Giovanni Paglioli:
In conclusion, on a linear acquisition device such has a CCD or CMOS, f ratio will not change the SNR on a giveng angle sampled that must be equal in both system, the only thing that will improve SNR on the same angle is the chance to acquire more photon flux and is the collecting area of the system. This is exactly identical to the statement that equal sampling in object space results in a signal strength ratio that is proportional to the square of the ratio of the lens diameters, and what is stated on John's slide 37:  |
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John Hayes:
Steeve Body:
Kay Ogetay:
Steeve Body:
Well from personal experience, running tak FSQ 106 f3.6 and the 135mm lense samyang at f2.8 as a dual rig, the FSQ capture way more light than the samyang at f2.8… the samyang is I think 65mm… so aperture seems in this case to make a really big difference…
That doesn't sound right Steeve. I'm pretty sure that's what you get, but not because of the aperture. Those are two very distinct optics. Samyang doesn't give the same performance at F2.8 that Tak does give at F3.6, not to mention your pixelscale differs. It's very clear that one gets better SNR for extended objects if the F ratio is lower, not just theoretically but fair comparisons also show this. But if you are targeting faint point sources like stars, then you need aperture --hence why we have large aperture scopes for research. But we also have Dragonfly array-like ultra-fast systems for extended objects with low surface brightness.
*Actually it is quite straightforward to see how effective f-ratio increases SNR --and why it matters. Take the same two telescopes and make an array, the effective ratio is sqrt(2) times lower. But what you basically get is x2 the exposure time. It is the equivalent of exposing two times more, or you can have two of the same telescopes. Everyone agrees that two times the exposure time would give you sqrt(2) increase in SNR, but for some reason, F-ratio is still a discussion. In fact that's why they are both ~sqrt(N), because they are essentially equivalent. It's a known fact in science, hence we design things according to that.
It all depends on what you need. Both low f ratio and higher aperture for high focal length are great. It is the purpose and how you do it that matters.
Hey Ogetay,
Yes, on paper a lower f-ratio should collect more light per unit area on extended objects. But in real world use, why is the FSQ106 at f/3.6 consistently outperforms the Samyang at f/2.8 in the observation I have made on the same object (horsehead, orion etc...)?
The images were taken at the same time with the same amount of integration time... I understand it is different pixel scale... but even with that before doing this experiment I was certain that the samyang would yield better results on faint nebulosity than the FSQ.... I'm not seeing that....
Here is what I'm seeing
Steve,
1) Comparing stretched images isn't how you compare signals. You have to look at the actual ADU values for pixels that are looking at the same points in object space.
2) You have to use identical cameras with identical pixel size and responsivity.
3) You have to take into account the throughput of the optical system. That includes the transmission factors for each surface. It's important to understand that it is totally possible to have a very fast objective that has a lot of elements with poor coatings producing a lower signal than a slower system with fewer components that have very high performance coatings. This is one reason that the Dragonfly systems focus on getting lenses with the highest possible transmission. The equations that I provided in my presentation allow for differences in transmission factors.
John I did not want to look not corresponding @Steeve Body , I agree with what Dr. Hayes says here. I just wanted to say there isn't much I can add to his comment. As I mentioned in my earlier answer, it becomes much simpler to understand when you consider arrays. It's very clear that two same scopes/setups in 1 hour will result in better SNR than the same single scope in 1 hour, and that is due to the effective F-ratio being lower. This is equivalent to a single telescope exposing 2 hours. So unless one disagrees more exposure results in increased SNR, there is no argument against the F-ratio's impact here.
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John Hayes:
Steeve Body:
Kay Ogetay:
Steeve Body:
Well from personal experience, running tak FSQ 106 f3.6 and the 135mm lense samyang at f2.8 as a dual rig, the FSQ capture way more light than the samyang at f2.8… the samyang is I think 65mm… so aperture seems in this case to make a really big difference…
That doesn't sound right Steeve. I'm pretty sure that's what you get, but not because of the aperture. Those are two very distinct optics. Samyang doesn't give the same performance at F2.8 that Tak does give at F3.6, not to mention your pixelscale differs. It's very clear that one gets better SNR for extended objects if the F ratio is lower, not just theoretically but fair comparisons also show this. But if you are targeting faint point sources like stars, then you need aperture --hence why we have large aperture scopes for research. But we also have Dragonfly array-like ultra-fast systems for extended objects with low surface brightness.
