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Old 26-11-2010, 07:51 PM
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Originally Posted by ngcles View Post
Ganymede and Neptune are approximately similar sizes with Neptune a little bigger, but Ganymede has a brighter colour which presents as better surface brightness and the contrast on detail would, I'd think, be markedly better though neither would be described as a "walk in the park" basically due to size. I'd be sceptical too, though it's not outside the realms of possibility to see limited detail on Neptune in 28cm. BTW, what's an "IIRC" ??
Neptune is not only dimmer, but the changes in surface albedo would be very slight so to see that, if possible at all, would require very, very high power which would dim the planet too much, render it fuzzy (unless seeing is a 1 in a million - even then the limitations of the scope would apply) all of which works to further reduce one's ability to detect very slight difference of shading at such a small scale.

IIRC = If I remember correctly

Have seen Himalia -- tracked it down some time ago now it is by some distance the next easiest Jovian moon after the four big ones and not hard to see in 46cm in good conditions. The trick is finding the correct location/field and distinguishing it from a 14th-odd magnitude star. There is an ephemris here:

if anyone is interested.

It is obviously easiest at greatest elongation from Jupiter. If you can find Pluto, Himalia won't present many issues. It orbits well outside Callisto meaning you can exclude Jupiter from the field with ease during most of its orbit. Jupiter is heading north over the next few years which will make it harder as it is closer to the horizon and the air-mass you are looking through is greater. On the other hand, Sagittarius also presents problems of its own because of the blizzard of 14th mag stars it will be hiding among. Now could be a good time ...


Les D
cheers Les looks like an exotic challenge as few people ever seem to attempt to hunt Himalia down. Might try for it sometime.
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