FIRPO Quick Guide
Last modified on November 25, 1996 by
List of people to contact for
Before mounting FIRPO it is necessary to check the chopper throw. To do this
mount in FIRPO's place the special eye-piece equipment found in ChL's
cabinet (blue with a long steel handle on a side). The eyepiece
has a scale labelled in mm.
After having connected the programmed wave generator to the input of the
chopper electronic, one can measure the chopper throw amplitude,
remembering that for the 1.2 m. telescope the scale is 21 "/mm and
that for the 2.2 m. the scale is 9.4 "/mm.
It is possible to make these measurements using a diaphragm and a battery
light. The diaphragm is one of the small barrels (choose the one with
the smallest hole for this purpose), and must be inserted, with the
wide opening on top, into a special place close to the telescope
focus (push well). To access, open the small lid opposite to
the adapter's eye-piece.
The actual chopping direction is NS (complicated to change).
To change the modulation amplitude it is
necessary to act on the wave generator. Typical amplitudes used are
of order 0.5~V.
When FIRPO is in place (remember to remove the window cover!), you
can act on the micrometer screws attached to its mount, till you
maximize the signal. A good start is to have it as well centered
by eye as possible: no systematic deviations have been experienced.
The eye-piece adjustment comes next. In the last run
the values for the eye-piece offsets were:
15.27 (hor.) 15.20 (ver.) 2/6/96
If for some reason FIRPO is badly misaligned at the start and no IR
signal can be found, I recommend to:
remove the wide white panel from the side of the adapter, so
to access the chopping mirror.
using the finder, point a very bright star (1st mag).
if it's dark, put your head in the opening of 1). You should see the
spot of the star reflected on the side were FIRPO is mounted.
Use the paddle to bring the stellar image as close as possible to
the center of FIRPO's window.
without using the paddle, move only FIRPO (by its micrometer
screws) till you see some IR signal. Maximize it, then use
the paddle to maximize it again.
Wave-generator (HP 33120A)
There is an accompanying manual, if you wish to program it.
However, the FIRPO waveform has been stored and should be available.
To recall the stored waveform, after switch on press
recall, then select 1.
Alternatively: shift-Arb list and then Arb 1
Once this waveform is selected, you can change separately
Freq and Ampl. Don't forget to measure what is the offset
required to have the source at the center, with and without chopping.
It is typically of order 0.05-0.06 V.
When doing LO, DON'T FORGET to set the wave to DC!!!!
To obtain this, keep one of the keys in the upper row
(for instance, the square wave key)
pressed for more than two seconds.
When you arrive on CA, make sure the dewar has a good vacuum
(<=10-6). Start refilling both the inner and outer tanks with
LN2. The outer tank: typically, it needs
a first refill after 4-6 hours, and after that every 12 hours.
The inner tank should not need a serious refill. Only just before
you start pumping (typically the day after), make sure it's topped off.
FIRPO should be mounted with both tanks full of liquid nitrogen. Then
connect the vacuum line to the inner tank and open slowly the
micrometric valve. There should be an automatic opening system.In case
this does not work open the micrometer so to pump about 10 l/m.
(note: there is a leak in the vacuum pipes at the 1.2m and 2.2m,
so this value doesn't really mean much).
Readjust this value, first every 10-20min, then more slowly.
If the bypass is closed (recommended) at the start, open it
completely after the pressure is <50mb. Because of the
the leaks, there is little hope to go below this value anyway.
Things to check: ice on the dewar, on its window, and on the
rubber vacuum line.
Typically, the dewar should hold for 5+ days. If you need to refill,
account for at least 24 hours. First switch off the pump. Then let
air come in very slowly (better would be N2 vapour... if you can).
When the pressure is equalized with the atmosphere, plug off the
rubber vacuum line from the inner chamber. If you hear a big sucking
sound at this point, you may be in trouble. Check quickly that
no ice is being formed through the neck (use a long thin stick).
If you want to refill for another go, you can do so, and then
start pumping as before. Otherwise, if it's the end of the run,
keep the dewar upside down so as to let as much LN2 out as possible:
it will warm up more quickly.
ALWAYS REMEMBER TO SWITCH THE PREAMPLIFIER AS THE LAST THING (ON) OR
THE FIRST (OFF).
NEVER LEAVE THE TEST CONNECTIONS CONNECTED DURING
WHEN TOUCHING THE CABLES AND THE DEWAR, TAKE CARE TO DISCHARGE
Take a look at the
of the connections between FIRPO and the
various components in a typical set-up.
This is a view of the pre-amp
More pictures are available if you go
up by one page.
At the beginning of the run, one should check that the bias
and the JFET are working correctly. For this, connect the
G2-test output to an oscilloscope, and
on the right side of the preamp. there is a special connector which
should be plugged into a multimeter (voltage, range 0-20V).
At switch-on, quickly check that the bias level is NEGATIVE
(typically -40 to -80 mV). On the multimeter,
the voltage on pins 1-2 should increase exponentially toward
positive values of about 0.5 V. If the voltage becomes negative,
or the bias is positive, switch immediately off: you may have a problem!
Otherwise, switch the pre-amp off, remove the cables, and
connect the regular analog-output to DC out. You can check that
the detector is alive by moving filters, or observing microphonics
when you hit the dewar gently with your fingers or some small object.
measured at the 2.2m are of order 8V for K=0.
Also check the batteries (should have 18V or more across the bridges).
Check the waveform on the scope, when you are chopping on a
star with magnitude comparable to what you want. If the waveform
does not have square angles, you can act on the small grey trimmer
in the lower right part of the pre-amp box (open it if it's closed)
till the wave is squared. Typically, it is necessary to make
changes when going from K>3-4 to stars with K~1. Also when you
have a lot of background (long wavelengths).
Filters (as of March 96)
LUNOC + other software
The most recent version of lunoc is from September 95.
It should not
have bugs, but in case you can use the older version which is
also OK. There is now a version of reformat (from raw data
to ASCII) which runs under Unix.
All this stuff is available from /home/selene1/luna/lunoc
for users with an account in Arcetri. Otherwise, contact A.R.
news lists for updates on the situation.
If you use the TM-5000 Notebook:
The Linux side is still under development (boot with FD).
The DOS side (regular boot) starts with Windows 95. There
you can find telnet, ftp (recommended ws_ftp95), gnuplot, latex,
and many astronomical programs (check under menu Avvio, Programs,
Astronomy). Please use when possible c:\luna for work,
or create your own directory.