Expert: Tom Whiting - 11/3/2005

Question
Hi,
Could you please give me a little information about Sir William Herschel and the Telescope he used to detect Uranus? I can't locate Uranus with my small 4-inch telescope can I? Does its detection also depend on the telescope's focal ratio? How? What can I do to observe Uranus?
Best regards,
Leila


Answer
Hi Leila,
Sorry, but I don't know a thing about Sir William Herschel's
telescope, or what he used to find Uranus in 1781....perhaps that information, being historical, can be found on the internet....in fact I am sure of it.   (But your other questions
about observing, I can answer).

Yes, it (Uranus) would be an easy target with a 4 inch scope, provided you know EXACTLY where it is...but I doubt that a 4 inch scope would show it as a disk...probably just a star-like point of greenish light.   (Uranus has a very slight greenish color in a scope of 6 inches or more).  No, detection does not depend on the focal ratio, (IT's A Naked-eye object! Any scope will show it....See Below) but the longer the focal length, the higher the magnification you could obtain with the same eyepiece.

Actually, Herschel would have not even needed a telescope
at all, since Uranus is a faint naked-eye starlike object in a clear, dark, sky at magnitude +5.7....so it's easily visible IF you are in dark, limiting magnitude 6.0 skies.
I've seen it naked-eye many times, this past week in fact....so ANY binoculars or telescope (or any optical aid) will show it at 5.7 magnitude.
But if you want to see it as a tiny disk (it's about 4 arc-seconds in diameter) you'll need at least a 6 inch mirror.
Currently Uranus is naked eye about 2/3 the way between
the naked eye stars Lambda Aquarii and Sigma Aquarii.....but if you don't have a decent star atlas OR don't know where those stars are in Aquarius, then I (and no-one) can't help you find it because that's the only reference points (those two stars) available for finding Uranus.
So you have to know where Aquarius is, then those 2 stars
in Aquarius....basically, the same way we find ANY dim,
or below naked eye object, in the night sky....you have to
first know your naked-eye sky before you can proceed any
farther in observational astronomy.  

Just like you can't find the Armory in Columbus, Ohio until
you first know where the area Ohio (constellation equivalent)
is, then the city of Columbus (naked-eye star equivalent),
then you offset 5 miles to the south to find the non-naked
eye Armory.....we do exactly the same thing in observational
astronomy.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA