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Old 20-02-2018, 07:34 PM
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F-ratio myth

I have been puzzled by recent posts about Fast optics (eg F3.8) having the ability to capture data in less time than a slower optic (eg F8.0) .

This old chestnut was well addressed by Stan Moore....and I still have to agree with him.

Aperture is king.

Having recently dusted off my FSQ and its delightful F5 optics, I can say with certainty it does not gather data very quickly...its humble four inch aperture is not exactly a light bucket.

At F5.5 I have another optic I like to use (with five inches of aperture) which demonstrably gathers photons faster than my FSQ....despite being half an f-stop “slower”

I suspect the f-ratio myth persists as a carry-over from photographic lenses....which vary aperture at a constant focal length.

...not something you’d do with a telescope optic, unless you want to purposely reduce the aperture with a field-stop, then wonder when you removed the stop, why things got brighter!

This is not to say fast telescope optics with fine pixels is a folly.

Far from it!

Mechanically smaller (hence less taxing on the mount) plus they give good sampling and a wider field compared to the same aperture in a longer focal length. The big acreage CCD’s required to get the same field are not cheap! (or the filters/field correctors)

...but if you think more flux will fall into a smaller pipe...regardless of the F-ratio...

As they said in “The Castle”....tell ‘em their dreamin’.
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Old 20-02-2018, 07:56 PM
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Well, aperture is half the story, Peter. The other half is image scale, combining focal length and pixel size. A small aperture scope can be faster than a huge aperture provided you're collecting the photons in big enough buckets (and are happy with much less resolution with a larger FOV as the upside.)

I do agree that the f-ratio of a scope is meaningless by itself...

Cheers,
Rick.
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Old 20-02-2018, 07:58 PM
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However, if I put a reducer on my modest 4" f/6 scope and make it work at f/4.5, while keeping the same camera, I should be capturing data more quickly. Or I do misunderstand something fundamental here?

Some other factors that come to my mind that might be worthwhile consideration are refractor vs reflector with similar apertures (loss of light at mirrors), and working at really fast f-ratios with 3nm filters (wavelength shift).
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Old 20-02-2018, 08:05 PM
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Originally Posted by RickS View Post
Well, aperture is half the story, Peter. The other half is image scale, combining focal length and pixel size. A small aperture scope can be faster than a huge aperture provided you're collecting the photons in big enough buckets (and are happy with much less resolution with a larger FOV as the upside.)

I do agree that the f-ratio of a scope is meaningless by itself...

Cheers,
Rick.
I hear what you are saying Rick...my point however is a 10" can only gather 10" worth of flux.

Optimal sampling will help better detect what's hitting the focal plane...it just won't give you more photons down the pipe
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Old 20-02-2018, 08:09 PM
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However, if I put a reducer on my modest 4" f/6 scope and make it work at f/4.5, while keeping the same camera, I should be capturing data more quickly. Or I do misunderstand something fundamental here?
That's correct. You have kept the aperture the same but increased the image scale. You're stuffing more photons into each pixel. Thanks for illustrating my point perfectly, Suavi

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Originally Posted by Slawomir View Post
Some other factors that come to my mind that might be worthwhile consideration are refractor vs reflector with similar apertures (loss of light at mirrors), and working at really fast f-ratios with 3nm filters (wavelength shift).
Optical efficiency does matter (light loss through reflection vs lenses) but it's not a large effect. Central obstruction in some reflecting designs is a bigger consideration.

Really fast scopes and NB filters can be an issue, but a different one.

Cheers,
Rick.
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Old 20-02-2018, 08:10 PM
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However, if I put a reducer on my modest 4" f/6 scope and make it work at f/4.5, while keeping the same camera, I should be capturing data more quickly. Or I do misunderstand something fundamental here?........
All you will get is a wider field of view over the same sensor. Unless you increase the aperture you will not be collecting more flux.

Google Stan Moore F ratio myth
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Old 20-02-2018, 08:17 PM
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I noticed this recently while imaging with a mate with a larger refractor, ~2x the objective area. My scope f/5.5, his f/7, same camera (within manufacturing tolerances), same gain, same exposure time to hit "sky limited".

f-ratio had me fooled there for a while. It may work well for camera lenses, but comparing telescopes with differing light collecting areas has to be the way of the world, or should that be, the sky?
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Old 20-02-2018, 08:18 PM
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I hear what you are saying Rick...my point however is a 10" can only gather 10" worth of flux.

Optimal sampling will help better detect what's hitting the focal plane...it just won't give you more photons down the pipe
Hi Peter,

It is certainly true that the total number of pixels captured and funnelled into the image circle is determined by aperture.

If you want maximum resolution then you need large aperture to collect data quickly.

If you're looking for a wide field and are happy with much lower resolution then a smaller scope can do just as well or better.

Cheers,
Rick.
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Old 20-02-2018, 08:25 PM
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All you will get is a wider field of view over the same sensor. Unless you increase the aperture you will not be collecting more flux.
But you will be channelling more flux into every pixel and increasing the SNR in individual pixels more quickly.
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Old 20-02-2018, 08:35 PM
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If you're looking for a wide field and are happy with much lower resolution then a smaller scope can do just as well or better.

Cheers,
Rick.
Nup. They don't.

Please google Stan Moore's analysis with a good look at the F12.4 vs F3.9 data.
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Old 20-02-2018, 08:40 PM
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Please google Stan Moore's analysis with a good look at the F12.4 vs F3.9 data.
I have read it before a few times. His idea of "object SNR" doesn't convince me. Simple maths and physics does...

Cheers,
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Old 20-02-2018, 08:57 PM
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I have read it before a few times. His idea of "object SNR" doesn't convince me. Simple maths and physics does...

Cheers,
Rick.
The relationship is not that simple.

