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Originally Posted by davidpretorius
great stuff. i want to do something similiar down the track. what exposure time would you limit yourself to do a large mosaic?
it must be a fine balance to get structure, but not billions of stars burning out the image!!!
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This is something that is poorly understood.The brightness of an image of a point source (a star) depends on the size of the aperture independant of F number.The brightness of an image of an extended object such as a nebula,comet aurora depends on the F number.So the smaller the F number ie F2.8 is four times brighter than F5.6 for extended objects but if both lenses or optic have the same aperture the stars are the same brightness.Obviously the greater the aperture the better the resolution due to diffraction limits.
For photography small F number is better.If i put a 30mm eyepiece on my 300mm F2.8 the resultant exit pupil would be about 11mm in diameter!So more than 50% of the light would not even enter the average eye (7mm at best).
For visual observing low F numbers are not all that important for this reason.Aperture is all that matters.The Dob enthusiasts are correct!A bit over zealous but still correct.The trick is to match the magnification to the aperture so the exit pupil of the eyepiece is about 7mm for young people and 6mm to 5mm for us oldies.
So to get back to your query,why this lens works so well it has an aperture of 107mm and is F2.8 and it has as good a quality as man can produce at this time.I am not in competition with the Hubble or any other pro telescope.Just like all you visual observers I want to look/take pictures in real time.That is when we can depending on weather etc. but we are involved.Excuse the pun,Astronomy is not a spectator sport!
Bert
