Centering on bright objects using dispersed light

This procedure saves the time associated moving the grating to the imaging position, but it only efficient if the dispersed spectrum can be detected with high signal to noise in short integrations. It's good for standards.

This works if:

a.

a clear spectrum can be seen in a short integration time and

b.

if you can get the star nearly centered in the slit "visually" using the acquisition TV or by other methods.

1.

Establish guiding, either on the object itself or on an offset guide star.

2.

Take a spectrum of the object and set the display and integration time so you can clearly see it on the display.

3.

Run the macro peak (in /ua10/irs/scripts).

4.

Start up imexam at the same time and use the j key to get a gaussian fit to the spatial profile of the spectrum and measure its amplitude. The macro takes six spectra at the positions:

-.5 -.3 -.1 +.1 +.3 +.5 arcsec (offsets with respect to starting position)

in that order. The telescope returns to the starting position (0.0) when it finishes.

By examining the signal as a function of the offset, you can easily determine what offsets are required to center the star.

5.

Make required offsets (with follow on).

6.

Repeat peak to confirm that the star is centered.