Dome Dimensions

[OzDave]

After some thought and to-ing and fro-ing on things, I've decided that a DIY dome is the way to go. It will not be a geodesic design, and will be a simpler ribbed design with curved gords.

I am drawing up the plans now with Sketchup but I'm wondering wgat suitable dimensions would be.

It has to house a NEQ6 + MN190.

I was thinking of having an outer diameter dome of 2m but I am now thinking this might fit but would be very cramped to move around in without bumping the scope or just to observe anything.

So what would people recommend as a minimum dome diameter?

Also, slit width - would a 60cm slit be enough?

David

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[keithatrochdale]

Hi

The answer has to be a big as you can, 10' x 10' should give you room, but you may want to make it 10' x 12' to give you 2' for a bench across one end.

You can always start bigger if you want (28')!

[The Sailor]

A grain silo!!!

Jim

[teleskopjo]

I am doing mine, 2.9m diameter onto a room 3m x 5m.

[pete_l]

The problem with domes is that ... they're dome-shaped. So the highest point is at the centre, typically where your scope will be. So depending on how tall you are compared to the height of your dome, you'll have to build it higher, so that you can move around the 'scope without having to stoop the whole time.

So far as size goes, have a look at some garden sheds. 6' square might sound like a lot, but go stand in a 6-foot shed, it's pretty small. For a bit of elbow room, personally I'd go for about 10' diameter as a minimum, just so you have space to move around in the dark without bumping into things, or tripping over.

The other problem with domes is that they do limit you to a single telescope. If you decide that you want to branch out, maybe have a visual Newt and an imaging refractor - both on the go at the same time you won't be able to fit them inside a single dome. While domes do look nice, sometimes a roll-off-roof shed is more practical and can fit more scopes inside (isn't that the ultimate goal ?)

[The Sailor]

My shed is 6 x 7 f, the flat roof is 6 x 7.5 f, overhang on one end to hold the dome mechanism. The dome itself is half ball shape and is just short of 6m, the pier, mount and OTA's are of enough height to give a good clearance all round. I have put a platform floor in to give me hand access to all the equipment on the mount, all other equipment is located in the add on room.

Jim

[JamesF]

And you've got a "pier cosy" too

James

[The Sailor]

Have to say James that my obsy has gone through many changes, just hope it is at an end and I can get on with looking up at the star rather than down at screws. As a matter of interest I had the dome made local, went to a fairground repair works and found this dome mould and it was perfect size so got what you see made for £450, all the rest I done myself .

Jim

[nytecam]

I am drawing up the plans now with Sketchup but I'm wondering wgat suitable dimensions would be. It has to house a NEQ6 + MN190. I was thinking of having an outer diameter dome of 2m but I am now thinking this might fit but would be very cramped to move around in without bumping the scope or just to observe anything. So what would people recommend as a minimum dome diameter? Also, slit width - would a 60cm slit be enough? David

Some queries ?? How will you use the ob?? - imaging or viewing, what's a NEQ6 + MN190 [size/mount German/forks] - latter needs less space. The 'slit' should 'accomodate' scope/finder/guider with maybe 50% clearance in all orientations. Third of dome diam is good

[OzDave]

Hi

The answer has to be a big as you can, 10' x 10' should give you room, but you may want to make it 10' x 12' to give you 2' for a bench across one end.

You can always start bigger if you want (28')!

Hmmm, yeah, I wasn't thinking quite that big!!! The wife already thinks a 2m dome would be on the large side...

But 10'x10' would be about 3m, which is what I'm thinking I might need, just for overhead clearance. Although, depending on how high I make the walls (I was thinking 1.2m), I can get away with a smaller dome.

[OzDave]

The problem with domes is that ... they're dome-shaped. So the highest point is at the centre, typically where your scope will be. So depending on how tall you are compared to the height of your dome, you'll have to build it higher, so that you can move around the 'scope without having to stoop the whole time.

So far as size goes, have a look at some garden sheds. 6' square might sound like a lot, but go stand in a 6-foot shed, it's pretty small. For a bit of elbow room, personally I'd go for about 10' diameter as a minimum, just so you have space to move around in the dark without bumping into things, or tripping over.

The other problem with domes is that they do limit you to a single telescope. If you decide that you want to branch out, maybe have a visual Newt and an imaging refractor - both on the go at the same time you won't be able to fit them inside a single dome. While domes do look nice, sometimes a roll-off-roof shed is more practical and can fit more scopes inside (isn't that the ultimate goal ?)

