To make it simple, let's think about a 75mm aperture and a 150mm aperture and let's make both systems the same focal length, say 600mm. That would be F/4 for the larger scope and F/8 for the smaller. Use the same sensor so the FOV remains the same. Shoot an object for an hour in the small scope and 15 mi. on the large one. Both images will have the same density but you'll find that the image from the larger aperture will have tighter stars and better detail and contrast. Plus, you did it in less time.
It's also important to remember, and I keep saying this. Using average seeing as a hard stop is putting needless limits on your work. The thing about average seeing is well, it's an average. Some nights better, some nights worse. Also there is a lot of variation within a given night. If you're willing to keep subs short and cull as needed, you can beat the seeing to a certain degree and benefit even more from the larger aperture. Systems and flows optimized for average conditions will give you just that, dependable but average results.
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Dan H. M.:
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? The benefit of a large aparture is that it collects more photons from the Galaxy you are imaging. This means that a 14" F/11 SCT needs shorter exposure time than a 8" F/2 RASA if your goal is a nice close-up of the Sunflower Galaxy for example.
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Photographic brightness depends on the focal ratio and the diameter of the telescope, the value of the photographic brightness ratio is given by (D x D)/(Fr x Fr) where D is the diameter and Fr the focal ratio. A telescope with a 100 mm aperture at F/5 has the same photographic brightness as a 140 mm telescope at F/7. Of course the quality of the lenses is decisive in the comparison.
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Marco Luigi Tassi:
Photographic brightness depends on the focal ratio and the diameter of the telescope, the value of the photographic brightness ratio is given by (D x D)/(Fr x Fr) where D is the diameter and Fr the focal ratio. A telescope with a 100 mm aperture at F/5 has the same photographic brightness as a 140 mm telescope at F/7. Of course the quality of the lenses is decisive in the comparison. Doesn't make any sense. The units for brightness have dimensions of Luminosity/Length^2. The formula you're posting have dimensions of L^2, that is area. If it is a ratio of equal units it should be dimensionless. Maybe you meant (D1/FN1)/(D2/FN2)? P.S.: Possibly (D1^2/FL1)/(D2^2/FL2) makes even more sense.
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Erlend Langsrud:
Dan H. M.:
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?
The benefit of a large aparture is that it collects more photons from the Galaxy you are imaging. This means that a 14" F/11 SCT needs shorter exposure time than a 8" F/2 RASA if your goal is a nice close-up of the Sunflower Galaxy for example.
Right, that's true in theory but when does it necessarily translate into actual image quality? If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture?
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Dan H. M.:
Right, that's true in theory but when does it necessarily translate into actual image quality? If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture? *Yes it does, by the wagonload. Incidentally, I'd given an arm and a leg for a site with an average seeing of 1", even in the Acatama it doesn't get this good.
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Dan H. M.:
If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture? Singal is different that resolution. If seeing does not support a certain image scale, you would bin or resample to a coarser image scale. At that coarser pixel scale (measured in arcsec/px), the larger scope will have a considerable signal (and hence SNR) advantage over a smaller scope at that same image scale.
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Dan H. M.:
What benefit would a larger aperture have in such conditions over something with a diffraction limit in the range of 1 arcsecond? If you are going to image a single target (say M81) and it fits on your sensor with the larger aperture scope, the larger aperture will collect more photons from that target regardless of f ratio. It will also resolve dimmer stars and details even in poor seeing. F ratio will more determine exposure length to bring the sky background well above the read noise, but once there, aperture gives more signal of the target per overall session length. Over the last week, I've imaged M81/82 with my 10" f4 newt and last night with a 91mm f4.9 refractor. Both in poor seeing, so the star sizes in arcseconds were pretty similar. Viewing the raw single 60 second images from both at the same image scale, the image from the 10" had far and above more dim detail and much less background noise (much more than the f ratio differences would explain). Also resolved dimmer stars and details as expected. I'll try to put a visual example like I did earlier to clearly show this.
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Rick Krejci:
Over the last week, I've imaged M81/82 with my 10" f4 newt and last night with a 91mm f4.9 refractor. Both in poor seeing, so the star sizes in arcseconds were pretty similar. Viewing the raw single 60 second images from both at the same image scale, the image from the 10" had far and above more dim detail and much less background noise (much more than the f ratio differences would explain). Also resolved dimmer stars and details as expected. I'll try to put a visual example like I did earlier to clearly show this. This is exactly what a number of us having been saying all along (with the math to prove it). Neglecting things like transmission losses, assuming similar QE and low or negligible read noise - at common image scale and integration time, aperture will be the main thing that matters, by far. It cannot be otherwise, because if you could get the same result with a small scope as a large scope at common image scale, you would, in essence, be building the equivalent of a perpetual motion machine, a violation of the First Law of Thermodynamics.
