Fast Scope vs Large Aperture [Deep Sky] Acquisition techniques · Clayton Ostler · ... · 125 · 5204 · 17

I tried to get some feedback on another forum but it was crickets so I am going to try here where there is good conversation.

So I have literally spent hours reading about this the last few days. I am not nearly as smart as many of the people in this conversation. But here are some statements I want to make and let others react to. 

Focal Ratio: What this actually represents is the intensity of the gathered light on the focal point based on the total available light ie Aperture
(I think most of call this speed, and in the case of AP, this light intensity is focused on the camera sensor)

Stay with me before you start to rant. Example

If I have 2 80mm telescopes one is f10 and the other is f5 the equal aperture ensures that I gather the same amount of total photos in the same amount of time, however an F10 will produce less focused/intensity of the respective patch of sky "light" on the camera sensor than that of the F5 scope. 

Yes I get the FOV changes too, but to illustrate the point I am going to pretend that we don't care about the FOV and that the sensor and pixel size are constants, to make it even easier and to limit comments about "image scale and resolution"  lets say this is a fixed sensor size and a single pixel we are looking at. 

So far I haven't said anything controversial, but….. its not the whole story. 

For the sake of conversation lets assume we are going to capture 60 seconds of light onto a single fixed size camera sensor. 

An F5 scope with 100mm of Aperture and and F5 scope with 40mm have very different quantities of light (total photons) to deal with as a starting point. 
The speed or ability to intensify the available light is the same,  but the available photos to focus/condense/intensify onto the sensor is very very different. 

For a single star or consistent light source this might be less relevant because the light is a single point and generally constant enough that the difference is negligible, but for extended objects, like nebula and galaxies, the light is not constant and is not single point.  

This makes a huge difference. So for the given time of 60 seconds we want as many photons as possible and then we want to focus/condense/intensify these photons onto the respective sensor as efficiently as possible. 

Drastic Example: An f4 20mm scope does not have nearly the available light to focus/intensify/condense onto a sensor as and F7 200mm scope. One would be crazy to suggest that the small 40mm it will capture better images than the F7 200 just "because its faster".

I get that the angle and speed are better on the f4, so it does much better at intensifying the light that is has, but by nature of physics it has less light to work with.

No telescope or camera can create an image out of light that is doesn't have, and a larger aperture is the only way to get more photons to work with. 

The statement that only f ratio matters, only applies to constant light sources, like individual star, however most of what we capture in space and look at,  is not a constant light source and it not single point. so we need the biggest buckets to capture photos we can get while still being practical and efficient allowing us to focus/condense/intensify this light onto the camera sensor.  

 There is way better math than what I am suggesting right now,  but lets use common sense and photography numbers. 

In photography a measure of light is called an f-stop but its very similar to a focal ratio (speed)

Fact: Aperture is expediential, this means doubling the surface area of an objective or mirror will double the available photos and drop the required f-stop by 1 to get the same quantity of photons in the same time if all other factors remain the same. 

Increasing the F-stop (or focal ratio) 

F-Stops are also loosely expediential, Changing from an F2 to F2.8 essentially halves the intensity of the photons reaching the sensor. 

These 2 things offset each other, (Don't start in on image scale and resolution at this point of the conversation, it just muddies the water)

Sure we want speed, but we also want as many photons as possible, because time is the limiting factor.  All the speed on the planet cant intensify light that isn't there.  

The offsets between aperture and focal ratio are complicated to measure, but they are real. 

Lets not pretend that my FMA180 (40mm) F4.5 is going to capture/condense/intensify the same number of photons onto a camera sensor as the AT115EDT with a reducer at F5.5   

You can tell yourself whatever you want to feel good about your smaller imaging scope, but physics are not changeable, we cant capture photons that we don't have, not matter how fast the scope is. 

