Observatory wall height

[pbrowne]

Hi

I'm trying to work out the height that the wall of my small observatory should be for a 200cm x 200cm shed. This will house a Mach1GTO with a WO FLT-132 on a concrete pier 20.3 cm in diameter (8") and 122cm high (48 inches as recommended to me by George Whitney at Astro-Physics given the equipment above and my height of 5' 11").

At this stage I'm looking at existing zinc-alum shed modified by the manufacturer.

http://www.customshedsperth.com.au/garden-sheds

The roof will slide down on both sides from the centre ridge (rather than sliding the whole roof to one end of the shed, which saves a lot of space), eventually to be remotely controlled. I'll be working in my study, so the observatory only needs to be big enough to accommodate the equipment and it's arcs.

I found a formula for calculating the wall height at http://astronomy.mdodd.com/files/Cal...all_Height.pdf (http://astronomy.mdodd.com/files/Calculating_Observatory_Wall_Height .pdf)

"H elev = Tan(A elev) * D wall
EXAMPLE
Choose or measure A elev minimum desired or possible (if limited by trees) elevation (assume 25º)
Measure D wall distance from telescope center to wall (assume 60”)
Measure H scope height of telescope centerline above floor
Calculate H elev
Tan( 25º) = 0.466
H elev = 0.466 * 60
H elev = 27.96”
Wall may be 27.96” above telescope centerline height (H scope)
Add H scope to H elev for total wall height"


Problem is that formula puts the wall height way too high at 255cm. The image below shows my plan of the observatory in the shed with a wall height of 200cm, and includes the correct scale for the pier, mount and telescope. I'm probably looking at the lowest elevation of 25 to 30 degrees. So the wall height I image would need to be lower than 200cm.

http://130.95.21.121/pbrowne/Ob.jpg

Any advice welcome!

Peter

[Merlin66]

Peter,
The formula is correct - it would give you a 25 degree angle above your telescope to the wall height.
Looking at your sketch, if it's to scale and the telescope will fit in the roof section as shown, then you can use the supplied 2mtr wall height and get a lower horizon.

[pbrowne]

Thanks, a lower wall height than the formula derives at would be better. I have modified my drawing to a 1.8m wall height (that's the standard wall height on the shed), but with a higher 100 cm roof.

Also, I would probably just go with sliding the whole roof off towards one end of the shed...it will take up more of a footprint, but it would be cheaper to manufacture and automate the sliding of the roof (for a set and go to bed setup..with perhaps some cloud/rain monitoring). It would also give me more flexibility with the 180 degree line of sight (without the roof ridge supports).

http://130.95.21.121/pbrowne/Ob2.jpg

[Merlin66]

Peter,
You need to look at the strength of the roof and roof panels, to allow the roof to safely slide over the telescope.
When I did the TSO (Tin Shed Observatory) I had the telescope sitting just below the wall height and left the gable ends attached to the roof (for rigidity).

[rogerg]

Personally I don't like splitting the roof in the middle - leaves room for water leaks over the equipment and other complications. Like you later said rolling off one direction is easier for automation etc.

Wall height has many factors to consider:
- do you need to protect the equipment from wind or light?
- do you need the walls higher for security reasons?
- are other objects such as trees going to obstruct your horizon anyway, rendering a low wall height superficial?
- how high do you need your roof pitch for instrument clearance? For example do you want to have to park your telescope before closing the roof? This can have na impact on your wall height, depending on how much wall you include in your roof.

Your sketch how's the telescope quite high. I think the mai pivot point of mine is more in the 1.5m mark not 2.08m. That makes for quite a tall shed to not have the roof hit the equipment I'd think?

[gregbradley]

One thing to consider also is wind. If your walls are too low or the pier too high your scope can be affected by wind.

If you don't get wind where the observatory is - great. But if you do then having the end of the scope below the wall is an advantage when its windy short of getting a dome.

My dark site can be windy at times and its cost me imaging time. An FSQ 106 laughs at this as it ends up being well below the walls and is not affected by wind.

But years ago a closed tube RCOS 12.5 inch ended up being in the back of my car too often as winds about 10kmh would wreck the images.

Also consider you may end up changing your chosen scope over time as a lot end up with longer focal length scopes as the desire to image galaxies takes hold.

So 2.55 metres should be quite high. I think both my observatories walls are about 2.3 metres. One never gets wind even when its howling the other does as its quite exposed.

