Quote:
Originally Posted by BLiTZWiNG
..... the length of the OTA is 1113mm. At a guess I'd say that the focal length is probably 1000.
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OK, the distance you need to measure is from the primary mirror surface to about the upper third of the draw tube in the focusser, doing a right angle turn at the surface of the secondary mirror. I think you'll find it is close enough to 1200mm - an f6. (The other options are f5 - 1000mm and f4 - 800mm so you are just trying to get to the nearest 200mm. Eg, if you measure 1172mm - it's probably 1200mm focal length.) There isn't a straightforward way to find that spot in empty air where the primary mirror focuses, hence my suggestion of using the upper part of the draw tube - that should get you close enough to 1200, or 1000, or rarely 800.
(don't scratch the primary mirror with the end of the tape measure!! Measure from the centre spot position to be safe - a scratch or two there does not matter to your viewing. With the secondary mirror, try to not touch any of the surface.)