Aperture, F-ratio myth and telescope basics

[astroboof (Steve McN)]

If you had an aperture the size of Qld, but also had a FL that was equal to the average distance from our moon, what kind of light collection and inherent FOV quality would you think we might gain?

So, how long is a piece of string.

[RickS (Rick)]

Originally Posted by codemonkey

The main thing I was confused about is why does a bigger aperture not mean light from different sources rather than more light from the same sources.

Lee,

I think the thing you are missing is that we are so far from these objects that the light rays from them are effectively parallel. If you're imaging M8 then increasing your aperture gives you an increase in the number of photons you collect from M8 which is proportional to the increased area.

Cheers,
Rick.

[Shiraz (Ray)]

Originally Posted by julianh72

The last two comparison images at the bottom of the page http://www.stanmooreastro.com/f_ratio_myth.htm are very misleading. If one was indeed taken at f/12.4 and the other at f/3.9, then they have been scaled differently, or were taken with different sized sensors. For example, if the f/12.4 image is the full capture over the sensor, then the f/3.9 image is roughly a 1/3 x 1/3 crop of the whole image. (Either that, or the sensor used for the f/3.9 image is only 1/3 the size of the f/12.4 image sensor.)

Using a focal reducer on a long focal length telescope lets you capture a bigger sky area on the same sensor, but doesn't alter the brightness of point sources.
[Edited to correct an overly general statement]

my guess is that the long fl image was taken at 3x3 binning Julian - that would explain the results.

[Peter.M]

Originally Posted by codemonkey

Now if the bigger aperture means that more light is focused into an area of the same size, I can see why the intensity would increase...

For a given camera, focal length dictates field of view, nothing else. An 800mm FL f2 scope with an aperture of 400mm will have the same field of view as an 800mm FL f10 scope with an aperture of 80mm. In these systems, the only difference in the final image will be the light collecting area defined by its clear aperture. I think you would do well to google a ray trace of a telescope, what is happening is light from the star or nebula or whatever you image hits EVERY part of your mirror and it is focused onto the sensor. That is why bigger mirrors (for a set FL) will give brighter images.

Another way to show this physically, would be to put a longer focal length eyepiece into your scope. going from a 30mm to a 3mm (increasing magnification) makes the background darker.

[Steffen]

Originally Posted by codemonkey

Stan Moore has an interesting article on the F-ratio Myth basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

I'm guessing Stan uses his DSLR in full-auto

The refusal by most people to internalise (or at least accept) simple geometrical relations is why mobile phone cameras are eating DSLRs' lunch. Also, because they're smaller and always with you, but that reason doesn't further my argument

[Geoff45 (Geoff)]

There is a nice article here http://www.stark-labs.com/help/blog/...ioAperture.php by Craig Stark. He discusses Stan Moore's conclusions and puts a somewhat different, but not necessarily contradictory, slant on things. His contention is that focal ratio does make a difference, but it is not as simple as "half the focal ratio makes your scope 4x faster"
Geoff

[Shiraz (Ray)]

Originally Posted by ghsmith45

There is a nice article here http://www.stark-labs.com/help/blog/...ioAperture.php by Craig Stark. He discusses Stan Moore's conclusions and puts a somewhat different, but not necessarily contradictory, slant on things. His contention is that focal ratio does make a difference, but it is not as simple as "half the focal ratio makes your scope 4x faster"
Geoff

But in this series of slides, he supports the "1/2FNo gives 4x faster" result and points out that Stan's paper was not based on pixel SNR - lots of other interesting stuff as well.http://www.stark-labs.com/craig/reso...s_SNR_RTMC.pdf

[gregbradley]

Good link Ray.

This is why I got an AP RHA 305 F3.8 ad has been my exact experience with it so far. Its also why I am keen to use the reducer on the CDK17 when I can.

A 5 minute Ha image using a 5nm Ha at 1x1 binning is probably as smooth and noise free as a 20minute F7 image.

It speeds up imaging by a factor of 3 or 4 at an educated guess over the more typical imaging at F7/8. Its probably about 2-3 times as fast as an FSQ106 even with a narrower FOV. Couple that with nearly 80% QE of Sony ICX sensors and you've got a very fast imaging system which reduces the problem of the variable of astrophotography weather (cloud, moon, wind).

