Background

The DEIMOS CCD dewar is cooled by liquid nitrogen (LN) cryogen in order to maintain the detectors at their ideal operating temperateres of -115 C. This webpage describes adopted practices in achieving accepable dewar vacuum, timescale for cooling the dewar after a warm fill and for warming the dewar when cryogen is exhausted.

Configuration

The DEIMOS CCD dewar actually consists of three distinct volumes:

the LN can, a moderately-sized volume which is largely occupied by the LN reservoir;
the connector, a small volume which connects the two larger volumes;
the detector vessel, a large volume which contains the detector mosaic.

Please note that the vessels are not interconnected and must be pumped down separately.

Acceptable Vacuum

Given the value of the DEIMOS detector mosaic, care must be taken to prevent contamination of the dewar environment. The system includes two ion pumps to remove volatile substances from the dewar environment. Prior to cooling the dewar from ambient dome temperatures (near 0 C) to detector operating temperatures (near -115 C), the three separate dewar vessels must be pumped until a vacuum of 1×10^-7 Torr. The smaller volumes can be effectively evacuated within 24 hours using a good vaccum pump, but the main detector vessel will typically require several days to a week of pumping to achieve the target value.

Note that to 2 meters of vacuum pipe are required to connect the vacuum pumps to the DEIMOS dewars

Cooldown Curve

The following plot shows the behavior of the dewar temperature following a warm fill. The dewar can typically achieve the standard operarting temperature within about three hours after the initial cryo fill.

Click to enlarge

Warmup Curve

The DEIMOS dewar has a hold time of about 44 hours. In the event that lack of access to the summit forces an LN fill to be skipped, it may be possible to extend the hold time slightly by resetting the temperature setpoint of the detector system below the nominal value of -115 C via

	modify -s deiccd tempset=-140

Doing so will effectively disable the dewar heater and may reduce the rate of LN consumption slightly.

Click to enlarge

If the dewar warms above -100 C, then it is possible that volatiles will begin to escape from the ion pump and will contaminate the dewar. In these circumstances, it is best to allow the dewar to warm to ambient temperature and pump the dewar.