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)
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)
