Image credit: Timothy Vollmer
For some, a gleaming white fibreglass dome is the very epitome of amateur astronomy – a glistening beacon that makes a clear statement to all who see her. For others, the subtlety of a roll-off observatory is what is required – a private space for extraordinary tasks carried out almost in secret… where the neighbours hardly notice the difference between this and any other garden shed. If you’re in either of these polar camps, you’ve probably already made your mind up between these two types… but if you’re feeling undecided, this is a fairly technical comparison between the dome and the roll-off roof.
Before I begin, let me put my cards on the table right away: I design timber observatories and to date, these have all had roll-off roofs (more’s the pity, as I would like a dome for the portfolio). However, for some customers the right choice is a dome and we have never turned these customers away – only taken the time to explain the differences between the two styles of roof. We have designs on the drawing board for domed-roof observatories and we’re very happy to provide timber buildings with domes on top where that is the best choice. If a complete dome building with matching sidewalls is best, we’ll gladly refer our customers to the most appropriate supplier, with whom we have several good working relationships. So I consider myself unbiased – your mileage may vary – but comments are welcome (add them below).
Finally, I should say that I am comparing what I would describe as “amateur” observatories here – individual ‘garden’ observatories rather than architect-designed buildings such as professional observatories and university buildings. Furthermore, I’ve kept my analysis to the current UK market’s “fully supplied and installed” offerings, broadly in the sub-£15,000 bracket. Hopefully some of the points here will be helpful to self-builders too.
So – on to the comparison – I will break this down into sections for ease.
Moisture / Condensation
This is one area in which roll-off-roof observatories tend to win, and it’s simply a matter of materials. Your average roll-off-roof is made of timber, which is a generally good insulator and is usually warm to the touch. It’s this warmth that makes the difference here: when the roof is colder than the air inside the observatory, condensation will form that will often drip on your equipment. Likewise, if the equipment is colder than the air, condensation will form there too. Either, of course, is pretty bad for your optics (mould being the major issue) and awful for your electronics and any steel parts which will begin to corrode. Domes made of GRP (Glass-Reinforced Plastic, a.k.a. Fibreglass) tend to suffer worst from this – their thin skin is usually white on the outside, which helps to radiate heat away, causing its temperature to drop and moisture will bead on the inside. Other types of dome, e.g. plywood, or rotation-moulded plastic (such as the SkyShed POD) which includes an insulating layer of air, don’t have the same problem. Mention should be made here of the walls too: it is important that any observatory has good inside air-flow as this keeps moisture and temperature levels consistent with the outdoor air, reducing condensation. GRP falls down here too: air flow is minimal and as a result, the air inside the observatory will warm up faster than everything else when the Sun hits the dome in the morning. The result of course is more condensation clinging to your (still cold) telescope. The worst thing you can do is bolt your GRP dome straight down to a concrete base… you just sealed the floor too!
Roll-off-roof observatories are typically built entirely from timber and usually have pretty good airflow around the bottom of the roof – but it’s still possible to get this wrong. It’s very important to have a suspended floor that isn’t sitting on a concrete pad: good airflow at the base of the observatory is just as important as it is at the top. If you look at our designs, you will see that they sit about six inches off the ground; the main reason for this is moisture performance. You can’t have too much air-flow in an observatory!
Both domes and roll-offs can perform very well, or very poorly in this regard, and it all comes down to design. If you’re thinking about a dome, you might very well be considering choosing a colour other than white, in order to “blend in” to its surroundings. Don’t. White is very good at not absorbing heat from sunlight and your equipment will thank you for avoiding the extremes of temperature that any darker colour will give you on a hot day! Remember that air-flow in domes usually isn’t great, so any heat will stay in there too. The best domes are white on the outside (radiating heat away) and black on the inside (absorbing heat from inside). That said, single-wall domes (e.g. GRP domes) aren’t very good at insulating and the inside temperature of the observatory will vary consistently with the temperature outside as long as no direct solar heating is taking place – so expect your equipment to be at the same temperature as the air for most of the year. Rotation-moulded plastic domes have better insulating properties, so may stay slightly cooler on hot days, but not much, and if they are well ventilated they should be at the ambient air temperature.
As I said, roll-offs, like domes, can perform very well or very poorly here. It comes down mainly to the choice of materials and the design. Again, good air-flow is of paramount importance, but so is the choice of materials for the roof. Materials with a low thermal mass are best, as well as good insulators, as they keep any heat on the outside. A black felt roof will generate (and hold) a lot of heat, so if you use felt there needs to be a good thick board separating that from the inside of the observatory. Better still, some rigid insulation in the observatory roof will further serve to keep temperatures sensible. We use EPDM which is better than felt as it has a much lower thermal mass – but the materials underneath are still important, as is ventilation.
