Couple of questions on Pulsar domes

[old_eyes]

Not truly a post on DIY observatories, but I did not know where else to put it.

I am considering a Pulsar full-height dome. Most installations I have seen on the web use a concrete base. I would have thought this large thermal mass would get in the way of cooling down and getting rid of air currents. Would it not be better to mount the dome on a wooden decking platform? Is there any reason why this is a bad idea?

Second question - I currently use a side by side imaging and guide scope rig. When thinking about such a rig and the slot size on a dome, do you assume is is equivalent to an OTA with the same diameter as the distance across the two scopes?

Newbie questions for observatory owners, and I am hoping someone can answer. Thanks.

[PhotoGav]

I thoroughly promote the Pulsar dome. I have only had mine for a couple of months, but, now that a few teething issues have been dealt with, it is the best upgrade that I have made so far. Mine is on a concrete base, a 1 x 1 x 1 m central pier base and a 150mm deep, 2.4m diameter round pad. There is no evidence to suggest that the concrete base is anything put the perfect stable base on which to fix the dome. I find that the telescope is generally at ambient temperature all the time and there is basically no cool down time at all. I just press the button to start the computer, open up the shutter and off we go. So much easier than setting everything up every time. You could of course use a wooden decking, but you would still need the concrete pier base for maximum stability.

As for the side by side system you are running, yes, you are limited by the width of the aperture, 600mm, through which all equipment can 'see'. So, if you have a rig that is wider than that, it will be eclipsed by the dome wall. I run SGP to control everything and that is coping very well with slaving the dome to the mount, though I have so far only used a small telescope. The slaving becomes more critical as you approach the 600mm width - the shutter aperture must be placed very precisely to enable the equipment to see through. That is more than possible, but might take just a bit more tweaking of the dome slave values to achieve.

What ever you decide, good luck and you certainly won't regret an observatory, whatever the flavour!

[old_eyes]

Thanks for the advice. How are you guiding? OAG or guidescope? My main imaging scope is an 80ED, so with a similar guidescope there should be plenty of room with a smallish side by side bar.

old_eyes

[PhotoGav]

I guide with the OAG part of my QSI CCD. I am sure that you would be fine with the side by side set up you are suggesting, so long as the side by side bar is not huge!

[old_eyes]

Of course! I missed it was the QSI 683. I am jealous!

[Oddsocks]

17 hours ago, old_eyes said:

Would it not be better to mount the dome on a wooden decking platform? Is there any reason why this is a bad idea?

I had Pulsar dome installed this Spring on a concrete base, the Observatory has not been used through the Summer due to travelling but is being commissioned now so no real data yet on thermal cooldown rates but if you look at professional observatories of the last few hundred years they are built on concrete or brick and not on decking! 

The key is good ventillation. There were some condensation problems initially with my Pulsar dome and builders carrying out restoration work on my home while I was away fitted a solar powered stainless-steel ships yacht's extractor fan in the southern wall, this runs all day and contains a small rechargeable battery that keeps the fan running for an hour after sundown. 

On an observing night the shutter and entry door would be opened at the onset of twilight and imaging or observing won't begin for at least another hour and this should be sufficient to allow most thermals to diminish to a negligible level.

During the day the vast mass of concrete, base and pier block, are shaded from direct sun being inside the observatory so the concrete mass will be closer to average ambient then say a concrete path or roadway that would sit in full sun all day. Once the observatory is open and the pad begins to cool the rate of heat transfer would be quite low, concrete is not a great thermal conductor and with a low transfer rate thermals set up inside the observatory should not really be that great.

Last couple of points regarding a concrete base, you won't be leaving the floor inside the observatory plain concrete, not a good idea, expensive gear does get dropped and it wont bounce on concrete! Most have some kind of floor covering and the interlocking 10mm thick foam rubber tiles that many use both insulate the floor against thermal transfer from concrete pad to inside dome as well as providing excellent protection against damage to falling items. If you do go for a concrete base then if possible design it to be no bigger than 150mm wider than the dome to keep the unshaded concrete to a minimum and also to minimise the area that standing water can accumulate, good drainage is essential all round the dome to prevent water creeping under the dome walls. I made my concrete pad octagonal being a bit lazy and not wanting to spend ages on the shuttering for the concrete pour but I think the the bases made with circular shuttering look really neat and meet the requirement of minimum exposed concrete to sun and standing water

 

Regarding decking I can be more certain since my previous Skyshed Pod was on decking and it was a continual problem with water leaks, rotting timbers, insect infestation as well as the occasional rat or mouse problem.