One can also sample appropriately with a bigger 'scope....I also suspect my view (and Stan's) is vindicated by the professional push is toward ever larger aperture optical telescopes (e.g. 30 metres) rather than trying to telecompress 4 metre class instruments.

Guess we'll have to agree to disagree.
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Old 20-02-2018, 09:01 PM
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It is a little more complicated than simple "Bigger is Better" because as has been mentioned, it is all about flux. Aperture, FR and pixel size all come into play.
The difference between the FSQ and AP.
AP captures 69% more photons but is only 21% slower (5 v 5.5).

The difference between the the FSQ and Alluna.
Alluna captures 16x the amount of photons but is only 2.56x slower (5 v 8).

Both of those examples are considering a similar pixel size. If the pixel size is changed things also change. Putting an ASI183 (2.4 micron pixels) on the Alluna for SUPER HIGH RESOLUTION imaging and putting that up against the FSQ (keeping the KAF-16803) changes things slightly.

The ASI183 has 14x smaller surface area (pixel size wise) than the KAF-16803. Forget the fact that the Alluna is imaging at 0.155"/pixel now, the FSQ is now getting 2.25x better signal than the 16" Alluna as the photons are now being spread very thinly among the tiny pixels. Realistically it's going to be closer to 2x SNR as the IMX183 sensor has a near 90% QE against the 60% KAF-16803.

It is not a great real world example but you get the idea. It is not just about the amount of flux entering the system (the raw aperture) but the way the photons are spread (f/ratio and pixel size).
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Old 20-02-2018, 09:03 PM
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Quote:
Originally Posted by Peter Ward View Post
The relationship is not that simple.

One can also sample appropriately with a bigger 'scope....I also suspect my view (and Stan's) is vindicated by the professional push is toward ever larger aperture optical telescopes (e.g. 30 metres) rather than trying to telecompress 4 metre class instruments.

Guess we'll have to agree to disagree.
It all depends on what it is you're trying to accomplish. The perfect example of this is with the DragonFly Project. It is the only system on Earth that can go past Mag 30 and it only uses "small" Canon lens'.
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Old 20-02-2018, 09:22 PM
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It all depends on what it is you're trying to accomplish. The perfect example of this is with the DragonFly Project. It is the only system on Earth that can go past Mag 30 and it only uses "small" Canon lens'.
That's a bit of a stretch ....DragonFly works not because of it's fast f-ratio, but because of the extremely low scattering from those beautiful Canon coatings
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Old 20-02-2018, 09:36 PM
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The relationship is not that simple.
There's certainly a lot more to it than the original premise that aperture is all important.

Quote:
Originally Posted by Peter Ward View Post
One can also sample appropriately with a bigger 'scope....I also suspect my view (and Stan's) is vindicated by the professional push is toward ever larger aperture optical telescopes (e.g. 30 metres) rather than trying to telecompress 4 metre class instruments.
One of the main reasons for large aperture professional scopes is increased resolution (which is directly related to aperture.) If it was being done to increase the number of photons captured then there would be no point in optical interferometry.
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Old 20-02-2018, 09:50 PM
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It is a little more complicated than simple "Bigger is Better" because as has been mentioned, it is all about flux. Aperture, FR and pixel size all come into play.
The difference between the FSQ and AP.
AP captures 69% more photons but is only 21% slower (5 v 5.5).

The difference between the the FSQ and Alluna.
Alluna captures 16x the amount of photons but is only 2.56x slower (5 v 8).

Both of those examples are considering a similar pixel size. If the pixel size is changed things also change. Putting an ASI183 (2.4 micron pixels) on the Alluna for SUPER HIGH RESOLUTION imaging and putting that up against the FSQ (keeping the KAF-16803) changes things slightly.

The ASI183 has 14x smaller surface area (pixel size wise) than the KAF-16803. Forget the fact that the Alluna is imaging at 0.155"/pixel now, the FSQ is now getting 2.25x better signal than the 16" Alluna as the photons are now being spread very thinly among the tiny pixels. Realistically it's going to be closer to 2x SNR as the IMX183 sensor has a near 90% QE against the 60% KAF-16803.

It is not a great real world example but you get the idea. It is not just about the amount of flux entering the system (the raw aperture) but the way the photons are spread (f/ratio and pixel size).
I ran similar numbers hence have no problem with the arithmetic you've presented...but the elephant in the room is: why would one sample at a bit over 1/10th of a pixel ? or even 1/100th of pixel?

Reduction to an absurd conclusion does not make a lot of sense when so many other factors will start coming into play by doing so (e.g. pixels sooo small to have well depth of 100 electrons )

Sure, a well sampled small "fast" system can give great results..but the same sampling rules can also be applied to larger (albeit optically slower) systems, that gather buckets more light with higher resolution.
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Old 20-02-2018, 09:53 PM
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I ran similar numbers hence have no problem with the arithmetic you've presented...but the elephant in the room is: why would one sample at a bit over 1/10th of a pixel ? or even 1/100th of pixel?

Reduction to an absurd conclusion does not make a lot of sense when so many other factors will start coming into play by doing so (e.g. pixels sooo small to have well depth of 100 electrons )
Moving to Chile any time soon?
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Old 20-02-2018, 10:09 PM
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Moving to Chile any time soon?
Ah....yes....I wish.....Chart32
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Old 20-02-2018, 10:23 PM
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Fortunately for me I have two telescopes that I can compare directly. One is a 14" SCT using Hyperstar and has a focal length of 711mm. The second is a 4" Refractor with a focal length of 714mm so they are both effectively the same.

I happen to know that one of the scopes requires much less of an exposure to get a high signal shot of a 14th mag asteroid than the other using the same camera. That's simple physics, photons per pixel. I even had a paper published in the Journal for Occultation Astronomy on the subject
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