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I always thought less, but longer low ISO images would be better then more, shorter high ISO ones, but it seems not to be. I once did a single 1/2 hour ISO 200 shot of the Lagoon, it was nowhere near as good as stacking two 15 minute images at ISO 1600. It should have contained the same amount of "information" of the image but no matter how much fiddling I couldnt get anywhere near the detail as the 2 ISO 1600 shots stacked. I mentioned at Lostock that I deliberately move the scope slightly between each exposure (by moving the guidestar pick off prism in the off axis guider slightly). This means that when I stack the images, each one is moved slightly, this sees that the same actual pixels arent stacked on top of each other, as I think some of the noise ocurrs in the same pixels each time. Moving them smears out this noise,(at least it seems to work here), so I will always shoot deep sky stuff at ISO 1600 filtered, however unfiltered, ISO 400 (5 to 10 mines each) seems to give best results, as at ISO 1600 the stars "bloat" a bit without the filter to cut them back.
Scott
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