Calculating Offsets from one Hexabundle to another

The script Hector_Offsets will calculate the required telescope offsets to move a target between two Hexabundles. It can also be used to calculate the offset from the centre of a plate to a given Hexabundle. This is useful for performing SNAFUs at the telescope, for example, when we generally don’t have a guide bundle located exactly in the middle of the plate.

Usage

From the command line, call Hector_Offsets followed by the name of a Hector tile file and two hexabundle names:

Hector_Offsets /path/to/Tile/file GS1 GS2

where GS1 refers to guide bundle 1 and GS2 refers to guide bundle 2. Other hexabundles are referred to by their letter.

To find the offset between the centre of the plate and a given hexabundle, use the string ‘centre’:

Hector_Offsets /path/to/Tile/file centre A

The output will be something like the following:

Moving from the centre of the plate to Hexabundle GS1:
        --> offset 3.7 arcseconds S and 0.1 arcseconds E

Note that these offsets are in the correct direction to read out to the night assistant- e.g. in this case, Hexabundle GS1 is actually North and West of the centre, so the telescope must be moved South and East.

Details

The code uses the legacy function plate2sky from the sami package to convert a distance in millimetres on the plate to offsets in RA/DEC. Whilst it does a bit better than using a simple linear plate scale, we’ve found it to only be approximately correct for large offsets (e.g. from one side of the plate to another). Use with caution!