Checking Detectors

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.

For details of the various kinds of intermittent noise that affect the various pixels, see the (slightly dated) document qualitative noise characteristics...

Contact Last updated Feb 11, 2000.