Re: Planetary imaging with fast Newtonian

On 18 Oct 2005 06:40:21 -0700, "jonisaacs@xxxxxxx" <jonisaacs@xxxxxxx>
wrote:

>>>Put another way, if you take an image with a 10"
>f/10 scope for one minute, and you take an image with a 10" f/5 scope
>for one minute, you can either double the size of the latter or halve
>the size of the former, and the two images will be identical (as long
>as you preserve the total signal count).
>====
>
>I don't quite buy this one. For a given camera the field of view and
>aperture, the field of view is determined by the focal ratio. For a
>given camera and aperture the resolution per pixel also determined by
>the focal ratio. Double the size of the F5 image and you do not gain
>the information that the F10 scope has, nor halving the image size of
>the F10 scope does not give you the same field of view.

Talking about "information" adds another level of complexity. To keep
the discussion limited to exposure and S/N, I'm making a couple of
assumptions in my previous statement. First, the image scale with the
shorter scope is at least critically sampled (thus the f/10 image is
oversampled). Second, I'm talking about imaging an object which fits
completely on the detector at both f/5 and f/10. By making these
assumptions matters of resolution and field of view can be ignored.

Of course in practice, you wouldn't want to ignore these, and that is
precisely why an imager would never remotely consider focal ratio when
selecting a scope. First, the focal length would be chosen to provide
the desired balance between image scale (which could mean resolution)
and field of view for the sort of targets he intends to image. Then the
aperture would be chosen- typically as large as possible, limited only
by resources or the physical ability to manage the equipment. There
would be no reason at all to consider focal ratio.

In your statements above, you are placing constraints of the aperture in
order to discuss focal ratio. I see no reason for this. Focal ratio is a
derivative specification that by itself tells you little. I see no
reason to think in terms of "for a given aperture, field of view is
controlled by focal ratio" when it is much simpler to just say field of
view is controlled by focal length. It isn't that your statements are
inaccurate, only needlessly complicated.


>Back to exposure though,...
>
>It is handy to ignore the possibility of saturating the CCD because
>that takes away the issue of "over exposure." However it does seem to
>be a problem with at least some cameras, my old 12 bit camera
>definitely has issues with blooming. It seems lots of effort goes into
>avoiding "blooming" which I believe is caused by over filled bins. An
>F10 scope puts less light on each pixel so a longer exposure would seem
>possible prior to running into problems with blooming.

Blooming is caused by overfilled pixels, and isn't related to how many
bits are used (in fact, most CCDs used by amateurs have an intrinsic
dynamic range of about 12 bits- even those that happen to use 16-bit A/D
converters). Blooming and saturation at the pixel level are manageable
by taking shorter exposures and combining them. This works because the
chip is just accumulating photoelectrons linearly (you can't do this
effectively with film). Using multiple exposures does reduce the S/N
because you introduce additional readout noise, but this is usually made
minor by controlling the subexposure time and the number of
subexposures.

You can also saturate the signal without saturating the sensor itself-
something more likely to occur with a 12-bit camera than a 16-bit
camera. Again, however, it just means you need to combine shorter
exposures.

That said, you are correct that you will get saturation four times
faster with the camera on an f/5 scope than you will on the f/10 scope
(assuming they have the same aperture), although there will be no
difference in the S/N up to that point. But that obvious fact isn't
really of much use- you still wouldn't ordinarily use focal ratio to
choose your equipment or to make any exposure calculations.

_________________________________________________

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

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