Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
1llusive Senior Member • Posts: 1,571
Re: What I think is missing from the discussion

kiwi2 wrote:

1llusive wrote:

100mm aperture, 500mm focal length = f/5

80mm aperture, 500mm focal length = f/6.25

The 100mm aperture telescope/lens will not only concentrate more photons onto the sensor per square mm, it will also resolve more detail. This is the real effect of a larger aperture. This would in fact capture more signal per second of exposure, backing up Roger Clark's statements.

Please feel free to tell me where I am wrong about this.

Of course. And we already know it would provide more light for a given exposure time because of the f/5 over the f/6.25. Just like with any other camera lens in the history of photography. I haven't said otherwise.

What Roger would have us believe is that aperture, not f-ratio is what dictates the amount of light reaching the sensor...

rnclark wrote:
"DPreview changes focal length while keeping f-ratio constant. That means lens aperture area changes between cameras, thus the light delivered to the sensor is changing, and that means the amount of photon shot noise is different due to the lens, not the sensor"

Let's play with those numbers a bit more to see if you can get the idea.

Give that 80mm aperture a 400mm focal length and it would become f/5 as well. It would record the same brightness of extended objects (like a the dpreview test charts) in the same amount of time only with less magnification/resolution. A galaxy or nebula would be just as bright, only smaller in the frame.

Or give that 100mm aperture a 2500mm focal length like the 24" telescope I was mentioning earlier and it would become f/25. You would magnify the galaxy over the frame but it would be very dim and impossible to see and take very long exposure times to record. The f/25 tells us straight away it's too slow. That is, the aperture is too small for the focal length. However, increase that 100mm aperture to 500mm and it would become f/5 as well. Same relative brightness with the given exposure time as before (when it had a 500mm focal length) but now with more frame filling magnification.

Bigger telescopes give you longer focal lengths and more magnification. It's f-ratio tells you how bright things will look (at that focal length) and what kind of exposure times you'll need.

To be precise, Roger stated "the light delivered to the sensor is changing". He didn't say the "amount" of light. So maybe both of you are correct, and you are misinterpreting him because he could have worded his response better? Or maybe he misspoke? I don't know. I won't try to put words in his mouth. I just think it's possible you're talking past each other.

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