Facility Seeing Control
A major component of the UKIRT Upgrades Programme was the identification, understanding, and where possible, control of “Facility Seeing” effects generated by the telescope itself and its enclosure.
To attempt to understanding these effects required extensive instrumentation of the telescope, in particular to determine air and structural temperatures. Temperature probes are accordingly installed in a variety of locations. Their outputs can be examined here. These include inside and outside air temperatures (measured by carefully calibrated aspirating, radiation-shielded, thermometers), mirror surface and air temperatures, truss steelwork temperatures (used to correct focus changes with temperature) and top-end hub and air temperatures.
Robust measurements of pure seeing effects are are a likely product of the the autofocus mode of the fast guider, in which the latter operates in a mode exactly analogous to a Differential Image Motion Monitor, or DIMM.
This is caused by the presence of warmer air inside the dome mixing with colder outside air, generally in the vicinity of the dome aperture.
Schlieren effects in the telescope beam, seen in short-exposure pupil images using a knife-edge in the bottom-end wavefront sensor (WFS), demonstrated the existence of turbulent shear between warm dome air and colder outside air blowing past in the wind. At UKIRT this has been addressed in two ways.
- The building is equipped with a powerful air-extractor system which draws ~10 dome volumes per hour through the plant room in the basement (for cooling purposes) and vents this air through a tunnel. This can be set to draw directly from the external air or (most usually) through the dome, thereby ensuring ~10 changes per hour. (If the outside air is markedly warmer than that in the dome it may be preferable not to draw it inside.) The extracted air is drawn into the coudé room through floor apertures surrounding the north and south columns and in the centre of the floor.
- The dome is equipped with a Dome Ventilation System (DVS) of 16 weather-tight closeable apertures, each 1.8m square and equipped with a roller door closure and horizontal louvres to direct the airflow internally. In median Mauna Kea winds (~7 m/s) any two of these flush ~65 dome volumes per hour through the telescope environment. The open DVS can be well seen in this image.
This is caused by the surrounding air being cooler than the primary mirror, which then generates rising convection currents when the airflow near the mirror is slow-moving.
The occurrence of mirror seeing was demonstrated in 1993 by examining the dispersion of Zernike terms (i.e., the main aberrations), in repetitive sets of WFS measures. Their RMS got larger as exposures were reduced, even at constant S/N, demonstrating that the effect was one of seeing.The RMS also reduced when the telescope was pointed away from the zenith, as would be expected if it was caused by convection currents rising from the primary mirror. Finally the RMS was markedly reduced by blowing dome air over the primary mirror with a fan, so as to remove the (relatively slow) convection currents completely.
On UKIRT two measures have been undertaken to reduce mirror seeing.
- The louvres in the lower halves of the DVS apertures (which are ~4.5 m above the primary mirror: see figure ) are tilted so as to direct the wind-driven airflow downwards towards the mirror. As a result, in moderate (>10 mph) east and west winds a marked breeze can be felt at floor level and convection from the primary mirror is expected to be removed.
- The primary mirror has been also equipped with a cooling and ventilation system. 5 kW of cooling power is provided by a chiller in the plant room and conveyed to the mirror cell by a glycol circulation system. This feeds a heat exchanger which is used to cool air drawn in and circulated by a powerful low-vibration axial fan. The cooled air passes into a plenum and thence through outward-directed radial nozzles around the inner periphery of the mirror, 15 over the optical surface, and six under the lower surface. During the day this air is confined inside the mirror skirt and protective covers and is expected to be able to cool the mirror by ~4°C in a ten-hour day.
- If the mirror nevertheless does get warmer than the ambient air the cooling system makes it possible to neutralize the heat injected by the axial fan and to ventilate the upper surface with air at ambient temperature.
At the time of writing the mirror cooling system is undergoing checkout and initial fault correction. In operation it is intended to keep the mirror cooler than the dome ambient air as much of the time as possible, bearing in mind the risk of forming ice on its surface. Clearly the temperature will be kept above the dew-point as much as possible (heating is also available against condensation problems).
These are caused by individual objects which are warmer or cooler then the surrounding air, and thereby generate convection currents.
Typical local heat sources are electrical power supplies, which in UKIRT’s case release about 1,200 W below the mirror cell. As a result a rising plume is sometimes observed in the central hole. Local heat sources are normally diluted into insignificance (except from the point of view of the overall dome air thermal budget) by brisk ventilation; electronics cabinets are equipped with effective exhaust fans to promote immediate dilution of the warm exhaust air.
The largest local heat source is the topend itself, which is heated during the day, mainly by radiation from the inside of the upper dome which can reach 30°C. The excess temperature of the steelwork over the ambient air early in the night can reach 12°C; circulating air thought the upper dome using fans on the South Col was found to reduce this by a third. Experiments are proceeding with the goal of doubling this effect.
Conversely, in light winds the topend hub can cool by radiation to space until it is several degrees colder than its surroundings. Schlieren images have in the past shown of a plume of colder air descending from the old (pre-upgrade) top-end hub. The effect has been minimized in the new topend, which is painted with Lo-Mit proprietary low-emissivity paint.
Many telescope domes are painted white, which rejects sunlight and emits in the infrared, thus reducing day-time heating. At night, however, the white paint supercools efficiently and these telescopes must contend with negative plumes of cold air falling from the sides of their dome shutters into the dome. Such plumes have never been observed at UKIRT, probably because the dome is of unpainted aluminium, with properties rather similar to Lo-Mit, and supercooling is therefore much less dramatic than for a white-painted (high-emissivity) dome.