*Actually it is quite straightforward to see how effective f-ratio increases SNR --and why it matters. Take the same two telescopes and make an array, the effective ratio is sqrt(2) times lower. But what you basically get is x2 the exposure time. It is the equivalent of exposing two times more, or you can have two of the same telescopes. Everyone agrees that two times the exposure time would give you sqrt(2) increase in SNR, but for some reason, F-ratio is still a discussion. In fact that's why they are both ~sqrt(N), because they are essentially equivalent. It's a known fact in science, hence we design things according to that.
It all depends on what you need. Both low f ratio and higher aperture for high focal length are great. It is the purpose and how you do it that matters.
Hey Ogetay,
Yes, on paper a lower f-ratio should collect more light per unit area on extended objects. But in real world use, why is the FSQ106 at f/3.6 consistently outperforms the Samyang at f/2.8 in the observation I have made on the same object (horsehead, orion etc...)?
The images were taken at the same time with the same amount of integration time... I understand it is different pixel scale... but even with that before doing this experiment I was certain that the samyang would yield better results on faint nebulosity than the FSQ.... I'm not seeing that....
Here is what I'm seeing
Steve,
1) Comparing stretched images isn't how you compare signals. You have to look at the actual ADU values for pixels that are looking at the same points in object space.
2) You have to use identical cameras with identical pixel size and responsivity.
3) You have to take into account the throughput of the optical system. That includes the transmission factors for each surface. It's important to understand that it is totally possible to have a very fast objective that has a lot of elements with poor coatings producing a lower signal than a slower system with fewer components that have very high performance coatings. This is one reason that the Dragonfly systems focus on getting lenses with the highest possible transmission. The equations that I provided in my presentation allow for differences in transmission factors.
John Thanks for your input, John. I’m definitely seeing that while the Samyang is nominally ‘faster,’ real-world transmission and coatings play a huge role. In our setup, we used two slightly different cameras (IMX455 Player One Zeus on the Tak vs. IMX571 2600mm on the 135mm) plus different filters (3 nm vs. 6 nm), which also complicates the comparison. The Samyang has 11 optical elements, and even if they’re good, they can’t seem to match the Tak’s coatings and design efficiency, so the combination of all these factors lead to an actual throughput that is quite a bit lower than I expected. I was hoping the Samyang rig would at least equal the Tak in SNR for wide-field surveys of faint objects, but it seems that all these factors (optical quality, transmission, number of elements, filters) can overshadow the theoretical advantage of a lower f-ratio. Your insights really confirm that it’s about total system performance, not just the nominal speed. Thanks again for helping me understand this better!
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Clayton, I see some great answers by others. Yet, I would like to point your attention to this interactive guide to light by Bartosz Ciechanowski. It's very illuminating when it comes to understanding F-ratio/aperture. It teaches about travel vectors, irradiance, radiance, angles, luminance. The guide is not only beautiful but also easy to understand. As a bonus, there is a similar one about cameras and lenses, again from the same author. p.s. If the above spikes your interest, check all the resources I have put together over the years.
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Steeve Body:
I was hoping the Samyang rig would at least equal the Tak in SNR for wide-field surveys of faint objects, but it seems that all these factors (optical quality, transmission, number of elements, filters) can overshadow the theoretical advantage of a lower f-ratio. Your insights really confirm that it’s about total system performance, not just the nominal speed. Thanks again for helping me understand this better! Your are most likely hobbling that poor lens with that Ha filter whose transmittance is going to be rather poor, even at f/2.8, compared to the Tak @ ~ f/3.6. And of course 14 air-to-glass surfaces don't exactly help either, even with the best coatings in the world (and Samyang's are pretty darn good).
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There are lots of different coating used for different reasons but in broad terms you can figure on a 1% loss with every air to glass surface in your lens system. Cemented surfaces don't count in the same way as far as I know. I believe it's also wavelength dependent so things can get very complicated if you want to calculate the true efficiency of a complex optical system.
