RC's and Newts and Theory

[sjastro]

Originally Posted by Satchmo

Steve

The case where 4 stars at F8 get merged into one at F4 seems like an a quite esoteric case and just further confuses me..I think we are just getting bogged down about signal to noise.

I'm highlighting an extreme case for pixel S/N ratio as opposed to object S/N ratio to show that S/N ratio is independent of f/ratio for a constant aperture.

Ultimately it doesn't matter whether the source of the photons on a single pixel is from an extended object or a point source.

The point is the photon noise which is the statistical variation of the signal is independent of f/ratio for a fixed aperture.

Can you turn your explanation towards imaging diffuse objects..galaxies and nebulae.

The general consensus seems to be that the brighter the image or the longer the exposure the better the signal to noise ratio. F4 will record faster with better signal to noise on very faint diffuse objects.

You are not recording any faster at f/4. The amount of information in the image is a function of the diameter of the mirror (or lens). You are collecting the same number of photons at f/8 at the same exposure.

The important difference is in the pixel scale. The effects of camera noise such as read out noise and thermal noise can be more easily discerned in faint objects as one increases the f/ratio. That's because the pixel scale allows the noise to be resolved in the image.

That's probably where the confusion lies. The apparent lesser noise in a smaller f/ratio image is falsely attributed to a stronger signal.

Stan Moores two pictures at F4 and F12 show this clearly..for a 10 minute exposure there is less detail and definition in the diffuse areas and the the F12 shot looks dimmer and grainy. There is a slight very impovement in the sharpness of the stars. The F12 instrum,ent would only show 1/9 the sky area of the F4, so it doesn't appear to be a very good trade off.

The only criticism I have of Stan's article is that he has rescaled the images. That makes it very difficult to make direct comparisons. In fact by down sampling the f/12.4 image he has probably made the image look smoother than what it actually is.

Stan however has made the very valid point the f/12.4 doesn't need 10 times the imaging time of the f/3.9 scope unless a photograhic emulsion is used.

I'm not sure how this stellar s/n theory translates to diffuse objects that aren't point sources. Will I have no gain in image depth/contrast imaging faint nebulae and galaxies at F4 rather than F8?

Hopefully I've anwsered that.

Regards

Steven

[coldspace]

Originally Posted by sjastro

I'm highlighting an extreme case for pixel S/N ratio as opposed to object S/N ratio to show that S/N ratio is independent of f/ratio for a constant aperture.

Ultimately it doesn't matter whether the source of the photons on a single pixel is from an extended object or a point source.

The point is the photon noise which is the statistical variation of the signal is independent of f/ratio for a fixed aperture.

You are not recording any faster at f/4. The amount of information in the image is a function of the diameter of the mirror (or lens). You are collecting the same number of photons at f/8 at the same exposure.

The important difference is in the pixel scale. The effects of camera noise such as read out noise and thermal noise can be more easily discerned in faint objects as one increases the f/ratio. That's because the pixel scale allows the noise to be resolved in the image.

That's probably where the confusion lies. The apparent lesser noise in a smaller f/ratio image is falsely attributed to a stronger signal.

The only criticism I have of Stan's article is that he has rescaled the images. That makes it very difficult to make direct comparisons. In fact by down sampling the f/12.4 image he has probably made the image look smoother than what it actually is.

Stan however has made the very valid point the f/12.4 doesn't need 10 times the imaging time of the f/3.9 scope unless a photograhic emulsion is used.

Hopefully I've anwsered that.

Regards

Steven

Thanks for the explanation Steven,

Now I understand why with my video system because its so sensitive and maximum exposures are only 56 seconds it shows alot of noise at the longer focal length that I try so this in turn cause degration to my images.
And at the shorter focal lengths the image seems alot better as noise is less apparant.
Is this right?

Matt.

[sjastro]

Originally Posted by coldspace

Thanks for the explanation Steven,

Now I understand why with my video system because its so sensitive and maximum exposures are only 56 seconds it shows alot of noise at the longer focal length that I try so this in turn cause degration to my images.
And at the shorter focal lengths the image seems alot better as noise is less apparant.
Is this right?

Matt.

That's correct Matt.

Also since you are using very short exposure times, your images are probably dominated by camera read noise instead of photon noise.

Regards

Steven

[Satchmo]

Originally Posted by sjastro

You are not recording any faster at f/4. The amount of information in the image is a function of the diameter of the mirror (or lens). You are collecting the same number of photons at f/8 at the same exposure.

The important difference is in the pixel scale. The effects of camera noise such as read out noise and thermal noise can be more easily discerned in faint objects as one increases the f/ratio. That's because the pixel scale allows the noise to be resolved in the image.

That's probably where the confusion lies. The apparent lesser noise in a smaller f/ratio image is falsely attributed to a stronger signal.

OK, So is field of view the only reason that we should choose one F ratio over another or would choosing apparent less noise be valid also.

[sjastro]

Originally Posted by Satchmo

OK, So is field of view the only reason that we should choose one F ratio over another or would choosing apparent less noise be valid also.

The advantages of using a lower f/ratio are

(1) Larger FOV.
(2) Image quality is less dependent on seeing conditions.
(3) Easier to guide for long exposures.
(4) Camera related noise is less apparent due to higher pixel scale.

The disadvantages of a lower f/ratio are.

(1) Potential loss of resolution using too high a pixel scale.
(2) Images could be undersampled which reduces processing latitude.
(3) Possible reduction in optical quality (ie increased star spot sizes, off axis aberrations etc).

Regards

Steven

[gregbradley]

Nice and clear explanation Steven. Thanks mate.

That is what I have observed to be true.

Greg.

