Add Lateral Offset for Side-by-side rig with slotted dome

I’ve searched the forums and there are several others who have requested this, so there is definitely some interest in it.

I’m using a side-by-side rig in a slotted-dome observatory. I’m only using one scope at a time, so each scope should have no trouble seeing through the slot. But, since each scope is offset from the centerline of the Dec axis, there are some orientations where a scope cannot get a full, clear view through the slot.

This is a well known issue, and is easily solved by adding a GEM “lateral offset” value to the dome azimuth calculations. Here is a link to an article discussing the issue:

The article also has a link to the computer code that handles the calculations, including the lateral offset:

I hope you will consider adding this parameter to your dome setup screen, and to the dome azimuth calculation. It will make it possible to use a larger side-by-side rig with a slotted dome – something which is becoming very common.

Hi Dan,

thanks. We have so many array system with the dome management but this is the second time I receive this request. I will add for sure.

All the best

1 Like

Thanks, Leo!

If I understand the array system correctly, I think people are using multiple scopes at the same time. In that case, ALL the scopes need a clear view at the same time, so an offset would not help. In that setup, you need to consider all the scopes as if they were one large one. All the scopes have to be small enough to fit well within the dome-slot as a group.

A side by side is a bit different. Each scope can be quite large, as long as you’re only using one at a time. I have a 10" RC on one side, and a Skywatcher Esprit 100 on the other. Each can easily view through the slot if the lateral offset is used in the calculation. But I will never be able to use the two scopes at the same time.

In any case, I look forward to seeing this feature in Voyager when you have time to get to it. Thanks again!


I don’t think there are differences at all, the mechanics and the mount are always the same and both have to see the sky at the same time and many have bigger telescopes than yours. I would say that the basic problem is the opening of the dome which is not sufficient for the type of dislocation of the telescopes. In order not to touch the existing models I will add a third one for those who want to use this feature.

Hi Leo.

Yes, the mount and mechanics are the same. But the difference is how many scopes are in use at the same time. With an array, multiple scopes are used at the same time. So the view through the slot must be large enough to handle a single, much larger scope, that covers the view of all the other scopes. See my diagram.

In a side-by-side setup, many people use only one scope at a time. If both scopes are used at the same time, then it is the same as an array. But if only is used at a time it is different. In this case, a larger slot is not needed. You only need the Lateral Offset value to use one scope and get a full view of the sky.

In the side-by-side, you can ignore one scope. The geometry is different from the array because the one scope being used is offset to the side of the Dec axis. Note that since only one scope is being used, an array version of the software is not needed.


I understand now, thanks.

All the best