Focus camera lens

 The final step in setting up the instrument is to focus the camera lens. The camera lens must be adjusted to focus the collimated beam coming through the etalon onto the CCD detector. This is done by imaging the calibration lamp and adjusting the camera focus to provide the sharpest rings possible. As you may recall from the Fabry-Perot instrumental profile, the monochromatic rings in a Fabry-Perot image have a width that decreases with the radius of the ring. If you are using the Palomar ET-50 etalon, this trend is easily visible. Consequently, the outer rings in an image are usually best for focusing purposes.

To prepare to optimize the focus, first use the cameras command to check that the binning pattern for the CCD is set to 1 × 1, to optimize resolution. If you have not already done so, you should also prepare a directory in scr:[ccd] in which to save images:

The frame command is used to initialize the frame counter; it should probably be reset at the beginning of each night.

Next, update the on-line filter list with the names of any non-calibration filters that have been installed:

Remember to make sure that there are no duplicate initial letters among the filter names, and that names on the 200-inch telescope are given in capital letters. Finally, place the calibration filter in the optical path, turn on the calibration lamp, set the etalon spacing to the fiducial value of 100, and start taking exposures:

Move the camera lens focus arm to one extreme of its movement. Using the coarse scale drawn on the piece of tape fastened to the instrument tube (Figure 17), increment the focus by a couple of steps at a time, taking an image at each position. Use the optimal exposure time determined from the earlier calibration images. The RINGCHECK script or other FIGARO program may be applied to each image to measure the width of one of the outer rings. The image containing the minimal ring width indicates the optimal focus position. To obtain a more accurate value, you may want to repeat this procedure around the minimum, using smaller steps in the camera lens focus. Always move the focus lever in the same direction when setting the focus, to avoid possible backlash. In the case of the Palomar ET-50 etalon and CCD #15 or #16, the optimal focus should result in a FWHM of ~3-4 pixels for the centermost (7th) ring.

  

Figure 17: The focus plate on the mounted Fabry-Perot instrument tube. The plate may be slid horizontally along the tube, to align its left end with markings on the coarse scale on the piece of tape.

As is evident, the focus procedure is more easily accomplished with two persons: one staying in the dome adjusting the focus of the instrument, the other repeatedly taking an exposure at each setting.

If you are unable to find a clear minimum in the ring width, the camera lens focus arm was probably not placed in the proper position on the camera lens (i.e., Figure 4). Unfortunately, the instrument must be partially disassembled to rectify this problem. If you find this procedure necessary, also check the seating of various components (e.g., the camera lens) when reassembling the instrument, as this will also affect one's ability to focus the camera lens.