Dither Patterns

Dither patterns at the telescope are performed in detector coordinates, ie up/down/left/right (which we’ll call x,y), and NOT in RA/Dec offsets. Therefore if a particular object orientation is desired, a suitable Position Angle (PA) must be chosen so that the dithers coincide with the detector axes.

Dither patterns have a fixed offset (or step) size, given in arcseconds.

Please note that the first exposure is taken at x,y (0,0) prior to any dithers being applied. The dither patterns are designed to return to (0,0) at the end, so if more dithers are required, the pattern can be repeated.

The list of patterns already implemented is shown below. If you would like a different pattern, please email your Liaison SA or salthelp.

Dither patterns available:

Each pair of numbers indicates the offset command the telescope receives in detector (x,y) coordinates in units of offset size for each step. An ‘E’ indicates an Exposure is taken.

Horizontal-only dithers:

2×1: E, (1,0),E, (-1,0) – start in the centre, keeping y fixed, toggle between centre and offset to the right.
3×1: E, (1,0), E, (-2,0),E, (1,0) – ie keeping y fixed, start in the centre, exposure to plus x, exposure to -x, go back to centre if needed.

Vertical-only dithers:

1×2: E, (0,1),E, (0,-1) – start in the centre, keeping x fixed, toggle between centre and offset to the top.
1×3: E, (0,1),E, (0,-2),E, (0,1) – start in centre, expose to +y, expose at -y, back to centre if needed.
1×4: E, (0,1),E, (0,1),E, (0,1),E, (0,-1),E, (0,-1),E, (0,-1)
1×5: E, (0,1),E, (0,1),E,(0,1),E,(0,1),E,(0,-1),E,(0,-1),E,(0,-1),E,(0,-1)

Other patterns:

2×2: E,(1,0),E, (0,1),E,(-1,0),E,(0,-1) – after initial exposure, offset to the right, up, left and down.
2×3: E,(1,0),E,(0,1),E,(0,1),E,(-1,0),E,(0,-1),E,(0,-1) – after initial exposure, offset to the right, up, up again, left, down, and down again.
2×4: E,(0,1),E,(0,1),E,(0,1),E(1,0),E,(0,-1),E,(0,-1),E,(0,-1),E,(-1,0) – after initial exposure, offset up, up, up again, right, down, down, down again and left.
3×2: E,(1,0),E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(0,-1) – after initial exposure, offset to the right, right again, up, left, left again, down.
3×3: E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(0,-1),E,(0,-1),E,(1,0),E,(1,0),E,(-1,1) – spiral pattern. After the initial exposure, offset to the right, up, left, left again, down, down again, right, right again and offset back to the centre.
4×2: E,(1,0),E,(1,0),E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(-1,0),E,(0,-1) – after initial exposure, offset to the right, right again, and right again, move up, offset left, left again and left again, move down back to the beginning.
4×3: E,(1,0),E,(1,0),E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(-1,0),E,(0,1),E,(1,0),E,(1,0),E,(1,0),E,(-3,-2) – after initial exposure, 3 offsets to the right, up, 3 offsets to the left, up, 3 offsets to the right and back to the start.
4×4: E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(0,-1),E,(0,-1),E,(1,0),E,(1,0),E,(1,0),E,(0,1),E,(0,1),E,(0,1),E,(-1,0),E,(-1,0),E,(-1,0),E,(1,-2) – spiral pattern.
5×2: E,(1,0),E,(1,0),E,(1,0),E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(-1,0),E,(-1,0),E,(0,-1) – after inital exposure, step 4 times to the right, then offset up, step 4 times to the left and down.
8×2: E,(1,0),E,(1,0),E,(1,0),E,(1,0),E,(1,0),E,(1,0),E,(1,0),E,(0,1),E,(-1,0),E,(-1,0),E,(-1,0),E,(-1,0),E,(-1,0),E,(-1,0),E,(-1,0),E,(0,-1) – after initial exposure, step 7 times to the right, move up, step 7 times to the left and move down.