Greg

[pbrowne]

Thanks Roger, Ken and Greg,

Yes, the telescope quite clearly has to be below the top of the walls. The alternative is not to include the front gable in the sliding roof, which negates the advantage of sliding the whole roof (also presents sealing problems). Since this will be more or less a remote setup (albeit in my backyard), I could probably reduce the pier height to 65 cm, and with the mounting plates and bolts etc, this will bring the telescope pivot point down to 150cm. This would clear a 2M wall height. Also having a flatter roof will be structurally stronger, lighter and more wind resistant. Wouldn't allow for a 150 scope down the track, however I'll be primarily doing narrowband anyway given it's in inner suburb in Perth. I have updated the plan:

http://130.95.21.121/pbrowne/Ob3.jpg

Thanks all for your advice!

Peter

[rogerg]

I've heard that having a flatter roof is worse for wind, because there is less pressure on top of it relative to underneath and so the roof can more easily lift of in strong wind when a gust gets under it's edges. That's in relation to general building construction, not observatories specifically. I'm not sure how true it is, I'm not a structural engineer :)

[pbrowne]

That sounds logical, though the company that I'm looking at has the same low roofs for all of their standard sheds.

http://www.customshedsperth.com.au/garden-sheds

The rails in which the wheels sit will probably firmly anchor the roof, though having some wind shielding around the top of the wall might be an idea. Still, the shed won't be in an exposed location, sitting next to a brick wall...still another thing to consider:question:

[Merlin66]

Peter,
You show the telescope "parked" pointing to the pole...this is unnecessary...
You can park the scope East-West and save some height.
I have EQMod on my NEQ6 - it does this for me, semi-remotely.

[bugeater]

Sorry i havent read your post in detail, but just make sure you account for the height of the scope when it is at its lowest i.e. on its side/ready for a meridian flip. Takes a bit of height off. I didn't account for this so will probably have to fashion a taller pier at some point. In a small shed it can make quite a difference.

[ZeroID]

My 2.7 x 1.8 Shed started with the supplied flat roof and pivot removal. It 'flew' away twice in high winds but I partially put that down to the roof slope direction and the prevailing winds. The Pivot idea did not work well either but I had space restrictions.
I built a new ply roof that split across the shortest dimension and slides apart East and West. It's secured by 4 latches pinned internally and is quite stable. ( It's also in my back yard ). I created a 100mm overlap where they meet and have had only a few drips come through which I could make a modification to fix but can't be bothered.
It's advantage is I can open up in either direction and just enough to see the sky I need to see. It also covers the PC area at one end.
I strengthened the wall top edge with 150 x 25 timber ( Fence palings actually - cheap and effective ) and some cross bracing in corners. I also added a few down the corners.
The outside rails which support the two roof sections screw through into the top wood and go out to outrigger supports, fence on one side, post on other. I oly push the roof back far enough to see my target, last nght with Eta C I had most of the west side still closed across.
I run most of it from a PC inside using Team Viewer. I only need to go out there to start up and shut down. The roof is not automated.

[pbrowne]

Good point. One of the main advantages of a permanent setup is not to have to redo a polar alignment. As someone who is new to this, would there be the ability to go from park back to the recorded polar alignment automatically? Would I need to do any other alignment?

Peter

[Merlin66]

With the NEQ6/EQMod you can nominate a "Custom Park" position.
You then fire up the mount and without any further alignment issues GOTO to your target.
I don't know your Mach mount, but I'm sure there must be a similar option.

[glend]

A skillion roof can provide an adequate peak for parking pointed at the South Celestial Pole (which is the way mine works). This makes it easy to take the scope off the mount, if switching scopes and putting on cameras, etc; is also assists in setting up the balance as you can easily cant the scope from East to West with the roof closed. The only issue with the Skillion roof is that the peak has to be high enough to clear the end of your scope, and you may have to release the clutch and swing it to the east or west momentarily when sliding the roof back (as the blunt face of the skillion (the gable base) will have to clear the scope as it moves to the rear - not an issue for me.

[pbrowne]

The Mach1GTO has a 'resume from parked' function which works even after power off (at least that's what I read elsewhere...) enabling a GTO without further alignments. So I have updated my plan in case it helps someone else...unless there are some problems I'm not aware of...:eyepop:
http://130.95.21.121/pbrowne/Ob4.jpg

[Merlin66]

Peter,
I think by the time to add the additional framing (to stiffen everything up and support the rails for the roof) and the extra framing in the roof to support the wheels, you can probably add at least another 100mm to the "effective" wall height - this may allow you to raise the pier that bit more....

[ZeroID]

I just fixed my stiffening inside the walls parallel with the top of the existing panels. Added a flat rail surface to cover both surfaces. No wheels, roof just slides easily with a wee bit of wax on the rail. No weather seal problem either .. :thumbsup: