Fast Scope vs Large Aperture [Deep Sky] Acquisition techniques · Clayton Ostler · ... · 125 · 5202 · 17

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This hobby tends to attract high end people, those with enough, drive, intellect and money to make imaging happen. It is always interesting to watch these discussions develop, usually the opinions here reveal the facts.

There are among us a number of very skilled optical engineers, I have known John for years and John is one of them. I second John's comments here and also recommend taking some time to go through his presentation. The radiometry of astrographs often defies common sense, seems to defy logic and often personal experience, But it is sound... I think you will find John’s presentation well worth your time..
find it at:  https://www.youtube.com/watch?v=HiJoqQp1qFI.

Dave Erickson:
This hobby tends to attract high end people, those with enough, drive, intellect and money to make imaging happen. It is always interesting to watch these discussions develop, usually the opinions here reveal the facts.

There are among us a number of very skilled optical engineers, I have known John for years and John is one of them. I second John's comments here and also recommend taking some time to go through his presentation. The radiometry of astrographs often defies common sense, seems to defy logic and often personal experience, But it is sound... I think you will find John’s presentation well worth your time..
find it at:  https://www.youtube.com/watch?v=HiJoqQp1qFI.

Thanks Dave.  And...for those of you who might not know, Dave is another one of the very highly skilled (and very talented) optical engineers on this site.  He is absolutely right that there are a lot of very accomplished folks on this site with incredible technical depth--far beyond my own.  We are all in it because we love imaging, astronomy, and working on getting better at something that is hard.  I am personally extremely impressed that so many folks without a technical background are involved with this hobby and over the years I've been blown away at how hard many of you work to understand the underlying physics, optics, engineering and statistical theory surrounding a lot of this stuff.  I try to put together presentations for various imaging venues that will help provide a baseline for discussions like this one.  As I said, if I ever screw it up, tell me.  If you don't understand it, ask.  I'm happy to try to explain it better.  I really cringe then I see misinformation and old wive's tales spreading like wildfire so let's try to avoid that.

John

I'm confused about the answers I get here. I was encouraged to download John Hayes's Excel sheet and play with it because I was thinking about this the "wrong way".

Well, I did. The Excel sheet only confirms every claim I have made in this thread.

- A C14 is indeed better than a RASA 8 (less noise @ same exposure time and sampling)

- A fast scope is indeed faster than a slow scope at the same focal length.

I can't see where Hayes I disagree with me.
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Arun H:

John Hayes:
I really cringe then I see misinformation and old wive's tales spreading like wildfire so let's try to avoid that.

Thanks John for your efforts and and also for your patience in previous discussions on these type of topics. The camera equation and your AIC presentation are,IMO, must reads and must understands for anyone making any type of claim here.

I think part of the problem is the quest for one magic number that says X is better than Y. It is much better to take the time to develop an intuitive understanding, based on optics and physics, of how all these things work together to produce an image (f ratio, aperture, pixel size, sensor dimensions, QE, %T etc.).

After watching that video three times, yes three almost four hours of my life. I can't say I understand a lot, you just have to understand that I'm not a physics guy. I'm a computer scientist and things that you say everyone should understand or simple equations for me are not simple. But. After watching this a few times I'm kind of disheartened because it basically says that increasing aperture will not really increase my signal except for in the case of single point objects. I don't think most astrophotographers struggle with capturing stars. My 40 mm FMA 180 capture stars fine it's all of the extended objects that we work harder to capture. After watching the video I believe I understand that that signal isn't really captured any better from larger aperture. I really hope I misunderstand. I understand that two telescopes with the same f ratio maintain the same signal strength, that is basically because they have different focal lengths. I'll keep reading and trying to figure this out. I apologize if I frustrated anyone I'm really not trying to make anybody mad, and I understand that I'm dealing with people a lot smarter than I am. If the upside of more aperture is limited to increase magnitude only I kind of feel like I just wasted a couple thousand dollars. But I'm not really seeing that in real life experience.

