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Seeing width in the Cassini Division

Started by EdZ, 03/28/2003 12:15PM
Posted 03/28/2003 12:15PM Opening Post
Way back in early March this discussion gathered quite a few posts and here I link back to the original question.

http://www.astromart.com/messages.asp?message_id=52452&page=

I did some reading at the time and posted my opinions. Back then I posted a three part thread to cover what I found. Questions were raised and I left that thread hanging at that time saying I needed to do some additional reading. I have since done so.

I cannot possible post here all the info I have gathered, but I will post my conclusions. Hopefully in the very near future I will edit this post and provide a link to the complete article.

The original question was:
Aperture to resolve Cassini Division into gap?
can the Cassini Division be resolved into a *small* gap with a 4" aperture?

Some other questions raised as a result of the original question included:
What size scope and magnification is required to resolve the "gap" so that it can be readily perceived as having width?
Why can we see the Cassini division with scopes that would seem too small to resolve its width?
How does the Cassini division fit the criteria of an extended object?
Can we use point source criteria to judge the measurements of extended objects?

I would encourage you to return to this location for a link to the complete article, however the following post is the last page of the complete article, my conclusions.

thanks, edz

4-1-03 here is the link to the complete researched article

http://www.cloudynights.com/Observation/cassini.htm




Posted 03/28/2003 12:20PM #1
Conclusions (edited to fit)
Utilizing point source limiting criteria for determination of potential to see an extended source is a difficult application, but I believe it is a valid approach.

Although the rings easily fit the extended object criteria of images larger than the detecting element, the width of the Cassini division does not. The width is small, even smaller than the limit of resolution of many small scopes. With exception to its linear dimension, Cassini is a small, very high contrast image requiring fine resolution that fits the definition of a point source.

Extended objects are easier seen if high contrast is present.

The Cassini division separates the brightest portion of the B ring from the brightest portion of the A ring. The dark Cassini division is bounded on either side by the brightest features within the ring system. The Cassini division may just be the highest contrast extended planetary feature in the entire Solar system.

The extent that diffraction causes the light sources to infringe on the dark space is the question left unanswered. A larger aperture will provide a smaller diffraction disk. Greater magnification will reduce the extent of the diffraction fringe. High contrast will improve the ability to see the division. And finally, an added linear dimension to the feature helps make the image easier to see.

Any feature that does not have angular dimension greater than the limit of the telescope’s ability to resolve cannot be observed as anything larger than the Airy disk. Since this is the smallest image spot size that can be achieved by a given telescope, no matter how large that image is magnified, it cannot be construed as having width. You would not be magnifying a resolved feature. You would simply be magnifying the smallest image spot size.

A telescope with an aperture that will produce an Airy disk size smaller than the observed feature is required to see the feature as having dimension. The smallest telescope that significantly exceeds the necessary criteria is a 9” scope. I suspect it takes at least a 9” scope to achieve true dimensional effect.

The telescope will realize potential resolution with a magnification of 1300D. For a 9” scope 1300D produces a magnification of 297x. As the aperture of the telescope increases, and the limit of resolution increases, the magnification needed may be even less than 1300D to achieve dimensional resolution of the feature.

Some observers may achieve eye resolution of 3 arc minutes or 180”, in which case they could perceive dimensional width at a magnification of 180” / 0.7” = 257x. It is more likely that most observers have eye resolution in a middle of a range needed and would require 4 arc minutes or 240”, resulting in a necessary magnification of 240” / 0.7” or 342x to see dimension in this feature.