MODS Mask Exchange Procedure

MODS has a multi-slit mask cassette that accepts up to 24 masks mounted in special frames. The masks are laser cut in thin spherical shells made of Nicolloy coated with copper oxide black.

This document describes the procedures for exchanging slit masks.

Advance Preparations

Before the mask exchange is done:
  • the MOS scientist should have created the new slitmask tables, which will be named according to the convention, slitmask_<PARTNER>_<DDMmmYY>.tab, e.g. These tables contain, for each mask, its position in the cassette (1-12 are occupied by the permanent long-slit and engineering masks while 13-24 are for the custom masks), mask ID (of the form ID5xxxxx), and a brief description of the mask (up to 32 character description).
    • the table for MODS1 will be on the mods1 computer in /home/mods/Config/MODS1/00Masks, and
    • the table for MODS2, on the mods2 computer in /home/mods/Config/MODS2/00Masks.
  • the ISp should have:
    • inserted the masks into frames.
    • checked the masks for recast or other defects, following the procedure: Inspection of Cut Masks
    • prepared the Mask Box (see below), placing the frames such that the holes for the loading tool face up. Place labels on the frames with the slot position, mask ID, Partner, and start date of science run. Leave several empty slots in the box to place masks that are to be unloaded.

Preparations immediately before the exchange:

Rotator Service Positions

The instrument rotator service positions for mask load/unload for the two instruments are as follows:
  • MODS1: 105 deg
  • MODS2: 285 deg
These angles are recommended for easy access with a lift, although another rotator position which allows safe access to the mask hatch is acceptable.

Prepare the Mask Lists, Cart & Tools

  1. Before going to the dome, prepare 2 lists:
    1. the current mask configuration with the masks to be removed highlighted. This is the file which, for MODS1, is on the mods1 computer in the directory /home/mods/Config/MODS1, and, for MODS2, on the mods2 computer in /home/mods/Config/MODS2.
    2. the cassette assignments for the new masks, with the masks to be inserted highlighted. This list is the new slitmask_<PARTNER>_<DDMmmYY>.tab file, located, for MODS1, on the mods1 computer in /home/mods/Config/MODS1/00Masks and, for MODS2, on the mods2 computer in /home/mods/Config/MODS2/00Masks.
  2. Prepare the cart and necessary tools. Along with the box of masks to be inserted, check that the red and blue collimator covers are on the cart. You will also need the mask loading tool, a laptop, and a long Ethernet cable.

Prepare the Telescope and Instrument

  1. Lock out telescope elevation at zenith.
  2. Position the LDG and RDG rotators so that the mask hatch is safely accessibly via a scissor lift while at zenith. The recommended rotator positions are:
    105o for MODS1
    285o for MODS2
  3. Lock out the rotator.
  4. At MODS, open the bottom hatch with the 4 T-handle latches and stand up inside. Put on the red and blue collimator covers. These covers must be lifted over the collimators flat, so there is no chance an edge would scrape the collimator. The collimator covers protect the mirrors in case you drop a mask or the mask loading tool.
  5. Position a lift under the mask hatch. Put the mask box containing the masks to be loaded, the mask loading tool, the list of masks to be removed, list of masks to be loaded, a flashlight/headlamp, table, and a laptop connected to the network on the lift.
  6. Finally, insure that no one else is running the instrument.
    1. Open a terminal window on the laptop and type ssh lbto@obsN (where N can be 2,3 or 4).
    2. As lbto on obsN, type mods1 status or mods2 status. If the modsUI is not Stopped, call an ISA.