Clayton Ostler:
After watching this a few times I'm kind of disheartened because it basically says that increasing aperture will not really increase my signal except for in the case of single point objects.

That is not what he is saying, or if you think that's what he is saying, you have misunderstood him.

The flaw is in looking at focal ratio (or aperture) in isolation, without considering other parameters. 

• An increased aperture scope will, without question, capture more photons from a given area of object - for the sake of argument, let us say a fixed region in space around M-81 - than a smaller aperture scope. Aperture and only aperture dictates what number of photons per unit time are collected from this fixed area of object.
• Focal length dictates how these photons are spread out on the sensor plane, or the area on the sensor plane the image of a given object occupies. A short focal length scope will concentrate whatever captured photons on a smaller image area of the sensor than a long focal length scope. The area occupied by an object on the sensor plane does NOT depend on aperture, only on focal length.

What the above two means is that, while a large aperture scope may in fact collect more photons from a given object area in space, if it has a long focal length, these collected photons may result in smaller irradiance on the focal plane than a small aperture scope of short focal length because the larger number of captured photons are also spread out over a much larger sensor area.

The way to make use of the increased light gathering ability of large aperture scopes is to use larger pixels. When you do this, and when in fact you select your pixel size such that in both cases a single pixel represents the same area of object, then the signal captured with the larger scope will in fact be much greater than the small aperture scope. This is one reason why people use large aperture scopes.

As an example - even though Hubble is an f/24 (!!!) scope, it has an enormous aperture (2.4 meters), but uses enormous pixels (21 micron).

Clayton Ostler:
If the upside of more aperture is limited to increase magnitude only I kind of feel like I just wasted a couple thousand dollars. But I'm not really seeing that in real life experience.

It is not, and your experience is trying to tell you that. So is John, and so are others here. If the only point in larger scopes were to capture data from stars faster, Obstech wouldn't be loaded full of gigantic telescopes now would it?

Arun H:
The way to make use of the increased light gathering ability of large aperture scopes is to use larger pixels.

I'll add to this that larger pixels could just mean downsampling of the image in post-processing (assuming low read noise cameras), roughly equivalent of binning in the old CCDs.

So, again, if a given subject fits in your field of view of your camera with a large and small aperture scope regardless of f ratio, if you view the results AT THE SAME IMAGE SCALE (i.e. sampling the image from the larger aperture to match the image from the smaller aperture or vice versa), the subject will have better S/N from the larger aperture scope per a total capture time (assuming your individual sub-exposure lengths ensure the sky background is well above the read noise for each) 

If those scopes have the same focal length, that means the larger scope has a "faster" f ratio by definition and will capture both the subject (because of the larger aperture) and sky background (because of the faster f ratio) faster since everything will be at the same image scale (because of the same focal length)

If those scopes have the same f ratio, then the larger aperture one will have a longer focal length.  At 100% scale, the sky background will be similarly noisy between the 2 (because of the same f ratio).   Downsampling the image from the larger aperture scope to match the scale of that of the smaller aperture scope, your sky background will now be less noisy (because of the "larger pixels"/downsampling) and the subject will will have better S/N as well (because of the larger aperture and "larger pixels"/downsampling)

Clayton,

You've mentioned that you'd prefer to avoid a math-based answer, so let me answer by using an analogy that will be familiar to you.  If you've set up a video projector or spent time in movie theaters of differing screen sizes, you are intuitively aware that for a given brightness of the light source, a bigger projection area gets dimmer on the screen. You can set your home video projector to zoom in or out to create a bigger dimmer picture on the wall, or a brighter smaller one. The photons per second is the same either way, but they are spread out to a greater or lesser area on the wall, lowering or raising the photons per second per unit of area. This is why folks above are telling you that the FOV matters.

Yes, a given telescope aperture yields some absolute number of photons per unit of imaging time. But a longer focal length optic spreads those photons over a larger imaging area, and your camera sensor has pixels of a given size, so each sees a smaller portion of the object and receives fewer pixels per unit time vs. imaging with the same camera through a shorter focal length optic. 

