Closed Cycle coolers?

[Maki Sekiguchi]

Dear Paul Jorden,

 We have a bit of experinces in using a small Stering cycle cooler (SCC)
for our mosaic camera. (We did not get enoguh experince. We abandoned SCC 
eventally becase we needed more cooling power than SCC could provide.) The 
followings are my observation of SCC:

(1) They work fine. (This is important.)

(2) The cooling head must be tied to a certain mass to minimize the
positional vibration, OR, CCD's must be mecanically detouched from the
head. In our case, the head was mechanically mountaned on the dewar wall
and did not cause any problems.

(3) It generates heat. This especially true for a compact SCC near a 
dewar. The oppsoite side of the head and a compressor become hot. These 
parts should be air-cooled in order to get good efficiency. (But it also 
depends on the heat-load in a dewar.) Often it requires a separate 
power-supply unit that generate more heat.

(4) It has a life time as Fred Harris alreay mentioned. The typical life 
time (Mean Time befre Failure) of a compact SCC ranges from 2000 hours to 
5000 hours. (Note that 1 year (day+night) is 8760 hours.) I was told that 
some expensive? SCCs for space application have a life time over 20000. 
They have life-time because they have moving parts that wear out. (I was 
told these worn-out parts generate contamination in the system and the 
performance will degrade eventally.)

(5) Using LN2 is probably better in terms of keeping vaccum clean. We 
could not get enough experince to tell.

There are many applications that use SCC's these days. Most of comertial 
IR cameras use SCC's as well as in each missile head which uses IR 
sensor. Surprisingly (or reasonablly?) ground-based astronomer do not use 
SCC's very often.

As you know, Subaru, 8.3m Japanese telescope, is under construction at 
Mauna Kea. NAO of Japan already set a policy that all the cameras at 
Subaru MUST use some kind of coolers in stead of LN2 unless special reason 
for LN2. NAO believe that this would minimize the dewar mainteanace. All 
visible camera will use one or two SCC's. They will be replaced or 
maintained before the expected life-time. 

	I hope this information helps.
	Reagrds,

      Maki Sekiguchi

          maki at indy.mtk.nao.ac.jp
          National Astronomical Observatory
          Dep. of Optial & IR