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I find the concept of Etendue useful here. The SNR depends on the amount of light falling on each pixel in a given time E = QE x d^2*s^2 where QE is the efficiency of light delivery to the sensor, D is the diameter of the lens or mirror and s is is just the image scale in arcseconds-per-pixel. E is a metric that quantifies the light delivered to each pixel in a given time. So - for example - if two set ups are imaging at the same image scale (same focal length and camera) and one is a 12 inch reflector and the other a 6 inch then the 12 will image 4x more quickly than than the 6 inch and deliver 2X the SNR in the same imaging time. Discussed here on cloudy nights https://www.cloudynights.com/topic/660553-etendue-calculator |
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Tim Hawkes:
I find the concept of Etendue useful here. The SNR depends on the amount of light falling on each pixel in a given time
E = QE x d^2*s^2
where QE is the efficiency of light delivery to the sensor, D is the diameter of the lens or mirror and s is is just the image scale in arcseconds-per-pixel.
E is a metric that quantifies the light delivered to each pixel in a given time. So - for example - if two set ups are imaging at the same image scale (same focal length and camera) and one is a 12 inch reflector and the other a 6 inch then the 12 will image 4x more quickly than than the 6 inch and deliver 2X the SNR in the same imaging time.
Discussed here on cloudy nights
https://www.cloudynights.com/topic/660553-etendue-calculator If Entendue is equivalent to how well my optical train will do for imaging, I think I just wasted 2k on a big refractor, that the fma180 can out perform. Someone talk me off the cliff.
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Clayton Ostler:
If Entendue is equivalent to how well my optical train will do for imaging, I think I just wasted 2k on a big refractor, that the fma180 can out perform. Someone talk me off the cliff. Before selling off your refractor, go back and read what Tim wrote: If two set ups are imaging at the same image scale (same focal length and camera) and one is a 12 inch reflector and the other a 6 inch then the 12 will image 4x more quickly than than the 6 inch and deliver 2X the SNR in the same imaging time." Comparison of Entendue between two systems is only meaningful if the image scale is the same. A 50 mm f/1.8 lens that you can buy for $125 will do even better than your FM 180 in terms of delivering light to a pixel. Would you buy that instead  ?
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Arun H:
Clayton Ostler:
If Entendue is equivalent to how well my optical train will do for imaging, I think I just wasted 2k on a big refractor, that the fma180 can out perform. Someone talk me off the cliff.
Before selling off your refractor, go back and read what Tim wrote:
If two set ups are imaging at the same image scale (same focal length and camera) and one is a 12 inch reflector and the other a 6 inch then the 12 will image 4x more quickly than than the 6 inch and deliver 2X the SNR in the same imaging time."
Comparison of Entendue between two systems is only meaningful if the image scale is the same. A 50 mm f/1.8 lens that you can buy for $125 will do even better than your FM 180 in terms of delivering light to a pixel. Would you buy that instead ? Part of the problem is I'm not smart enough to understand everything that Tim wrote. Here's a couple things to consider. I only get one camera there's no way I'm going to talk my wife into getting more than one. I have a hard time getting her convinced that I need more than one scope hahaha. So pixel size is not going to change. After doing a fair amount of reading I really thought I could get better images and better performance going from a 60 mm F5 to a 115 mm f7 and then using a reducer to get it down to an f 5.6. This all seemed reasonable to me. My main goal was to be able to better image smaller galaxies and nebula. I would use the 60 mm for wider field and my big bad megascope for the other. Because the skies have been so terrible I don't really have any real-world experience comparing these. But common sense says that the aperture being that much larger should be able to capture that many more photons, although the angle is not going to be as optimal between the F5 and the F5.6, the sheer quantity of light should offset that and even improve it. However typing in these numbers it doesn't look like that. It looks like my new megascope performs slightly less than my cheaper 60 mm. And I'm not even using the FMA 180 in this scenario which seems to perform even better. Sorry if I sound dramatic, I can just think of a lot of stuff that I could have dropped 2 Grand on. I'm not ready to run out and sell it yet but I'm also not near as giddy about using it as I was before.
