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Most flatteners have a recommended distance between the flattener (usually measured from the the thread starts, not the edge of the flattener) and the imaging plane. The distances for Canon's are ~45-46mm (have a look on google). You need to allow the thickness of the T-adapter as well (usually about 10mm). Therefore a standard DSLR & t-adapter will give you a minimum of 55mm from back of the flattener to the imaging plane.
Varying this distance changes the "focus" of the flattener and usually the degree of "reduction". Get it right and you'll be a happy astrophotographer. Get it wrong and you'll look like you're in the cockpit of the Millennium Falcon
as hyperdrive is engaged.
DT
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