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I agree with H. I'd go the 200mm F2.8 prime. Bound to be the better lens.
Sigma 70-200mm 2.8 gets good reviews but its getting pretty large and heavy and zooms are often a compromise. For astro work you really want little chromatic aberration wide open and no coma wide open as an ideal.
You can use stop down rings to achieve a higher F ratio and not get diffraction spikes.
Cheaper choices are Pentax 67 300mm, 200mm. I use a Pentax 67 165mm F2.8 and its spectacular wide open on a huge 16803 chip. A cheap $30 ebay adapter and you are away. The 165mm lens costs hardly anything on Ebay.
Greg.
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