Aperture, F-Ratio, and Exposure Time

Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4 reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Thanks,
Stephen Paul

"Stephen Paul" wrote in message
...
Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the
previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4
reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Hi,
The exposure time depends on square of the f/ratio and the aperture is
practiclaly without importance because the f/ratio represents somewhere the
"brightness of the object".
E.g. For a similar diameter, a f/1.5 aperture is 28.44 times faster than a
f/8 bec. (8/1.5)^2.
So the most important in your calculation is the f/ratio not the diameter.
The diameter will only affect the resolution (if the seeing is fine).
Other examples at
http://www.astrosurf.com/lombry/digi...-examples3.htm
See also
http://www.astrosurf.com/lombry/repo...0time%20accord
ing%20the%20fratio

Thierry
http://www.astrosurf.com/lombry


Thanks,
Stephen Paul

Hello,

To determine exposure time, focal ratio is the important number. An F/5
scope will always require longer exposure time than an F/4, regardless of
the aperture. It is expected that the F4 scope will require less exposure
time than the F7.5 scope.

Focal length determines image scale. Given the same camera, the same
object, and the same sky conditions, a 12" F4 will require the exact same
exposure time as an 8" F4, but 8" will have a wider field due to it's
shorter focal length.

I hope that this helps,
-Wade

"Stephen Paul" wrote in message
...
Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the
previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4
reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Thanks,
Stephen Paul

I thought for a point source (star) and a plate scale (focal length) matched so
that
all the starlight falls in one pixel the exposure goes with the inverse of the
area or D^2
and for an extended object (sky, moon etc.) the exposure goes with 1/F#^2

Dan

"Wade A. Hilmo" wrote:

Hello,

To determine exposure time, focal ratio is the important number. An F/5
scope will always require longer exposure time than an F/4, regardless of
the aperture. It is expected that the F4 scope will require less exposure
time than the F7.5 scope.

Focal length determines image scale. Given the same camera, the same
object, and the same sky conditions, a 12" F4 will require the exact same
exposure time as an 8" F4, but 8" will have a wider field due to it's
shorter focal length.

I hope that this helps,
-Wade

"Stephen Paul" wrote in message
...
Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the
previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4
reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Thanks,
Stephen Paul

"Stephen Paul" wrote in message ...
Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4 reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

F ratio determines exposure time. Aperture
determines resolution. Put slightly differently
for the same f ratio a bigger lens/mirror
will give a bigger image for the same exposure.

--

Hilton Evans
---------------------------------------------------------------
Lon -71° 04' 35.3"
Lat +42° 11' 06.7"
---------------------------------------------------------------
Webcam Astroimaging
http://home.earthlink.net/~hiltoneva...troimaging.htm
---------------------------------------------------------------
ChemPen Chemical Structure Software
http://www.chempensoftware.com

On Wed, 23 Mar 2005 10:28:09 -0500, "Stephen Paul"
wrote:

Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4 reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

The f/4 system will deliver an image of equivalent exposure to the f/5
system in less time.

But focal ratio is not a very useful concept when you are talking about
astronomical imaging. There are several reasons for that. One is that
focal ratio by itself doesn't give you much information. The most
important thing you need to know is the focal length, followed by the
aperture (then, of course, you can calculate the focal ratio). But the
main issue is that you don't compose your shots based on focal ratio-
you compose them based on focal length. When you plan a shot, is your
fundamental concern the resolution and field of view, or is it how long
the exposure will be? I would hope it is the first, and that's why you
need to be concerned with the focal length only. In your example above,
the f/4 system requires a shorter exposure time- but only because the
image is smaller on the sensor. The two systems do not produce
equivalent images, and simply comparing them on the basis of one value-
focal ratio- is misleading.

A good astroimager is concerned first and foremost with image scale-
finding a balance between field of view and resolution. Once this
calculation is made (which is determined solely by focal length), he
will select a system with the largest aperture he can. Of course, once
the focal length is fixed, increasing the aperture increases the amount
of light, and therefore decreases the exposure time.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Hilton Evans" wrote in message
.net...
"Stephen Paul" wrote in message
...

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4
reflector
(800mm fl).

What is the relationship?

F ratio determines exposure time. Aperture
determines resolution. Put slightly differently
for the same f ratio a bigger lens/mirror
will give a bigger image for the same exposure.

That was too easy. :-)

FWIW, I do also understand there is an important relationship between
CMOS/CCD sensor pixel size, and telescope focal length. I was only here
trying to ascertain the exposure time relationship with aperture and ratio.

Thanks a lot,
-Stephen

"Stephen Paul" wrote in message
...
Picking up a related conversation we left off a while ago (or at least I did),
I'd like to discuss the subject header a bit further. In the previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4 reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which is
going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Thanks,
Stephen Paul


In Astronomy, because of the low light levels, even for planets for most
telescopes, f/ratio doesn't matter as much as aperture. This is probably how
daytime and even nighttime photographers get it wrong when it comes to taking
Astronomy images. I'm not saying f/ratio doesn't count when it comes to
exposure times, but it's a secondary consideration. The main contribution that
f/ratio has to astronomy imaging is to adjust the field of view and consequently
the magnification that can be achieved. So for instance, if you want details on
small deep sky objects, the planets or the moon, you need to consider the
f/ratio along with the aperture. So in the specific instance of web cameras and
planet imaging, you need to have focal length (not necessarily focal ratio) to
increase image size, but if you don't have enough aperture to make a bright
enough image, then the exposure times are not going to be long enough to assist
in eliminating seeing distortion through stacking.

Hope this helps a little.
--
Sincerely,
--- Dave
----------------------------------------------------------------------
It don't mean a thing
unless it has that certain "je ne sais quoi"
Duke Ellington
----------------------------------------------------------------------

"Chris L Peterson" wrote in message
...
On Wed, 23 Mar 2005 10:28:09 -0500, "Stephen Paul"
wrote:

A good astroimager is concerned first and foremost with image scale-

So I'm a bad astroimager? ;-)

I should append the title of the thread to include, "with a DSLR".

I'm using an uncooled, IR modified Canon 300D. My first concern is keeping
noise down, and that means the shortest possible exposure times, at the
lowest possible ISO setting.

I'm not doing high resolution deep sky stuff.

What I need to know first, is what makes for a short exposure. Once I know
that, I can adjust any other parameter to get the required image scale (to
the capacity of my mount).

-Stephen

"I'm using an uncooled, IR modified Canon 300D. My first concern is
keeping
noise down, and that means the shortest possible exposure times, at the

lowest possible ISO setting."

Google Christian Buil

He shows that using ISO 400 and exposing to the sky fog limit AND THEN
doing astro-imaging post processing give a better signal to noise ratio
than
using either higher or lower ISO. I think he indicates that 5 minutes
is
optimal for S/N.

Astro-image processing is more involved that dSLR-typical
postprocessing.
It requires dark frame subtraction, flat fielding, and a couple of
other steps
before you get to regular dSLR postprocessing.

"What I need to know first, is what makes for a short exposure."

F/ratio, the bigger (lower numerically) the shorter the exposure. I can
get
pretty useful constelation images with 30 seconds and F/4 at 24mm FL.
A shot of the Big dipper at 24mm and 30 seconds on my 20D gives an
image where both M51 and M101 can be seen above the background noise
from a nice medium dark site (but not as nice as a real dark site like
Ft Davis).

Mitch