Wind Protection & Weather
The skies are rarely clear in high winds – even less so at night. However if you use a long refractor or a large Newtonian telescope, shelter from the wind will be important if there is a stiff breeze, because otherwise buffeting from wind will upset your view and ruin your photographs. There’s no doubt that when in use, a dome gives a good deal more protection from wind than a roll-off roof.
When the observatory is closed, it’s again worth considering the strength and performance of the observatory. With either type, there is no strength to be gained from the walls of the observatory, as the dome or rolling roof must be independent of the walls in order to move. Key things to look out for are a way of securing the roof when closed, to ensure it doesn’t lift. Generally, owing to their weight and lower overall height, roll-off roofs are less likely to be dislodged by wind, but take no chances.
For many years I’ve been repairing Celestron SCTs and on one occasion I was called to a school in Cheam, Surrey. According to the staff member I spoke to, their rooftop dome had actually blown off its sidewalls in the wind, resulting in a deluge of rain entering the observatory. The Celestron CPC inside had been the unfortunate recipient of some six litres of water… inside the optical tube! I was able to restore it to its former glory, but it was a salutary lesson for them in making sure everything is secure.
Rain ingress is another weather issue to be aware of. I mentioned earlier that airflow is import, and it is – this is what helps keep the telescope free of condensation. However, make sure that you don’t have big gaps that allow rain to be blown in onto your equipment. Last month I visited a school in South Wales with this exact problem… a telescope that I had installed there two years ago was already substantially corroded because their observatory roof (built by some enthusiastic members of their PTA) was rather gappy, allowing a sprinkling of rain to be blown onto the telescope. Some domes also aren’t particularly brilliant in this regard either – if you are considering a dome that is supplied in sections, make sure you pay close attention to the application of sealant, and any rubber seals – this is ‘make or break’ for weather-tightness and far harder to put right later if you get it wrong during assembly.
Finally, some mention must be made of dew control. It’s worth saying that the only way to properly prevent dew building up on your optics and ruining your view is to install a dew heater system on the telescope. However, your observatory can play a minor role here too.
Dew is formed on a telescope when the temperature of the instrument falls below the temperature of the surrounding air (just like a bathroom mirror – if it is cold, condensation will form on it when you run a warm shower). The mechanism for telescopes to get cold is by radiating their heat away to the (much colder) sky. Anything that shields the telescope from the sky will, to a certain extent, slow down the cooling process and delay the dew condensing on it (this is how a ‘dew shield’ works) – so you can see that a dome can be helpful for this, too. That said, when there is a breeze, a dome can actually speed up the formation of dew on your optics, by shielding the telescope from the (drying) wind….. so it really just depends on the conditions. Dew that does form on your equipment (including the outside of the optical tube, and the mount) will always take longer to evaporate away inside a closed building – so once again, airflow is important for allowing your equipment to dry rapidly after you close the roof.
Surely you can have an observatory any size you like? Perhaps not. There are a few issues at play here so let’s take them one-at-a-time.
Firstly, you will need to fit your equipment in the observatory, and see out at the same time. The dome has a unique advantage here: the base of the dome can be lower than the telescope, allowing you to view all the way from horizon to zenith. Because only the slot opens, the telescope can protrude into the dome without fear of banging into it. A roll-off-roof observatory will rarely grant views all the way to the horizon in all directions – more usually, you would be able to see horizon to the South (and/or maybe the East and West) but not to the North. To facilitate all-round views of the horizon, you’d need a height-adjustable telescope (or pier) so that you can raise it once the roof is open…. the problem is simply that the middle of the telescope must be lower than the eaves of the observatory, so that the roof can safely pass over it. So it’s a win for the dome.
Second, you need to think about height from the outside. Whilst some are willing to say that observatories are “not a permanent structure” the law is a very grey area here. In reality it comes down to the discretion of your neighbours (in not complaining), and the local planners as to whether they believe that your observatory constitutes “development” under section 56 of the Town & Country Planning Act 1990. Assuming you’re not in an AONB or National Park, permitted development is limited to a height of 2.5m if your observatory will be located within 2m of your property boundary. If not, the maximum height of a double-eaved building is 4m…. but a dome doesn’t fall within the definition of double-eaved…. which is quite a quandary. I’d recommend you apply for planning permission (which should not be unreasonably withheld) in any case if your overall height will exceed 2.5m, but whatever you do, make sure your neighbours are happy and you’ll save yourself many issues down the line. Most roll-off roof observatories fall inside the 2.5m limit (and other requirements), so no planning consent is usually required.
