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Sep 2, 2018

Pier offset for LX200 12" Classic?


Just wondering what others have done. I see that in the drawings of the observatory there is a mention of offsetting the pier by about 10 inches. I have a home-made pier that I'm using with a 12" LX200 classic and the wedge mount. I my last observatory I did find that the space between the wall and back of the scope was kind of tight when pointing toward the south. The 10 inch offset shown in the diagram should give some nice extra headroom back there. I'm thinking of going with the 10 inch offset to the south of center of the observatory to give some more equal (or at least more!) room when pointed south.

Any comments or experience with this from anyone out there?



Sep 2, 2018

Tom, that image is very misleading. When pointing at zenith, the offset of the optical axis of your scope from the center of your pier (on an equatorial mount) is only the distance between the center of your pier and the point where the RA and DEC axes of your mount intersect. The dome opens several inches past zenith so it should be enough to accommodate even large scopes. If you offset the dome too far, you could have a problem on the south side of your dome accommodating long imaging trains on the back end of your scope - especially with larger refractors.


Your mount specs should indicate the distance of the RA & DEC axes intersection from the center of your mount (easily measured also) - I would go with that number to offset your dome (assuming your mount is mounted in the center of your pier).

Sep 3, 2018

Yeah, that image is kind of confusing. I just know that in my last observatory I had the pier excactly centered and found things kind of cramped between the back of the scope and the dome. I had wished I offset it at that time, so I want to make sure I do it right this time. I'm thinking if I point the scope directly overhead to the zenith, I should offset it just enough to have the scope itself in the middle of the dome and just whatever offset gets it there. I'm at latitude 45 (Sturgeon Bay, WI) so the scope will be about halfway 44.8 degrees on the wedge when polar aligned, so take that into consideration also.


I'm at the decision point of deciding about the offset placement. 11' SCT and equatorial mount. Steve, it sounds like 'the point where the RA and DEC axes of the mount intersect' should be in the center of the dome and the pier should be to the South (in the northern hemisphere). Is this correct?

Yes, that's correct. For most mounts, the offset is only a few (2 - 3) inches. The image on the NexDome site is very misleading as it appears to include the height of the OTA above the saddle-plate in determining the pier offset which you never want to do.


However, in reality it doesn't have to be perfect since you can account for the final geometry when you setup the dome-control software. The dome software will then rotate the shutter to whatever point is necessary to accommodate the OTA when pointing at zenith (or anywhere else). Remember, the only way to point an OTA on an EQ mount at zenith is to first rotate the RA axis 90deg East or West so the OTA will never be in the center of the dome.

Gotcha. Thanks for the prompt reply.

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