Decking really is meant for sitting on in removable chairs etc, it does not like things left on top of it as water will collect under said objects and in no time begin to rot the decking.

With decking, the timbers expand and contract a huge ammount with the seasons, properly installed, a gap of several millimeters is left between each decking plank and it's neigbour to allow for seasonal expansion, this gap is a good route for water to track below the dome walls and start to rot the timber within a few months of fitting. Decking timbers are probably the cheapest low grade "flooring" you can get, it is certainly not allowed for internal house construction, cut from the tree heartwood after all the good stuff is removed for other purposes it often has tightly radiused end grain which means the planks warp width-ways and continually move as the moisture content changes through the year. Any extenal silicone sealant used to seal the observatory base to the timber decking is continually pulled away from the joint by this timber movement so you will be chasing leaks. Inside the observatory the gaps between the planks allow free movement of creepy-crawlies as well as high water vapour movement from the ground below and the space below the decking is a great place for a rats nest. Any timbers that do warp or rot badly will need the observatory jacking up so that the plank can be extracted and replaced which means the entire observatory timber-dome seal has to be cut away and redone.

If you try to install a water barrier sheet above the decking and below any internal dome flooring this just allows the decking timbers to become sodden with condensed vapour from the ground and if you place the barrier sheet between the decking beams and planks this just does the same to the beams, seems you just can't win.

There is no doubt that a small dome looks good on decking but as a practical proposition I found it a continual headache and given my experience decking would not even be a serious proposition.

HTH.

William.

 

[old_eyes]

Fabulous and detailed advice William. Thanks very much.

Your experience with the SkyPod is alarming and the point about condensation soaking hte timbers one I had not considered. It does seem concrete with a floor covering is more practical. Time to think further.

As you have much experience in the challange of home observatories, could I ask how you supply power to the dome? I want to have a mini-consumer unit in the dome and I was wondering how to place/fix it to the curving wall.

[PhotoGav]

I have a mains power cable running under the garden that arrives inside the dome through a conduit. I fixed a small-ish panel of mdf board to the wall with a no-nails like product, forget exactly which, and the spark wired it all in. So far, so good! Please excuse my 'cable management'!!

[Oddsocks]

On 04/10/2016 at 09:49, old_eyes said:

could I ask how you supply power to the dome? I want to have a mini-consumer unit in the dome and I was wondering how to place/fix it to the curving wall.

Having worked with industrial high voltage power supplies in medical systems, up to 200 kV, in often damp operating theatre environments I'm all too aware of the dangers of dampness and electricity!

I did my own installation but not being domestic qualified I still had to have it certified by council building control.

Power out to the dome was by buried armoured two core, live and neutral only, with a two metre earth rod at the dome and the cable armour grounded at the house supply end only, insulated at the dome end. All dome earthing is via the grounding rod including the steel pier. Consumer unit, light, light switch and sockets all IP66 waterproof rated, consumer unit and sockets all individually RCD protected, overkill really but I already had the RCD protected sockets for another cancelled job.

The Pulsar dome has an inward fitting dome-wall flange so it is just possible for some rain water drips to be blown inside the dome as well as condensation drips from the dome roof to track down the to the dome-wall flange so it is not a good idea to mount a consumer unit directly on the wall as any drips from the flange may fall on it.

My Pulsar dome has a single bay and if you have a bay you could fix the consumer unit inside the bay on the flat wall, but I wanted my bay free for computer and networking so I mounted my consumer unit, three double sockets, twin network ports and earth distribution box all on a piece of varnished 3/4 ply 1 mtr tall and around 300mm wide. The plywood panel is screwed to a block of wood on the floor set 60mm away from the wall, there is a stand-off wood block screwed to the dome wall flange-web three quarters of the height of the wall.

Difficult to visualize but imagine a free standing plank set away from the wall and screwed to a wood block on the the floor with a single wood block supporting the plank near the top. This means no screw holes in the dome exterior wall just a single bolt hole internally where each wall segment meets its neighbour and forms an internal webing joint.

By standing a panel inside the dome any dripping water from the dome-wall flange falls to the ground behind the panel and being a flat surface the consumer unit and sockets are simple to fit.

