ALFOSC internal focus monitoring

Instructions

1. Put in the small pinhole in the beam.
   The small pinhole is not the same as the default pinhole that is usually mounted (for aligning grisms). With the pinhole one can measure the focus in Up/Down and in Left/Right orientation.

2. First determine Up/Down Hartmann shifts: put in the filter wheel 2 Hartmann masks, one with the top half covered, one with the bottom half covered.
   With the Up/Down masks we focus the instrument in the spectral direction for horizontal slits. If the instrument is not in focus, the pinhole image will shift in Y-position for the two Hartmann masks.
3. Set internal focus to 2000 (first guess)
4. Put UP Hartmann in beam (whichever you call UP)
5. Switch on quickexam
6. Take a 0.2 sec exp using Helium light
7. Measure the pinhole position in quickexam (use 'r'), and note it down
8. Put DOWN Hartmann in beam
9. Take another 0.2 sec exp using Helium light
10. Measure the new pinhole position in quickexam (use 'r'), and note it down
11. From the Y-positions of the pinhole, determine Hartmann pixel-shift UP minus DOWN, and write it down

12. Set internal focus to 1500 (second guess)
13. Repeat steps 4) to 11) and determine new Hartmann pixel shift
14. From the two determined Up minus Down shift values, determine how many focus units correspond to how many pixels Hartmann shift.
    Note: we found previously that 500 focus steps correspond to a Hartmann shift of about 0.23 pixel (the sign depends on how you define UP and DOWN).
15. Now you know what focus adjustment is needed to obtain a zero-pixel Hartmann shift: apply the adjustment to the focus and iterate steps 4) to 11) to find further smaller adjustments.
    The pinhole is in focus when you reach a zero Hartmann shift. Final accuracy: about 20 focus units.

16. Now, manually rotate the Hartmann masks by 90 degrees in the wheel, so that they become Left/Right Hartmann masks.
    With the Left/Right masks we focus the instrument in the spectral direction for vertical slits. If the instrument is not in focus, the pinhole image will shift in X-position for the two Hartmann masks.
17. Set internal focus to the best value determined above
18. Repeat steps 4) to 11) and determine new Hartmann pixel shift for LEFT minus RIGHT, and apply correction to the internal focus (you might apply it with the wrong sign if you ar not careful). Iterate until you reach LEFT-RIGHT shift=0 pixels.
    If the internal focus for Left/Right is different than for Up/Down this means that ALFOSC has astigmatism.

19. Update the above table for new Up/Down and Left/Right internal focus values.
20. You may want to discuss editing the default focus value in the software. See instructions.

The above procedure will focus the aperture wheel items onto the detector. It has been investigated that flipping a MOS mask in the aperture wheel makes no significant change to the internal focus. For spectroscopy, a grism has to be added to the beam which will slightly change the internal focus. However, this grism-dependent focus change does not seem to compromise the spectral resolution. Nevertheless, for optimal spectral resolution a Hartmann test with a slit and the grism in the beam can be done, using the pixel shifts of the arc lines.

Any other optics in the beam of ALFOSC, such as filters or calcite blocks, will also change the internal focus of the instrument. For ALFOSC imaging modes we usually choose to compensate for this by changing the telescope focus. However, for slit spectroscopy with added optics (e.g. spectropolarimetry) one has to adjust the internal focus.