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Originally Posted by freeview
Based on the comments I read from this thread and some of the experiments wejones has done, I became skeptical of the angle readout scale on the offset dish. For reference, I am installing a FortecStar FC90U.
I mounted a pole through the dish U-bolt mount and measured the angle using an adjustable set-square (a handy tool to measure angles like the protractor). If I make the pole parallel to the LNB arm, where it attaches to the back of the dish, I read about 20 degrees on the *dish's* readout scale. I then angled the pole to have a readout of Zero on the *dish's* scale. At this point my adjustable *set-square* reads 17 Degrees. Neither of these measurements seem to correlate with the 24.62 Degrees specified by the manufacturer. Does the LNB arm and holder angle make the offset angle verification not possible by simple means? Ideally I would like to use my adjustable set square, as it has 0.5 degree resolution -- but, for that to work, it is necessary to understand the offset angle concept and how the dish manufacturer has established the 0 degree mark on the readout scale
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I did something similar to what you described above. There are a couple additional pictures in the URL I mentioned above, ie:
http://eskerridge.com/bj/FC90-sg2100.html . My dish came with a different mount, ie I got the pole mount version. My mount, however apparently came from an FC80, and was off a bit but was pretty close to being at the proper angle. My guess is that the U-bolt version is a generic mount, and is even further off. However I am also not convinced that any part of the dish mount is really appropriate for making these measurements. On my dish, the lnb arm is pretty close to being perpindicular to the "apparent" aim of the dish, so I was hoping that it would be a good place to make measurements to confirm the scale on the mount, but I've seen some dishes where the mount surfaces on the back of the dish are off center, so I'm not sure that the manufacturers really made any attempt to make these surfaces perpindicular to the apparent aim. WHen I measured the offset angle using the lnbf arm, I think I got 23.5 degrees, but when I estimated it using the mirrors, the angle was closer to the 24.6 specification for the dish. At least with the FC90P, it is close to being accurate, ie within a degree.
However, the setting of this dish elevation scale only needs to be approximate, because in all liklihood you'll end up changing it when you peak on a satellite anyway. When I set up my system, I set the angle on my motor mount, before I ever set it up, and have never touched it again. I also set up the dish elevation to match what I measured in my experiments here, however I quickly started changing this setting during alignment. However once I was aligned, I looked at where it had ended up, and it was within a degree of where it was supposed to be. So the FC90P seems to come with a mount that is relatively accurate, while apparently that isn't true for the FC90U version.