Aperture vs Focal Ratio to determine exposure time


topboxman
 

Please see the following Cloudy Nights thread about aperture vs focal ratio:

https://www.cloudynights.com/topic/617363-speed-of-optics-example/

Personally I believe focal ratio helps determine imaging speed and aperture determines SNR. I know it gets complicated when optics mix with camera like image scale.

Let me know what you think?

Peter


Donald Rudny
 

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.


Roland Christen
 

I'm with Stan Moore up to a point. Aperture certainly makes a difference, more than F-ratio, but other things also affect your final image: resolution of small objects, whether or not you are undersampled, what field of view you wish to capture, etc.

Rolando



-----Original Message-----
From: Donald Rudny mkea13800@... [ap-gto]
To: ap-gto <ap-gto@...>
Sent: Sat, May 12, 2018 1:58 pm
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.




topboxman
 

Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter


---In ap-gto@..., <mkea13800@...> wrote :

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.


Bill Long
 

When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea was to show newer folks what happens when you simply change the aperture and hold all of the other values constant.






From: ap-gto@... on behalf of pnagy@... [ap-gto]
Sent: Saturday, May 12, 2018 1:17 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter



---In ap-gto@..., wrote :

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.


topboxman
 

Now you can sell your TOA 130 and get Absolute Encoder Kit.

Peter


---In ap-gto@..., <bill@...> wrote :

When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea was to show newer folks what happens when you simply change the aperture and hold all of the other values constant.






From: ap-gto@... <ap-gto@...> on behalf of pnagy@... [ap-gto] <ap-gto@...>
Sent: Saturday, May 12, 2018 1:17 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter



---In ap-gto@..., <mkea13800@...> wrote :

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.


Joe Renzetti <axnyslie@...>
 

I have done the same objects with the same camera and aperture at f/2, f/6.3, and f/10. It makes a very significant difference. It is not a "myth"

Joe

On Sat, May 12, 2018 at 5:21 PM, pnagy@... [ap-gto] <ap-gto@...> wrote:
 

Now you can sell your TOA 130 and get Absolute Encoder Kit.

Peter



---In ap-gto@..., wrote :

When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea was to show newer folks what happens when you simply change the aperture and hold all of the other values constant.






From: ap-gto@... <ap-gto@...> on behalf of pnagy@... [ap-gto] <ap-gto@...>
Sent: Saturday, May 12, 2018 1:17 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter



---In ap-gto@..., wrote :

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.



Bill Long
 

Hah! Not sure I will. With PEMPro I get less than 1" of PE and with PHD properly configured the mount pretty much slips into the background. I use Roland's method of calibrating the settings and it works wonderfully.


The one thing I found was with "fast switching for large deflections" disabled, I had some problems. Re-enabling it seems to have solved them and my mount is back to how it was when I first got it. 




From: ap-gto@... on behalf of pnagy@... [ap-gto]
Sent: Saturday, May 12, 2018 2:21 PM
To: ap-gto@...
Subject: Re: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Now you can sell your TOA 130 and get Absolute Encoder Kit.

Peter



---In ap-gto@..., wrote :

When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea was to show newer folks what happens when you simply change the aperture and hold all of the other values constant.






From: ap-gto@... on behalf of pnagy@... [ap-gto]
Sent: Saturday, May 12, 2018 1:17 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter



---In ap-gto@..., wrote :

Peter,

I’ve been researching this recently and found this article by Craig Stark that I also posted in the EAA section of CN.  I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn’t require much more exposure for the same object and certainly not 25 times more.  Stan Moore also has an article referred to in Craig’s article that states f ratio doesn’t matter for digital cameras as long as the signal stays well above the read noise.  It’s aperture that makes the difference.  It’s different for film.  Great topic.  I’ll be interesting to see what others think.


Ray Gralak
 

Hi Bill,

This topic has certainly been a hot potato many times over the years. :-)

There are two variables, aperture and focal length...

In the one case you tested, keeping focal length constant but decreasing focal ratio of course means that more aperture is used.
Light collection thus goes up approximately to the square of the f-ratio.

Alternatively, if you hold aperture fixed and lower the f-ratio you will get a wider field of view and for extended objects like
nebula and even galaxies exposure time is reduced with the lower f/ratio at the expense of image scale. For example, a 5" f/5 and
10" f/5 and exposed M51 each for 1 minute, the background noise and contrast would look about the same, but the image scale of the
5" scope would be about 1/2 the size of the 10" F/5.

