FIRPO Quick Guide

Last modified on November 25, 1996 by A. Richichi


List of people to contact for emergency help.

Alignment

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:
  1. remove the wide white panel from the side of the adapter, so to access the chopping mirror.
  2. using the finder, point a very bright star (1st mag).
  3. 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.
  4. 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.

Cryogenics

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.

Electrical Connections

ALWAYS REMEMBER TO SWITCH THE PREAMPLIFIER AS THE LAST THING (ON) OR THE FIRST (OFF).
NEVER LEAVE THE TEST CONNECTIONS CONNECTED DURING OBSERVATIONS.
WHEN TOUCHING THE CABLES AND THE DEWAR, TAKE CARE TO DISCHARGE YOURSELF.
Take a look at the scheme of the connections between FIRPO and the various components in a typical set-up. This is a view of the pre-amp circuit. 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. Typical zero-points 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)

  1. J
  2. H
  3. K (wide)
  4. L
  5. M
  6. open
  7. K (narrow)
  8. (new J?)
  9. narrow 3.1um
  12. closed

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. Check the message and 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, gsview, netscape, and many astronomical programs (check under menu Avvio, Programs, Astronomy). Please use when possible c:\luna for work, or create your own directory.

Back to the following homepages: Lunar Occultations, FIRPO. Comments to luna@arcetri.astro.it