I have a related issue. I have a brand new STXL-6303e and am unable to cool it below -35C (delta around 40C). Even if I work my way down to -35, then continue in small steps, it stays at -35 at 40-50% power. I'm using the SBIG cabling and a 15 amp extension lead. No water-cooling.
You might want to start a separate thread. Also, that camera's cooling delta is 60C, so if it's cooling to -35C, and ambient is 25C, it's on spec. You didnt mention ambient temperature, and didnt mention if you are using the supplied power supply.
I just split this from the other thread, because it is not related. The STXL cameras limit the cooler to -35C maximum absolute temperature, for a variety of reasons. If you too much lower than this you may start to get frost despite the desiccant working perfectly. At -35C the dark current of a 6303 is below 0.01 e-/pixel/second. At that rate a 10 minute exposure will accumulate about 5 electrons dark current, and the noise component of that will be less than 3. That's still way below the read noise.
