Instructions for pumping LFC before cool-down. You should start pumping on
the day before you want to mount it on the telescope, so that you can fill
the dewar about 24 hours before it needs to be used. The LFC takes about 24
hours to reach operating temperature.
The LFC should be pumped only with a dry turbomolecular pump system. This
is a special pump that uses a turbine combined with an oil-free backing
pump. It is used to make sure that no oil (which is present in the other
Palomar pumps) can get into the dewar and contaminate the CCDs.
These instructions assume that the LFC is in its normal state. That is, it
has simply warmed up in between runs, but is still close to vacuum
pressure. (NOTE: the LFC should never be opened up to atmospheric pressure
without first consulting Mark Metzger). Do not attempt to pump the LFC
before it has completely warmed up: this can lead to Very Bad Things.
Connect the convectron gauge (made by Granville-Phillips) to the
convectron mounted on the LFC. It is opposite the valve, and has a blue
plastic connector. Plug in the gauge. You should see it reading some
number of milliTorr, but it should not be above 2 Torr. (mTorr readings
are when the small LED in the corner is illuminated.) Plug in the
Remove the "blank" (cover) from the valve opening on the LFC. DO NOT
OPEN THE VALVE YET! There is a piece of tape on the quick release
clamp, for now, that makes sure it does not pop open: save this or use a
new piece when you replace it at the end.
Connect the hose to the LFC valve, using the centering ring (which has a
Viton rubber gasket on the outside that makes the seal). You will have to
remove a blank and a centering ring from the end of the hose as well. Do
not touch with your fingers any part of the inside of the hose, the
inside of the valve, or the metal part of the centering ring. Handle the
centering ring only by the edges of the Viton. When properly connected,
the quick clamp should close fairly easily, but with a small amount of
force in the middle which eases at the end and clicks closed. Note that
you are connecting to the end of the hose that has a reducer; the end
without the reducer should connect to the valve on the pump in the same
manner. In all cases, try to keep the blanks clean: don't touch the
inner part of the inside. Even natural grease from your fingers can
contaminate vacuum parts, and they're difficult to clean.
Make sure the valve on the top of the turbo pump is open to the hose
(several turns counter-clockwise, but be sure not to turn the valve hard
after it reaches the counter-clockwise stop.
Turn on the turbopump by pressing the Start/Stop button under the LCD.
You should hear the backing pump start (and notice the hose being evacuated,
which is important), and hear the turbopump start to spin up.
Leave the pump run for about 5 minutes. This will ensure that the turbo
has had enough time to spin up. When the turbo is fully spun up, you
should see on the pump display something like "Normal Operation" or "75K
Once the pump is in normal operation and up to speed, open the valve
on the LFC about 3 turns counterclockwise. You should immediately see
the pressure gauge start to drop.
Leave the pump run for about 3 hours. You should see the pressure drop
steadily, but slow down toward the end. You want to achieve a pressure
of 2 mTorr or less, sometimes it will reach "0 with a dot (LED)", which
is about 0.5 mTorr—this is ideal.
NOTE: You should check the pump periodically to make sure that the
turbo is operating properly at 75 KRPM. The turbo has been known
to crap out in the past, reducing it's speed to 45 KRPM and
causing the pump to work less efficiently. If this happens,
immediately close off the valve to the dewar, and follow the
procedure for turning off the pump. Then, power the pump back on
and try again. This may seem a little silly, but it works.
Before cooling LFC, nitrogen must be flowing through the instrument to
prevent the possibility of a "Frost Ring" on the window.
When the pressure is less than 2 mTorr or 3 hours has expired, whichever
is later, you can start filling the dewar. Start putting LN2 into the
dewar, and once the flow is started and you see a lot of gas blowing out,
close the vacuum valve on the LFC. You may see the pressure rise briefly,
but once the can starts to cool down a bit, the pressure should drop to 0
mTorr, and the LED in the corner to turn out (0 without a dot). Continue
filling the dewar—it takes a lot of nitrogen!
