The telescope can be guided either by the finders or by the instrument. This is specified in the <guidingmode> value in the Phase 2 visit description.


The finders are recommended for short exposures of up to TBD seconds. Beyond that, we start to see drift due to unmodelled terms in the relative pointing of the instrument and finders, probably originating in the secondary mount.

Typically, the guiding with the finders contributes about 1.5 arcsec FWHM to the image quality. However, this is skewed by a small number of images with very poor guiding. We believe that these are caused by backlash in the declination axis, which makes it difficult for the telescope to change guiding direction. Throwing out the worst 10% of images reduces the contribution to about 1.1 arcsec FWHM.

The guider performance is given in the header of each image as the value of the EGDRMTE record. The value is the RMS error in radians. To convert to FWHM, assuming a Gaussian distribution of errors, multiply by 1.665.

For more details of guiding with the finders, see Watson et al. (2012).



Last modified 2 years ago Last modified on May 21, 2015 4:45:13 AM