The RA dither dimension and the DEC dither dimension are the number of positions in the rectangular grid in RA and DEC respectively. The RA dimension and DEC dimension correspond to the number of rows/columns in the grid pattern. The total number of positions in the grid will be the product of the RA and DEC dimensions. These must be integer numbers. If either of these numbers is even then there will not be a position in the rectangular grid at an offset of 0”, 0”.
The RA Offset and the DEC Offset are the amounts by which the grid centers will be separated. ***The New Object button should be clicked every time that one slews to a new target.*** This centers the telescope on the field and will ensure that dither patterns started at the new object begin at the correct field center. The first step forward (or the last step backwards) in a grid pattern will then move the telescope in both RA and DEC. The amount of the motion will be such that the telescope will be pointed at the North-East corner position of the grid. Subsequent Forward motions will only be in one axis, moving first to the West, then moving “down” (i.e. south) one row and moving back East. The offsets will continue moving East and West until the last position in the grid is reached, at which point further Forward motions will not be permitted.
The number of coadds
is set by default to be 1. Any number <1 will be ignored and the default
setting of 1 will be used. If the number of coadds is 1 then each click of the Forward
button will send a command to the TCS to move the telescope to the next
position in the offsets file. If the number of coadds is >1 then the Forward
button will increment a counter within the program without sending a command to
the TCS. You must click forward for every coadd at every position. It will also
send a message to the console from which the GUI was started and write a
message to the logfile. This allows the observer to make multiple observations
at the same position without having to keep track of how many coadds have been
done. As long as the Forward button is clicked before each observation,
the telescope will be offset to the next position at the correct time. This allows a simple rhythm of “forward,
expose, forward, expose, …”, so hopefully you won’t have to worry about losing
track of where you are – the program will keep track for you!
The Forward and Back buttons are used to offset the telescope either one “position” forward in the observing sequence (this may mean incrementing the coadds counter) or one position back. The Back button does not take into account the number of coadds that have been done at any observing position and reset the coadds counter to 0.
N.B. BE VERY CAUTIOUS ABOUT USING THE BACK BUTTON WHEN
YOU ARE DOING COADDS! It may be
confusing if one makes an error when using 2 or more coadds. If one clicks on
the Forward button twice, before the last coadd at a particular
position, the TCS would be sent a command to offset to the next position in the
grid. If the Back button is then clicked the telescope will be offset
back, but the next click of the Forward button will not offset the
telescope it will simply increment the number of coadds.