As you are filling, double-check that the LFC valve is CLOSED.
Close the valve on the top of the turbopump.
Turn off the power to the pump by pressing the start/stop button again.
You should hear the backing pump shut off and the turbo start to spin
Disconnect the hose from the LFC, and replace the blank to close off the
valve and keep it clean. It will be slightly difficult to break the
first connection, as there will still be a vacuum in the hose.
Once the dewar is full, you're done! The dewar hold time is >24 hours,
so it should only need filling once a day, and in the morning is best.
However, after the initial fill it may be prudent to top things off
again on the afternoon of the first night, since more LN2 will be burned
off while cooling down the focal plane. When performing the top-off,
first check the vacuum with the convectron to make sure everything is
happy (i.e., reading zero, no LEDs)
DO NOT VENT THE LFC TO ATMOSPHERIC PRESSURE. If you need to shut down
the pump for some reason while you are running, close both valves first.
Don't vent the turbopump to atmosphere while the turbo is spinning
rapidly. This can damage the rotor.
Under rare circumstances, the backing pump has been seen to turn off due
to overheating. When this happens, you will notice the pressure on the GP
pressure gauge rising, and will rise to over 400 mTorr. This is not good:
gas from the turbopump is seeping back into the dewar. If this does happen,
close both valves, and turn off the pump for 5 minutes. Then start again at
step 3 above.
Don't pump the LFC when it is cold. If you check the pressure with
the Granville-Phillips gauge and the pressure is above 1 mTorr when
the LFC is cold, it needs to be pumped again. However, you need to let
the instrument warm up completely before pumping, and this takes more than
24 hours. If the cold pressure ever gets above 1 mTorr, contact Mark Metzger
If for some reason the dewar does not pump to below 2 mTorr warm, even
after 3 hours, something is leaking. Perhaps the hose is not properly
connected. Before checking this, though, CLOSE BOTH VALVES! You don't want
to let air into the dewar, the primary reason being that we don't want water
(even the vapor in the air) or any other suspended particles/oils/colognes
2. Installing LFC in Prime Focus
Consider installing filters before the lift—it's easier on the ground.
Bring the following items up into the cage:
The temperature controller, mounted on a small V-plate
Fiber modem and associated power supply
2 sets of fiber pairs
1 small screwdriver
1 AC power cable for temp controller
1 temp controller cable
Set the counter on the photometric pedestal to 7500.
Install LFC mechanically. The silver box strapped to the dewar should be
facing the North side.
Install the V-plate with the temperature controller.
NOTE: There are 2 power strips mounted on LFC. One is along the south side,
paralell to the edge. The other is mounted diagonally. NEVER
change the power scheme on the diagonally mounted strip. These feed
the CCD power supplies, and changing these or adding other loads
could add noise.
Run the AC power cable from the temperature controller to the power strip
parallel to the south edge of LFC.
The dry nitrogen hose is clear, and should be rolled up and stuffed
behind the fiber patch panel in the PF cage. Unroll it and attach
it to the spigot located on the side on the structure containing the
shutter and filter wheel. the spigot is near the set of connectors
for the cables running into the box.
Attach the fiber modem to the 25 pin D-connector located on the motor
controller box. This connector should be labeled "RS-232". Plug
the power for the fiber modem into the non-diagonal (non-CCD) power
Run fibers from the patch panel to LFC. Fibers 1 & 2 from the panel
should run to the CCD electronics box, and fibers 3 & 4 should run
to the fiber modem.
Verify that both power strips are switched OFF.
IMPORTANT: Verify that the high-voltage power (located on the CCD electronics
box, labeled "HV") is turned OFF. AS OF 2000/09/22, this
switch should ALWAYS remain off. (In the next engineering
cycle, this switch will be internally disabled, but for now it
should not be used.)
Plug the CCD power strip into the isobar outlet on the wall of the PF
cage. It should be on one of the outlets labeled "Filter Bank 3".
Plug the other power strip into the isobar, filter bank 1.