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Arun H:
This is exactly what a number of us having been saying all along I've been part of that chorus as well. Just wanted to give another concrete example.
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Ok, it's making sense now. I did a comparison of two systems, one of which I own, and can see that the larger aperture should capture significantly more signal.
One is an EdgeHD 8" at f/10 with an ASI2400MC, the other is a Tak e130D with the 1.5x extender at f/5 paired with an IMX585 camera. If I were to resample the EdgeHD's data by 2x, that would essentially make it an f/5 system. But the signal strength is not the same between the two despite similar image scales because the pixel size is so much larger in the EdgeHD system. And even if I put the same camera on both and resample the Epsilon's data to match the Edge's image scale, the Edge should still be more than 2x faster.
This doesn't tell the full story, of course. The Epsilon with the extender has a very fine spot size whereas I'm basically massacring the Edge's spot size by resampling the data so much. The Edge also isn't as suited to FF, but that's why I think it's more for galaxies and PNs.
Interesting stuff.
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Dan H. M.:
Erlend Langsrud:
Dan H. M.:
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?
The benefit of a large aparture is that it collects more photons from the Galaxy you are imaging. This means that a 14" F/11 SCT needs shorter exposure time than a 8" F/2 RASA if your goal is a nice close-up of the Sunflower Galaxy for example.
Right, that's true in theory but when does it necessarily translate into actual image quality? If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture? Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope. Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
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Erlend Langsrud:
Dan H. M.:
Erlend Langsrud:
Dan H. M.:
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?
The benefit of a large aparture is that it collects more photons from the Galaxy you are imaging. This means that a 14" F/11 SCT needs shorter exposure time than a 8" F/2 RASA if your goal is a nice close-up of the Sunflower Galaxy for example.
Right, that's true in theory but when does it necessarily translate into actual image quality? If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture?
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose. As a plus, consider that deconvolution will greatly increase the details of that image. To make an EFFECTIVE deconvolution, You need quite an oversampled image, the one that is undersampled will take no benefit (or also may generate artifacts) from deconvolution. We all use to make stack statistics on our images which are normally dithered and, becouse of that, needs to be registered on a reference. In this process of moving and resampling images You loose details and informations. If You want to be shure to not lose informations during the process and since we are not in a monodimensional sine-wave realm of the Nyquist rule, We need to sample almost 3.3/3.5 times the FWHM to ensure that.
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Giovanni Paglioli:
Erlend Langsrud:
Dan H. M.:
Erlend Langsrud:
Dan H. M.:
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?
The benefit of a large aparture is that it collects more photons from the Galaxy you are imaging. This means that a 14" F/11 SCT needs shorter exposure time than a 8" F/2 RASA if your goal is a nice close-up of the Sunflower Galaxy for example.
Right, that's true in theory but when does it necessarily translate into actual image quality? If seeing in a given location never gets better than 1" (probably worse) over the course of an entire season, is there any visible benefit to the larger aperture?
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
As a plus, consider that deconvolution will greatly increase the details of that image. To make an EFFECTIVE deconvolution, You need quite an oversampled image, the one that is undersampled will take no benefit (or also may generate artifacts) from deconvolution. We all use to make stack statistics on our images which are normally dithered and, becouse of that, needs to be registered on a reference. In this process of moving and resampling images You loose details and informations. If You want to be shure to not lose informations during the process and since we are not in a monodimensional sine-wave realm of the Nyquist rule, We need to sample almost 3.3/3.5 times the FWHM to ensure that. I don't think it's an accurate statement to say undersampled images take no benefit from deconvolution. Especially when drizzled the detail in an undersampled image can be greatly enhanced with deconvolution.
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I am not a fan of low focal ratios being referred to as faster. For the same aperture, I they are wider field/lower "plate" scale than slower beams.
Depends entirely on what you wish to image. Ultimately it depends on SNR per desired resolution element.
Indeed I findi the term "faster" somewhat misleading as "faster" telescope, is also harder to collimate and keep in focus/collimation than it slower counterparts.
