Dithering

Figure 2.1: HSC dithering patterns. a): the pre-defined 5-point pattern; b): example of a circular pattern for $N=5$ (TDITH=$\theta $, RDITH=$r_D$, NDITH=$N$).

Image ndith5

The chips of the HSC are separated by small gaps (up to 53 arcsec) and some parts have defects2.1, so to avoid having a target fall in such gap or bad chip, and to ensure complete coverage of the whole field of view, dithering is used. Dithering procedures for HSC are similar to the ones available for Suprime-Cam2.2. In particular, there is a pre-defined 5-point dither pattern, a customized N-position circular pattern, and no-dither option is allowed as well. The pre-defined 5-point and 5-position circular patterns are recommended. Only one single exposure is taken on one position, which means that the recommended patterns will result in 5 exposures for a given field.

In the pre-defined 5-point dither, the user defines the steps dRA and dDEC, in arcsec, and the telescope points according to the scheme in Figure 2.1a. In the circular case, the user defines the radius $r_{D}$ in arcsec, initial angle $\theta $ in degrees, and number of steps $N$ (5 recommended). Consecutive exposures are taken at positions that lay on a common circle of radius $r_{D}$, every $360^\circ / N$ from each other, as in Figure 2.1b. Note that this is not done by rotation of the instrument, but by moving the telescope; the position angle of the instrument remains constant. The suggested values are $120''$ for dRA, dDEC, and $r_{D}$; and $15^\circ$ and $10^{\circ}$ for $\theta $ in the case of $N\neq 5$ and $N=5$, respectively.

If one sequence, either pre-defined 5-point or circular, takes a lot of time, it may be split into two or more, using the parameters SKIP and STOP. For example, $\mathit{STOP}=3$ means that the telescope will observe only in the first three positions, $\mathit{SKIP}=3$ means that the telescope will start from the 4-th position, skipping the first three. $\mathit{SKIP}=1$ and $\mathit{STOP}=2$ will make the telescope move only to the second position. $\mathit{STOP}$ must always be larger than $\mathit{SKIP}$.

Without dithering, it may be necessary to introduce an offset to the target position, in order to avoid placing it in a gap between the CCDs.

See the following links for the details:

https://www.subarutelescope.org/Observing/Instruments/HSC/ope.html.