But, if you compare 1 minute images using the same sensor from a 10" f/5 and a 10" f/10, the 10" f/10 will look far noisier because
a smaller area of the sky is spread over the same sensor.

I think Stan Moore states that you can down-sample such an image to 1/4 the size and get a similar S/N. Well, if you do the F10's
image will certainly not be better because the light is spread over more pixels which results in more collective readout and photon
noise. And, the image from the 10" f/5 has collected light from 4x the area of the sky, so it obviously collected a LOT more light
even though the aperture is the same between the two scopes.

So, for imaging *extended* objects (e.g., h-alpha nebula regions), I think f-ratio matters. No matter which sensor you use, I think
that more light is collected *per*pixel with a faster f-ratio no matter what the aperture. I think that this results in less
exposure time to get a usable image.

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Saturday, May 12, 2018 1:54 PM
To: ap-gto@...
Subject: Re: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm
aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same
target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would
spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea
was to show newer folks what happens when you simply change the aperture and hold all of the other values
constant.









________________________________

From: ap-gto@... <ap-gto@...> on behalf of pnagy@... [ap-gto] <ap-
gto@...>
Sent: Saturday, May 12, 2018 1:17 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get
similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel
sizes which changes image scale, then I can believe it.

I find Stan Moore's article very misleading and poorly written.

Peter




---In ap-gto@..., <mkea13800@...> wrote :


Peter,

I've been researching this recently and found this article by Craig Stark that I also posted in the EAA section of
CN. I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn't require much
more exposure for the same object and certainly not 25 times more. Stan Moore also has an article referred to in
Craig's article that states f ratio doesn't matter for digital cameras as long as the signal stays well above the read
noise. It's aperture that makes the difference. It's different for film. Great topic. I'll be interesting to see what
others think.

http://www.stark-labs.com/help/blog/files/FratioAperture.php
<https://eur03.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.stark-
labs.com%2Fhelp%2Fblog%2Ffiles%2FFratioAperture.php&data=02%7C01%7C%7Cd3f231c8215c4999fdc808d
5b8457815%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636617530860540905&sdata=IB3JmJqv
bIyoYrlBaCB4%2FrNkYuZVZ9Gy%2FBWNP9JRh0Y%3D&reserved=0>


Don


Worsel
 

f ratio is just a calculation.  You cannot turn a dial and change the f ratio, but leave everything else alone.  You have to change either aperture or focal length.  


Bryan


Ray Gralak
 

f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.
But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Saturday, May 12, 2018 3:21 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.


Bryan


Roland Christen
 

Size wise the Airy disc stays the same as far as microns (or pixel size). Resolution wise it reduces in arc seconds as the aperture increases.

Rolando



-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto]
To: ap-gto
Sent: Sat, May 12, 2018 5:45 pm
Subject: RE: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time

> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> have to change either aperture or focal length.

But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Saturday, May 12, 2018 3:21 PM
> To: ap-gto@...
> Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
>
>
> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> have to change either aperture or focal length.
>
>
> Bryan
>
>



------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@...>
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Bill Long
 

I completely understand this all. My post there was not for my own edification. :) It spiraled out of control though... 




From: ap-gto@... on behalf of 'Ray Gralak (Groups)' groups3@... [ap-gto]
Sent: Saturday, May 12, 2018 2:50 PM
To: ap-gto@...
Subject: RE: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
 
 

Hi Bill,

This topic has certainly been a hot potato many times over the years. :-)

There are two variables, aperture and focal length...

In the one case you tested, keeping focal length constant but decreasing focal ratio of course means that more aperture is used.
Light collection thus goes up approximately to the square of the f-ratio.

Alternatively, if you hold aperture fixed and lower the f-ratio you will get a wider field of view and for extended objects like
nebula and even galaxies exposure time is reduced with the lower f/ratio at the expense of image scale. For example, a 5" f/5 and
10" f/5 and exposed M51 each for 1 minute, the background noise and contrast would look about the same, but the image scale of the
5" scope would be about 1/2 the size of the 10" F/5.

But, if you compare 1 minute images using the same sensor from a 10" f/5 and a 10" f/10, the 10" f/10 will look far noisier because
a smaller area of the sky is spread over the same sensor.

I think Stan Moore states that you can down-sample such an image to 1/4 the size and get a similar S/N. Well, if you do the F10's
image will certainly not be better because the light is spread over more pixels which results in more collective readout and photon
noise. And, the image from the 10" f/5 has collected light from 4x the area of the sky, so it obviously collected a LOT more light
even though the aperture is the same between the two scopes.

