When the Helium-3 in SPARO has been cycled and the detectors are
near 300 mK in temperature, several tests can be done to make sure
that the system is alive and the detectors are active. The bias
should be turned on to turn the detectors on. The bias switches are
located on the preamps.
One possible test is to look for "hand signals". This is
difficult when SPARO is mounted on Viper.
Another quick and easy way to check out the detectors is
by gently tapping the dewar to see if microphonics can be seen.
Microphonic noise can only be seen if the detectors are operating.
Looking at detector noise is another way to see that the system is
operating. The detector noise is larger than the JFET noise and has
approximately a value of (1/8) inch of hash for most pixels when the
system is configured as follows:
Note that the exact value of the noise will depend on the "responsivity"
(volts per watt) of the detectors, which gets bigger for lower temperatures.
The temperature varies with time and with "loading" (ie looking at sky
vs. blankoff wheel). Also, the sky will make a contribution to the
noise, due to quantization of sky background ("photon noise"), and also
to variations in the background level due to moving clouds of water
vapour ("sky noise"). But for decent observing conditions, the value
(1/8)-inch will be approximately correct.
- preamp gain = 10,000,
- global attenuation of the data program = 1000
- strip chart attenuation = 8
- chops/frame = 2
For details of the various kinds of intermittent noise that affect the
various pixels, see the (slightly dated) document
qualitative noise characteristics...
Contact email@example.com. Last updated Feb 11, 2000.