On occassion it is necessary to open up Gemini
for repairs. The procedure is not difficult, but
is time consuming and should be done carefully so as
to not get dust and dirt into the dewar or damage
- Disconnect polarimeter motor and limit cables
from the polarimeter module.
- Remove polarimeter module. It is held in place
with six 1/4-20 bolts. The lengths of the bolts
vary so make sure you note which holes take the shorter
- Disconnect cables from the back side of Gemini. Make
sure everything is properly labeled first so that cables
can be properly reconnected later.
- Remove back L-bar mounting bracket at the top of Gemini
(two 1/4-20 bolts and four larger bolts).
- Remove back frame. (many many 1/4-20 bolts, most are short - longer
bolts should be used for corners and mid-frame support, there are about
15 bolt holes that have no bolts, these are marked with brown tape).
- Rotate Gemini 90 degrees so that the front face of Gemini is
There is a lock bolt at the bottom of the rotation mount that needs to
be removed while rotating and put back in to stabilize the instrument
in the rotated position.
When rotating, be careful not to catch any of the remaining cables or
the Lakeshore temperature readout box.
- Remove external bar support on the bottom of Gemini (four large bolts).
This item is not symmetric, so make careful note which end is towards
the front of Gemini).
- Unscrew all but the corner vacuum dewar housing bolts (these are all
1/4-20 bolts). Loosen the corner bolts.
- Have two people support the vacuum dewar housing with the wood
Gemini support panel. The third person will unscrew the four corner
bolts. When the housing is free, carefully lower it away from the
instrument. Store the housing out of the way, using Kim-wipes to
cover it to prevent dust or dirt from getting it in.
- Rotate Gemini 180 degrees so the back side of Gemini is facing upwards,
allowing easy access to the heat shield.
- Put on gloves so that oils from your hands do not get on the heat
shield or any components inside the dewar.
- Remove the copper grounding cable from the window ring.
- Remove window ring by unscrewing it from the heat shield.
- Loosen the screws holding the heat shield in place (these screws
should not be removed). There is one screw at the lower left corner
(when Gemini is upright) that needs be unscrewed more than the rest.
The screws behind the insultator supports need be unscrewed with a small
hex wrench (5/64, if I recall correctly) since you can't get your
fingers in there.
- Carefully remove the heat shield. Place someplace clean and out of
the way (usually it gets put on top of the vacuum housing).
- Do whatever work inside the dewar needs to be done.
Gemini Reassembly is mostly the reverse procedure of taking it apart. With
a few extra steps.
- Put on gloves.
- Make sure all electrical connections inside the dewar are firmly
- Make sure all mechanical components are firmly bolted in place.
- [Optional] Hook up all electronics and computer and test operation
of all motors and detectors while warm and components accessible.
- Thoroughly clean vacuum grease off of dewar vacuum housing, o-ring,
and anywhere else you might find stray grease.
- Carefully install heat shield. Tighten all its locking screws after it
is properly fitted in place (it is a snug fit).
- Install window ring. Reattach copper braided grounding strap.
- Wipe down exterior of heat shield with alcohol to clean off any
oils or dust.
- Rotate Gemini 180 degrees so front is facing upwards.
- Carefully coat o-ring and channel with vacuum grease.
- Clean interior of vacuum housing with alcohol if necessary to clean
off grease, oil, or dust.
- Place vacuum housing on wooden Gemini platform and align with Gemini.
- Have two people lift the dewar housing into place. It must be butted
up against the top (when Gemini is upright) of the external frame for
it to be properly aligned with the bolt holes and for the dewar window
mount to clear. The third person
should install the four corner screws. Onces the screws are in place
the remaining screws should be installed loosely.
- Tighten the dewar housing screws. Note, one should use a pattern
for tightening the screws so that a proper vacuum seal is achieved.
For example, tighten every third screw and continue around in that
pattern until all screws are tight.
- Install external bar support frame on bottom of Gemini.
- Rotate Gemini so it is upright.
- Install back frame.
- Install top L-bar mounting bracket.
- Reconnect cables to back of Gemini electronics.
- Install polarimeter module.
- Reconnect polarimeter motor drive and limit cables.
- [Optional] Reconnect all other cables and test Gemini while warm.
- Put on vacuum pump and start pumping the vacuum. Bake getter once
vacuum has gotten down to 1e-1 or 5e-2 torr.
- Cool down Gemini to operating temperature.
- Test Gemini operations (motor motion and detector operation).
Last modified: Sat Mar 6 11:30:13 PST 2010