[AZ-Observing] Io and Ganymede
- From: "Frank Kraljic" <fjkraljic@xxxxxxxx>
- To: "AZ Observing List" <az-observing@xxxxxxxxxxxxx>,"Amastro" <amastro@xxxxxxxxxxxxxxx>
- Date: Thu, 14 Feb 2002 09:09:34 -0700
Monday night, I noticed a shadow riding the north perimeter of the SEB, and
a slightly darker-than-the-belt "point"
crossing the CM. I assumed it was Io given the size and placement of the
other three moons, and confirmed its identity using the "planet drawer" at
http://ringmaster.arc.nasa.gov/tools/tools.html. Unfortunately, not much
detail could be ascertained even at 560x, until ten-fifteen minutes before
egress. Just as the moon entered the edge of Jupiter's foreshortened limb,
Io's disc became more apparent, but still subdued in contrast against
Jupiter bright background. Io split into two parts: a bright equatorial
region, and tan-colored poles; the south pole larger and more prominent.
With the seeing very good, I moved the telescope east to Ganymede and
punched the magnification to 1000x. With a little atmospheric flicker and a
slightly dimmed disc, subtle albedo features became prominent--more
prominent than I have ever witnessed. Jupiter itself could not handle the
high magnification in contrast, however strange enough, the moons managed
the magnification fine (a function of their brightness).
I sketched both instances and they are posted at:
http://www.psiaz.com/polakis/kraljic/GanymedeDrawing.jpg
http://www.psiaz.com/polakis/kraljic/IoTransitDrawing.jpg
The CM value for Ganymede was calculated by Guide 7.0 and confirmed using
the JPL Horizons Ephemeris Generator: http://ssd.jpl.nasa.gov/cgi-bin/eph.
I also used a bitmap produced by Guide (courtesy Sam Herchak) to compare my
drawing to visible features. Thanks to Tom Polakis for posting the pictures
on his website.
The next favorable Io transit is on the evening of the 18th (Ingress:
3:55UT; Egress: 6:09UT). Hopefully, the seeing will be as good as those
witnessed on the 11th. After swapping eyepieces during the transit, I
figured a minimum magnification of 500x was required to separate Io.
-FRANK
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