The panel was transferred from my old Skyshed POD and attached are a couple of pictures from when it was installed there, the only change being the stand-off support is nearer the top of the panel in the Pulsar and in the Skyshed the stand-off is fixed to one of the POD wall through bolts, in the Pulsar it is fixed directly to the turned-in wall web where the individual wall panels meet.

The pictures were taken before the network cables were terminated so are just seen looping out of the surface mount box, in the Pulsar these are now neatly terminated.

 

 

Edited June 29, 2017 by Oddsocks
Added photos

[PhotoGav]

There you have it - the two ends of the dome electrical install spectrum!!! That's quite some job you've done there @Oddsocks! I hope the dome is now fully functional after your travels and you are enjoying the upgrade from the SkyPod.

[old_eyes]

@ PhotoGav and Oddsocks.

Thanks. Both elegant and neat solutions. Oddsocks; your point about not touching the walls is well made and a good warning. I like your separate block idea.

[Oddsocks]

55 minutes ago, PhotoGav said:

I hope the dome is now fully functional after your travels and you are enjoying the upgrade from the SkyPod.

Thanks Gav.

Had a great summer travelling and on the last few days staying with the partners in laws in Switzerland, supposed to be out sailing today but tipping down so just reading eating and drinking today!

Will be home this weekend and then the fun starts getting to grips with ACP and setting up the Paramount MX sky model PEC and polar alignment, reckon that will take me a month (at least).

58 minutes ago, old_eyes said:

Oddsocks; your point about not touching the walls is well made and a good warning. I like your separate block idea.

Best of luck with your dome old_eyes.

Only other tip I can give re a concrete pad if you go that way is to make sure the surface is properly sealed before the dome is installed otherwise the sealant they use around the base will not stick, I was lucky that the builders were watching out for me while away and they found lots of water inside the dome one morning after the sealant had peeled away from the dusty concrete pad, the builders lifted the dome in one piece and sealed the concrete with Ronseal Concrete Sealant, which is a solvent based acrylic polymer that penetrates deeply into the concrete slab and dries in a few hours, then they lowered the observatory back onto the pad and redid the silicone rubber sealant, that was back in May and since then it has remained dry.

[PhotoGav]

I heeded Oddsocks' advice regarding sealing the concrete base and now that I have put a good thick bead of silicon around the base, the inside of the dome floor remains dry even after the harshest of downpours. It is slightly prone to a bit of moisture being trapped underneath the waterproof membrane under the rubber floor tiles, but that seems to be quite normal and to be expected. I have a dehumidifier that is set to run during the daytime to keep humidty down to 40%. Nothing has ever been wet within the working area of the dome, so far!

[cloudnine]

I'm a bit late to the party, but my pulsar dome was installed just over one month ago on a DIY concrete base. I love it! It has transformed my observing as (a) I am naturally lazy, (b) the equipment is set-up and ready to go, and (c) being shielded from the breeze make for more comfortable and hence longer observing sessions!

 

Based on the evidence of reading the temperature inside the dome at the start and end of observing sessions, where the temperature drop over the course of a couple of hours is only about 1 degree C, I reckon that the dome is thin enough and the concrete shielded enough from direct sunlight that reaching thermal equilibrium isn't a major issue... It could be just the time of year... maybe in the depths of winter it might be more noticeable.  

As others have mentioned, sealing the concrete base is important, primarily to avoid the surface dusting up resulting in concrete dust settling on your nice polished surfaces ... not a good combination when it comes to cleaning optics. 

Attached is an image showing my non-permanent cable routing ... The channel is back-filled with pea-shingle to avoid mice entering the building! The channel is also handy in case of flooding. Notice the differing levels of concrete... an example of my poor handiwork! The obsy copes well with this.

Martin

[DAVE AMENDALL]

Hi, I made the mistake of having my 2.2 metre observatory erected on decking instead of concrete. The decking was down two years before the Obsey was installed a year ago. So far I have had no problems with dampness or condensation, but after reading the postings, eventually rot must come. I will then have to have the observatory lifted in one piece placed on the lawn at the side and a proper concrete base installed,and the observatory placed on the  new base. The observatory is  heavier duty than a Pulsar so will be heavier to lift,but will deal with that when the time comes. The cheapest is the always the most expensive in the end

Dave

[steppenwolf]

I too have a Pulsar dome mounted on a concrete base and it does a fantastic job - highly recommended!