For me, 800mm to 1200mm focal length is the sweet spot for my imaging, matched to the seeing of my location.
Nevertheless I choose a f/6.4 300mm RC (binx2) over an f/4 300mm Newtonian because of ease of set-up, focussing and maintenance. At times I do wish I had a wider field-of-view, but I can alway mosaic 2-3 frames. [About the max PI can manage!]
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Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose. Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it. John
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John Hayes:
Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it.
John I am the original (OP) of this thread. I now have enough confidence based on research and reading that what you're saying is not entirely true. I've seen your spreadsheet I've looked at the formulas. But I'm going to give you a really drastic example. If in a 10-second period you can gather 1000 photons from a really small aperture scope, or you could gather 10 million photons from a larger aperture scope. Regardless of what you do with those 1,000 photons. You could do more with the 10 million even if they are less optimized. I've seen way too many suggestions that speed is all that matters. I don't agree with it , and real world experience shows that that is not true. I'm not disregarding that you can optimize the light path by having a fabulous focal ratio, but if you only have so much to work with as a starting point. I know everyone has their own opinions and I really appreciate everyone's comments on this. After all the reading I've done now I don't need some crazy formula that I don't really understand. Common Sense says that limiting the amount of photons collected will also limit the total signal.
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John Hayes:
Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it.
John The way I see it, if you double the focal length AND the pixel size you get same number of photons in each pixel. The result should be the same. You achieve pretty much the same by scaling down the image of a long FL scope, making each new pixel the average of 4 pixels. Hubble is F/24 btw.
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John Hayes:
Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it.
John I'd like to add that I think there are several correct ways to look at this, as well as some wrong ways. The f-ratio certainly says something about how grainy an image of an extended object will look after a given exposure time. Every daytime photographer knows this works in practice. Nonetheless, I think the number of photons per arcsec sky is what matters to bring out details of a galaxy that fits the FOV.
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Clayton Ostler:
John Hayes:
Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it.
John
I am the original (OP) of this thread. I now have enough confidence based on research and reading that what you're saying is not entirely true. I've seen your spreadsheet I've looked at the formulas. But I'm going to give you a really drastic example.
If in a 10-second period you can gather 1000 photons from a really small aperture scope, or you could gather 10 million photons from a larger aperture scope. Regardless of what you do with those 1,000 photons. You could do more with the 10 million even if they are less optimized. I've seen way too many suggestions that speed is all that matters. I don't agree with it , and real world experience shows that that is not true. I'm not disregarding that you can optimize the light path by having a fabulous focal ratio, but if you only have so much to work with as a starting point.
I know everyone has their own opinions and I really appreciate everyone's comments on this.
After all the reading I've done now I don't need some crazy formula that I don't really understand. Common Sense says that limiting the amount of photons collected will also limit the total signal. Ok. If you want to rely on "common sense" and ignore the well established science of radiometry and the presentation that I posted that clearly lays out how this stuff works, then there isn't anything I can do to help you understand anything any better. I just have to wonder why even bother to ask a question if you don’t really want to make an effort to understand the answer? John
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John Hayes:
Clayton Ostler:
John Hayes:
Erlend Langsrud:
Yes. First of all because the large scope collects more photons per hour. This advantage can be offset by using a long integration times with a smaller scope.
Secondly, even if the seeing is 2", the blur from the telescope will add to the blur from the atmosphere. The advantage of a big scope (14" vs 4") when it comes to resolution might be small in practice, but it is not zero. I want to take the sharpest possible image of a galaxy, I would use the biggest scope i dispose.
Yes, a larger aperture collects more photons/sec; however, that is the wrong way to think about it. What you do with those photons to generate the signal is what counts. When I gave my AIC presentation, I posted an Excel spreadsheet that makes it easy to compare signals between various systems. I suggest that you download that spreadsheet and experiment with it.
John
I am the original (OP) of this thread. I now have enough confidence based on research and reading that what you're saying is not entirely true. I've seen your spreadsheet I've looked at the formulas. But I'm going to give you a really drastic example.
If in a 10-second period you can gather 1000 photons from a really small aperture scope, or you could gather 10 million photons from a larger aperture scope. Regardless of what you do with those 1,000 photons. You could do more with the 10 million even if they are less optimized. I've seen way too many suggestions that speed is all that matters. I don't agree with it , and real world experience shows that that is not true. I'm not disregarding that you can optimize the light path by having a fabulous focal ratio, but if you only have so much to work with as a starting point.