So, for imaging *extended* objects (e.g., h-alpha nebula regions), I think f-ratio matters. No matter which sensor you use, I think
that more light is collected *per*pixel with a faster f-ratio no matter what the aperture. I think that this results in less
exposure time to get a usable image.

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Saturday, May 12, 2018 1:54 PM
> To: ap-gto@...
> Subject: Re: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
>
>
> When I posted the data on this thread (I am the OP) I was just trying to show the difference between a 130mm
> aperture refractor at 1000mm focal length, and a 254mm aperture Newt at 1000mm focal length, on the same
> target with the same camera (and of course my trusty AP1100 mount, but that isnt relevant). I had no idea it would
> spark so much discussion about the topic, although a lot of it has been very interesting to see unfold. The idea
> was to show newer folks what happens when you simply change the aperture and hold all of the other values
> constant.
>
>
>
>
>
>
>
>
>
> ________________________________
>
> From: ap-gto@... on behalf of pnagy@... [ap-gto] > gto@...>
> Sent: Saturday, May 12, 2018 1:17 PM
> To: ap-gto@...
> Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
>
>
> Are you using same camera at F/10 and F/2? If you are using same camera, I find it hard to believe you get
> similar performance at similar exposure times at f/10 and f/2. If you are using different cameras with different pixel
> sizes which changes image scale, then I can believe it.
>
> I find Stan Moore's article very misleading and poorly written.
>
> Peter
>
>
>
>
> ---In ap-gto@..., wrote :
>
>
> Peter,
>
> I've been researching this recently and found this article by Craig Stark that I also posted in the EAA section of
> CN. I was interested because I found that using my C11 at f/10 versus f/2 with Hyperstar doesn't require much
> more exposure for the same object and certainly not 25 times more. Stan Moore also has an article referred to in
> Craig's article that states f ratio doesn't matter for digital cameras as long as the signal stays well above the read
> noise. It's aperture that makes the difference. It's different for film. Great topic. I'll be interesting to see what
> others think.
>
> http://www.stark-labs.com/help/blog/files/FratioAperture.php
> > labs.com%2Fhelp%2Fblog%2Ffiles%2FFratioAperture.php&data=02%7C01%7C%7Cd3f231c8215c4999fdc808d
> 5b8457815%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636617530860540905&sdata=IB3JmJqv
> bIyoYrlBaCB4%2FrNkYuZVZ9Gy%2FBWNP9JRh0Y%3D&reserved=0>
>
>
> Don
>
>


Ray Gralak
 

Size wise the Airy disc stays the same as far as microns (or pixel size).
Resolution wise it reduces in arc seconds as the aperture increases.

Rolando
Yes, that means that despite having the same size Airy disks, the 200" F/3 scope I mentioned will out-resolve the 2" F/3 scope, as it should.

But another interesting point is that at any given aperture the maximum theoretical resolution is the same at all f-ratios. That is true because numerical f-ratio is directly proportional to Airy disk size. Decreasing f/ratio decreases Airy disk size, so smaller pixels could be utilized to recover resolution.

So, at any given aperture faster (lower numerically) f-ratio telescopes can take advantage of smaller pixel cameras to get equal resolution to a higher f-ratio telescope *and* yet capture a larger sky area on the same size sensor.

So, for imaging it is usually better to choose the "fastest" scope at any given aperture. You can make up the resolution by choosing a camera with smaller pixels, assuming one is available. Of course there are practical limits to how fast a scope can be made as well as the field flatness and quality of the image.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Saturday, May 12, 2018 5:14 PM
To: ap-gto@...
Subject: Re: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



Size wise the Airy disc stays the same as far as microns (or pixel size). Resolution wise it reduces in arc seconds
as the aperture increases.

Rolando




-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Sat, May 12, 2018 5:45 pm
Subject: RE: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time

f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.
But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to
f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-
physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


-----Original Message-----
From: ap-gto@... [mailto:ap-gto@... <mailto:ap-gto@...?> ]
Sent: Saturday, May 12, 2018 3:21 PM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
have to change either aperture or focal length.


Bryan



------------------------------------
Posted by: "Ray Gralak &#92;(Groups&#92;)" <groups3@...>
------------------------------------

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Worsel
 

Ray

The Airy disc size is a function only of wavelength and aperture. If we consider the ability to separate (resolve) two close objects, using the Rayleigh Criteria, and make some assumptions, then minimum angular separation equation can be manipulated to show the f ratio.  This is because both aperture and focal length are in the underlying equation, due to the assumptions about the angular separation.  

