Collimation Troubleshooting

The LBCs are focused and collimated differently from the Gregorian instruments such as MODS and LUCI. First, collimation must be achieved by moving and bending the primary mirror, which is the only component of the system that can be adjusted, and, second, the collimation procedure makes use of on-axis pupil images and therefore cannot be done while taking science data. Because the prime-focus LBCs are at a fast f/1.47 beam, the image quality is very sensitive to changes, to the mirror and steel temperatures, for example.

There is a collimation lookup table which adjusts the mirror position as a function of elevation and temperature, but it cannot account for everything and collimation should be checked every 30-40 minutes, depending on the stability of the night. When the mirrors are not in good thermal equilibrium, it can be extremely difficult to achieve and maintain collimation and, in these conditions, it is not advisable to observe with the LBCs. If a cold/warm front has come through or this is the first night open after having been closed for a while, it is likely that the mirrors will be far from equilibrium. Ask your OSA and ISA for advice on this matter.

Focus and collimation is done using either of the IDL procedures: dohybrid or dofpia. Usually it is done at the field of your “co-pointing” star, but you can also focus and collimate on your science field provided there are enough bright stars in the collimation region that covers rows Y=1201-2608 of chip 2.

• In a terminal window, start IDL.
• Once the telescope is on-target, at the IDL prompt, type either dohybrid or dofpia.
  • Run dohybrid if it is the first collimation of the night or in the middle of the night after active optics has been cleared.
  • Run dofpia otherwise, because it is significantly faster.
• Both routines run in a loop, where they send initial focus (z4) and spherical (z11) aberrations to help the fitting, and then iterate through the procedure:

1. 1. take a 16-sec or 32-sec (/x2 keyword, see below) pupil image
   2. analyze it (dohybrid has a first stage which uses a different algorithm). This will produce some IDL windows as shown here.
   3. send the measured aberrations to the primary mirror, and
   4. repeat from 1 until the convergence criteria are met. The convergence criteria depend on the seeing which FPIA estimates (“es“) and are:
      • z4, z5, z6, z11 <= 250nm; z7,z8 < 400nm, z22<50nm, if es <=0.8
      • z4,z5,z6,z11 <= 250*(es/0.8); z7,z8 <= 400*(es/0.8); z22<50*(es/0.8)

• Final z4 and z11 corrections are sent to remove the aberrations injected to help fitting.
• You are now ready to obtain the pair of images used for co-pointing or to observe your science field. Don’t forget to reload your co-point or science OB!

There are 2 ways to break out of the loop:

• S (shift-S) is more graceful way, to be used if you think FPIA is nearly converged and you don’t want to wait another iteration. S will send the final z4 and z11 corrections to remove the injected aberrations and, hopefully, give you a well-collimated image.
• Cntl-C  for when you want just to stop it ‘here and now’. After Cntl-C, to remove the initial z4 and z11 corrections, you should run dofpia or dohybrid with the backout keyword. Use “up-arrow” and then append the keyword, /backout, IDL> dohybrid, /backout  

Command-line options for dohybrid and dofpia

Commonly used command-line options for dohybrid and dofpia are:

• • /X2 – double the automatic OB exposure times from 16 sec to 32 sec. Use /X2 for clouds, high dome seeing, or fields with faint stars. Don’t use /X2 when at very high elevation (above ~86 deg, TBC).
  • /First – (dofpia only) sets RedInitial and BlueInitial to larger values appropriate to poor seeing (dofpia only). Before dohybrid was developed, this keyword was also for the first collimation of the night.
  • /BackOut – causes DOFPIA to remove initial Z11 values sent to the primary mirrors.Use this to remove the extra Z11 if something like clouds made the loop fail. Simply replay your previous DOFPIA command with the /BackOut flag added. No images will be taken as the program exits after removing the corrections.
  • /RedOnly – only work on LBC-Red
  • /BlueOnly – only work on LBC-Blue
  • /askme –  interactive mode which prompts for “y”/”n”/”z22”. “y” causes the corrections printed to the terminal to be sent, “n” does not send anything and “z22” will set the z22 correction to 0 and remove its crosstalk terms which are significant: z4 (z4cross = z22* -8) and (z11cross = -1.75 *z22 for LBCB or -2.4 *z11 for LBCR).

Background

Some commonly seen problems and corrective actions are listed below. The first two deal with warnings or failures that might be seen immediately; the remaining ones document problems with convergence.

Warning. Data files did not appear in /newdata, Please double check and then press: Q to exit or any other key to continue. This occurs when the pupil image did not get written into the /newdata directory within the timeout period (99 sec?).

Check:

• list the contents of /newdata: ls -ltr /newdata will order contents by the time of creation, with the last file written at the end of the list.
• If the file is not in /newdata, even after waiting 1-2 more minutes, there may be a bigger problem with the data flow, and the OSA and ISA should be contacted.
• Note that if you hit any key and the new file is still not there, the old pair of pupil images will be analyzed, and the corrections sent, again. BackOut only works to remove the initial Z4 and Z11 corrections, it does not undo any other set of corrections.

No Good Pupils Found No pupils were found above the brightness threshold.

• Are there clouds or is it still twilight? You may need to run dofpia or dohybrid in darker conditions or use the /X2 option to take longer exposures. Run dofpia, /backout or dohybrid, /backout to remove the initial Z4 and Z11 corrections sent.
• Do the pupil images show a lot of coma? (see below).

Collimation: Focus (z4) is oscillating wildly. The pupil illumination pattern can sometimes develop a ring near the center, and when this is bright enough, FPIA fits this ring and treats it as the outer diameter upon which the focus (z4) correction is based. FPIA thinks the pupil is too small and sends a negative z4 correction which makes the pupil larger. But in the next image with these larger pupils, the intensity of this ring is diluted and the outer diameter is correctly fit and indicates that the pupils should be smaller. But then the ring is brighter, and so it goes on…

The development of this ring, a sign of higher order spherical aberration most likely due to the mirrors not being in thermal equilibrium, is the main problem.

• How far apart are the glass and ambient temperatures? If greater than 1 degree, consider switching instruments.
• Examine the z11 and z22 corrections leading up to this, using the [s|d]xHiZ (z22) or [s|d]xLoZ (z11) tabs in LBTplot.
• Consider switching to another instrument (MODS or LUCI) if the mirrors are not in equilibrium, but if you must go on, then:
  • clear active optics and try again;
  • if that doesn’t work, exit dofpia and ask the OSA to add z22 or z11 before restarting. The by-eye active optics sheet can help with z11 and the model pupils shown here with z22.

Figure 1: A dofpia series during which a bright inner ring built up, to the point where the ring itself was fit as the outer diameter and FPIA entered into a focus oscillation.

Strong coma: dohybrid ought to remove strong beginning-of-the night coma. But in case it does not, you may need to ask the OSA to send a coma correction, using this by-eye Active Optics cartoon as a guide. The series of pupil images below arose from this situation where dohybrid did not correct coma sufficiently so that dofpia (which starts with the image 015408) sees only the bright spot and sends a large focus correction. In the hugely out of focus image (015542), you can that there is strong coma. When the coma is strong enough that dofpia cannot handle it, the corrections required are ~1500-2000 nm.