Also, no one has mentioned transmission ratio vs. focal ratio. While the size of the focused field of view is governed by the focal ratio, if your optic has a central obstruction, the photons per unit time arriving at the camera sensor is lower that for an unobstructed aperture. When professional cinematographers calculate proper exposure, they are keeping this in mind and using the optic's specified transmission ratio. They'll use its focal ratio to know the field of view they will be capturing onto a give size film frame or imaging sensor. For example, Celestron calls its RASA-11 an f/2.2 optic, but as far as exposure is concerned, it's a T/2.5 optic because of its central obstruction. Nonetheless, it's a very fast imager.

(Asides: Also, these signal photons do not arrive like clockwork; each arrives independently with a time-randomized spread of arrival times: a mathy thing called a Poisson distribution. While these are true signal photons, the randomness appears as exposure variation--a form of noise cured by stacking more total exposure time--in each pixel of each subframe. When conventional non-astro photographers say that "high-pixel count sensors are noisier" or "don't work as well in nightscene photography", they are noticing the effect of this Poisson noise distribution, since the pixels are smaller, which enhances the effect of the randomness of the arrival times. To this noise you also are dealing with the non-signal noise sources of atmospheric glow, of thermal noise in the sensor pixels, and of readout and quantization noise in the process of analog to digital conversion. But we'll ignore all those since if you are just comparing the effects of one optic vs. another. But they all impact signal to noise ratio of your image.)

I dunno, does this answer assist you?

Regards,

Jim

First, Thank you so much for the well written answer. The fact that someone would write this, just to help another random guy out, is incredible.   

I love the projector example, and it is something I do understand. 
It also matches the reason why an f2 50 mm and f2 100 mm scope have the same brightness, which I think I now comfortably understand. 

I also now understand a couple items that are absolutes. 

1. Increased Aperture is the only way to increase to total volume of photons
2. Increased or decreased focal length is the variable that determines the given brightness of a light source. Because of equations I dont really understand but fully accept, I get the light spreads as it travels and the longer it travels the more it spreads making the signal weaker, and vice-versa. 
3. Focal Length is the ONLY contributor to FOV, I cant change the FOV by pumping more photons into something. 
4. Aperture is the ONLY contributor to total volume of Photons, I cant collect photons I dont have. 

But... The combined ratio of Aperture to Focal Length, does affect the given brightness, and this is why we are all hung up on Focal Ratio. 

Recap:
Adding Aperture increases photons 
Adding focal length decreases their brightness

                   and the inverse is equally true

Reducing Aperture decreases photons
Reducing  focal length increases their brightness. 

I think what I am seeing here is a bit of cross talk and splitting hairs, that has caused my confusion. 

I am going to say something controversial but it helps me understand in my mind at least. 

Focal Ratio, is just as the name it represents, the ratio of Aperture to Focal Length, for me its not really a "Thing" it is just a ratio/name. Representing what I consider the 2 important parts of this whole thing. 

The important parts of this whole analysis are the total Aperture and the total Focal Length and then we look at how the "Ratio" between the two affects the given brightness.  

Maybe this is why some people say "It all comes down to FOV"  because if you start with a desired FOV and determined that your goal is a specific FOV,

Then start working backwards from there, you could easily say "Focal Ratio is the most important thing, while maintaining the required FOV" 

Example: 
For a specific FOV on a fixed size sensor,  I can get this same FOV from ANY scope that has the required Focal Length, An F2 500mm and an F10 100mm have the same FOV.
But if I want to increase the brightness of this same FOV the only option is to up the Aperture. (Which in turn lowers the respective focal length if I am to keep the same FOV)
These 2 items are what represent the whole "Focal Ratio" fueling this long conversation. 

I think I have a better grasp on this now.   If I am misunderstanding, or saying anything incorrect please correct me.  

Let me give a scenario I am still trying to figure out though.........

Lets take F5 60mm and F7 115mm (if this seems oddly specific, its because it was what I used for my test)
Use same ASI533MC camera with 3.76 size pixel and photograph the whirlpool galaxy M51. Lets say 1 180 second exposure. 

