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'Fast' scopes and exposure time.


CanesVenatici

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Simple? Here's a go:

F ratio determines how fast the image builds up on the chip.

Aperture determines optical resolution.

Focal length and pixel size determine image scale in pixels.

Although aperture determines limiting optical resolution, the resolution in a digital image also depends on focal length and pixel size.k

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Apipolilogies for going on and on and on about this...

I sat and thought about f-ratio and it's meaning a few weeks back and went Aha! after quite a bit of pondering. I just lack the ability to express myself clearly, and in no way can I put a formula behind my argument.

My conclusion was that any same f-ratio telescope has exactly the same photon count per surface area in the focal plane looking at the same sky. This means that your CCD - if fully illuminated - will expose at the same rate moved between two different sized same f-ratio scopes.

Any low f-ratio scope will be faster than any high f-ratio scope. Move the same chip to a high f-ratio scope and it will expose slower. In any of them.

Useful insight browsing telescope shops and websites.

One chap who did the same thinking - Steve Cannistra, fellow amateur - whom I don't know btw, came to the same insight. Luckily he is clever enough to put the formulas to it. It ends this particular section with a few simple examples - simple after the theory is understood at least.

Here is the link for anyone still reading this post:

f ratio and signal to noise

Give it a moment's thought and play perhaps with a CCD calculator after reading. (I recommend the New Astronomy Press one for its ease of use and many nice targets to compare FOV on.)

The confusion seem to set in when overlooking the fact that the resulting image at the focal plane also changes size as we increase the aperture. Double aperture means double the size at the focal plane too, so more light comes in but is then more spread out towards the focal plane in perfect proportion - so per per surface area unit there is no change - it evens out. Which equals same exposure if you maintain f-ratio.

Another good writeup for visual observers can be found here:

(Go down to 'Theories and practice' and click 'explain this'.)

Telescope Equations

And with that I'll try to keep my gob shut on the subject forever haha. PM me when you go Aha!

All the best!

/Jesper

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This picture pretty much sums up the difference between fast and slow -

fratio.gif

As for the why faster scopes allow for shorter exposures, it can be likened to a projector. Imagine holding a projector just a matter of inches from a screen, the image would be small and bright. Now hold the same projector (giving out the same amount of light) 5 meters away from the screen, the image would appear dimmer. It's not quite the same, but that's about the closest analogy i can come up with.

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This is not a sign of believing in a "myth", it's a display of lack of understanding of f-ratio.
On the contrary, it is propagated by people who say that only f-ratio determines the speed of imaging. By that argument an 8" f5 and an 8m f5 should be the same!
Not sure what you mean by image here. Size, saturation FOV?
I guess I mean depth and resolution - FOV is, of course, what F-ratio is really all about.
Leave them on the screen and take a few steps back, then a few more... At some point, maybe 10 or 15 meters away (You're in the kitchen now...) the resolution of your eyes come into play and you hit max and beyond. Now looking back at the two Strawman galaxies you cannot tell one from the other. They are identical to your struggling eyes.
No - you will see more faint objects on the one with the longer exposure, unless you go so far away that you loose the ability to see the faintest objects on the image, in which case this is a pointless argument!
Pixels are confusing ...
The trouble is that it is all about pixels. An ideal photon counting detector knows nothing about pixels - it would just tell you which point on the sky each photon came from. Then f-ratio really is totally irrelevant, apart from determining FOV (and even that is dependent on the optical design). It it only because we shoehorn things into pixels that the scale at the focal plane starts to have an apparent effect. And pixels are things we have control over - so you cannot ignore them.

NigelM

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Quote:

This is not a sign of believing in a "myth", it's a display of lack of understanding of f-ratio.

On the contrary, it is propagated by people who say that only f-ratio determines the speed of imaging. By that argument an 8" f5 and an 8m f5 should be the same!

Nobody says that, or nobody I have spoken to. The two scopes do not take the same picture because they have radically different focal lengths. The comparison is, so far as I can see, pure nonesense because the picture is the picture is the picture. How long does THE SAME picture take to get? It goes as the square of the F ratio. Can you imagine daytime photographers comparing a fisheye with a telephoto as if they were the same? (Hey using a fisheye, I could get Michael Schumacher's eyeball at the end of the Hangar Straight, on a cloudy day, at one thousandth of a second!) The F ratio myth would be a myth if anybody actually uttered it, but they don't! So the F ratio myth is a ... Uh-Oh ... myth! This one will run and run...

Olly

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