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Formula to calculate useful focal length?
Is there a formula or recommendation for the maximum useful focal length for a given aperature when imaging with a toucam? I have been using my 8" f/6 Newt to image Saturn. I only have a 2x barlow but would like to get a bigger image scale. The question is, do I get a 3, 4 or 5X powermate/barlow? I see a lot of people use a 5x Powermate but I don't think I have the light gathering power with my 8" Newt. So I'm debating between the 3x or 4x. Will I have enough light gathering power when using the 4x or would I have to really crank up the gain and brightness settings?
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