Is the larger aperture worth the money?

[dph1nm]

So I capture say an ADU of 10k on point x of a nebula in 15 minutes at f3.9, to capture the same 10k at the same point will take 45 mins at f6.8..... And at f8 that would take me 64 minutes.

This all depends what you mean by "point X"! If you talk in terms of surface brightness (i.e. signal per square arcsecond) then there is no difference in the time taken with different f-ratio.

NigelM

[swag72]

This all depends what you mean by "point X"! If you talk in terms of surface brightness (i.e. signal per square arcsecond) then there is no difference in the time taken with different f-ratio.

NigelM

How about if I meant point x in the frame - So at f3.9 and 330mm there's a lot surrounding point x, at f8 and 1600mm there's less space surrounding point x in the frame, but in each frame I want that point to be of the same brightness .......... does that make sense? Because if I want to get the same signal per square arcsecond that's going to be a big headaache at 1600mm!!!

[ollypenrice]

Hang on, there is a certain flux or signal per square arcsecond coming from 'x' part of the sky. This doesn't care whether we are looking at it or not and it doesnt get brighter or dimmer when we change our scopes. However, if we are trying to capture that signal and put x square arcseconds of sky onto y square mm of chip (which means we are at a fixed focal length) we will surely obtain more signal on our chip from a large aperture than from a small.

Olly

[ollypenrice]

This all depends what you mean by "point X"! If you talk in terms of surface brightness (i.e. signal per square arcsecond) then there is no difference in the time taken with different f-ratio.

NigelM

Awe, c'mon, give us more!

lly

[sharkmelley]

Sara's spreadsheet is correct.  When imaging at a slow f-ratio there is not so much light reaching each pixel and so, for a given exposure time, the slow f-ratio image will appear noisier.

Mark

[dph1nm]

When imaging at a slow f-ratio there is not so much light reaching each pixel and so, for a given exposure time, the slow f-ratio image will appear noisier.When imaging at a slow f-ratio there is not so much light reaching each pixel and so, for a given exposure time, the slow f-ratio image will appear noisier.

Correct, providing the emphasis is on the word 'appear'. In fact, assuming we are talking about equal aperture scopes, the s/n per unit area on the sky (or per astrophysical object, if you like) will be the same (unless you have a significant read-noise contribution per pixel, in which case the quick f-ratio will do somewhat better). To put it another way, you could bin up the 'slow' image and get the same s/n per pixel as the 'quick' image.

NigelM

[dph1nm]

Because if I want to get the same signal per square arcsecond that's going to be a big headaache at 1600mm!!!

The signal per sq. arcsec. is only dependent on the aperture, not the focal length.

NigelM

[sharkmelley]

So NigelM, to paraphrase what you are saying, if I may.  Two scopes with the same aperture will collect the same number of photons for a given object.  However the longer focal length scope (slower F-ratio) will spread that signal over a larger number of pixel sites (if the same camera is being used) because of the higher magnification.  So, looking at the image on a "per pixel" basis, the longer focal length image will appear noisier because of the reduced signal.  But if it can be binned so the object size in pixels is the same as the fast F-ratio scope then the 2 images will be equivalent.

This is the main argument used to bust the "F-ratio myth" and it is true as long as slower F-ratio image doesn't start hitting other limits such as read noise (which may be the case the CCD imaging) or thermal noise (which is often the case for DSLR imaging).

Also, I suspect that in general, the user of the longer focal length scope will not want to bin the data, making the image size smaller.

Mark

[ollypenrice]

Once again the debate centres on whether we vary the aperture or vary the focal length. Please don't let's vary both at once! 

Olly

[wuthton]

Once again the debate centres on whether we vary the aperture or vary the focal length. Please don't let's vary both at once!

Olly

I knew there would be trouble from uttering the A word in the imaging section.

[swag72]

That's it ............ I'm lost!!!

[ollypenrice]

That's it ............ I'm lost!!!

No you're not. I think NigelM is introducing the concept of object noise, which is the unavoidabe degradation of information from the object en route to your objective. There's nowt we can do about that miduck. But as imagers we have lots of experience of the noise in our systems and we know that a given field of view in our cameras will benefit from more signal, obtainable less expensively by taking more time or more expensively by adding aperture or, best of all, by adding both. 

Olly

[sharkmelley]

That's it ............ I'm lost!!!

Don't worry - your original intuition was correct.  Taking the sensor as a whole, the sensor will collect fewer photons at a slower f-ratio. So the image that you display on your screen will have a noisier appearance than the same length exposure taken at a fast f-ratio.

If you want, you can start binning that data to improve the signal to noise ratio but you end up with a smaller image.

Mark