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RE: Measuring Focal Length

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Posted by Robert May on May 29, 2000 21:26:19 UTC

The last thing to do when putting a telescope tube together is to mount the primary mirror. This is so you can actually test where the primary needs to go for the proper focuser height. Use a very short FL EP (4.6mm) for the in end of the focuser and use a barlow if you have one to verify the distance. You may want to put several holes in for the primary as you will want to mount the primary closer to the secondary for doing photos as the focal plane of a camera is a lot further out than any EP.
You also want to use a targer that is at least a mile away if you do the testing in the daylight as the closer that your target is, the more the focus point will be off from what it is with the stars.

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