On Thu, 06 Jul 2006 00:48:21 GMT, "The Partridgefamily"
wrote:

Well, I couldn't resist and, even though it has been raining here for days,
I was able to set up the scope in an enclosed area and focus on the same
distant object with various configurations. After taking images with a
webcam and two barlows, both located after the JMI focuser, I was able to
use the Photoshop "ruler" to measure the distance between the same two
points on the distant object (which was a telephone pole). Assuming that
without the focuser attached, the image is going to be at F10, here's what I
found for various configurations:

That is probably not a safe assumption. Of course, as others have
pointed out it doesn't matter much what your exact focal length is for
visual work.

If you know the size of the object you are imaging, and the size of your
pixels, you can work out your exact focal length using

F = 206265 * w / A

where A is the angular size of your object in arcseconds, and w is the
width of a pixel on your webcam, in the same units as the focal length,
F (typically mm). This is an easy measurement to make on a pair of
bright stars or on Jupiter. You can also calculate the angular size of a
terrestrial object if you know its size and distance, of course.

_________________________________________________

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