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Hi again,
to work out your magnification divide the size eyepiece in mm into the focal length of the telescope. eg=900mmfl divided by 20mm eyepiece=45 x magnification. This would be okay as I said for a start for deep sky objects. For planets a 10mm will give 90x and add a 2x barlow to double that to 180x. But get a good barlow otherwise image will not be very sharp and clear. Or get a 5mm eyepiece instead of barlow and that will give you 180x magnification also. The SR4 would give you 225 x magnification, try that first before getting 5mm. If image is not sharp, or the mount not steady enough for good viewing at that magnification then get the 5mm.
Hope I explained it okay for you.
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