Attach the temp controller cable to the rear of the temperature
controller. the smaller of the 2 round connectors should go to port
A, and the bannana plug will only fit in two out of the three holes
(the spacing of the three holes on the back is not the same). Don't
worry about the polarity of the bannana plug, it is connected to a
resistor so it shouldn't matter. Attach the cable to the side of
the PF cage with one of the velcro ties for strain relief.
Ground yourself...run your hand along portions of the PF cage with bare
metal (no paint), the cases of power outlets, and the outside of the
CCD electronics box.
Once you're sure you're well grounded, attach the round connector at the
other end of the temperature cable to the dewar. The mating
connector for this is located on the dewar itself, underneath the
Turn on the non-CCD power supply (along the south edge).
Turn on the temperature controller. You will see 2 numbers displayed.
One of these should be -85.00. This is the "set point", or target
temperature at which we run the CCDs. The other number is the
actual temperature. Next to the numbers, there is a scale labeled
"50%" and "100%". This indicates if the focal plane heater is
working, and if so, at what percent of capacity. There is a button
marked "heater" on the panel display which toggles the heater
on/off. Press this until you're convinced the heater is on. This
should be clear from looking at the scale. If the set point is
something other than −85.00, something may be amiss, and it is
probably better to leave the heater off if in doubt. Note
that in normal operation, the temperature may read as warm as −70°C.
At this point you're ready to start up LFC. To execute the startup
procedure, refer to the HTML documentation of the LFC
and power-up procedures for performing a cold start.
3. Swapping Filters
The LFC filters are each mounted in a custom built cartridge
designed to slide into place and lock in the filter wheel. The filters
should never be removed from their cartridges. Because the filters have
varying thicknesses, some are considerably heavier than others. One
therefore needs to take care when mounting filters in the wheel, so as not
to put the wheel grossly out of balance. This can lead to slippage in the
mechanics at high airmass. A brief summary of the filter weights:
We will be attempting to bring the lighter filters up to ~2.5 lbs to avoid
any filter wheel imbalance.
We try to mount heavy filters opposite one another. Usually observers will
want u'g'r'i' or g'r'i'z'. In these cases, we mount as below:
0 1 3 2
u' r' i' g'
g' i' z' r'
u' Rs Is g'
Where the 0123 corresponds to the positions in the filter wheel. Just try
not to mount u' and g' in adjacent filter slots opposite i' and z'. This is
asking for trouble.
To change a filter,
Open the filter port on the side of the shutter/filter wheel box. The port
is located on the same side of the cage as the temperature controller,
and can be accessed by loosening its 4 captive screws and pulling off the
DO NOT PUSH THE FILTER WHEEL BY HAND to move it to the desired
If the LFC is powered up, ask someone in the control room use
the software to move it for you. If you must move the wheel
from the PF cage, power off the filter wheel motor. Then, turn
the gray plastic knob on top of the filter wheel motor (in the
indicated direction ONLY) to move the wheel into it's proper
location. You should be able to feel the mechanical detent
close to the appropriate position of the wheel.
After powering the filter wheel back on, you should initiate a warm
*** start-up sequence as documented in Mark Metzger's LFC HTML
The filter slots are labeled 0-4 on the wheel, which you can see from the
port. once the correct slot is in view, remove the filter there.
This is done by loosening the captive screw you see on the slot.
Once the screw is loose, reach into the slot and slide the filter
out. It can be tricky to get the filter started on it's slide.
Tricks I've found helpful are to pull gently on the captive screw to
pull until you can grab onto the filter, or to reach your fingers
around under the large thin ring and try to poke the filter out. If
you haven't done this before, it's probably best to find someone who
is more experienced to teach you the ropes in person. The "feel" is
Once you get the filter started, it should slide out easily. Then slide
the new filter into the slot in reverse fashion, and attach the
After installing filters, you will want to issue a filter home
command (in mcdcom) to properly re-align the wheel in the beam.
You will also want to modify the filter names by editing the file
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