As has been noted for some time,
molecular contaminants have been slowly building up on a filter that shields
the CCD chips in the Chandra X-ray Observatory’s Advanced CCD Imaging
Spectrometer (ACIS) from optical and ultraviolet light. The effect of this
buildup has been to gradually decrease the efficiency of the detector at low
X-ray energies to roughly 60% of their value at launch in July of 1999. Observations
at medium and high X-ray energies are affected by less than 10%.

Observations of objects that radiate primarily low energy X-rays, such as
the coronas of stars and some supernova remnants, will take longer to reach
the same sensitivity as an uncontaminated detector. The great majority of Chandra
observations rely on medium and high-energy X-rays for which the contamination
effects are minimal.

The contaminants are thought to be hydrocarbon substances whose source could
be a Chandra spacecraft lubricant. Based on detailed analyses and extensive
experience with other NASA missions, the Chandra team anticipated that molecular
contaminants would slowly be released from material in the spacecraft when
it was in orbit. The observed level of contamination is consistent with the
high end of the expected range. To deal with this possibility, a heater system
was incorporated into the design of ACIS to allow the contaminants to be baked-off
as needed while in orbit, as is done routinely on other missions. The Chandra
team is currently performing detailed analysis in preparation for a possible
bakeout planned for April or May 2004.

Since the build-up of additional contaminants is rather slow, it is important
to analyze the nature of the contaminant to the extent possible, to optimize
the bakeout process and to make sure there were no additional risks to be encountered
during the bakeout. Even though analysis and previous experience indicate very
little risk in this regard, a very conservative approach has been adopted,
and various laboratory tests using spare components are underway.

Chandra’s other detector, the High Resolution Camera, and the mirror
assembly are maintained at relatively warm temperatures (10 degrees Celsius,
and 20 degrees Celsius, respectively) so they have experienced very little
build-up of contaminants. By comparison, ACIS’s CCDs operate -120 Celsius,
while its filters are maintained between -60 and
-50 degrees Celsius.