updated 20 January, 1999

This is the new procedure for setting theDIS grating zerostep positions. NOTE: The first part needs to beperformed each time the DIS PC is rebooted or the Mirella software isrestarted (i.e. every afternoon during checkout).

I. Tell the DIS which gratings are in andthe zerostep positions.

Specify which gratings are in eachbay. In an mcnode session, tell the DIS which gratings are loadedwith the command

dis_define_gratings

This command reads the information in thefile /home/apotop/MC/gratings and issues the following mccommands:

dis grating n=1 type=low (e.g.)

(The choices for n are 1, and2 (for the low and high bays respectively). The choices fortype are low, medium, and high. Note, youcannot abbreviate "medium".)

and

dis grating n=1 col=b zerostep=280(e.g.)

The choices for n are 1, and2 (i.e. low or high res bay). The choices for col areb, and r.

If the gratings have not been taken out,skip part II.

II. Measure the zerostep positions - Youmust do this if the gratings have been taken out.

Edit the grating names in the file/home/apotop/MC/gratings (e.g. low, medium, high). Don't abbreviatethe grating names.

Turn on an arc lamp. Use the Neon lamp forthe red grating and the helium lamp for the blue grating. In Remark,set the central wavelength to 7245 for the red grating and 4472 forthe blue grating. These two lines correspond to bright lines in Neonand Helium, respectively. Look at the APO lamp spectra plots toidentify these lines. Take a spectrum with the appropriate lamp.Measure the position of the 7245 (for red) or 4472 (for blue) line onthe image. You can use implot to do this. On the red image, the 7245line should be on pixel 400 (the center of the chip). On the blueimage, the 4472 line should be on pixel 256 (the center of the chip).If it is not, subtract the position of the line from theposition of the center of the chip (e.g. if the 7245 line is at pixel350.2,

400 pix - 350.2 pix = +49.8 pix

Multiply this number by the step/pixelscale for that color grating. The step/pixel scales are:

blue    +0.30 step/pixred     -0.19 step/pix

Keep track of the signs.

e.g.,

+49.8 pix * (-.19 step/pix) = -9.5steps

Round the result off to the nearest wholenumber. If the result is negative subtract it from the zerostep valuefor that grating in the gratings file. If the result is positive, addit to the zerostep value for that grating in the gratings file. Saveyour changes and reissue the mc command dis_define_gratings.This will read in the new values. In Remark, click sync and reset thewavelength center. Take another spectrum to check that you have thecorrect wavelength center.

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