NIRIM will hold a good fill for 24hrs, and since it takes a long time to fill (over half an hour, even when filling regularly), it is recommended that it be filled in the afternoon before flats or dinner. As long as it is not tipped over, you should be ok. You need to fill both the inner and the outer chambers (there are two fill tubes on the dewar).
If the dewar goes dry, and the optics warm up even slightly, it can take many hours to cool down to operating temperatures, not to mention to fill. Be very careful about fills and turning the dewar upside-down. Also, do NOT touch the dewar vacuum valve (see Figure 3). If that is loosened, it will cause the dewar to lose vacuum and warm up.
Also, the smaller fill tube is prone to
ice-plugs. It is recommended that you
keep a loose "plug" in the exhaust at all times.
Finallly, NIRIM is a LN2 HOG!! You'll go through a lot of LN2.
Figure 3. Snapshot of the NIRIM dewar mounted on WIYN. Note that the dewar is mounted with the fill tubes pointed UP towards the sky.