Purpose

This checklist leads you through the set procedure for the DEIMOS Flexure Compensation System (FCS).

For in-depth information about the FCS, please refer to additional notes on FCS operation.

Preparing DEIMOS for FCS setup

Clamp up slider. Even though gratings can be clamped at any rotator angle during the night observing, it is very important to make sure that the grating is clamped at a rotator angle of -30deg before taking an FCS reference. The focus script clamps the grating at -30deg, but in case of doubt, you can use the background menu to DEIMOS Engineering → Select Slider ... → Select Slider 3|4, or in a polo Xterm, run the command select_slider 3|4.
Configure spectrograph. Configure the instrument with the desired grating, tilt wavelength, and filter for acquiring spectra.
Close hatch. Make certain that the remotely-controlled hatch at the front of DEIMOS is closed.
Extinguish lamps. Make certain that all of the internal calibration lamps are turned off.
Insert slitmask. To reduce the amount of stray light that reaches the dewar, insert a slitmask. Any slitmask will do, even the focus holes mask or the longslit.
Idle tracking. Make sure that the FCS tracking loop is in the idle state. Check the FCS MODE status at the top of the DEIMOS FCS GUI. It should indicate Off and the FCS STATUS flag should indicate idle. If not, then click on the Stop FCS tracking button, and wait until the FCS STATUS displays idle.
Enable rotator. Check that the Rotator mode is Pos on the System overview area of the DEIMOS instrument control GUI. If the rotator mode is not Pos, then click on the Rotator Details... button to bring up the rotator GUI. On the the rotator GUI:
1. Click on Manual mode to enable the manual mode area.
2. Click on the Rotator mode pull down menu, select Pos and then Apply.
3. Click on Close to close the rotator GUI.

Taking the Reference Frame

The first set of instructions below pertain to the normal case of observing a slitmask without regard to previous observations. If you are re-observing a slitmask that was observed on a previous night of your run, then follow the matching procedure below. If you are re-observing a slitmask that was observed on a previous run, then follow the archival matching procedure below.

Normal (Non-Matching) Case

Use the FCS GUI to:

Reset FCS exposure time. On the FCS detector control area, change the Integration (s) and hit Apply. Normally 1 or 2 seconds should be sufficient. It is not recommendable to use an exposure time longer than 5 seconds.
Select mode. On the Reference control area, select New for the reference frame type.
Home FCS Stages. On the Reference control area, click on Prepare for reference. Wait for the message box at the bottom of the GUI to show
```
fcszero new successful. You are now ready to take reference images
```
Acquire reference image. When the FCS system is ready, click on Take a reference. The system will acquire an FCS exposure. A gray box will popup when the image has been acquired. Close the popup by hitting the OK button.
Measure peak flux. Draw a statistics box around the 2 spectra on the left half of the image display area by clicking and dragging with the right mouse button. Note the Max value on the Selection statistics area. Draw another statistics box around the 2 spectra on the right-hand side and again note the Max value.
Adjust exposure time. If the peak flux in each statistics box is at least 2000, the reference frame is acceptable; proceed to the next step. If not, then select a longer expoxure time and click on Take a reference button. The system will acquire an new FCS exposure. Repeat the previous step to measure the peak flux and continue to lengthen the exposure time until both sides have a peak flux over 2000.
Verify Tracking. Click on Start FCS tracking. The FCS Status should transition to Tracking and the State should transition to On Target. If FCS Status reports Lockout, ensure the shutter is open. If State still does not report OK, please contact your SA.
Repeat as needed. Repeat all steps in this section for each unique combination of grating, central wavelength, and filter you plan to use during the night.

Matching Against a Previous Night

Follow this procedure if you are re-observing a slitmask that was observed on a previous night of your run. Note: if you want to match against observations made on a previous observing run, please see the procedure below.

Use the FCS GUI to:

Verify match. On the Reference images area, ensure that the File (current reference file), Img (current reference image) and Log (FCS log file for the current configuration) are all defined. If they are not, then either the reference file from the previous night needs to be copied into the current FCS directory, or the instrument is not configured with exactly the right grating, wavelength, and filter to match the previous setup.

To copy manually the FCS reference file from a different date into you current FCS directory:
- Determine the UT date of the original observation.
- Copy the old reference files into the new FCS directory by issuing these commands on a polo terminal:
```
cdfcs
```
```
fcsref_retrieve date
```
  where date is of the form YYYYmmmDD (e.g., 2015dec08).
Select mode. On the Reference control area, select Match for the reference frame type.
Home FCS Stages. On the Reference control area, click on Prepare for reference. Wait for the message box at the bottom of the GUI to show:
```
fcszero new successful. You are now ready to take reference images
```
Acquire reference image. When the FCS system is ready, click on Take a reference . The system will acquire an FCS exposure. Note that it is advisable to take a new reference image, rather than using a previous one, because the focus is likely to have changed from the previous night, with the consequence of changing (slightly) the pixel scale. The system will take several minutes to reset the focus to the value employed in the old FCS setup, run FCS to achieve FCS lock against the old FCS reference image, restore the focus value, and acquire a new FCS reference exposure. Your indication that the procedure is done will be when the Img shown on the Reference images area changes. Besides, a gray box will popup when the image has been acquired. Close the popup by hitting the OK button.
Measure peak flux. Draw a statistics box around the 2 spectra on the left half of the image display area by clicking and dragging with the right mouse button. Note the Max value on the Selection statistics area. Draw another statistics box around the 2 spectra on the right-hand side and again note the Max value.
Adjust exposure time. If the peak flux in each statistics box is at least 2000, the reference frame is acceptable; proceed to the next step. If not, then select a longer expoxure time and click on Take a reference button. The system will acquire an new FCS exposure. Repeat the previous step to measure the peak flux and continue to lengthen the exposure time until both sides have a peak flux over 2000.
Verify Tracking. Click on Start FCS tracking. The FCS Status should transition to Tracking and the State should transition to On Target. If FCS Status reports Lockout, ensure the shutter is open. If State still does not report OK, please contact your SA.
Repeat as needed. Repeat all steps in this section for each unique combination of grating, central wavelength, and filter you plan to use during the night.

Matching Against a Previous Run

If an observer wishes to re-observe a slitmask from a previous DEIMOS run, the new and old data can be aligned to within a fraction of a pixel if the original FCS reference files are used to establish the alignment. We recommend that observers save their old FCS reference files if they intend to re-observe a mask in the future. However, we also try to maintain a repository of old FCS reference files at Keck as a backup in case the observer is unable to save their data.

Follow these steps to set up FCS to match against a reference image acquired on a previous run.

Determine the UT date of the original observation.
Copy the old reference files into the new FCS directory by issuing these commands on a polo terminal:
```
cdfcs
```
```
fcsref_retrieve date
```
where date is of the form YYYYmmmDD (e.g., 2019oct01).
Generally speaking, the focus in the old and new datasets will not be the same and thus you must use the match previous procedure (starting on step #2) to complete the setup.