From: Alan Friedman <alan@greatarrow.com>

Date: March 25, 2008 7:32:49 PM MDT

To: Astro_IIDC@yahoogroups.com, macastronomer@yahoogroups.com

Subject: dancing with the moons

Hi all,

Here is an animation prepared from images captured early on Easter morning:

http://www.avertedimagination.com/img_pages/dance_of_tethys.html

(follow the link below the image to compare a monochrome version of the observation.)

I was excited to have a night out under a clear sky after many months of clouds and cold. As I pored through video of saturn recorded in pretty good (6/10) seeing, I noticed a pair of bright and dusky spots moving in tandem. Had I known that a transit of Tethys was scheduled I would have planned the workflow differently. The four frames here are captured through a mix of red, green and IR filters - hence the luminosity variance seen in Saturn's cloud belts. Using a small number of frames (about 200 for each image) and a firm hand in processing I was able to isolate the disk and shadow of Tethys against Saturn - perhaps also Enceladus as it reached the darkened limb just to the left of Tethys' shadow.

A moon transit on Saturn is quite a bit more challenging to record than similar events on Jupiter. Saturn is twice as far from us and Tethys only a third the diameter of Europa (a fifth that of Ganymede.) Tethys subtends about one sixth of an arc second (Enceladus would be about half that size.) But as the plane of Saturn's system aligns with ours and the dances become more frequent events, I expect we will have a chance to see many more of these observations.

All the data in this imaging session were recorded and processed on my Mac using Astro IIDC v.4.0 - final processing of the animation was done with Adobe Photoshop CS3.

best wishes,

Alan