[Bassnut (Fred)]

So how does cam QE affect s/n ratio ?. For instance if the apature was reduced to capture half the number of photons and the cam QE doubled ,that doubles the photon conversion to electrons efficiency and produces same number of electrons, to I suppose give an equivalent exposure time.

Optical quality differences aside, is the S/N ratio (thermal, readout etc) the same?.

[CometGuy]

Fred,

In the worst case s/n ratio increases as a square root of increase in QE. The worst case being where the greatest contribution to image noise is from the sky background.

Here is a question for the forum: I want to photograph the horsehead nebula over an area of 1 degree wide field. Therefore I've determined I need 500mm focal length with the CCD I am using. Which of the following - well corrected optically - instruments would you choose?

a. 500mm focal length f10
b. 500mm focal length f2

Answer that and tell me whether you think focal ratio is not important..

T.

[Peter Ward]

Originally Posted by CometGuy

Which of the following - well corrected optically - instruments would you choose?

a. 500mm focal length f10
b. 500mm focal length f2

Answer that and tell me whether you think focal ratio is not important..

T.

This is a no brainer...50mm *aperture* vs 250mm...I pick the latter...but it wasn't because it was an F2 (though that would be cool....who makes such a neat optic?)

How about we do the numbers the another way? ie.

250mm aperture 500mm FL
2.5metre apetrure say make it 7500mm FL

[CometGuy]

Exactly...no brainer..f ratio is important

[Satchmo]

As I understand it now there is no point in paying for large fast highly corrected instruments . A 20" F5 with coma corrector will produce similar results to an exotic design 20" F2.5 in the same exposure time for 1/10 of the cost, with a narrower field of view.

[Terry B]

Originally Posted by CometGuy

Exactly...no brainer..f ratio is important

This makes no sense.
Optical physics states that resolution is proportional to diameter of the scope.
You need to compare your 500mm fl F10 scope ie 50mm diameter with the same scope with a focal reducer on it as this is what people do with their scopes ie a 50mm diameter f2 scope with a fl of 100mm.
I think I would rather use the 500mm fl scope rather than a 100mm mild telephoto lens.
If this is the case why do we bother with telescopes at all?
Just look at the sky with a camera lens.

[sjastro]

Originally Posted by Satchmo

As I understand it now there is no point in paying for large fast highly corrected instruments . A 20" F5 with coma corrector will produce similar results to an exotic design 20" F2.5 in the same exposure time for 1/10 of the cost, with a narrower field of view.

That's correct. The f/2.5 won't go any deeper.

As you know the rule doesn't apply to photographic emulsions. The f/5 will require 4X the exposure.

A good example of Quantum Mechanics at work.

Regards

Steven

[TrevorW]

I brought my RC because it looks good

[CometGuy]

Why doesn't this make sense?

You are assuming I am interested in ultimate resolution. What happens if I want to make a wide angle image of the milky way? Or what happens if I want to do a survey for Asteroids, comets or nova? I'll want the fastest focal ratio practical.

Aperture, focal length and the ratio between them are all important factors.

Terry

Originally Posted by Terry B

This makes no sense.
Optical physics states that resolution is proportional to diameter of the scope.
You need to compare your 500mm fl F10 scope ie 50mm diameter with the same scope with a focal reducer on it as this is what people do with their scopes ie a 50mm diameter f2 scope with a fl of 100mm.
I think I would rather use the 500mm fl scope rather than a 100mm mild telephoto lens.
If this is the case why do we bother with telescopes at all?
Just look at the sky with a camera lens.

[Peter Ward]

Originally Posted by CometGuy

Why doesn't this make sense?

I didn't pick the F2 because it was a F2....it was because you put up a 50mm vs 250mm *aperture*.....which wins every time

[dpastern (Dave Pastern)]

mmm whilst admitting I know jack about astro imaging, I find this all odd. When it comes to terrestial imaging, the exposure triangle (aperture, iso, shutter speed) all come into play. As an example, a 500mm f4 telephoto lens, used at 1/1000 second, ISO 400 and f4 will provide twice as much light as the same shutter speed and iso but f5.6. Why is it different for astro imaging? Logic indicates to me that f stop (at the same focal length ratio and aperture) *does* make a difference. Not so much in the amount of light hitting, but in the amount of time required to hit a particular level of exposure. I fail to see why astro imaging is any different to terrestial, digital or film be damnéd.

Dave

edit: so a f3 10" newt will gather light twice as fast as one that is one stop slower in terms of f ratio (but same aperture, i.e 10"). Am I just misunderstanding what you guys are all saying, or just completely off track here with my thinking?

[Peter Ward]

Originally Posted by dpastern

....

edit: so a f3 10" newt will gather light twice as fast as one that is one stop slower in terms of f ratio (but same aperture, i.e 10").

No it won't. Sure we've been haggling over details in the limit cases, but
a 10" is a 10" you will only get a marginal performance increase (using a CCD) with a faster system.

[dpastern (Dave Pastern)]

But (excuse my non astro logic here) doesn't make sense. It is completely the opposite to terrestial photography. Physics are the same (at least that's how I was taught) - photography and photons are the same at night or day. Sorry, I'm just having a very hard time wrapping my head around it all Peter.

Dave

[CometGuy]

Dave,

I don't think anyone is wrong, just looking at the it from different angles.

We all agree FOCAL RATIO = FOCAL LENGTH / APERTURE

For instance Stan Moores article states that f-ratio has little impact on image depth when APERTURE is kept constant. On that basis his article is sound. I would even add that as focal ratio is increased stellar limiting magnitude is improved since the sky background appears darker.

However, if you keep FOCAL LENGTH constant then varying focal ratio has a big effect as you are varying aperture. This is how photographers tend to think - i.e what focal length do I need to frame my subject. Of course in astrophotography we often find ourselves not having enough focal length.

Sorry, if I am getting over pedantic...

Terry