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Scott Stoness wrote:
I did some calculating and googling:
For the moon to fill the sensor you would need about 2500mm lens. http://apod.nasa.gov/apod/ap061228.html
Assuming you want a compositin with 1/3 space empty around moon = 1700mm
Adromedra as an example is 6x bigger than moon in apparent size = 277mm (300mm)
Using a crop sensor = 173mm (200mm)
If you wanted to shoot NGC 4517 with good resolution, which is 1/10 the size of adromedra, you would need a 2000mm lens on a crop camera.
So:
1) There are only about 3 galaxies that you could do a good job with - with a 200mm on crop
2) And perhaps 10 more (guessing) if you have 600mm
Anyone see problems with my logic.
Scott
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Your math seems fine, but keep in mind that "resolution" is arcseconds per pixel, not the amount of the sensor that's covered. For instance, using the same lens, you'll get much more resolution with a 7D than with a 10D (assuming your mount, guiding and stillness of the air conspire to allow you to maximize the resolution).
Like I said, you're going to be some combination of frustrated and disappointed if you limit your shooting to galaxies, since there are so few galaxies (especially in the northern sky) which have an apparent size sufficient for significant detail with short focal length. Broaden your scope (pun intended) to include nebulae, and you'll have a lot more fun.
Mark
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