Quote:
Originally Posted by Sharnbrook
G'day all,
From the Google Earth line, I shall be 7.3km away from the direct pass, and Starry Night shows that I shall just miss the transit of the Moon, but I shall try to capture it one way or another.
However, my dilemma is what exposure to use? HST has a magnitude of .22, and the Moon tonight is -11.78, giving a differential magnitude of 12.0. This equates to approximately 64000 times as bright. Presumably, all we are going to get is a black dot as the HST passes in front of the moon, so detail isn't a problem, so expose for the moon, and hope to catch a black dot?
OK. But if HST misses the moon, as I expect it will from my location, what exposure should I give for the HST? I intend using my 40D on an 80mm Celestron ED with a focal length of 600mm. This seems to give a field of view of about 1 deg, or twice the moon's diameter, so if I don't get a transit, I should get HST going past. However, if I expose at say 1 second, the light from the moon will flood the whole view, and I shall miss out on the HST.
Any "Bright" ideas?
|
Mike,
The moon is an extended object so the magnitude is a function of it's size. The actual surface brightness of the moon is equivalent to a house brick. The HST won't be so different from this in terms of surface brightness (maybe some of the reflective foil and solar panels will be brighter
).
So expose for the moon, the surface brightness of both the HST and the Moon will be similar. That in itself is a problem if the HST pass is visible, because the contrast between the light moon surface and the HST will be small
.
It would probably be better if the HST pass was not visible, so you would get a black silouette against the moon...
Al.