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Originally Posted by The_bluester
OK, this one even has my wife semi stumped (Trained as a professional photographer by the RAAF, who at that time even went into the chemical reactions going on in the film/paper to make it all work in film based photography) and she had to mumble about not quite recalling the techincalities of it.
F stop of a camera lens is easy to understand and easy to see why exposure times vary. Stop it down, reduce the apeture, reduce the amount of light falling on the media (Film, sensor, whatever) Open it up, more light, shorter exposure, simple.
How does it work with a scope? I can understand my CPC925 being an F10 system and that it will need exposures of X amount of time at prime focus etc. But how, if I get a fastar setup and convert it to F6 (I think) does it work to reduce exposures required to a fraction of the time when the objective that is gathering the light is precisely the same as before (Except it probably has a larger obstruction from replacing the secondary mirror with the fastar and the body of the camera)
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Yes, it works the same. To change the focal ratio of a telescope you put a focal reducer in, which takes the light from that same aperture put narrows it down to focus the same light on to a smaller area, reducing focal length (magnification) and making the focal ratio "faster".
Quote:
Originally Posted by The_bluester
Is it an inverse square thing where the light has had to travel further at F10 from the objective mirror to the camera media than with a fastar or other setup at a faster ratio? And if so, how does that work with folded optics like the 925? Apart from the fact that it just would not work, if I imaged at F2, straight off the objective, versus imaging at the prime focus at F10, the light clearly has not travelled five times as far!
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When you use something like Fastar, you are dramatically reducing the focal length, while keeping the aperture the same, so you are dramatically changing the focal ratio. A focal reducer might change the focal length by a multiplication of 0.8, but a Fastar option changes it by significantly more.
It's worth noting that, from what I understand, light sources which are a pinpoint at infinity do not become brighter when focal ratio is changed, so, the recorded brightness of a star will not change with focal ratio, it will only change with aperture. However a nebula will become brighter on the chip with a faster focal ratio. Now, this is what I've been told and I can see some logic there but I could also argue against it
I was convinced (somewhat) of this when I started delving in to Occultations (point sources of light moving in front of each other) and being convinced a F/4 8" is not as capable as a F/10 16" telescope for such purposes because its the aperture which matters, faster focal ratio wouldn't have an impact. Like I say, I could still argue otherwise so obviously don't fully understand it
Roger.