CFHT operating manual - Homepage Usask

ESPaDOnS: ccd readout modes and characteristics http://webast.ast.obs-mip.fr/magnetisme/espadons_new/ccd.html
ESPaDOnS
CCD readout modes and characteristics
CCD characteristics
The chip used for ESPaDOnS is a 2kx4.5k 0.0135mm square pixel ccd manufactured by eev (42-90 series). The one tested up to
now is an engineering grade with a rather high number of cosmetic defects. The science grade chip that cfht allocated to
ESPaDOnS (referred to as eev1 in cfht dialect) is supposedly much better for cosmetics. Cfht unofficially agreed that this
detector would be dedicated to ESPaDOnS as much as possible, and that it will remain mounted all the time on the instrument
to optimise the instrument stability and preserve the thermal and mechanical equilibrium within the instrument as much as
possible.
In order to cover a wide enough range of astrophysical applications, we decided to implement several readout speeds. This
flexibility is usually not offered on other cfht instruments, but we thought that ESPaDOnS users could greatly benefit from it.
For the brightest objects for which photon noise will dominate, achieving the smallest possible readout noise is not crucial; short
readout times are much more important, either to improve the overall duty cycle of the observing session (eg when short
exposures are required to avoid saturating the chip or to ensure a high frequency temporal monitoring). For the faintest objects
that are usually exposed for longer time chunks, having short readout times is much less critical; decreasing readout noise as
much as possible is in this case very important as it impacts very heavily on the final quality of the collected data.
To optimise observing time as much as possible, we also requested that ccd readout could be done as a background task while
setting up the instrument (eg changing the polarimeter configuration) for the next exposure. While this possibility exists already
in the cfht detector control software system (called detcom in cfht dialect), and is being used for other cfht instruments
(megaprime), it still appears as very unreliable, producing major system failures at random times and subsequent losses of
collected data.
Readout modes
Four readout modes were selected to cover all potential needs
of future users. The fastest reads out the full chip in 25s with a
readout noise of 7.5e, while the slowest achieves the lowest
possible noise of 2.5e, reading out the whole chip in 90s. For
each of these readout modes, we determined the noise by
measuring the rms deviation in various 100x100pxl portions of
the chip in a bias frame. The gain was measured by ratioing
slightly out-of-focus flat field images taken in identical
conditions and by computing the slope of the inverse variance
(ie the squared signal to noise ratio) as a function of adu
counts. The graph on the right shows one of such fits in the
particular case of the ’slow’ readout speed (points representing
measurements troughout the image while the full line depicts
the linear fit to the points). In all cases, good linearity was
observed up to the saturation level.
The following table summarises the measured characteristics of
each readout speed. Note however that these values are likely
to change slightly when the science grade eev chip is mounted
into the dewar.
speed gain (e/adu) noise (e) time (s) saturation (adu)
fast 1.85 7.4 25 58,000
normal 1.40 4.2 40 >65,535
1 of 2 08/07/04 11:31 PM