We could create an f1 5mm straw and point it at the sky, but it wont have near the photons available to condense/focus/intensify as a  f5 200mm 

I feel like there's a bit of unrealistic credit given to focal ratio, and suggesting its all that matters is missing something important (the total photons available to try and capture)

Assume a given target that fits in the field of view of a small aperture scope and a large aperture scope (lets say M66).   And assume a camera with very low read noise and thermal noise.

If you expose that object for a given time on each and view the results of that target at the same magnification, the one taken with the larger aperture scope will have better S/N and resolution at that same magnification independent of f ratio.    That "same magnification" part is important part.     I did this very experiment 20 years ago below.    The left (12.5" f4.5) and top-right (6" f3.3) are native resolution images and the bottom-right is the left image down-sampled to match the smaller aperture but faster f ratio scope.   The 6" was a Tak Epsilon 160 and the 12.5" scope was a homemade that wasn't all that great (you can see that in the left image).

If you have the same focal length (and thus magnification), the faster scope will give better results, but that's mainly because it would have a larger aperture.

Small scopes are great for wide field of views that larger scopes can't do in one shot.  

I'm not sure what you're driving at with your original post. Perhaps focusing (:-) your question down to a single sentence or paragraph would make it a bit easier to answer?

Tony Gondola:
I'm not sure what you're driving at with your original post. Perhaps focusing (:-) your question down to a single sentence or paragraph would make it a bit easier to answer?

Thats fair. 
Am I wasting money on buying a bigger aperture scope if it is slower than my faster/smaller aperture scope? 

Disregard FOV from the equation.

Enter etendue : Etendue - Wikipedia

Incidentally, the rate of arrival of photons is independent from the specifics of the optical system (assuming no diffusion or absorption takes place) or the aperture used to collect said flux. 

What matter in terms of efficiency (amount of photons collected in a given time) is the ratio of the solid angle subtended by two optical systems against the ratio of the square of the two apertures used to collect said flux.

Clayton Ostler:
I think what I am seeing here is that the signal (information) on the larger aperture is stronger, making a better SNR. Do you agree with that?

If you're talking sky background or extended nebulosity, the signal per area of the sensor (say, a pixel) is determined by f ratio.   My example makes it look like the sky background may be better with the left pic than the top-right both at full res, but that's probably where I set the black point or other variable.   It should be a little worse all else being equal.    But the lower-right image's sky background is far better than the upper-right.   

The smaller, faster scope will be able to capture deeper IFNs and faint nebulosity per given time at the entire image level because of it's faster f ratio, but as you zoom in the details fuzz up with a smaller scope.

The signal quality and resolution of  say a single galaxy as a whole is more determined by aperture, more meaningful for smaller targets.    Normally, longer, slower scopes will have more pixels on that target than faster, smaller scopes.   But if you bin or resample like I did, the target will be better resolved and have better signal with the larger scope.    But that's at the sacrifice of overall field of view.   But if your target fits with the larger scope, it will normally be better.    But you may have to expose longer individual subframes with a slower f ratio to get the sky background well above the read noise, which may be more challenging at a longer focal length.     So maybe 10x 5 minutes with the slower scope rather than 25x 2 minutes with the faster one.

So one's not inherently "better", they are just better at different things.  Which is why different scopes exist.

Clayton Ostler:

Tony Gondola:

Clayton Ostler:

Tony Gondola:
I'm not sure what you're driving at with your original post. Perhaps focusing (:-) your question down to a single sentence or paragraph would make it a bit easier to answer?

Thats fair. 
Am I wasting money on buying a bigger aperture scope if it is slower than my faster/smaller aperture scope? 

Disregard FOV from the equation.

Absolutely not! The idea of using the smaller aperture and just cropping in instead of going with a larger aperture, even if it's slower, just doesn't make any sense.

Thanks, although I still don't really understand I'm feeling better about the money I just spent.

You won't regret it...