This is an excellent point, and something I hadn't actually thought about. I was thinking that the walls might be 1.2m high and provide a 2x2m box and on top would sit a 2m diameter dome. That would mean, maximum overhead clearance would be 2.2m when standing in the middle, which you can't do because the pier and scope go there. I am 1.82m tall (6'2") so standing in the middle would give about 40cm overhead. A bit less due to the inside dimensions being smaller.

With a bit of trigonometry, it seems that my head would hit the dome ceiling at about 78cm from the centre of the floor. To extend this, I could make the walls a bit taller, or the dome a bit wider.

I had a chat with the wife and said the dome would be 2-3m in size and she picked up immediately on the 3m bit, so I am thinking that maybe I could manage 2.5m. And with that, I'd be able to stand 1.08m (of a possible 1.25m) from the centre before I hit the dome. I can probably manage with that. With a square base, I would build into the corners some triangular benches to store stuff on. Also, I am planning to have the control laptop remotely connected in the house. So really, once I set things up, I should just be able to leave it and sit in the house in relative comfort.

[OzDave]

Some queries ?? How will you use the ob?? - imaging or viewing, what's a NEQ6 + MN190 [size/mount German/forks] - latter needs less space. The 'slit' should 'accomodate' scope/finder/guider with maybe 50% clearance in all orientations. Third of dome diam is good

I intend to use the obsy primarily for imaging.

An NEQ6 is a German Equatorial mount. It is about 50cm in height I guess. The MN190 is a 190mm diameter Maksutov Newtonian scope.

So, my scope + finder + guider could be as much as 190 + 80 + spacing. So say 350mm. If we add 50% to that, we get 525mm. I was thinking about a slit of 50-60cm, so probably I should go for 60cm to be on the safe side.

That's with my current scopes of course. I may get a larger refractor in the future. Like a 120mm or 150mm of some description. I guess the thing with refractors is their length. Am I likely to have problems in a dome of 2.5m diameter with walls 1.2m high?

David

[astrosathya]

Hi Guys,

this may seem a really stupid question, but I think it is valid. Agreed that one of the ways is to build moulds and make the dome. However, if we were to go the traditional route, with ribs and slots built out of wood and then later covered with a suitable material, a problem arises. How do we calculate the arc length of the ribs? They cannot be the same length as in a hemisphere because the slot is in the way. Therefore, please could someone advice how do we calculate mathematically the dimensions of these parts?

Clear Skies,

Astrosathya

[JamesF]

One solution would be to build the frame without the slot so you don't have to work out the odd dimensions, then cut the slot afterwards.

James

[skybadger]

to The sailor

As a matter of interest I had the dome made local, went to a fairground repair works and found this dome mould and it was perfect size so got what you see made for £450,

what did you do with the mould ?

I mooted this in a recent observatory building post and noone has responded at all .

I have some autocad drawings that may help - basically I took a hemisphere and drew the slot into it, gives me all the figures of arc heights, lengths etc. Means the dme isn't quite as high as expected since the slot tends to be flat rather than curved across the top of the dome.

Mike

[Gina]

You could make a scale model - some people do that to check haow things fit together before building the full sized version.

[The Sailor]

Hi Mike, the mould is still there, I only paid for the fibreglassing of the dome. As it was round the slot was a bit of a pig to cut but got there in the end. The slot cutout was reused as the slot cover and works perfect, a rope pully afffair inside to finish it off.

Jim

[JamesF]

This might also be of interest:

http://oldsite.astro-sharp.com/dome_building_1.asp

James

[skybadger]

I don't agree that you are limited to a single telescope - I happily carry a cassegrain and a refractor on the same mount . The issue is that Newts have a significantly different viewing position than other scopes, so newts end up being mounted low needing low dome walls and the others ( end viewing) are mounted as high as poss. If you don't like stooping over a diagonal then mount them high enough to use at zenith without and take the hit of needing a stepladder for horizon viewing.

The MN190 is a 190mm diameter Maksutov Newtonian scope.

I may get a larger refractor in the future. Like a 120mm or 150mm of some description. I guess the thing with refractors is their length. Am I likely to have problems in a dome of 2.5m diameter with walls 1.2m high?