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In that same thread that Tim linked from CN are some words of wisdom from John Hayes: " Just remember that what you gain in irradiance with the smaller refractor, you lose in image scale (along with sampling in object space.) To increase both signal strength and image scale, you need a larger aperture. So, unfortunately, there is no free lunch. Larger aperture is always a benefit and that's why professional telescopes are so large." People can use all manner of fancy terms, but at a fundamental level, this is conservation of energy - a large telescope will always gather more light from a specific area of object than a small scope. Focal length dictates how that energy is translated into irradiance ("brightness") on the image plane and that combined with pixel size translates into signal. When normalized to a common image scale (through resampling after acquisition) your 115 mm will give you roughly 4x the signal as your 60mm. The only question you really need to ask is - does what you want to image fit on your sensor using the 115mm? A site like Telescopius will help you with that.
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Clayton Ostler:
Arun H:
Clayton Ostler:
If Entendue is equivalent to how well my optical train will do for imaging, I think I just wasted 2k on a big refractor, that the fma180 can out perform. Someone talk me off the cliff.
Before selling off your refractor, go back and read what Tim wrote:
If two set ups are imaging at the same image scale (same focal length and camera) and one is a 12 inch reflector and the other a 6 inch then the 12 will image 4x more quickly than than the 6 inch and deliver 2X the SNR in the same imaging time."
Comparison of Entendue between two systems is only meaningful if the image scale is the same. A 50 mm f/1.8 lens that you can buy for $125 will do even better than your FM 180 in terms of delivering light to a pixel. Would you buy that instead ?
Part of the problem is I'm not smart enough to understand everything that Tim wrote.
Here's a couple things to consider. I only get one camera there's no way I'm going to talk my wife into getting more than one. I have a hard time getting her convinced that I need more than one scope hahaha. So pixel size is not going to change. After doing a fair amount of reading I really thought I could get better images and better performance going from a 60 mm F5 to a 115 mm f7 and then using a reducer to get it down to an f 5.6. This all seemed reasonable to me. My main goal was to be able to better image smaller galaxies and nebula. I would use the 60 mm for wider field and my big bad megascope for the other.
Because the skies have been so terrible I don't really have any real-world experience comparing these. But common sense says that the aperture being that much larger should be able to capture that many more photons, although the angle is not going to be as optimal between the F5 and the F5.6, the sheer quantity of light should offset that and even improve it. However typing in these numbers it doesn't look like that. It looks like my new megascope performs slightly less than my cheaper 60 mm. And I'm not even using the FMA 180 in this scenario which seems to perform even better. Sorry if I sound dramatic, I can just think of a lot of stuff that I could have dropped 2 Grand on. I'm not ready to run out and sell it yet but I'm also not near as giddy about using it as I was before. This is going to be an exageration, but which would you prefer? This in let's say 4h  Or this in 30 minutes?  Yes, your new telescope is slightly slower, but you'll get so much better that out of it. From what you said, you're limited to 1 camera, so you can't vary FOV on the same scope without cropping. Cropping mean lower resolution and part of those extra photons from the faster scope are also getting cropped. Matching desired FOV & pixel scale to scope-camera combo is (in my opinion) more important than F ratio. You can always exposed for longer (don't worry, you'll get another clear night in 2 years  ) but you can't drizzle for more resolution that much.
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For the original OP. I think you have to be careful about words like quality or better because those terms are not precise. The image of a given extended object, galaxy or Ha cloud will be larger, dimmer and at twice the resolution in the larger scope. As the above tries to express, brighter with less detail is not what you're after. Yes, things would be better of you could match the pixel size to the scope for optimum sampling but even if that's not possible you can always bin. Try the larger scope, I think you'll be very pleased and will wonder what all the worry was about.
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Thanks everyone for the input. I've learned a lot from this discussion.
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I recently asked Grok 3.0 about speed vs aperture, as I recently bought an EdgeHD 8. My other scope being an 80mm f/6 refractor. All else being equal, yes, aperture makes a difference. My EdgeHD 8 at it's native f/10 still collects light faster than my f/6 refractor. About twice as fast when targeting a similar SNR. Grok thread: https://x.com/i/grok/share/kAW98cGpCbYJbAS8tRLZa2Hqw |
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Derek Vasselin:
I recently asked Grok 3.0 about speed vs aperture, as I recently bought an EdgeHD 8. My other scope being an 80mm f/6 refractor.
All else being equal, yes, aperture makes a difference.
My EdgeHD 8 at it's native f/10 still collects light faster than my f/6 refractor. About twice as fast when targeting a similar SNR.
