Camera focus

We emphasize that it is important to check the camera focus before each observing run. The same is true for the telescope focus (see the note at the end of this page).

We focus the camera on the first night of observing since this can change between observing runs even with the same detector. The camera focus depends only on the relationship of the camera to the CCD plane, and cares nothing about what is in the pupil or focal plane. Here are the stages.

1.    From the TAURUS control window, hit the period/DEL key to bring up the ":" prompt and then type

            load disk$user:[observer.icl_load]focus
            obeyw taurus focus 400

This puts the matrix mask in the aperture wheel into the beam. Read the comments as they go by on the control screen.

2.    Turn on the tungsten lamp.

3.    At the CCD window, type

            obj  camera focus
            ti 1
            win mitll_2inch
            run

4.    Now display the image in iraf with display. The one shown here is the best we ever got.

5.     Use imexam/a to get PSFs for the central 9 stars in the field. See the table below. You won't ever do better than this as the optical design does not allow for it with 100um mask holes. Remember that you can get nice surface plots with imexam/s, nice radial profiles with imexam/r, and nice contour plots with imexam/e.

cl> imexa
#   COL    LINE   COORDINATES
#     R    MAG    FLUX     SKY    PEAK    E   PA BETA ENCLOSED   MOFFAT DIRECT
 221.53  651.28 221.53 651.28
   7.60  11.94 167287.    296.  21293. 0.01   45 7.78     2.17     2.58   2.53
 401.79  649.76 401.79 649.76
   8.86  11.63 222222.    297.  22598. 0.02   59 4.65    2.44     3.06   2.95
 581.76  648.63 581.76 648.63
   7.77  11.85 181150.    296.  22053. 0.05   49 7.06     2.23     2.64   2.59
 220.42  471.12 220.42 471.12
   8.24  11.81 189071.    296.  21526. 0.01   33 9.52     2.31     2.79   2.75
 400.46  469.80 400.46 469.80
   7.72  11.94 167181.    296.  20815. 0.02   27 7.47     2.20     2.61   2.57
 580.27  468.55 580.27 468.55
   7.81  11.85 181281.    296.  22262. 0.04   50 7.61     2.23     2.63   2.60
 218.83  290.85 218.83 290.85
   8.26  11.80 190202.    296.  21064. 0.07   51 4.39     2.33     2.87   2.75
 399.01  289.80 399.01 289.80
   7.51  12.06 150513.    295.  19834. 0.01   41 4.18     2.12     2.60   2.50
 579.09  288.33 579.09 288.33
   6.79  12.28 122783.    295.  17934. 0.00   50 6.18     2.15     2.36   2.26
cl>
 

6.    I recommend you step from a focus of 400, in steps of 10 to a focus of 500. Your focus will be in the range 425 to 475. Once you find the best focus, set the focus back to 400, and then to your optimal value to avoid hysteresis.
 

Note:  Camera focus is more of a black art than you might imagine. We used to get focus=425 for MITLL2A, but this slipped to focus=480 after the matrix mask was taken out of its holder then put back in (reversed?).  If there is a slight bow in the mask, you get a non-optimal solution in that you are focussing on a point out of the focal plane. The telescope focus corrects for this. However, the light passing through the TTF is no longer perfectly collimated. We really do need stiff aperture masks.