Is it possible to get both? I'm currently a big fan of Newts, from f2.8 Sharpstar 15028HNT to F4 Skywatcher Quattro (8'', 10''). They are really a bit more friendly to light pollution area APs. I also have a RC8 and Edge11, now I may still use Edge11 for really small stuffs but won't use RC8 cause the data collection is really too slow.

Yingtian

Clayton Ostler:

Tony Gondola:
I'm not sure what you're driving at with your original post. Perhaps focusing (:-) your question down to a single sentence or paragraph would make it a bit easier to answer?

Thats fair. 
Am I wasting money on buying a bigger aperture scope if it is slower than my faster/smaller aperture scope? 

Disregard FOV from the equation.

Here's the thing: you simply cannot "disregard FOV". The FOV is the thing that makes all this math make sense. In your original post, you call out a theoretical f/5 100mm scope (500mm focal length) and an f/5 40mm scope (200mm focal length). You are correct in saying that they receive the same intensity of light per area, and this is precisely because of the larger field of view.

Consider for a second, a single telescope that is f/5 and has a 100mm aperture. You take this telescope and add a 2x barlow, which makes it f/10. The intensity of the light has decreased because the focal length has increased. The objective still receives the same number of photons per unit time, but it is more zoomed in, and as such the light is less intense. This is, I assume, why you have received animosity elsewhere for saying to disregard field of view. This is also the reason that the f/5 40mm has the same intensity of light as the f/5 100mm. Because the 40mm telescope only has 200mm focal length, it is zoomed out when compared to the 100mm that has 500mm focal length. This means that even though there are less photons, because the total irradiance projected onto the sensor is different, it balances out.

Charles Bracken explains this well in his book "The Deep Sky Imaging Primer", here is an excerpt from his section on this very topic:
"The claim is that it is a myth that lower focal ratios lead to shorter exposures. Because only aperture determines the number of photons gathered per unit time from a given object. The opposing position notes that focal ratio uniquely determines the total irradiance projected onto the sensor, regardless of aperture: an f/8 telephoto lens with an 80mm focal length and an f/8 refractor with an 800mm focal length will go equally 'deep', revealing the same dim features, but at different sizes. And experience bears this out, as anyone who has ever used a focal reducer knows that they clearly brighten the image. Both sides of this argument are correct, but they are defining different goals—brightness of an object (i.e., per angular area of sky, sometimes called 'object SNR') vs, brightness per spatial area as projected onto the sensor (i.e., per photosite)."

What you trade off when you go to a longer focal length with a larger aperture (in your case, f/7 instead of f/5) is more detail. In astrophotography, you have to balance SNR from focal ratio with detail from aperture. This is why long (f/15 or greater) SCT's are popular for planetary photography; the planets are very bright with lots of very fine detail, so the scale is tipped towards the side of detail and aperture over SNR. Your 60mm f/5 scope only packed a 300mm focal length, if you compare that to a massive but slow scope like an Edge HD 8 at native f/10, despite the image on the Edge HD being dimmer per unit time, it is more detailed because of the larger (200mm) aperture and longer (2000mm) focal length. These are the sorts of trade offs you have to make, and the key is finding the balance where you are most satisfied with both the detail in your images as well as the time required to bring out dim details.

In my club we recently did a comparison - horse head and flame shot with various small APOs vs larger ones such as my MN65 and a GSO 10” RC, with camera sensors matched to give similar FOV. 

In just 15 minutes the two larger scopes produced better shots than the smaller APOs did in an hour or two. Aperture does count.

Here area couple of references:

John Hayes's outstanding presentation:  https://www.youtube.com/watch?v=HiJoqQp1qFI

His description of the camera equation: https://www.astrobin.com/forum/post/163689/

My use of this equation in a specific case, including the effect of resizing the final image; I also worked out the case in microscopy in that same post: https://www.astrobin.com/forum/post/163764/

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.

An F/2 telescope is 25x faster than a F/10 scope with the same focal length.

A F/2 scope has a 5x wider field (25x larger area) field than a F/10 scope with the same aparture.

(All things equal)