You might find this link of some interest. Somewhere within the pages there is an image of my AC mains power connection.

Quote

do you assume is is equivalent to an OTA with the same diameter as the distance across the two scopes?

In essence, yes but if the two 'scopes are of different weights then they will be offset from the centre line so you need to measure the distance from the centre of the RA axis to the furthest extreme of the lighter instrument and then double that measurement to give you the total diameter allowing for imaging of both sides of the meridian. This shouldn't be too much of an issue but bear in mind that one telescope or the other may well be below the base of the aperture slit at some orientations.

There is no way of allowing for the offset as all the software that includes dome slaving assumes that the light cone passes through the centre line of the telescope. I have successfully used such a system with a WO 72mm Megrez and a WO FLT 98 mounted on a standard ADM dual bar and that was with my first Pulsar observatory (2.1meter) that had only a 500mm aperture slit - the current (2.2metre) one has a 600mm aperture slit.

[ollypenrice]

Would switching from side by side to piggyback be an option?

As for concrete, I don't think heat is an issue. We have a six tonne concrete base for our 4-scope remote hosting shed and daytime temperatures reaching the high thirties in summer. I think the effect on seeing is negligible. Likewise in one of our own 'in house' roll offs we imaged at 0.66"PP with a 14 inch deep sky rig and had no issues.

Olly

[steppenwolf]

Quote

Would switching from side by side to piggyback be an option?

Yes, this would work better (counterbalance issues aside) as the dynamic of the pair would be constant and setting the distance from the centre of the RA axis to the telescope's optical axis as the centre point between the extreme inside edge of the first telescope and the extreme outside edge of the second telescope would make both telescope 'look' through the aperture. I've always preferred a side by side arrangement though as it reduces the moment arm of the RA axis and keeps the weight closer to the centre of the mount.

[old_eyes]

Thanks for all the comments. Having had a look at my rig and where it balances, I think I might need a smaller side by side bar. The current Geoptik one has the scopes quite widely spaced and with the balancing offset I think it will look pretty wide. I went side-by-side rather than piggy back to keep the weight closer to the axis of rotation, but it does seem to make things more complicated with a dome.

Steve - interested in your point about the lack of software available for modelling a side by side rig. I wonder how much solid geometry I could dredge up from school?

Olly - I well remember your concrete bases with the run off sheds. I should have realised that if they worked in southern France in summer there wouldn't be much trouble in  Wales in winter!

[JayStar]

Old thread I know, but just wanted to ask whether you ended up with the install on concrete or decking?

I am thinking of replacing my SkyShed Pod.  That has been sitting on decking for well over 3 years with not a single problem.  No rot, not leaks, nothing.

For me I have to use the decking for the Pulsar (or NexDome) - hoping it's not an issue.

 

 

[Rusted]

Just a few thoughts: Domes are great!
Instant gratification for observing/imaging and instant cover when a passing shower arrives.

The rollers of my [raised] home made, plywood dome rest on heavy timbers and there are plywood obs. walls.
The floor is high quality, larch planking and is deliberately spaced for ventilation.

The gaps between the observatory floor joists are open to the outside air to maximise ventilation.
Thermal mass must be very low but still averages 3-5F offset.
This is the difference between the outside air temperature [in deep shade] and inside the dome.
An indoor/outdoor digital thermometer allows easy monitoring at low cost.

A concrete pad would and should be shaded from direct sunshine by the obsy. itself.
The pad's high thermal mass would act as a thermal flywheel.
Probably helping to even out the observatory's internal temperatures.

Careful sealing around the edges of a GRP dome will prevent wind blown, water ingress.
A covering of insulating foam panels over concrete sounds ideal. Though I have never tried them.

BTW: There are waterproof wood product sheets available in metric 8'x4'.
A "wooden" floor doesn't have to be softwood or terrace boarding. Man-made terrace boarding is also available.

An alternative to a complete concrete pad would be individual, cast concrete, pyramidal, carport anchors.
I used eight of these for my two storey building. Four more for the isolated timber pier.
This avoided the very serious problem of a massive concrete pad to be rid of on moving house!

My own ground floor is pea gravel over self-compacting gravel.
I added concrete slabs for the foot trafficked areas to avoid the gravel dust being carried upstairs.
Artificial lawn might make an amusing "alternative" floor covering for an observatory.