For Instrument Scientists and Engineers ONLY

The bias on the SPEC detector has been carefully optimized to give the best compromise between dark current and well depth. At the default setting, it yields a well-depth (within the 1% linearity limit) of 18,000 DN or 90,000 electrons, and a "true" dark current (assuming no recent exposure to light) of about 0.1 electrons/sec. In addition, this very low dark current level is stable and fairly uniform over the array. This default setting is highly recommended for almost all observing programs!

If the observing program absolutely requires greater pixel well depth and can afford significantly worse dark current, it is possible to change the SPEC detector bias to increase the well depth. The maximum safe limit is to double the detector bias, which doubles the well depth (to about 180,000 electrons). It also increases the dark current to an average of about 1.5 electrons/sec (a fifteen-fold increase!), and the dark current appearance on the chip becomes "blotchy" and unstable (the blotches move around with time).

Warning: Be Aware that the SPEC detector does not return to normal performance immediately after restoring the default bias setting. If the bias is raised and then lowered back to normal, the SPEC detector will need at least 2 or 3 hours to stabilize at nominal dark current levels. The time required for this stabilization is not well known, because we have very little experience with observing under different biases.

Here are the commands to change the bias setting. As usual, they can be issued from the nirspec account in any window with a waimea prompt.

• setbias lowdark   to restore the default bias setting.
• setbias deepwell   to double the bias, and double the well depth.
• setbias   without argument, displays the current bias setting.

Here is a table summarizing the properties of the two settings.

Technical note: for setting the DETBIAS keyword by hand, the value must be specified in digitization numbers. The scaling relation is 0.62 mv/DN.