My dome is 2.4m diameter on top of a 7 foot wall, carrying a 8" cass and 140mm refractor mounted on a tall pillar. The refractor just clears the inside of the dome when balanced. I did used to have a pier which was capable of being raised and lowered which might do the job you want.

Mike

[ollypenrice]

I can't help on dome diameters and tend to think they are the wrong shape for small observatories for the reasons mentioned above, notably that the height is in the middle. However, I have a 2 metre square observatory and a 2.2 metre square one. You'd be amazed by how much easier it is to operate in the larger one. A good trick is to add a protruding bay to give you table space off the main floorplan.

Olly

[The Sailor]

I have a number of OTA options for my mount and at the moment I have two refractors plus a PST, the mount sits half into the dome with the OTA's having 6" of space all round. As I use a diagonal on the main scope I am standing sideways to it so never a problem as far as space is concerned, in actual fact it is nice and comfortable since I put an extra floor in, this gave me an extra 18" in height and able to control everything well. I will admit I bought the shed first and the observatory took shape around that, maybe in the future it will be planned a lot better .

Jim

[opticalpath]

I have a MN190 on a german mount (Gemini G42+) housed in a square shed with a slide-off flat pent roof. Internal dimensions are 7' x 7' (2.13 x 2.13 m) - which resulted really from the decision to make the overhanging roof 8' (2.4m) square from two standard size boards. It would be quite comfortable if not for the storage cupboards I've installed (I was surprised at how much storage I wanted) and other bits and bobs. They make it a little cramped to walk around the scope. If doing it again, I would have the same size area but uncluttered for the scope alone, and have an additional bay with a fixed roof section for computers and storage.

Adrian

[OzDave]

So my current plan is to go with a 2.5m dome on a 2.5m square base with 1.2m walls. My primary use will be for imaging. I rarely observe. One goal is to try to keep the building size down so no one complains (i.e. wife). Otherwise I'd go for taller walls.

I understand what people say about the dome height and that a RoR would give more room, but my site is quite exposed to the wind and I am thinking a dome may help provide better shielding. With the square base, I intend to make at least one triangular bench in the corner.

It seems to me that a scope pointing at horizon due south would have to simply clear the bottom of the dome slit by some margin in order to enable imaging. This should not matter whether it is a big 'frac or a newt. Observing might be difficult in the case of a frac with the eyepiece being far too low, but as I say, observing is a rare occurance. Am I correct in my statements here?

I have started designing things a bit in SketchUp. I'll try to post an image here to see.

David

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[Keith Howlett]

Hi Dave,

Congratulations and I think you'll be glad you opted for the larger size. I have a 3m dome with a 1.3m lower wall. It's a very good size to get around the scope and there is plenty of head-room.

I widened the dome slit and made a new shutter a couple of years ago. Although marking up the curved surface needed some care I found that cutting the fibreglass was very easy with a power jigsaw and a fibreglass blade (it has an abrasive surface rather than teeth).

Cheers,

Keith

[nytecam]

Hi Guys.....if we were to go the traditional route, with ribs and slots built out of wood and then later covered with a suitable material, a problem arises. How do we calculate the arc length of the ribs? They cannot be the same length as in a hemisphere because the slot is in the way. Therefore, please could someone advice how do we calculate mathematically the dimensions of these parts? Clear Skies, Astrosathya

Can't provide a maths formula but a [traditionally wooden] dome has four components 1] base ring, 2] shutter 'hoop', 3] ribs and 4] gores [filling the ribs].

The shutter hoop is obviously of slightly greater radius than the dome for shutter clearance. The rib spacing is a matter of choice usually set by economic use of sheet material like ply. The rib outer radius is usually the same as the base ring outer radius usually cut with jigsaw from say 11mm? ply and stacked for overlaps or reinforcement of base ring.

As you can only bend a sheet material, like ply gores, in one [cylindrical] plane - the dome is not a true hemisphere but a multi-facetted version that is close! You can reduce the dome to just 8 facets and it's still visually identical to a hemisphere! 4 of the facets are simply rectangular ply sheets curved to cylinders + 4 infil corner gore which greatly minimises complex cutting - checkout my traditional dome @ http://home.freeuk.com/m.gavin/mydome.htm and Ron Johnson's 8-facet dome [with formula/dims] that I worked on @ http://home.freeuk.com/m.gavin/rondome.htm - good luck.