Using a deck as a stargazing platform, and keeping the timber sound

Plenty of New Zealand backyard observers never build an observatory. They carry the telescope out the back door and are observing on the deck in five minutes. A deck is level, dry underfoot on a dewy night and close to the kettle. It also has one big drawback worth understanding before you decide where the tripod goes.

The vibration problem

A timber deck is a springy structure: boards on joists, joists on bearers, bearers on piles. Every step flexes it slightly, and a telescope magnifies the result. At 200x on Saturn, someone walking across to bring you a coffee can set the image dancing for several seconds.

Some ways to live with it:

Set up close to a pile or bearer, not in the middle of a joist span. The deck is stiffest directly over its supports.
Stand still at the eyepiece. Even shifting your weight shows up at high power.
Use vibration pads under the tripod feet to damp short, sharp shakes.
Lower the magnification when people are moving about.

If you are building a new deck, or rebuilding an old one, there is a better answer: an isolated pier. This is a concrete footing and post (or steel column) that comes up through a gap in the decking without touching any of the framing. The deck can move all it likes and the telescope stays put. It is the same principle as the pier in a roll-off roof observatory, which we covered in planning a backyard observatory. Leave a clear gap of a centimetre or two around the pier and cover it with a loose collar so nothing bridges the two structures.

Any new or substantially altered deck has to meet the Building Code, and depending on its height above the ground it may need a building consent. The Building Performance guidance on building to the Code is a sensible starting point, and your council can confirm what applies to your site.

Lighting a deck without ruining your night vision

Decks tend to collect lights: step lights, festoon strings, a floodlight on the house wall. For observing, the goal is to be able to turn every one of them off, or at least to swap to something dim and red.

Put deck lights on their own switch, separate from the house exterior lights.
Low step lights aimed down at the treads are fine. Choose warm, dim, shielded fittings rather than bright cool-white strips.
A red head torch, or a red-filtered lamp at low brightness, is all you need for star charts and eyepiece changes.

Our earlier piece on outdoor lighting and light pollution goes into fitting choices in more detail.

Keeping outdoor timber in good shape

A deck that is soft, slippery or rotten is no good for anything, telescopes included. New Zealand's high UV levels are hard on exposed timber, and our wet winters do the rest.

Cleaning

Clean the deck at least once a year, ideally in spring. Sweep out the gaps, scrub off the grey surface layer and any green algae, and rinse well. Algae is a slip hazard on dewy nights, exactly when you are carrying optics in the dark. Go gently with a water blaster: too much pressure tears up the soft grain and leaves a furry surface.

Oiling and staining

Oils and stains wear fastest on the sunny, north-facing parts of a deck. Many NZ decks need a fresh coat every year or two, more often in full sun. The test is simple: sprinkle some water on the boards. If it beads, the finish is still working. If it soaks straight in and darkens the timber, it is time to recoat. Let the deck dry thoroughly before applying anything and follow the product's own instructions.

Treated pine and hardwoods

Most NZ decking is radiata pine treated to hazard class H3.2, which suits timber that is above ground and exposed to the weather. Timber that goes into the ground, such as piles, needs a higher treatment class (H5 for house and deck piles). Hardwood decking (kwila and similar) is denser and more stable, but it still greys and checks in the sun and still benefits from regular oiling. Whatever the timber, cut ends should be sealed, because that is where moisture gets in first.

Fixings near the coast

Salt air eats ordinary fixings. Within about 500 metres of the sea (the NZS 3604 high-corrosion zone), structural fixings are expected to be stainless steel, grade 304 at minimum, and grade 316 gives extra margin right on the coast. Rust streaks around nail heads are an early warning that the fixings are failing. The same salty air works on your mount and tripod, so wipe them down after a coastal night.

Checking for rot and borer

Once a year, walk the deck and look underneath where you can.

Press a screwdriver into boards and joist ends that look dark or stay damp. Sound timber resists; rotten timber lets the tip sink in.
Look for boards that flex more than their neighbours, which can point to a failing joist below.
In older untreated timber, such as a deck or steps that predate modern treatment, look for small round holes about a millimetre or two across, often with fine powdery dust nearby. These are typical borer exit holes and worth having a borer treatment specialist assess.
Check that the gaps between boards are still open. Decking gaps let water drain and air circulate; when they fill with leaf litter and dirt, the edges of the boards stay wet and start to decay.

Treat it like equipment

Treat the deck a little like a piece of equipment: keep it clean, keep the finish up, keep the lights switchable, and put the telescope where the structure is strongest. On a still, clear night it may be all the observatory you need.