Which camera do you have? Some sensors benefit from deep cooling eg the Kodak KAF CCDs. Others, like the Sony CCD, less so. The ATIK cameras I have used only cool ~25C below ambient so I run at -10C all year round which gives negligible thermal noise and saves me worrying about the risk of non matching darks/bias. Incidentally if you do cool to very low temps, watch out for ghost residual images in following exposures. This can happen for example if you take a faint spectrum image after an overexposed calibration lamp image. I have never seen it myself at -10C but it was a problem than plagued a pro-am campaign I was involved in where they were using KAF CCD cooled to a low temp.