The FOV is very different because of the 805 vs 300 focal lengths. But the general brightness, SNR ratio and image contrast, and detail from the F7 115 is just better to my eyes.  Not a little, but A LOT better.

What in the example above is different?

Clayton Ostler:

Clayton,

You've mentioned that you'd prefer to avoid a math-based answer, so let me answer by using an analogy that will be familiar to you.  If you've set up a video projector or spent time in movie theaters of differing screen sizes, you are intuitively aware that for a given brightness of the light source, a bigger projection area gets dimmer on the screen. You can set your home video projector to zoom in or out to create a bigger dimmer picture on the wall, or a brighter smaller one. The photons per second is the same either way, but they are spread out to a greater or lesser area on the wall, lowering or raising the photons per second per unit of area. This is why folks above are telling you that the FOV matters.

Yes, a given telescope aperture yields some absolute number of photons per unit of imaging time. But a longer focal length optic spreads those photons over a larger imaging area, and your camera sensor has pixels of a given size, so each sees a smaller portion of the object and receives fewer pixels per unit time vs. imaging with the same camera through a shorter focal length optic. 

Also, no one has mentioned transmission ratio vs. focal ratio. While the size of the focused field of view is governed by the focal ratio, if your optic has a central obstruction, the photons per unit time arriving at the camera sensor is lower that for an unobstructed aperture. When professional cinematographers calculate proper exposure, they are keeping this in mind and using the optic's specified transmission ratio. They'll use its focal ratio to know the field of view they will be capturing onto a give size film frame or imaging sensor. For example, Celestron calls its RASA-11 an f/2.2 optic, but as far as exposure is concerned, it's a T/2.5 optic because of its central obstruction. Nonetheless, it's a very fast imager.

(Asides: Also, these signal photons do not arrive like clockwork; each arrives independently with a time-randomized spread of arrival times: a mathy thing called a Poisson distribution. While these are true signal photons, the randomness appears as exposure variation--a form of noise cured by stacking more total exposure time--in each pixel of each subframe. When conventional non-astro photographers say that "high-pixel count sensors are noisier" or "don't work as well in nightscene photography", they are noticing the effect of this Poisson noise distribution, since the pixels are smaller, which enhances the effect of the randomness of the arrival times. To this noise you also are dealing with the non-signal noise sources of atmospheric glow, of thermal noise in the sensor pixels, and of readout and quantization noise in the process of analog to digital conversion. But we'll ignore all those since if you are just comparing the effects of one optic vs. another. But they all impact signal to noise ratio of your image.)

I dunno, does this answer assist you?

Regards,

Jim

First, Thank you so much for the well written answer. The fact that someone would write this, just to help another random guy out, is incredible.   

I love the projector example, and it is something I do understand. 
It also matches the reason why an f2 50 mm and f2 100 mm scope have the same brightness, which I think I now comfortably understand. 

I also now understand a couple items that are absolutes. 

1. Increased Aperture is the only way to increase to total volume of photons
2. Increased or decreased focal length is the variable that determines the given brightness of a light source. Because of equations I dont really understand but fully accept, I get the light spreads as it travels and the longer it travels the more it spreads making the signal weaker, and vice-versa. 
3. Focal Length is the ONLY contributor to FOV, I cant change the FOV by pumping more photons into something. 
4. Aperture is the ONLY contributor to total volume of Photons, I cant collect photons I dont have. 

But... The combined ratio of Aperture to Focal Length, does affect the given brightness, and this is why we are all hung up on Focal Ratio. 

Recap:
Adding Aperture increases photons 
Adding focal length decreases their brightness

                   and the inverse is equally true

Reducing Aperture decreases photons
Reducing  focal length increases their brightness. 

I think what I am seeing here is a bit of cross talk and splitting hairs, that has caused my confusion. 

I am going to say something controversial but it helps me understand in my mind at least. 

Focal Ratio, is just as the name it represents, the ratio of Aperture to Focal Length, for me its not really a "Thing" it is just a ratio/name. Representing what I consider the 2 important parts of this whole thing. 

