IRCAL (Dis)Assembly Instructions

or: "How to take it apart and put it back together without having any extra pieces left over."

By James Graham, Marshall Perrin, and Jamie Lloyd.

IRCAL Disassembly
Optics Changeouts
IRCAL Re-assembly
Appendix - Taking apart the filter wheel box

IRCAL Disassembly

Remove IRCAL from the AO bench. Be sure to use a grounding strap while disconnecting the cables, and IMMEDIATELY attach the preamp grounding plug to the preamp. Make sure the window cover is closed to prevent inadvertent fingerprints.
  1. Crack vacuum
  2. Flip dewar so that it rests on its handles
  3. Remove 16 9/64 screws on base and dismount base.
  4. Withdraw the base vertically so as not to snag the G-10 dog clutches It's nice if you leave on the waveplate/flexure metrology system bracket, which provides a mechanical reference when reinstalling it.
  5. Stow the base and cover with kim wipe. The o-ring should come off with the base.
  6. Remove 3/32 first radiation shield screws
  7. Remove 8 G-10 tab screws, being careful not to scratch the o-ring mating surface withthe screwdriver
  8. Remove 8 feed through screws. Remove the first radiation shield by lifting up on the G-10 tabs.
  9. Remove 3/32 second radiation shield screws and remove the 2nd radiation shield, again lifting by the G-10 tabs.
  10. You now have access to the optics

IRCAL Optics Changeouts

Changing Apertures

First disconnect the home switch wire by unplugging the small black connector partway along the wire. With a long hex driver, unscrew the three 7/64 screws which attach the wheel base to the optics table. Lift the mask mechanism out, being careful not to snag on the baffle tubes. Any mask may be replaced by loosening the beryllium copper clips and popping out the mask. Insert the new mask and re-tighten the clips.

To reinstall the aperture wheel, just slide it back into position and re-attach the screws. It helps to have someone hold the mask wheel properly aligned as you screw it in. If the aperture wheel is placed where is naturally sits, the feedthroughs rub on the shroud. To prevent this, hold the aperture wheel with a stiff clockwise rotation before screwing it down.

Specifications: Aperture masks must be 0.995" in diameter, 1/16" thick. Plate scale at this focal plane is 34.3 microns/detector pixel = 454 microns/arcsec.

Changing Filters

Removing the filter wheel is somewhat complicated. In order to lift it clear, the light baffle clamp on the front side must be loosened and slid away from the filter wheel box. Then loosen (but not remove) the box containing the final camera optics, and move it slightly away from the filter wheel box. After that is done you should be able to unscrew and lift up the filter wheel mechanism. The rear baffle on the filter box interlocks with the rear optics baffle box, necessitating that filter box it tilted so the baffle tube can be disengaged.

Individual filters are most easily changed via the access hatches on the filter box. By twirling the motor drive shaft with your fingers, you can rotate the appropriate filter into the aperture. Filters are held in via small beryllium copper clips, which can be removed via a small screwdriver. (Just loosen the clips enough to rotate them out of the way. This make it far easier to get them back into place after inserting the new filter.)

In order to see the filters in position, you will have to slide the cold pupil up out of the way. **Be sure to put it back down into position when done** Doing this is a useful way to check the orientation of grisms or prisms.

See Appendix for a discussion of actually taking the whole mechanism apart.

Specifications: Filters or holders should be circular, 1" in diameter. The 2nd filter wheel accomodates thick elements up to 1/2" thick. The 2st wheel only accomodates filters less than ??? thick.


While it's open, you might as well change the getter. Unscrew the box containing the getter, dump out the old stuff, and put in new pellets from the huge jar which is on the shelf in the instrument room. There are two different kinds of getter, so replace each with the appropriate same type.

The charcoal getter doesn't wear out unless it becomes contaminated. The zeolite permanently adsorbs water unless it is baked at extremely high temperatures. Given the small size of the getter charge, it pretty easy to just replace it. The charcoal is really messy, so best to leave it alone.

IRCAL Re-assembly

  1. Reverse 9-4. If any screws fall down between the dewar and first radiation shield, the easiest fix is to flip the dewar and gently shake it until the screw falls back out.
  2. Clean edge of dewar wall where o-ring seals with isopropyl alcohol. Clean o-ring and groove. Apply a thin coating of vacuum grease. Reseat o-ring
  3. Align G-10 cubes in the dog clutches and lower the base. This step is made much easier by first aligning all the dog clutches and cubes in the same orientation, e.g. with their sides parallel to the sides of the dewar.
  4. Tighten the screws 1/4 of a turn at a time, rotating one side at a time so that you uniformly apply pressure on the O-ring. If the outer dewar wall were to become tilted with respect to the dewar, the o-ring can pinch.
  5. Check for positive engagement of the motor feedthroughs by confirming motor motion. You can see the aperture mask moving by looking in the window, and you should be able to feel/hear the home position engage.
  6. Put back on the final outer dewar walls.
  7. Connect the pumping manifold to the pump using stainless flexible hose. Include the convectron vacuum guage using a T-piece. Make sure that the o-rings are free of dust and debris. Do not over-tighten the clamps. With the dewar valve closed pump down the fore line to a few mTorr (should take no more than a minute). If the line does not pump down promptly check the seals. When the fore line is fine open the dewar valve slowly (10-20 seconds to open). Leave the dewar pumping with the valve fully open for at least two hours at < 10 mTorr. Valve off, disconnect the fore line and begin cool down.
  8. To cool, use the white plastic funnel connected to the long narrow metal tube. Insert this *carefully* into the opening on top of the dewar, being careful to insert it vertically so as not to puncture the sides of the heat exchanger which are apparently quite thin. A cardboard box of the appropriate size may be used to prop the funnel up so its bottom is only about 1" inside the reservoir, which will make the fill *much* faster.

    Get LN2 from down in the basement near the CAT room. Pour the LN2 into the funnel and wait for it to drain out and boil off. Repeat this procedure until it stops boiling off inside of IRCAL; the pattern of escaping gases will change noticably at this point so it's not hard to tell when this is. You can also monitor the cooldown by connecting the temperature controller and reading out the internal thermocouples.

Appendix - Taking apart the filter wheel box

This is a potentially disasterous step because the filter wheels are spring-loaded and held in place by several hundred very small sapphire ball bearings which could easily be spring launched all over the room. Doing this is not necessary during normal operation or filter changout but may be required to repair broken home switches or make other repairs inside of the filter box. **Do this only after carefully understanding the assembly drawing.**

In order to open it safely requires two people. Place the filter wheel box horizontally on a table, with the removable side facing up. It's irregularly shaped so you will want to support it using some small flat objects like kimwipe boxes etc. One person should, by hand, hold the two pieces of the filter box together, while the other person unscrews them. Only once all of the screws are removed should the first person *slowly* release pressure on the top piece, allowing the springs to lift it up and away from the rest of the filter mechanism slowly and without everything flying apart. The top side can then be lifted off carefully by hand, exposing the wheels and other mechanisms inside. *DO NOT* unscrew the filter wheel shaft (which is the central machine screw on one side, and looks very tempting). The removable side is the other one.

Putting it back together requires a reversal of this process. In order to put the removable piece back in place, the wheels and springs have to be compressed. One trick that works well is to hold everything together by applying downward pressure through the open aperture hole onto the wheels, compressing the springs that way and allowing everything to be fit back together and screwed into place.

Elinor Gates
Last modified: Tue Feb 4 11:11:30 PST 2003