Standard domes are available in a sizes within a fairly narrow range, typically from 2.2m to 2.7m. There’s nothing smaller (and you wouldn’t want to go much smaller because there wouldn’t be much space in there with the telescope installed) and larger domes cost a great deal more than the standard sizes, as the engineering must be much stronger as the size of the dome increases. Roll-off roof observatories win here, because you can have any size and shape that you require. As the size increases, the price of a roll-off-roof observatory usually becomes even more competitive due to the economies of scale that can be achieved.
Finally, if you choose to buy a dome with side-walls, bear in mind that there aren’t many different sizes available there either… so there isn’t an option for a short dome or an especially tall one…. you get what you get. With many roll-off observatories, the story is the same – a standard wall height. Custom observatory designers (like me) will usually build to the exact height that you need, based on a combination of your own stature, and the type and size of equipment you will be using inside.
Of course you can go to all sorts of lengths to secure your observatory (including preventing people from accessing it in the first place) but here I will speak only of the standard security measures that are usually supplied.
Domes with side-walls usually come with a locking door, which uses a single-key “cabinet lock” type lock, sometimes integrated into the handle. Suffice to say these offer minimal security as they are easily forced or picked, and the door itself is made from plastic…. enough said. The dome itself has an opening flap, but this is usually secured from the inside and is thus safer. Equally, it should be impossible to remove a dome if the observatory is closed, as there shouldn’t be any fixings you can undo from outside. Some dome manufacturers supply additional clamps that you can use to add extra security if you are going away, or during high winds.
Roll-Off observatories vary by manufacturer – some provide basic locks or shed-style hasp-and-staple locks for the door, and the roof itself is almost always secured by one or more hasp-and-staple locks. Ours are supplied with a 5-lever mortice deadlock (or two) on the entrance door. Look carefully at the door and windows though – a great lock is fantastic but won’t be much use if the door is a flimsy shed-style door. Again, our locks are installed in exterior fire-doors to ensure security. Windows need to be designed so that the glass cannot be removed from the outside. So there are perhaps more opportunities to secure a roll-off roof observatory, but check what your manufacturer provides.
The other side of security is what IT professionals call “Security through obscurity”. It is arguable that a shed-like timber building attracts considerably less attention than a bright white dome of course… but remember that sheds get broken into every day. Most burglars wouldn’t know what to do with your fantastic telescope… but they might nick your computer when they break in looking for tools. Good locks and solid doors are the best defence!
Automation takes two forms in observatories – motors that can open and close the roof (applies to domes and roll-offs) and rotator motors (applicable only to domes). Clearly automation is simpler when it comes to a roll-off, because all you have to do is operate a motor to open and close the roof. However most dome rotators come with clever controllers and/or software that will ensure your dome’s observing slot is correctly aligned with your telescope, so it shouldn’t be problematic once it is set up.
Base & Location Considerations
For most domes, and most roll-off observatories, a level base is an absolute requirement. Most, but not all. There are ways to mount domes on ground that isn’t level (usually by laying a concrete pad ) and the same can be said for roll-off-roof observatories. Some (but very few) are designed to accommodate a slope, and it can even be beneficial – for instance it may allow extra headroom in a warm room. Speaking of which….
This is a knockout punch for the roll-off-roof observatory – as warm rooms simply aren’t available for domes with side-walls included. Some domes can be ordered with “bays” which are walls that stick out and can accommodate storage or even a small laptop table, but there really isn’t a way to create a proper warm room with a pure dome, other than to build a separate (preferably insulated) shed next to it. For the roll-off-roof observatory, warm rooms are straightforward to accommodate, either under the rolling roof, or to one side of the observatory. However if you put a dome on a timber observatory, you really can have the best of both worlds – though you may need to include a raised platform around the telescope in order to accommodate the necessary ceiling height in the warm room whilst still being able to see out.
It’s pretty clear from everything I’ve written that there are advantages to both types of observatory, and no one-size-fits-all solution. Whilst roll-off-roofs win on flexibility, automation, warm rooms and security, domes provide better all-round horizon-level views, provide shielding from wind and can take up less space.
 If you are lucky enough to live in an AONB or a National Park, you must check local planning regulations. These may limit the square footage of outbuildings, or regulate their impact on the ground. Often they will compel the owner to apply for planning consent where the observatory would otherwise have been ‘permitted development’. Similarly if your premises is non-residential, or if your observatory will be located in your front garden, permission will be required.
 Another way to place an dome on sloped ground is to put it on a wooden deck. Mount your pier to a sub-surface concrete block beneath the deck, coming up through it (don’t let the deck touch the pier). The level deck provides a sound base to which a dome can easily be fixed. This is a great way to get good airflow under a dome, too.