See Cameras under


A more accurate statement is

A 200" f/3 and a 2" f/3 can both resolve two objects separated by the same angular distance.

The former will be a 67" aperture (!) and the latter will be 6.7".  The Airy discs for each will be different, but the ability to resolve is the same.

Bryan


---In ap-gto@..., <groups3@...> wrote :

> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> have to change either aperture or focal length.

But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma



Ray Gralak
 

Brian,

The Airy disc size is a function only of wavelength and aperture.
The *resolution* resulting from the size of the Airy disk is a function of wavelength and aperture.

The actual physical size in microns of the Airy disk is proportional to F-ratio and is independent of aperture.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma


-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Sunday, May 13, 2018 8:23 AM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



Ray

The Airy disc size is a function only of wavelength and aperture. If we consider the ability to separate (resolve)
two close objects, using the Rayleigh Criteria, and make some assumptions, then minimum angular separation
equation can be manipulated to show the f ratio. This is because both aperture and focal length are in the
underlying equation, due to the assumptions about the angular separation.

See Cameras under

https://en.wikipedia.org/wiki/Airy_disk


A more accurate statement is

A 200" f/3 and a 2" f/3 can both resolve two objects separated by the same angular distance.


T he former will be a 67" aperture (!) and the latter will be 6.7". The Airy discs for each will be different, but the
ability to resolve is the same.

Bryan



---In ap-gto@..., <groups3@...> wrote :



> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone.
You
> have to change either aperture or focal length.



But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to
f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-
physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma





Ray Gralak
 

Brian,

To follow up on my last post, here's one link that shows the Airy disk size based on f-ratio:

https://www.edmundoptics.com/resources/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/

The Airy disk size (in microns) must vary with f-ratio so that the theoretical maximum resolution of an optic of given aperture stays constant independent of f-ratio.

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Sunday, May 13, 2018 8:23 AM
To: ap-gto@...
Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time



Ray

The Airy disc size is a function only of wavelength and aperture. If we consider the ability to separate (resolve)
two close objects, using the Rayleigh Criteria, and make some assumptions, then minimum angular separation
equation can be manipulated to show the f ratio. This is because both aperture and focal length are in the
underlying equation, due to the assumptions about the angular separation.

See Cameras under

https://en.wikipedia.org/wiki/Airy_disk


A more accurate statement is

A 200" f/3 and a 2" f/3 can both resolve two objects separated by the same angular distance.


T he former will be a 67" aperture (!) and the latter will be 6.7". The Airy discs for each will be different, but the
ability to resolve is the same.

Bryan



---In ap-gto@..., <groups3@...> wrote :



> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone.
You
> have to change either aperture or focal length.



But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to
f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-
physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma





Worsel
 

Figure 1: Diffraction Increases as the Imaging Lens Iris is Closed (f/# Increases)
In other words, it's the aperture.

P.S.  I suggest that we can agree to disagree and let this go.

Bryan


---In ap-gto@..., <groups3@...> wrote :

Brian,

To follow up on my last post, here's one link that shows the Airy disk size based on f-ratio:

https://www.edmundoptics.com/resources/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/

The Airy disk size (in microns) must vary with f-ratio so that the theoretical maximum resolution of an optic of given aperture stays constant independent of f-ratio.

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma



Cheng-Yang Tan
 

Hi guys,
   I think, at least for myself, it is much more intuitive to think of f/D as the number of photons we can catch in a pixel, i.e. f/D can be thought of as the number of photons caught per pixel.

So, given an f/D for a telescope and for the *same* camera, the image has to have identical brightness per pixel because the number of photons caught per pixel is the same. Therefore, intuitively, a telescope that has twice the diameter that I have it has got to collect 4 times the photons. But if they have the same f/D number, then to keep the number of photons per pixel the same, the image must be twice as big, i.e. 4 times the area. This immediately tells me that I have twice the resolution with the twice diameter telescope, but each pixel catches the same number of photons.

Similarly, Ray's example of using a smaller pixel size is immediately explained with the above intuitive argument. If the dimensions of the pixel is halved, this means that the number of photons per pixel is quartered and so the f/D number has to increase by 2. i.e. an f/D = 6 becomes equivalently an f/D=12.

