Posted By Alex McConahay
The real question is how far away does a terrestrial object have to be before it has substantially the same focuser position as something at infinity?
Specifically, we have rebuilt a 22 inch F4.2 or so Dob, and will be setting it up for the day for the first time this weekend. I know with the work we did on the mirror cell that we will probably have to shorten the truss tubes (since the mirror cell and collimation system is now an inch and a half "shorter" and the mirror is that much further back).
There is a cross on the hill a mile away that we can focus on. How much difference will there be between the focuser position for that cross and a star at infinity?
If it is essentially the same, then I can focus as well as possible to determine where the focuser would need to be, shorten the poles during the day, and observe that night. Other wise, I will have to check things out at night, and not really be able to observe until then next night after modifications.
By the way--anybody have any helpful hints about how to do this operation?