Mounting, Sky Access, and Pointing (NB: User Input Requested!)
Sky access NB: User Input is requested
IN BRIEF: UKIRT has DEC limits of +60° 07′ and -40°. The hour angle limits are approximately +4.5hr and -4.5hr, corresponding to an airmass of 2.13. The limits are in equinox of date.
FURTHER DETAILS: The mounting is an “English” yoke design, which is very solid and which permitted important cost saving by the use of inexpensive bearings. A three-dimensional diagram and description can be seen here. However the design physically limits the telescope’s access to the northern sky (in a very fundamental way: collision with the massive steel yoke) to declinations south of +60° 07′. To east, west and south the ultimate limits are set by collision with dome structures. NOTE: the DEC limits are in equinox of date or “Mount” coordinates. Precession may therefore take a source which was accessible in, e.g., 1950 (or even 1985!), out of reach.
The telescope safety systems set more conservative limits than the physical constraints. A mechanical safety system limits sky access by means of cone switches, which trigger when a steel ball in a cone of semi-angle 29° rolls out of the bottom of the cone to make an electrical contact. The accessible sky therefore subtends a cone of semi-angle 62°, centered on the zenith, and cut off at a northern dec limit of 60°. Hence, since UKIRT’s latitude is a little under 20° north, the current southern limit is a little over 42° south. (Somewhat more conservative software limits are normally the first to trigger, however.) The current hour angle limits occur at an airmass of approximately 2.13, corresponding to maximum hour angles of plus or minus ~4.5 hours
The southern limit is slightly softened by refraction and it is in principle possible to observe a source at -42°, albeit not for any great length of time. A working limit of -40° is therefore normally quoted. This permits sufficient time both to set up on a target and to complete a useful short programme. Targets further south than this are really only worth attempting if (a) they are scientifically urgent and (b) the required observations can be done very quickly.
FUTURE POSSIBILITY: Declinations down to -50° are now permitted (physically) by the more compact design of the new topend ring. It is planned that access down to -47° 30′ will eventually be made available, by rotating the cone switches northwards on the telescope (so that they trigger at a more southerly DEC), and adjusting the software limits accordingly. This will not only offer access to a currently unavailable section of the a sky (containing, inter alia, Centaurus A, Omega Centauri, NGC 3256, NGC 7552 and NGC 7852!) but will significantly extend the time available on objects at less extreme declinations. We are interested in assessing the level of demand for this change. Input is invited from potential users who have favorite targets falling in the range from -42° to -47° 30′ .
NOTE FOR USERS: Accessibility of potential targets can be verified using a facility elsewhere in these pages.
Pointing accuracy and target acquisition.
UKIRT’s mount is exceptionally sturdy, the yoke being particularly stiff by comparison with, e.g., the horseshoe or split-ring mounts of many of its more expensive contemporaries. It appears to be very free of inelastic deformations, and therefore the more readily modelled. UKIRT’s absolute pointing accuracy is consequently amongst the best of any telescopes and probably the best amongst equatorially-mounted instruments. (This is probably facilitated by the restricted sky coverage, which obviously puts fewer demands on a pointing model than for telescopes which must reach airmasses of 3 or more.) Pointing tests are carried out every month or two as opportunity arises.
When a recent pointing model solution is in place UKIRT regularly delivers RMS all-sky absolute accuracies around 1.”3, occasionally slightly better.
While this level of absolute pointing accuracy is very useful, target acquisition is normally done via a “nearstar”, currently chosen from the Carlsberg Meridian Catalogue (CMC). This contains about 1 star per square degree down to V ~11, with positions known to ~0.”1 at the present epoch. Telescope offsets from such stars over distances of order a degree will reliably locate a target to ~0.”3 in RA and DEC. For higher precision still, e.g. for locating invisible objects on a narrow spectrometer slit, crosshead offsets offer precision down to <0.”1, at least in principle.
UKIRT’s mounting structure is not rigidly attached to the concrete piers which support it, but sits freely on two sets of three ball-bearings under the N and S columns respectively. Thin brass pins, which shear under small lateral loads, assure this freedom, but allow the telescope to be accurately realigned after a quake has moved it relative to the piers, which happens several times per year. The (manual) realignment process takes 30 to 45 minutes.