Pointing, autoguiding, differential tracking, dithering

Information for WHT observers about pointing, blind-offsetting, autoguiding, tracking, differential tracking and dithering is summarised on this page. This page was written prior to the advent of WEAVE, but much of the information is relevant to observations with WEAVE. Further details can be found on the WHT quality-control pages.

Sections below:

Pointing and blind-offsetting
Autoguiding and tracking
Differential tracking
Dithering
Differential tracking with dithering

Pointing and blind-offsetting
The WHT typically points anywhere on the sky with rms accuracy 1.5 - 2.5 arcsec for an instrument at the Cassegrain or prime foci, and 2.5 - 3.5 arcsec for an instrument at one of the Nasmyth foci.

Acquisition of faint objects (e.g. r > 19 in dark of moon) into a spectrograph slit requires blind-offsetting from a brighter star. Blind-offsetting is accurate to < 0.2 arcsec rms over offsets of up to at least 20 arcmin (see the quality-control pages). For detailed advice about blind-offsetting, see the ISIS observer's guide section 6.2.1 (much of this advice is also relevant to blind-offsetting when observing with other instruments).

Autoguiding and tracking
Most observations are autoguided. The autoguider either intercepts light from a star outside the field of view of the main science camera (as at Cassegrain, or for prime-focus imaging), or intercepts part of the light from a star in that field of view, via a beamsplitter (often used at Nasmyth focus). When autoguiding, the telescope tracks with accuracy < 0.05 arcsec on timescales of minutes, and <~ 0.3 arcsec on a timescale ~ 1 hour (flexure etc). Further information can be found on the Cassegrain autoguider and autoguider detectors pages.

Observers do not need to select guide stars in advance of observing (except for AF2). The telescope operator will identify a suitable guide star when pointing the telescope, and will also acquire the guide star and close the guide loop.

For most instruments, autoguiding is in azimuth and elevation only. For AF2, guiding in rotation is also provided.

Without autoguiding, the WHT tracks with accuracy ~ 1 arcsec over 10 minutes, and on timescales of this order, the tracking drift is steady and monotonic. There's no apparent dependence of drift amplitude or direction on either azimuth or elevation, except that tracking is poorer (by a factor ~ 2) at high elevations (> 80 deg), where the speed of rotation in azimuth is fastest.

The rotators track accurately enough to ensure < 0.2 arcsec movement of a star at the edge of the field of view at any of the WHT's focal stations.

There are occasional reports of oscillations in azimuth and elevation and in the positions of the rotators, but these are rare (see the quality-control pages). Oscillations in azimuth and elevation (and for AF2 only, rotation) with period >~ 30 sec should be eliminated by autoguiding using short exposure times (faster oscillations will not be).

Differential tracking
The WHT can track targets moving at non-sidereal rates, e.g. solar-system targets. Differential rates in RA and Dec of dra sec/sec and ddec arcsec/sec respectively can be entered into the telescope control system (TCS) (manual here) with the command:

TCS> DIFF_RATES dra ddec

The allowed ranges of both dra and ddec are -100 to 100. Note the RA differential rate dra required by the TCS is coordinate motion in seconds change of RA per second of time, not coordinate motion in arcsec/sec (15 * dra) or sky motion (15 * cos(dec) * dra), which may be quoted by some ephemerides and/or used at other telescopes.

Note that a separate command, PROPER_MOTION, is used to enter proper motions, which determine the coordinates to which the telescope is pointed, but which don't provide differential tracking.

For at least ISIS, LIRIS and ACAM, the WHT is able to autoguide while differentially tracking, but only while the guide star remains in the ~ 1-arcmin field of view of the autoguider camera. After that, the target must be re-acquired. To maximise the time elapsing until the guide star reaches the edge of the field of view, the telescope operator will probably position the star initially at the opposite edge (he/she will first need to observe the direction of drift on the autoguider). Autoguiding while tracking has not been tested at other focal stations e.g. for imaging at prime-focus.

Observing the moon (e.g. the limb, to study the exosphere) poses a particular challenge, because the differential rates change rapidly. A command:

TCS> GOMOON

is available to point at the centre of the moon (from which the usual offsets can be made) and track (without autoguiding), taking into account the time-varying differential rates in each coordinate. This facility is not available for other celestial bodies.

Dithering
Scripts are available to dither in a raster pattern for imaging with ACAM or LIRIS, and to dither along the slit for spectroscopy with LIRIS. In each case, the script may be used with or without autoguiding, and the accuracy of pointing, within the dither pattern (and when repeating the dither pattern), will be << 1 arcsec.

E.g. observers requiring dithering when imaging with ACAM can use the multdither command, which steps the telescope through a spiral of positions with user-defined step size, e.g.:

If the dithering moves a guide star outside the readout window on the autoguider (e.g. if that window hasn't been made large enough), the dither script will probably pause and wait for the guide star to appear.

Differential tracking + dithering
Differential tracking and automatic dithering are not offered together. Visiting observers interested in dithered, autoguided, observations of a moving target may be able to dither manually, using differential tracking with autoguiding at each dither point. The interruptions for manual dithering will introduce small (< 1 arcsec) errors in the dither stepping.

Observers interested in using differential tracking with dithering in this way are advised to contact the relevant ING instrument specialist before applying for observing time.