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Camera Head Rotation

The camera head of the IPCS should be rotated until the spectral direction is aligned with the camera pixel direction of the IPCS. At present the camera head mechanism is unreliable, and this adjustment should only be done by RGO technical staff, who should then disconnect the power to the motor which drives the camera head to stop it from rotating of its own accord.

Set up the spectrograph and comparison lamp unit to give a narrow tungsten spectrum with reasonably uniform illumination along the spectrum:--

Open the shutter carefully in OVERSCAN mode to check the count rate, and adjust the comparison filters, slit width and grating angle if necessary.

Set up the IPCS using the ICL procedure IPCSFORMAT to have a window about 30 camera pixels wide with the spectrum from the narrow dekker in the centre, and with full (x8) resolution in the X direction and x2 or x4 resolution in the Y direction. Alternatively the ICL procedure IPCSSDC should do the automatically, although it may be necessary to adjust the start pixel of the window in the Y direction; see the `` IPCS-II Users' Manual'' for details of how to do this.

Open the shutter and take an exposure of at least 200 seconds to a reasonable count level. When the exposure has finished, at the DMS control window type:--

Y-FIND

Using the DMS mouse put the cursor a little way in from one end of the spectrum and press the space bar. The DMS will determine the centroid in the Y direction of the spectrum. Press the space bar again and repeat the process at the other end of the spectrum.

If the spectrum is clearly tilted then the camera head should be rotated. First, check that the power lead to the motor which drives the camera head rotation is connected. The camera head can then be rotated from the ICL interface by typing:--

IPROT <value>

where value in an integer between 0 and 65535 which represents the amount the camera head is to be rotated. Each step represents 4.4 seconds of arc, and the value is biased by 32768, so that 32760 rotates 8 steps in a negative direction and 32778 rotates 10 steps in a positive direction. A value of 32768 stops the rotation.

As a rough guide, a slope of 1 pixel over the length of the spectrum at x2 resolution implies that a rotation of 25 units is required, and to rotate the spectrum clockwise on the DMS display a negative rotation of the camera head is required.

This process should be rotated until the spectrum is straight to an accuracy of about 0.5 pixel slope at x2 resolution (or 1 pixel at x4 resolution). When the rotation is correct it is advisable to disconnect the power lead to the rotation drive motor.