The important parts of this whole analysis are the total Aperture and the total Focal Length and then we look at how the "Ratio" between the two affects the given brightness.  

Maybe this is why some people say "It all comes down to FOV"  because if you start with a desired FOV and determined that your goal is a specific FOV,

Then start working backwards from there, you could easily say "Focal Ratio is the most important thing, while maintaining the required FOV" 

Example: 
For a specific FOV on a fixed size sensor,  I can get this same FOV from ANY scope that has the required Focal Length, An F2 500mm and an F10 100mm have the same FOV.
But if I want to increase the brightness of this same FOV the only option is to up the Aperture. (Which in turn lowers the respective focal length if I am to keep the same FOV)
These 2 items are what represent the whole "Focal Ratio" fueling this long conversation. 

I think I have a better grasp on this now.   If I am misunderstanding, or saying anything incorrect please correct me.  

Let me give a scenario I am still trying to figure out though.........

Lets take F5 60mm and F7 115mm (if this seems oddly specific, its because it was what I used for my test)
Use same ASI533MC camera with 3.76 size pixel and photograph the whirlpool galaxy M51. Lets say 1 180 second exposure. 

The FOV is very different because of the 805 vs 300 focal lengths. But the general brightness, SNR ratio and image contrast, and detail from the F7 115 is just better to my eyes.  Not a little, but A LOT better.

What in the example above is different?

The difference might be is that you're not taking native resolution into account. Your 115mm has almost twice the resolving power of the 60mm. How much of that is captured depends on your sampling and seeing.

Rick Krejci:

Clayton Ostler:
Lets take F5 60mm and F7 115mm (if this seems oddly specific, its because it was what I used for my test)
Use same ASI533MC camera with 3.76 size pixel and photograph the whirlpool galaxy M51. Lets say 1 180 second exposure.

The FOV is very different because of the 805 vs 300 focal lengths. But the general brightness, SNR ratio and image contrast, and detail from the F7 115 is just better to my eyes. Not a little, but A LOT better.

What in the example above is different?

Are you viewing these images with M51 at the same size, or both at 100% native resolution on your screen?    At 100%, I'd assume the SNR of the f7 scope would be a little worse because of the f ratio being "slower".    But if you view M51 at the same scale side by side, the 115mm one will have better S/N and more detail because of the larger aperture.    The resampling negates the fact that the f ratio was slower by making "larger pixels".

Both at 100 percent. Obviously if I zoom in on the 60 mm scope pictures to get it to be the same size it's going to be blown up and scaled and that will never look good.

Clayton Ostler:
Focal Ratio, is just as the name it represents, the ratio of Aperture to Focal Length, for me its not really a "Thing" it is just a ratio/name. Representing what I consider the 2 important parts of this whole thing. 

The important parts of this whole analysis are the total Aperture and the total Focal Length and then we look at how the "Ratio" between the two affects the given brightness.

The reason focal ratio has the importance it does is that it is the factor that determines irradiance on the focal plane. And, in terrestrial photography, focal ratio is what is used to calculate exposure (for given ISO and shutter speed). A 35 mm focal length f/2 and 135 mm focal length f/2 lens will both give you identical exposure with the same camera. But everyone understands that these are two very different lenses used for very different purposes. On a full frame camera, a 135mm is a nice portrait lens, a 35mm is a nice walk around lens.

Astro work has similarities to this too. I'd use a short focal length scope for a widefield, and long focal length for galaxies or planetary nebulae. In each case, the focal length and sensor size determines the field of view. And for a fixed sensor and pre-determined field of view, the only way to increase signal for a given time is to increase aperture; which of course, also means a "faster" or smaller f ratio scope. Part of the confusion I think arises because fast, long focal length scopes are not very common (they are difficult and expensive to manufacture and correct for optical aberrations), but fast, long focal length camera lenses tend to be a bit more common. And of course, we tend to deal with much longer focal lengths in astro than in terrestrial photography which exacerbates the issue.