Exchange: Unload the old masks & Load the new ones

  1. Raise the scissors lift and open the mask hatch. Bring your laptop on the lift with you.
  2. From the laptop, login to one of the obs machines as LBTO by typing the following in a terminal:
    • ssh lbto@obsN (where N can be any 2,3 or 4).
  3. Retract any masks that may be in the focal plane by typing the following into your obsN terminal:
    • modsMask 1 reset (for MODS1), or
    • modsMask 2 reset (for MODS2).
  4. Open the flapper on the mask cassette. This carries the mask in-cassette sensor and must be folded down to access the mask load port.
  5. Remove and insert the masks:
    1. Move the cassette to position N (where N is the position with the mask to be removed) by typing, in the terminal logged into obsN:
      • modsMask 1 mask N (for MODS1), or
      • modsMask 2 mask N (for MODS2).
    2. Once the cassette has completed its move, use the load tool to remove the mask. Latch the mask tool and gently pull the mask out. Use both hands to ease the mask out.
    3. Put the mask in the mask box and unlatch the loading tool.
    4. Insert the vacant slot with the new mask. Use the loading tool to extract the new mask from the box. Make sure the tool is firmly gripping the mask frame before lifting it into the instrument, and verify again there is no mask in the cassette position. Carefully insert the mask. Note that there are two 'soft stops' and as it slides into position the detent will 'pull' it. At the last stop, the loading tool will be flush against the mask insertion guide.
    5. Release the mask tool and pull it back slowly - make sure the mask releases cleanly and does not pull out of the cassette even by a little.
    6. Repeat the above 5 steps until all masks have been removed and all masks have been inserted.
  6. Close the flapper
  7. In the terminal, type the following command:
    • modsMask 1 reset (for MODS1), or
    • modsMask 2 reset (for MODS2).
  8. Verify that the cassette mechanism resets without errors
  9. Drive the cassette through its full range of motion, from position number 24 to position number 1, and listen for abnormal sounds and verify correct operation.
    • modsMask 1 mask 24 (for MODS1) or modsMask 2 mask 24 (for MODS2) and wait for it to complete the move
    • modsMask 1 mask 1 (for MODS1) or modsMask 2 mask 1 (for MODS2)
  10. Visually inspect the cassette and make sure all the masks are well seated.

Clean up

  • Close the mask access hatch
  • Lower the scissor lift.
  • Remove the collimator covers and close the bottom hatch.
  • Put all boxes, covers, tools, etc. in their proper storage places.
  • Unlock the rotator and elevation drive.

Install new Slit Mask Tables

This is probably best done from an obsN machine, but can be done from any computer on the lbto network.
  • Open a terminal window and, in it, ssh to the mods1 or mods2 computer:
    • ssh -X mods@mods1 (or ssh -X mods@mods2)
  • Now, on the mods1 or mods2 computer, type the following:
  • cd Config/MODS1/00Masks (or cd Config/MODS2/00Masks)
  • ls to list contents of directory. Identify the table with the masks just loaded, (for example
  • cp ../ (and answer "y" to overwrite the old
    • Example: cp ../
    • Hit "Y" or "y" to overwrite old mask
  • cd (to return to the home directory, /home/mods/)
  • Type updateSlits
    • This loads the new mask table into the instrument server. If the MODS Control Panel GUI is running, it will need to be stopped and restarted to make these changes effective.
  • The Mask Exchange is NOT complete until "updateSlits" is typed

The script

  • The mask scientist will create the masksnap file for the ISps and place it in the directory on the OBS machines
  • The file is located in /home/lbto/ISp/maskSnaps/
  • The file name will be in the format of "" - for example:

