G'day, I have started excavations for our observatory and pier footings, finally a permanent setup - no more carrying.
The Obs. will be an elevated and Maxdome automated Sirius 3.5m housing a 14" LX200GPS on a Meade Ultrawedge at 36.8 deg South, but before I go too far and fill a bloody big hole with concrete I would like to check that I am on the right track in regards the pier location.
Having always used Alt/Az, this Equatorial stuff is a learning curve, I have modelled the setup in CAD, but with no practical experience I would appreciate some feedback/comments:
From what I understand, I should locate the Optical tube and Dec axes at the dome center, calculating back from here gives me a pier offset 275mm to the North.
Looking at future possibilities, Equatorial mounts do not appear to require this amount of offset (PME, PMX, etc) and given that the rigidity of a fabricated offset plate should be solvable by design:
If I was to place the pier central, would the 275mm offset plate mounted on top introduce a physical obstacle to observing with the Meade?
If I was to place the pier 275mm North, would an offset plate to the south then be an issue with the mount/counterweight shaft or more likely the scope on an Eq mount ?
The Sirius 3.5m should give us enough room to swing a cat, so clearance should not be an issue, but how does this offset or lack of it affect dome slot alignment (the 1000mm slot opens 400mm past zenith). Could this create an unresolvable alignment ?
And finally, what other practical issues have I missed ?
Thanks Phil,
I had seen that thread, but all it did was make me less sure of whether to offset or center the pier.
Maybe the answer is to halve the offset and then use a shorter offset plate.
My big concern is that if I change the mount in the future, we will have problems as Chrism found.
I am after some feedback as to what has worked and what has ended up being or could be a problem.
If I was to place the pier central, would the 275mm offset plate mounted on top introduce a physical obstacle to observing with the Meade?
If I was to place the pier 275mm North, would an offset plate to the south then be an issue with the mount/counterweight shaft or more likely the scope on an Eq mount ?
The Sirius 3.5m should give us enough room to swing a cat, so clearance should not be an issue, but how does this offset or lack of it affect dome slot alignment (the 1000mm slot opens 400mm past zenith). Could this create an unresolvable alignment ?
Thanks for your advice, Neil.
Hi Neil,
When I had the 8" Meade LX90 wedged on my pier, I recall there being some fairly awkward / neck-straining observing positions when viewing objects high in the South. I had to get up very 'close and personal' to the pier. I imagine that with your proposed set-up, the geometry would be the same, so I definitely would not want the mount off-set to the North of the pier, as would be the case with your 'central pier' option.
Your other option, ie to offset a future GEM back to the dome centre (from an offset North pier) is what I considered - albeit briefly - when swapping over to an equatorial mount. As you may have read, my pier is offset about 250 mm North of centre, and in hindsight, it would have been better to locate my pier centrally, but I designed it for the equipment of the day without much appreciation of GEMs. For purely aesthetic reasons, I would not offset a mount on a pier, and if you're going to be doing any visual observing, I suspect that it would be much better to have the mount line-up with the pier. The other reason I did not pursue the offset GEM was that I have a tall concrete pier and I did not want it to be in tension - ie. I wanted all the weight straight down on the pier. I don't believe that the counterweights or shaft would be an issue, as they're usually angled down to the South.
Given that you've got a 1000 mm slot aperture, you would be about 300 mm better off than I am (also with a 14" SCT), and as I also said previously, I can always find a dome position to get a clear view, but slaving the dome to the mount is going to be a challenge.
I started designing our obs a while back, initially based on a geodesic dome above a square room. Perceived difficulties attaching facets to the ring and slot frame as well as trying to get a stairway from a rectangular wall into a round aperture led me to settle on a ribbed structure with 24 gores on a round room with a spiral staircase.
Then I discovered IIS and saw your thread, wow , that is what I had visualized.
To speed things up a bit we then settled on a sirius 3.5 dome only, but before all the ducks lined up a secondhand 3.5 came up which we grabbed.
The plan is to now elevate the sirius on the round room.
So our column will be probably be about the same height as yours. I would not have thought that the comparatively small load of a scope and mount would put much tension on a reinforced column suitable for supporting a shopping centre carpark !, although I have not yet waded through the calculations of an offset load.
I am hoping that the ascom-maxdome will handle scope/dome geometry as you can enter all sorts of offsets
So our column will be probably be about the same height as yours. I would not have thought that the comparatively small load of a scope and mount would put much tension on a reinforced column suitable for supporting a shopping centre carpark !, although I have not yet waded through the calculations of an offset load.
I am hoping that the ascom-maxdome will handle scope/dome geometry as you can enter all sorts of offsets.
Neil,
Your CAD drawings look great. All my drawings were just sketches literally done on the blank backs of pages of an article that I printed!
I didn't do any calcs on the pier for the overhanging weight, but I do know that concrete is very weak in tension, and I also thought that any off-centre load could potentially put a lean on the pier which would mess up the polar alignment. So, the easy way was to put the weight (which could be in the order of 100 kg) directly on top.
I can't say if the ascom-maxdome software will handle all the necessary parameters - it would also need to include slot width.
Another observation is that once you include the floor opening for a spiral staircase, you start chewing into your precious space upstairs. With my setup, the tightest position for viewing through the eyepiece is when targeting a Northern object a little West of the meridian. The top post of the ballustrade makes it tight for the observing stool, but that's easily fixed by turning the EP to one side or the other. A forked mount is somewhat more compact than a GEM.
If you're coming down this way, you're welcome to make an arrangement to drop in.
Chris,
Thanks for the notes about the stair opening, looking at my posted snapshot I noticed that I had included the wrong stair layer - it was a construction layer that does not line up with the floor opening.
My rationale was that with the pier offset to the North there would be less room in this quadrant, therefore the opening is in the South.
Is that a good idea ?.
Looking at a couple of automation descriptions and algorithms, surprisingly they do not take account of dome slot or aperture width. They just try to align the aperture center line with the scope.
When you had the LX90, was it alt/az or equatorially mounted ?.
In trying to answer my own questions, I am doing a bit more CAD modelling with a Paramount ME as a comparison.
At 36.8s, I have an offset of 47mm to the Dec/RA axes, but unlike the LX where this lines up with the OAT center, it appears to me that you then have to add the OTA radius and mounting rings ?.
I would agree that a stair opening in the South makes sense if you do offset your pier to the North. That's what I did, and the pier ended up roughly in the centre of the 'circle' left after the floor cutout, which was 600 mm wide by 92 deg of arc, was made.
With my current GEM, there is a surprisingly large horizontal offset between the OTA axis and the pier. As I think I said in an earlier post, when the scope is pointing North, I'm guessing that it would be 600 mm or more offset from the pier centre line. Hence the dome slot needs to be considerably 'off' North to line up with the scope.
Cheers,
Chris
Last edited by ChrisM; 14-05-2012 at 08:39 PM.
Reason: typo
, that is what I had visualized. 