Grok thread:
https://x.com/i/grok/share/kAW98cGpCbYJbAS8tRLZa2Hqw This very much aligns with what I was thinking. It just makes common sense more photons equals stronger signal. Unfortunately every time I do the math with these other complex formulas that I don't seem to understand, hahaha it never works out that way. I just have to believe that with all the really smart people in this hobby if smaller scopes could perform as good as larger scopes, we would have stopped buying larger scopes a long time ago. I mean those guys at NASA think about this all day long if they could figure out how to do it on an FMA 180 then they would have.
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And this is literally this first math I've understood in this string 100%. Nice work. https://x.com/i/grok/share/kaw98cgpcbyjbas8trlza2hqw |
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I will try to make it really easy as a concept: You have to collect a quantity of energy from an area and that energy is costant in form of a flux of photons. Your acquiring system has some failures and collect also something non related to that flux. You can have a "perfect system" able to not introduce any error and capable of gather all the flux that is coming. Even in this case You can't "invent" energy that is not present! The time You spent to collect such energy is not relevant since You can always sum or median or wathever You want, make statistic to improve the SNR BUT, the flux is not coming in a linear fashion but with a poisson distribution meaning that even on a perfect system and with no errors related to the sky noise, each exposure will be different just becouse of the nature of the incoming signal. The only way to increase such precision in the measurement (SNR for each shot) is to increase the chance of capturing such a poisson distributed flux of energy and that is augmenting the collecting area. The total energy emitted per angle of sky is fixed and measured by the photometry and We cannot change that in any way. Faster f ratio collect light faster ONLY from a source that has no area (a point) but photometry takes into account the energy present in an area (better to say angle of sky). So in each pixel sampling THE SAME area of sky You can't in any way increase the energy phisically present in that area. Since the energy is coming from a multiple sources is to be considered the sum of all the sources emitting in that area, this means You coluld have a subject of a brighter magnitude (more photon flux) and others much more dim (less photon flux). If a source has a flux of 100 photons/sec in that area You can't collect more than that whatever system You use. If sources have 0.01 photons per second You have to make almost 100" exposure to have the chance to collect that single photon! If You have a scope with a larger collecting area, than the flux is multiplied by that area so You have the chance to collect that small energy much more efficiently and that's the only thing that matter reguard to the Physics of the emitted energy. The system to be "characterized" is just introducing errors in the measurement that has to be taken in consideration for the precision of the measure, remember, in digital this imprecision or uncertanty of the measure is the NOISE in digital terms. Hope this is more helpful…  |
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Giovanni Paglioli:
I will try to make it really easy as a concept: You have to collect a quantity of energy from an area and that energy is costant in form of a flux of photons. Your acquiring system has some failures and collect also something non related to that flux. You can have a "perfect system" able to not introduce any error and capable of gather all the flux that is coming. Even in this case You can't "invent" energy that is not present! The time You spent to collect such energy is not relevant since You can always sum or median or wathever You want, make statistic to improve the SNR BUT, the flux is not coming in a linear fashion but with a poisson distribution meaning that even on a perfect system and with no errors related to the sky noise, each exposure will be different just becouse of the nature of the incoming signal. The only way to increase such precision in the measurement (SNR for each shot) is to increase the chance of capturing such a poisson distributed flux of energy and that is augmenting the collecting area. The total energy emitted per angle of sky is fixed and measured by the photometry and We cannot change that in any way. Faster f ratio collect light faster ONLY from a source that has no area (a point) but photometry takes into account the energy present in an area (better to say angle of sky). So in each pixel sampling THE SAME area of sky You can't in any way increase the energy phisically present in that area. Since the energy is coming from a multiple sources is to be considered the sum of all the sources emitting in that area, this means You coluld have a subject of a brighter magnitude (more photon flux) and others much more dim (less photon flux). If a source has a flux of 100 photons/sec in that area You can't collect more than that whatever system You use. If sources have 0.01 photons per second You have to make almost 100" exposure to have the chance to collect that single photon! If You have a scope with a larger collecting area, than the flux is multiplied by that area so You have the chance to collect that small energy much more efficiently and that's the only thing that matter reguard to the Physics of the emitted energy. The system to be "characterized" is just introducing errors in the measurement that has to be taken in consideration for the precision of the measure, remember, in digital this imprecision or uncertanty of the measure is the NOISE in digital terms.
