I got it. You install head on tripod.
Check first with good ball head.
Great idea. I'll do this tonight.
I took my 501 head off the tripod, attached it to the sliding mechanism, and that's where there is still play. I think it would probably work fine with any other head. The uneven surface of the 501 mount seems to be the culprit.
May be you can make short video showing the problem?
I think you want to attach it to the center of the slider and use one tripod?
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