I know everyone has their own opinions and I really appreciate everyone's comments on this.
After all the reading I've done now I don't need some crazy formula that I don't really understand. Common Sense says that limiting the amount of photons collected will also limit the total signal.
Ok. If you want to rely on "common sense" and ignore the well established science of radiometry and the presentation that I posted that clearly lays out how this stuff works, then there isn't anything I can do to help you understand anything any better. I just have to wonder why even bother to ask a question if you don’t really want to make an effort to understand the answer?
John I'm not at all disregarding your comments and I have watched your presentation. There's some things that don't add up and I would love a common sense explanation without a giant formula that I don't really follow. I recognize that makes me sound dumb and I'm okay with that. But here's the reality, I don't see any way that scattered light photons can be more condensed onto a sensor solely because the focal ratio is better. If I only have 10 photons to focus and I'm perfect at focusing them on a sensor, I can still only capture 10 photons, in contrast if I have a million photons and I'm really bad at capturing them and can only condense 100,000 of them, I still have way more photons hitting my sensor. Every time I look at these formulas my naive understanding is that they represent that light travels in a straight line and that the photons are lined up, but they're not they're scattered and they don't come in evenly. In addition to that I don't see any way that I can make 10 photons into an image better than I can make 100,000 photons into an image. Every time I read these formulas they suggest the way that you solve that is by extended exposure time to get more photons. What I'm saying is in the same amount of time there is no way that 10 photons can be better than 100,000. I understand that I'm not arguing science and that doesn't line up with the way a lot of the scientists on this channel think. If someone can explain it to me in common terms, which I called common sense and I'm sorry if that's offensive. I would love to understand it, but please don't show me a formula on the efficiency of light paths. I've seen the presentation and to be completely honest I don't follow most of it probably because I'm not really a super smart guy
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Science is often - in fact usually - not common sense. Quantum theory and general relativity cannot be understood using the tools of common sense. But both are true. For millenia common sense dictated that we live on a flat earth. It is necessary to go a bit further to really understand things. Once you do then it fits and you will grasp it intuitively. So don't give up on the math too easily
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Clayton Ostler:
. If I only have 10 photons to focus and I'm perfect at focusing them on a sensor, I can still only capture 10 photons, in contrast if I have a million photons and I'm really bad at capturing them and can only condense 100,000 of them, I still have way more photons hitting my sensor. To understand the fallacy of this argument, a simple thought experiment is useful. If all that mattered was the number of photons entering the OTA - one does not need a lens. One can simply take a big large tube, let us say 50" in diameter, no glass, no mirrors, nothing. Such a device would "collect"many, many, many times more photons than a 115 mm telescope; it would be incredibly bad at focusing them, but, by your argument, this does not matter. Put a sensor at the bottom of that big, large tube. By your argument, it should outperform the 115mm, correct?
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Here's an actual comparison that I think very clearly shows what you can expect. The image on the left was taken with a 200mm aperture at F/10, uv/ir cut. The one on the right was taken with a 150mm f/12, dual band Ha Oiii filter. Please ignore the density difference as the 200mm image has a lot less integration time. What this image shows is the resolution difference. Not a perfect comparison by a long shot, it does give you an idea of what happens when you increase the aperture.  |
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andrea tasselli:
Marco Luigi Tassi:
Photographic brightness depends on the focal ratio and the diameter of the telescope, the value of the photographic brightness ratio is given by (D x D)/(Fr x Fr) where D is the diameter and Fr the focal ratio. A telescope with a 100 mm aperture at F/5 has the same photographic brightness as a 140 mm telescope at F/7. Of course the quality of the lenses is decisive in the comparison.
Doesn't make any sense. The units for brightness have dimensions of Luminosity/Length^2. The formula you're posting have dimensions of L^2, that is area. If it is a ratio of equal units it should be dimensionless. Maybe you meant (D1/FN1)/(D2/FN2)?
P.S.: Possibly (D1^2/FL1)/(D2^2/FL2) makes even more sense. *** The formula indicated is not my theory but an indication of the INAF in their article regarding the photographic brightness of telescopes. What I tested, using the exposure time necessary to obtain a 90% peak on the histogram of a flat frame, is the brightness of a 130mm/F5.6 telescope compared to a 110mm/F4.8. The 110mm was brighter because the exposure time was less than the 130mm. From this we can deduce that the focal ratio, in photography, overwhelms the aperture. ***
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