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Generating relative astrometry from LDSS-2 images

It is possible to generate the astrometric information from positions measured on a direct image taken with LDSS-2. This is most likely to be of use when you are at the telescope and wish to improve on astrometry obtained by other means. The main complication with data obtained this way is that the optical distortion in LDSS-2 must be accounted for in transforming from the detector focal plane to the telescope focal plane. This means that the procedure should not be used with image data obtained with other instrumentation unless the distortion is recalibrated within LEXT.

This method is discussed here although it is unlikely that you will be in a position to do this before you get to the telescope. The main steps are as follows.

• Take a direct image with LDSS-2 though a filter for which you have already worked out the distortion correction from a matrix mask exposure (see Section 6.1). You will need to record the position of the field centre and the position angle used for the original direct image since this information will be used for re-acquiring the field for spectroscopy.

• Use LEXT procedure COORDS_TO_MASK to generate a MASK list. COORDS_TO_MASK can take input either as a list of coordinates produced by a previous WRITE command or an LDSS-2 direct image. If an image is used as input, the coordinates are derived interactively in the procedure using LEXT's graphical editing/selection capability. The relevant distortion correction data derived from matrix mask observations must be available (see Section 6.1). The output is a MASK list containing the relative positions of the targets. Optionally a field centre in celestial coordinates may be provided. If so, the MASK list produced will include the assumed RA and Decs of the positions in addition to the relative sky coordinates in arcsec.

• Generate a catalogue file from the MASK list and edit as appropriate. This MASK list is the internal representation of a catalogue file. You can then either proceed with the mask design or output the MASK list to a catalogue file with WRITE/CAT for further processing.