Stacking Procedure

Before you can go on sky, you stack the mirrors. Stacking is the process we do in order to align the mirrors to the best of our ability. A number of factors go into the behavior of the mirror. The truss the mirror is supported by is made out of a steel alloy, which expands and contracts quite a bit depending on the temperature. How cool the dome and the truss is compared to the external temperature when you first open can greatly affect your stack. Wind speed is another major factor. A high enough wind speed will put pressure on the mirrors and thus make stacking much more difficult. Using the louvers in a strategic way can really help with the success of your stack. They allow external air to flow in quickly, helping the truss temperature, but if the winds are over say, 10mph, maybe opening the louvers 45 degrees will help with your stack. Every TO has slightly different tried and true techniques with how they use the louvers. It is definitely recommended to other operators about what has worked well for them in the past. You want to complete the stack before astronomical twilight (18 degree) and maintain that stack throughout the night. Some nights, this can take up to an hour; on good nights it can take about 35 minutes. Different tasks carried out during the day can greatly affect your stack so it is wise to check the HET daily status report sent out by the site manager. Some things to look for would be to check if the primary was hand washed, sensors & mirrors removed, piston work, mirror coating swaps etc. In addition to day activities, if a large temperature drop or front is expected you generally want to stack when conditions are steady. There are 91 mirrors, each doing its own thing. Each mirror has 6 sensors trying to correct for thermal expansion, contraction, and instrumentation. The point is to shine a narrow light down at the mirrors and move each mirror until the return spots are all at the same spot in the HEFI view, ie, stacked. The term stack comes from stacking the light path from each mirror top to the same spot.

What you need (if you have to set it up refer to the Ops Eng manual):

• Dome and structure should be at CCAS. Make sure the tracker is sent lower right or lower left (go out and look, or look at last move in green font in the TCS GUI command status box). Open louvers.
• SAMS (Segment Alignment Maintenance System), make sure TRSE (Target RMS sensor error) Operation Control is set to a number above the RSE

• SCS (Segment control system) in closed loop with SAMS (Move authority set to SAMS)

• MARS (Mirror Alignment Recovery System)

• HEFI (Hartmann Extra Focal Instrument) is on in high for initial alignment.

• AOA/Wavescope is on in the APC GUI.

• weather GUIs and smoke start work

Procedure

1) Switch to Hefi view on video 6. In the HEFI GUI, move HEFI mirror IN and light source to high. A blurb of light on the overhead screen should appear; this is the array. If it doesn’t appear, the dome or the CCAS shutter is closed. It could also be that it is out of frame. The light can be moved around from the MARS GUI on the X and Y coordinate system. Once you have centered the HEFI spot move on to step 2.

2) Note the time as your “start stacking time” in the night report.

3) Center the array (blurb) in HEFI view. Using the MARS GUI on the right-hand-side towards the middle of the page you can enter x and y values and move those directions via the Left, Right, Up, Down X-Y Step Motion buttons. Remember that in HEFI view the horizontal axis is fixed but the vertical is inverted. Therefore, if you want to move the image up you must command the y-value to go down. Steps in this view are normally several hundred. When HEFI is well centered move on the step 4.

4) Switch to AOA. To see the spot images (rather than the HEFI blurb), we need to switch to the AOA by viewing video 2 and setting HEFI mirror out and HEFI light off.

On the MARS GUI, 2 shutters towards the bottom should be open (the REF and the HET). Once AOA software is properly brought up click on view spot images in the manual alignment for HET.

Line up the center dots (Mirror 43) upon each other using the same MARS X-Y Step Motion buttons (much smaller steps than the HEFI view—about 25-100).

5) On Mars, click the DMI Stage to In

Under the Focus area, click the green DMI button. It will make a measurement to the center of the primary. In order for this to work the Hefi mirror must be out since the HEFI pick off mirror blocks the pathway to DMI.

Whatever value appears in the box next to the DMI button, change the focus by that amount. (Negative means move focus in and positive values mean out).

*Troubleshooting: If you receive a reading that is -/+ ve 99999.99 try adjusting the focus in or out while watching the dots on AOA (hefi mirror back in) by about 2000 microns and then retry the DMI. This is a default number that appears when the DMI cannot make a calculation because the primary is so out of focus. If this still doesn’t give you a reading, check the temperature from the last stack (previous night) and if it’s way different than the current temp then you might need to make a substantial GROC correction. GROC is covered in the independent tasks section.

Repeat this procedure a couple of times until the DMI focus is within 10 microns of 0.

Now move the DMI mirror out by clicking the green Out button beside DMI Stage.

6) Recheck that the center mirror is aligned in the AOA view using video 2 with the reference shutter from MARS open; close the ref shutter in MARS and type HETStackOne or s1 at the prompt in the AOA/wavescope computer. This will be the first iteration of your stack (if the wind is above 15 mph during your stack AND you see “dancing” spots, you may want to type ‘fixedap’ in the AOA terminal). During high winds, feathering the louvers to 45o or less may improve the quality of the stack, or closing the louvers to about 10 o in the direction of the high winds to minimize them.

7) Once AOA finishes its iteration, on the MARS GUI click on the View Last Measured Tips/Tilts? and it will bring up a diagram of the mirrors. Red is out of spec, green is in spec and grey is not in the calculation and thus unmoved. The vectors indicate relative sizes of the needed moves. The goal will be to get all the mirrors “in the green”.

-Another way to determine whether the calculated tip/tilts are going to produce a good stack is to look at the very bottom of the output from HETStackOne and see if the tip/tilt corrections are very close to zero (final number on the bottom right). Keep iterating and do not move the tracker until we can get the bottom right number of the tip tilts close to 0.05. There are 3 numbers to pay attention to before deciding to move the tracker. The Tip Tilt scale on SAMS must be down to a 0.3 or 0.2 and the RSE on the Tip Tilt must be close to 0.05 on SAMS in addition to 0.05 on the bottom right of a completed HetStackOne? (s1).

8) Once the above 3 numbers have reached their goal (exception: when it is very windy and the fixedap is on, sometimes the goal RSE on SAMS of 0.05 can go up to x ~ 0.10.) Once achieved, and once most of the mirrors are “in the green” it is time to move the tracker to catch the mirrors hidden from view by Lefting the tracker. On the TCS GUI, Tracker tab ->Preset position tab -> lower left preset arrow. This preset will move the tracker all the way to the other side. It is safe to Slew. Align the center spot using your reference shutter and movements in MARS (the motion of the tracker is more than enough to move your test spots off) and run the s1 command in AOA and continue the iterations until, again, the above 3 numbers are satisfactory and we’re mostly “in the green.”

9) Right Tracker. In a ratio of R-L-R the number of iterations executed is around

3-3-4. The numbers are not significant, but realize that this last “tightening up” is crucial to the stack measurement so take your time and do a good job.

10) Save position in SCS and SAMS. Once the final iteration has settled (RSE on Tip tilt ~ 0.10, T/T error ~.10), move SAMS from Operate to Standby mode. Click the Set Reference and select “New Reference Preserve Mode” and OK when asked. When the mode is not grayed any more, move from Standby to Operate. You should witness the TRSE (green line) drop significantly. When SAMS begins to level out move over to SCS, and click the Save Pos button. You may have to wait until SCS is done “moving.” Save the file that appears highlighted.

11) If there is time, inform the RA that you are about 5 minutes from a completed stack and go to measure the stack.

For Self Stacking Procedure see this link.