Previous: Target acquisition with a multiaperture mask
Up: Observing procedure
Next: Calibration exposures
Previous Page: Further refinements to acquisition
Next Page: Calibration exposures

Target acquisition with a longslit

You need to consult this section only if you are using one of the supplied longslit masks (Section 1.2). If you are using a multiaperture mask that contains one (long) slit which also has fiducial holes custom-made for the target field, then you should use the multiaperture acquisition procedure in Section 5.3.

The procedure here is as follows:

(a)

Follow steps (1)--(4) in the standard multiaperture acquisition sequence (Section 5.3).

(b)

Take a direct image through the slit with white light (or sky) illumination (You may have prepared one earlier).

(c)

Take a direct image of the sky with no mask.

(d)

Use LEXT to work out the position of the target(s) and the position on the longslit where you wish to place the target (for example, using LEXT function CURSOR/GFIT.

(e)

Calculate the distance you need to move the target to put it on the slit in arcsec (,) in the LEXT coordinate frame. For example, if -positions measured on the Tektronix CCD (in pixels) are 567.4 for the slit and 578.2 for the target, the offset is arcsec.

(f)

If you also want to adjust the position angle of the slit to place it accurately on two or more targets, you need to calculate the angle offset in degrees which is positive if you need to move the line joining the targets clockwise to align them with the slit direction. For example, if you want to place the slit on two targets with positions (571.9,606.9) and (570.7,315.6) , .

This assumes that the slit is precisely aligned with the columns of the detector. If it is not for any reason, you will need to include this misalignment in the angle calculation. Note that this also assumes that the targets are roughly symmetrically disposed about the centre of the field. If this is not the case, a small lateral displacement might be introduced when the angle is changed. To cure this you would need to solve for and together rather than independently.

(g)

Move the telescope using the command

TWEAK

as described in step (7) in the standard multiaperture acquisition sequence (Section 5.3). Note that if you BLIND_OFFSET from a pointing recalibration star (step (2) of the standard multiaperture acquisition sequence) the TWEAK is likely to be small

(h)

Repeat steps (c) and (d) to check that the target field is now correct. If not, repeat steps (e) to (g).

Complications may arise if the size of the required telescope offset is large because:

• the effect of optical distortion will need to be accounted for (Section 4.8) in which case it will be necessary either to undistort the coordinates of the slit and target (or the images themselves; Section 6.1) or to do the offset in two steps so that the last one is a small offset to account empirically for the effects of distortion.

• the telescope offset can only be done in open-loop in which case the offset should be done in two steps: an initial open-loop TWEAK followed by a small closed-loop TWEAK after the position of the target has been redetermined.