Hopefully I did the above right :)

cytan






On Sunday, May 13, 2018, 1:29:17 AM CDT, 'Ray Gralak (Groups)' groups3@... [ap-gto] wrote:


 

> Size wise the Airy disc stays the same as far as microns (or pixel size).
> Resolution wise it reduces in arc seconds as the aperture increases.
>
> Rolando

Yes, that means that despite having the same size Airy disks, the 200" F/3 scope I mentioned will out-resolve the 2" F/3 scope, as it should.

But another interesting point is that at any given aperture the maximum theoretical resolution is the same at all f-ratios. That is true because numerical f-ratio is directly proportional to Airy disk size. Decreasing f/ratio decreases Airy disk size, so smaller pixels could be utilized to recover resolution.

So, at any given aperture faster (lower numerically) f-ratio telescopes can take advantage of smaller pixel cameras to get equal resolution to a higher f-ratio telescope *and* yet capture a larger sky area on the same size sensor.

So, for imaging it is usually better to choose the "fastest" scope at any given aperture. You can make up the resolution by choosing a camera with smaller pixels, assuming one is available. Of course there are practical limits to how fast a scope can be made as well as the field flatness and quality of the image.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Saturday, May 12, 2018 5:14 PM
> To: ap-gto@...
> Subject: Re: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
>
>
> Size wise the Airy disc stays the same as far as microns (or pixel size). Resolution wise it reduces in arc seconds
> as the aperture increases.
>
> Rolando
>
>
>
>
> -----Original Message-----
> From: 'Ray Gralak (Groups)' groups3@... [ap-gto]
> To: ap-gto > Sent: Sat, May 12, 2018 5:45 pm
> Subject: RE: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
> > f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> > have to change either aperture or focal length.
>
> But, f-ratio has some importance in physics.
>
> For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to
> f-ratio? It gets smaller proportional to f-ratio.
>
> So, a 200" f/3 has the same size Airy disk as a 2" f/3.
>
> -Ray Gralak
> Author of APCC (Astro-Physics Command Center): http://www.astro-
> physics.com/index.htm?products/accessories/software/apcc/apcc
> Author of PEMPro V3: http://www.ccdware.com
> Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
> Author of PulseGuide: http://www.pulseguide.com
> Author of Sigma: http://www.gralak.com/sigma
>
>
> > -----Original Message-----
> > From: ap-gto@... [mailto:ap-gto@... ]
> > Sent: Saturday, May 12, 2018 3:21 PM
> > To: ap-gto@...
> > Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
> >
> >
> >
> > f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> > have to change either aperture or focal length.
> >
> >
> > Bryan
> >
> >
>
>
>
> ------------------------------------
> Posted by: "Ray Gralak \(Groups\)"
> ------------------------------------
>
> To UNSUBSCRIBE, or for general information on the ap-gto list
> see http://groups.yahoo.com/group/ap-gto
> ------------------------------------
>
> Yahoo Groups Links
>
>
>
>
>
>


dan kowall
 

It is important to remember that the number of photons collected from an object is solely dependent upon the aperture of the telescope.
If you are going to 'compare' images of an object made with equal aperture telescopes, one with a lower f-ratio and one with a higher f-ratio then you should compare sky-limited subexposures. These two groups of subexposures will obviously have different times to become sky-limited but both will have buried the readout and photon noise and both will have collected equal numbers of object photons.

dan kowall


--------------------------------------------

On Sat, 5/12/18, 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...> wrote:





.... I think Stan Moore states that you can down-sample such an
image to 1/4 the size and get a similar S/N. Well, if you do
the F10's

image will certainly not be better because the light is
spread over more pixels which results in more collective
readout and photon

noise. And, the image from the 10" f/5 has collected
light from 4x the area of the sky, so it obviously collected
a LOT more light

even though the aperture is the same between the two
scopes.



So, for imaging *extended* objects (e.g., h-alpha nebula
regions), I think f-ratio matters. No matter which sensor
you use, I think

that more light is collected *per*pixel with a faster
f-ratio no matter what the aperture. I think that this
results in less

exposure time to get a usable image.



Best regards,



-Ray Gralak

Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc


Author of PEMPro V3: http://www.ccdware.com

Author of Astro-Physics V2 ASCOM Driver:
http://www.gralak.com/apdriver

Author of PulseGuide: http://www.pulseguide.com

Author of Sigma: http://www.gralak.com/sigma




Posted by: "Ray Gralak &#92;(Groups&#92;)"
<groups3@...>




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