolutely nothing about exposure time.


>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.

But it isn't. It is dependent on aperture alone. The fact that the focal
ratio happens to change as you change aperture is irrelevant. Knowing
the aperture you can make meaningful calculations of S/N, and therefore
plan reasonable exposure times. You cannot do that if you only know the
focal ratio.

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

Of course- with any two you can calculate the other. But in general
there is no need for the focal ratio- it isn't a useful metric for
either visual astronomers or for imagers. It is marginally useful for
film imagers, mainly for historical reasons, but also because
"brightness" affects S/N when using a highly non-linear recording medium
like film.


>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.

It doesn't affect the exposure length at all when you are imaging. A
larger image with a lower photon density and a smaller image with a
higher photon density (call that "brightness") have the same S/N ratio
if they are made with the same aperture. Changing the image scale
doesn't affect that. Put another way, if you take an image with a 10"
f/10 scope for one minute, and you take an image with a 10" f/5 scope
for one minute, you can either double the size of the latter or halve
the size of the former, and the two images will be identical (as long as
you preserve the total signal count).


>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.

They will. But the difference is purely based on the aperture difference
between the scopes.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

  • 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: 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. ... Clearly in this case S/N is dependent on the focal ratio, ...
    (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)