[vsnet-chat 2111] Re: faster read-out is really better ?

[aah@nofs.navy.mil]

Ulisse Munari gave a more complete description of their
spectrograph CCD readout scheme.  If I understand it
properly, then I understand why they use such a long
read time.  Like I said earlier, as you shorten the
read time, you increase the readnoise.  However, as
you lengthen the read time, you will eventually come
to the point where you start losing correlation between
the two correlated-double-sample reads, and readnoise
again increases (or at least reaches an asymptotic
limit).
  I think the main reason for Munari's long read time
is contained in the statement:
>...to achieve the
>100 m/sec EXTERNAL precision in the radial velocities of suitable
>cool stellar targets, charge transfer wake effects must be taken
>under strict control (particoularly with the strong lines in the
>comparison spectrum). I have seen clear degradation in the
>radial velocity accuracy with faster read-outs.
  That is, the longer read time permits proper waveform shaping
and control of CTE effects; this is not the same as getting
minimum read noise.
  I fully agree and support his other comments regarding filtered
observations rather than unfiltered ones!
Arne

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp