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Bintel have reducers / flatteners. The Orion brand has either but not both in one. I have the Orion Field flattener for my ED80 and it works well. But if you want both a flattener and reducer there's the Tele-vue TRF-2008 which I have heard good things about.
As for ISO speed, every camera model is different. My old Nikon likes ISO 400 and in fact shows fainter stars at ISO 400 than 1600. But your Canon will be different. Take a couple of pics say at 3 minutes or whatever you think is suitable for your setup. One at ISO 400, one at ISO 800 and another at ISO 1600. Stretch the slower ISO pics until they match the higher ISO pics. Place them side by side on the screen and zoom in to some faint stars. Whichever ISO shows the faintest stars wins. At least that's how I do it.
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