Re: RC or just LX???
- From: Chris L Peterson <clp@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 25 Sep 2006 15:53:38 GMT
On 25 Sep 2006 08:34:01 -0700, "atasselli@xxxxxxxxxxx"
<atasselli@xxxxxxxxxxx> wrote:
In your arguments here you seem to assume that read-out noise is
neglible with respect to the signal whereas it mght be not, as well as
cosmic ray noise.
No, I have been quite careful to point out that focal ratio is
unimportant only in the case where readout noise becomes insignificant.
In reality, however, it is usually possible (especially with aesthetic
imaging) to set the minimum exposure time so that readout noise is not a
factor. I don't see that "cosmic ray noise" is really ever important;
the effects of cosmic rays are largely invisible in any properly median
combined image set.
This also applies to longer exposures if the dynamic
range of the object being images is very large. Besides, saying that F/
ratio isn't important in imaging you're simply ignoring that fact the
for any given camera size different F/ ratios will determine different
imaged areas. Practical issues also arise, as in focusing and tracking
which ultimately lead to a better S/N.
As I see it, focal ratio is a secondary term. Under most circumstances,
it doesn't provide much useful information. For instance, you would
never suggest to an imager that he get an f/x scope as opposed to an f/y
scope. You would first determine the focal length, since that is almost
always the single most important spec. From there, you would seek as
much aperture as possible. Of course, the focal ratio will decrease as
that aperture increases, but the actual value of that focal ratio isn't
important.
The thing is, most discussions of focal ratio start with the
qualification "all other things being kept the same", and that can't
ever be done. You can't have two otherwise identical optical systems
that differ only in focal ratio. Focal ratio comparisons tend to be
apples vs oranges.
Focal ratio can be an important consideration in special cases. As I
already mentioned, it may be important in some photometric or
astrometric applications where the exposure time is limited. In those
cases, pixel noise may become more important than object noise (this is
never the case with aesthetic imaging), and the researcher will be
willing to trade away resolution by reducing the focal length, thereby
reducing the focal ratio and improving the pixel S/N. Focal ratio is
also an important factor in assessing optical system performance; low
focal ratios (with their consequent high cone angles) may exceed the
acceptable input angles for some eyepieces, instruments, or filters. But
realistically, in everyday usage by the vast majority of aesthetic
imagers, focal ratio is a useless concept.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.
- References:
- RC or just LX???
- From: Andre
- Re: RC or just LX???
- From: MitchAlsup
- Re: RC or just LX???
- From: Chris L Peterson
- Re: RC or just LX???
- From: atasselli@xxxxxxxxxxx
- Re: RC or just LX???
- From: Chris L Peterson
- Re: RC or just LX???
- From: atasselli@xxxxxxxxxxx
- Re: RC or just LX???
- From: Chris L Peterson
- Re: RC or just LX???
- From: atasselli@xxxxxxxxxxx
- RC or just LX???
- Prev by Date: Re: RC or just LX???
- Next by Date: Re: Exit Pupil Question
- Previous by thread: Re: RC or just LX???
- Next by thread: Trashbag in orbit?
- Index(es):
Relevant Pages
|
