Pointing and Focus mechanics

Pointing and focus observations are made of sources chosen from the JCMT Pointing Catalogue. For heterodyne work we mainly take CO spectra of bright asymptotic giant branch stars; for SCUBA-2 work we take images of (mainly)  blazars.


Any systematic errors in the pointing model will generate large-scale error patterns over the sky. Pointings need to be made, therefore, often enough to follow and compensate for these errors. In practice, this is well accomplished by hourly pointings when a science field is being observed for multiple hours (‘tracked’), but certainly a new pointing is required before each new science observation.

Heterodyne pointing observations are made by taking spectra in the direction of the nominal position of source and then towards four positions offset by half of the beamsize.  (Heterodyne pointing and focus observations are not calibrated by hot and cold load measurements).

The results of an example (HARP) pointing experiment are shown below. The inset (blue) spectra correspond to the positions within the ‘five-point’ indicated by the small arrow.  The color scale used in displaying the fivepoint usually relates white with the strongest spectral line strength


The line appears strongest in the bottom-most of the 5 positions. The extent of the line is contributed interactively by the TSS (see the red vertical lines in the figure below):


. . and the pointing software derives the line intensities at each of the 5 positions and fits a Gaussian in both directions to determine the location of the peak line strength


  The resulting pointing correction is then applied.


During a focus observation, spectra are taken with the secondary mirror at each of 7 positions  spaced by 0.3mm along the Z-axis, which is the direction between the primary and secondary surfaces.  The extent of the spectral line is established


and the integrated line strength of each spectrum is extracted by the fitting of a Gaussian to the resulting (uncalibrated) intensity values:


yielding the Z-position where the signal strength is the strongest.

Similar observations when adjusting the SMU position (by 1.0mm) along the X- (up-down) and Y- axes allows for total optimization of the SMU configuration (‘focus’!). We typically do ‘focusses’ in Z, X & Y at the start of every night, again after 2 and maybe 4 hours, but less frequently in the second half of the night as the telescope settles, thermally.

In Z we aim to be within about lambda/10 or about 0.1 mm, noting that 0.3 mm ~ lambda/4 is a significant offset. In X and Y we are less sensitive – our standard step is 1 mm or about lambda. After 3 steps the signal is almost gone. So in these cases you would like to be better than say lambda/3 or so.

SCUBA-2 pointings

are short daisy observatons analyzed for the image centroid.

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