Visually Inspecting the MODS MOS masks on an OBS machine

  • Logged into an obs machine as lbto, start up MODS1 or MODS2.
  • Bring up a terminal and start the MODS User Interface by typing: mods1 start gui or, for MODS2, mods2 start gui. A gui like the following will appear:
  • In the MODS Control Panel, click on the Setup icon on the left sidebar to display the MODS Instrument Setup screen
    • Click “GetDate” on the right side of the Instrument Setup Screen to update the UT date in the image file names:
      • If it is before 17:00MST (00UT), the date will be the same as last night's UT and you should click "refresh" to increment the image counter by one, so the mask images will not have the same filenames as any data taken the previous night.
      • If it is after 17:00MST, then the UT will be the same as that for the following night, and the image counter will be 0001.
      • Click “Apply” on the right side to store the date and starting index number, and then
    • On the left side of the Setup Sceen, edit as needed the names of Observer, Support Staff, any comments, and click "Apply" and then "Save".
  • Bring up a new terminal and open the mods image display, type: modsDisp for MODS1, or mods2Disp for MODS2. This will launch two read-only ds9 windows to display the most recent Blue and Red images.
  • Bring up a 3rd terminal to run scripts.
    • Wake mods up:
      • cd ~modseng/modsScripts/Support/
      • execBinoMODS - This will wake up BOTH MODS simultaneously
      • If only one MODS is available: execMODS --mods1 OR execMODS --mods2
      • Watch the progress on the MODS control panel and in the terminal as it runs. It takes about a minute. Watch for any errors, and make sure the laser turns on at full strength
  • Run the mask snap.
    • Make sure that the telescope is at zenith or at a position with elevation 60deg with a PA of 0, and will remain there for the duration of your masksnap. This script should take about 20 minutes if all 12 slots are filled, but less time for fewer masks. You will be asked to confirm this condition is met, but good to make sure before you get started. Open a new terminal and type:
    • cd /home/lbto/ISp/maskSnaps/
    • To run the script on both MODS1 and MODS2 simultaneously: execBinoMODS
      • for example: execBinoMODS
      • IF there are two different MODS masksnap files (for example there are different masks in each MODS) - then type:
      • execBinoMODS
    • Watch the progress. If the script fails with an IMCSlock timeout and "r" does not resolve it, the laser may not be on). The basic troubleshooting in the cookbook treats this, and the Advanced troubleshooting describes better the remedy which is to turn the irlaser off, back on and enable the irlaser (it may not work the first time).
    • Watch the ds9 displays. Pick a camera, and check the slits as the images stream in. If time allows, slits can be confirmed in the images of slits in the second camera but things do stream by quickly. The Red images will display first with the blue following a few seconds after. Change the stretch and scale so that the slits are well defined. Check that the number etched in the mask matched the number title assigned to the ds9 image. This confirms that updateSlits was run correctly and the masks are in the correct positions. If any signs of debris or castoff is found, record the image number, rough location of the obscuration, and mask ID. All images can be found in /newdata and reviewed in more detail with more time after the script has completed.
    • Any masks noted to have debris or cast off will have to be pulled and fixed following the mask exchange procedures and inspected on a light table.
  • Put MODS to Sleep when done.
    • cd ~modseng/modsScripts/Support/
    • execBinoMODS mods2
    • IF only ONE MODS is available then type:
      • execMODS --mods1 for MODS1 or execMODS --mods1 for MODS2
    • Once mods is put to sleep, all windows and GUIs for mods can be exited and closed.

Update the wiki (slit2wiki)

slit2wiki may be run on either mods1 or mods2 and will merge the files that are in /home/mods/Config/MODS1 and /home/mods/Config/MODS2 into a wiki-formatted table. For simplicity, the following steps assume you are running it on mods1:
  • Earlier, the new MODS1 slitmask table, slitmask_<PARTNER>_<DDMmmYY>.tab was copied to /home/mods/Config/MODS1/
  • If the MODS1 and MODS2 slitmask tables are identical, then copy the new table to /home/mods/Config/MODS1/, by typing:
    • cd /home/mods/Config/MODS1
    • cp ../MODS2
  • If the MODS1 and MODS2 slitmask tables are NOT identical, then you need to copy the MODS2 table from the mods2 computer. Type:
    • cd /home/mods/Config/MODS2
    • scp mods@mods2:/home/mods/Config/MODS2/ . (don't forget the ".")
  • Now, create the Twiki-formatted merged table, newtab.txt, by typing:
    • cd
    • slit2wiki newtab.txt
  • Finally, paste this table into the Twiki.
    • cat newtab.txt
    • Click-and-drag to copy the entire file, from the %TABLE to the last line, which starts with | 24 | into a buffer.
    • Go to: and scroll to the bottom to click "Edit wiki text" and enter the raw editing mode.
    • Position the cursor at the end of the existing table and paste the buffer here. Delete the old table and click "Save" at the bottom left.

Updated November 19, 2019 -Barry Rothberg
Topic attachments
I Attachment Action Size Date Who Comment
maskbox.jpgjpg maskbox.jpg manage 594 K 18 Oct 2018 - 21:48 OlgaKuhn "Mask Box" containing framed MODS masks
Topic revision: r28 - 16 Dec 2019, OlgaKuhn
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