Hope this is more helpful...  Thanks for this. In my limited understanding. We are saying that regardless of how efficient we are with the photons ( ie focal ratio or QE). You can't capture photons you don't have. And that is only increased by adding aperture.
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Warning, this will sound opinionated, I want to add something to this string that I feel is important for other newbies to this concept. Many people much smarter than I refer to Etendue as an important formula in this equation. After waiting a couple hours of videos and reading up on this, I want to point out that Etendue is only a measurement for how much the light will spread out, not a measurement for the total light arriving. An F5 50MM aperture and an F5 500MM aperture have the same Etendue calculations. For me this number and calculation is no more useful than the F ratio number because its change it literally the same. I am actually semi bothered by this as a reference in if the analysis of one telescope being better than than another for a specific astrophotography situation. Really, this is just a very smart way of saying "f ratio is all that matters" if you buy into the Etendue calculations. For me it was just a lot of math to say "look at the f ratio", Etendue or étendue ( /ˌeɪtɒnˈduː/; French pronunciation: [etɑ̃dy]) is a property of light in an optical system, which characterizes how "spread out" the light is in area and angle Notice this doesnt seem to calcuate total light just how spread out it is, which is important but is far from the whole picture. If I have an army of 5 and its spread out across a battlefield its still a pretty weak army, if I have an army of 5000 and its equally spread out, it could still be pretty powerful. Not saying focal ratio means nothing, but there seems to be an instant bias towards (speed is what matters most for AP) is flat out wrong.
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To convince yourself of how much speed dosen't or does matter, try doing some imaging at high F/ratios. I used to have a very nice 150mm F/12 Maksutove that I used for lunar imaging. Just for fun and against a lot of advice I tried some deep sky imaging with it. It actually did really well with the big issue being seeing and guiding rather than signal. The thing to remember is that yes, overall signal is lower but so is the sky background so the difference in S/N won't be as great as you might expect.
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Thanks Tony, Last night I a did very non scientific test of pointing the bigger scope at m81 for 5 60 sec exposures and then moving my camera over to the smaller and faster scope pointing it at m81 for 5 60 sec exposures,
Sure the FOV is very different, but the image quality and perceived brightness seems much better for the bigger scope. Im feeling good about things.
Again, no science behind this and then the clouds rolled in……
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Clayton Ostler:
Thanks Tony, Last night I a did very non scientific test of pointing the bigger scope at m81 for 5 60 sec exposures and then moving my camera over to the smaller and faster scope pointing it at m81 for 5 60 sec exposures,
Sure the FOV is very different, but the image quality and perceived brightness seems much better for the bigger scope. Im feeling good about things.
Again, no science behind this and then the clouds rolled in...... Sometimes the seeds of confusion ride on the wings of complexity...
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Clayton Ostler:
Thanks for this. In my limited understanding. We are saying that regardless of how efficient we are with the photons ( ie focal ratio or QE). You can't capture photons you don't have. And that is only increased by adding aperture. Congratulations! You've developed a fundamental and intuitive understanding of this. This really isn't that complicated, but the use of fancy quantities can make it more so. In terrestrial photography, people are used to "fast" lenses translating to smaller exposure times. But there is the implicit understanding that a 35 mm f/2 lens is a very different beast than a 200 mm f/2 (and priced very differently!). They serve very different purposes. One does not use a 35mm f/2 lens when the appropriate tool is a 200 mm f/2 lens. The same is true in astro work as well. a 40 mm aperture f/4 scope is a very different thing than a 200 mm aperture f/4 Newtonian and experienced astrophotographers would know that one serves a very different purpose than the other. I feel like, just like in terrestrial work, you need to start with what you want to image. If you wish to image a distant galaxy, you would need a very different scope than a Milky Way widefield or even a large nebula. Attempting to use single terms or single formulae to compare two very different things simply confuses the issue. Both your 60mm and your 115 mm will be of use to you - for different targets.
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Speaking for those of us in the Northeast USA, seeing seldom approaches the Rayleigh limit of larger apertures, and virtually never during galaxy season (i.e., when a large aperture would have its greatest use). What benefit would a larger aperture have in such conditions over something with a diffraction limit in the range of 1 arcsecond?
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