A martyr to solar gain.

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Wednesday: I went up to the dome do some proper measuring using a Laser-Line thermal measuring gun. This shoots a laser dot as a pointing guide and measures the surface temperature around the projected spot. Note that the further away the gun is from the surface the wider the measuring area. Think of it like a cone spreading out from the muzzle of the "gun." Best pin-point accuracy is within a couple of feet but usually unnecessary in practice. The manufacturers missed a trick in not providing further angled spots to indicate the measured field width.

The air temperature inside and outside the dome was 85F/80F today in bright sunshine with thin, high cloud casting only occasional, weak shadows. The outside thermal sensor is hidden in an open section under the NE veranda floor. It hangs freely in mid air where the sun can't ever reach. This veranda floor area is either in the shade of the adjoining shed roof or that of the dome or its supporting building. I have two other free air, shaded sensors on other, digital thermometers and they usually all agree with each other.

Since both buildings are largely constructed of timber and clad with plywood there is no thermal build up. Nor thermal storage to affect the air temperature readings in the shade. The inside dome thermometer head is mounted on the north side of the telescope pier. About four feet from the observatory floor and never in direct sunlight.

The highest dome panels, perpendicular to the sun, measured 130F /55C [!!!] on the inside measured at about 8' distance. The further down the dome the lower the panel temperature readings. The further away from the sun's direction the lower the panel temperatures. Which is exactly as expected.

The bottom dome panels to the SE [sunny side] measured 115F. The bare, unpainted, 15mm plywood walls, again to the south east, showed 100F. The northerly observatory walls measured just 84F. The northerly, lower dome panels 90F. The observatory walls, being vertical, are not perpendicular to the sun.

Bearing in mind today's air temperature in the low 80sF, the almost constant sunshine and the green paint it is no surprise how hot the dome panels really are. If the sun goes behind a cloud the panel temperatures would drop rapidly. Plywood has poor thermal storage capacity. Experience shows that the dome cools quickly when thick clouds intervene and the slit is open to allow free air movement.

Options to reduce inside temperatures include increased air movement through the walls. I have some large louvered panels which could be set into obs. wall panels to make a through draft in a breeze. The observation slit provides a large area for heat to escape right up to and over the zenith. The downside is that it is often uncomfortable to sit in a breeze. So the louvered panels would need to be fully sealed when not actually required.

I have tall, decorative, deciduous shrubs growing in front of the southerly and south western side of the observatory. These have already reached the veranda floor in height and will grow and thicken further over time. A similar shade shrub would need a large pot to grow in the SE corner between the obs. building and the shed. A conifer might be better and could be rolled about in its pot. To increase or reduce the amount of shade or simply to adjust for growth in girth.

Shading the dome itself would be a better idea but very difficult to easily arrange. The dome slit must obviously be opened for seeing out. Not to mention rotated to follow the sun. The veranda fence could be covered with shade netting but its shadow would only cover the veranda floor at this time of year. Making the exercise completely and utterly pointless. The veranda floor shades the top half of the lower floor of the obs. building. Nor very useful at reducing the upstairs obs. temperatures. Where it really matters.

More difficult and advanced shading could involve fixing louvers on the upstairs obs. walls to cast solid shadow but allow air movement. Some adjustment of the individual louver's angle would allow the degree of shade to be matched to solar altitude. It would only be useful on the southerly run of walls as the sun never [ever] reaches the northerly arc.

Placing louvers on the outside of the dome would be ridiculously complicated by the changing angles to the vertical and horizontal. An exo-skeleton of standoffs would allow shade netting to be fitted to the outside of the dome. Though it would not be a remotely easy exercise. Not least because of the difficulty in reaching most of the dome. Laying the net directly on the dome panels would probably just trap the heat inside the net. Fortunately net has very low drag in winds. Not sure about countless louvers in a gale.

I once considered a roll-off dome, on rails, but the telescopes project so high and wide that they would not clear the present observing slit as the dome was pushed away Westwards from the obs. building. The dome could be split in two and be moved both east and west. The difficulty of achieving this safely would be monumental. Or just, plain, old mental.

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