Greg.

[Geoff45 (Geoff)]

Originally Posted by codemonkey

Stan Moore has an interesting article on the F-ratio Myth basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.

He doesn't actually say the focal ratio doesn't matter: He says

Thus there is an actual relationship between S/N and f-ratio, but it is not the simple characterization of the “f-ratio myth”.

Geoff

[codemonkey (Lee)]

Thanks everyone!

Originally Posted by ghsmith45

He doesn't actually say the focal ratio doesn't matter: He says

Thus there is an actual relationship between S/N and f-ratio, but it is not the simple characterization of the “f-ratio myth”.

Geoff

Eh, he says a number of things, including:

"Focal length (and thus f-ratio) has absolutely no effect on the number of photons collected and delivered."

Either way, I derailed my own thread by mentioning that article. I didn't realise it would be so inflammatory.

[Geoff45 (Geoff)]

Originally Posted by codemonkey

Thanks everyone!



Eh, he says a number of things, including:

"Focal length (and thus f-ratio) has absolutely no effect on the number of photons collected and delivered."

Either way, I derailed my own thread by mentioning that article. I didn't realise it would be so inflammatory.

Well the statement in quotes is obviously true. There is no way that f ratio can affect the number of photons hitting and hence being transmitted by the objective. What happens after the photons are transmitted by the objective is where the disagreement arises.

Nothing wrong with robust or inflammatory discussion. Just treat it as all good fun!

[gregbradley]

Originally Posted by ghsmith45

Well the statement in quotes is obviously true. What happens after the photons pass through the objective is the problem.

And how it reaches pixels and then how those pixels translate into a signal and how many photons miss the detector.

Greg.

[codemonkey (Lee)]

Thanks for the clarification, really appreciate it.

[RickS (Rick)]

You're a computer geek like me IIRC, Lee? It's not very complicated if you have some basic maths and science skills and give it a little thought. Take a look at the Ray/Shiraz model here: http://www.iceinspace.com.au/forum/s...d.php?t=136008

I came to effectively the same conclusion myself. Clear aperture determines how many photons you capture from a DSO and then image scale will tell you how they get stuffed into pixels. Optical efficiency plays a smaller role determining if some of the photons don't make it to the sensor and then QE tells you how many photons get recorded. Add a basic understanding of SNR and the sources of noise (shot noise, read noise, dark current noise) and you have all the tools you need.

Cheers,
Rick.

[codemonkey (Lee)]

Thanks Rick :-)

I understand the role of aperture, focal length, central obstruction size, QE and pixel size and their combinatory effect on SNR/acquisition time; the part that I've been scratching my head about is the mechanics of telescopes and whether a change in aperture impacts the way light is captured from point sources vs diffuse sources. This was answered in the first response and it's simple and makes sense.

The other thing was whether (all else equal) a bigger mirror should mean you're capturing more light but from areas previously outside your FOV. Still a bit hazy on this one. Seems pretty obvious that it should, but all that seems to be discussed is how a bigger aperture = more light (implication being from the same source), and only the focal length affects the FOV.

[RobC (Rob)]

Originally Posted by codemonkey

The other thing was whether (all else equal) a bigger mirror should mean you're capturing more light but from areas previously outside your FOV. Still a bit hazy on this one. Seems pretty obvious that it should, but all that seems to be discussed is how a bigger aperture = more light (implication being from the same source), and only the focal length affects the FOV.

Changing your objective/mirror size is not changing your field of view but changing the amount of focused light ( or photons ) hitting your sensor from the same field of view.

I think you are getting confused over the amount of stray/scattered light outside of your FOV hitting your sensor. That's is why baffling/flocking is used inside of your telescope tube.

You double your objective/mirror size for the same focal length and your quadruple the amount of light hitting your sensor.

[codemonkey (Lee)]

Originally Posted by RobC

Changing your objective/mirror size is not changing your field of view but changing the amount of focused light ( or photons ) hitting your sensor from the same field of view.

I think you are getting confused over the amount of stray/scattered light outside of your FOV hitting your sensor. That's is why baffling/flocking is used inside of your telescope tube.

You double your objective/mirror size for the same focal length and your quadruple the amount of light hitting your sensor.

I think that's it... I think I actually get it now. Thanks Rob!