Re: Aperture vs Focal Ratio to determine exposure time

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):
Author of PEMPro V3:
Author of Astro-Physics V2 ASCOM Driver:
Author of PulseGuide:
Author of 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


From: ap-gto@... <ap-gto@...> on behalf of pnagy@... [ap-gto] <ap-
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.


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


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.


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