[vsnet-chat 2094] faster read-out is really better ?

[Ulisse Munari <munari@astras.pd.astro.it>]

my experience with professional CCD is that the faster you read
the CCD, the noiser the data you collect. To preserve the excellent
cosmetic of the 1024x1024 CCD we have mounted on our Echelle 
spectrograph, we read it out in 3 minutes and 22 sec (plus storing time
which is however light-speed on workstation hard-disks), and in this
way the intrinsic 0.5% accurate cosmetic is fully preserved.
Reading it faster, you appreciably increase the noise in the
data and therefore you decrese the accuracy of the data you collected.

It is a metter of balace: more data of poorer quality, or less data
but intrinsically more accurate ?

It depends on what are you looking at: flickering in the hot spot of
accretion disks (which dimensions are less than 1 light second) develop
on time scale of a few seconds, so time resolution is vital and you have
just to hope the the flickering amplitude is larger than the noise in your
data, otherwise you are either lost. On the other hand, following the
eclipse of a binary require the outmost accuracy in the data but rarely
a time resolution better than a few minutes.

There is one additional aspect to consider, among many others:
if you leave the telescope unattended, I suspect the field of view 
drifts across the CCD, with the variable and the comparison stars
migrating over regions of higher-and-lower response of the chip
(them being caused by intrinsic pixel-to-pixel variation in CCD 
response, by vignetting effects and by randomly distributed
shadows by the dust grains sticked to the CCD's window or on the
filters). Unless you have FULL CONTROLL over the flat-fielding
of your CCD, the expected errors may be QUITE LARGE.
If you cannot properly and timely flat-field your data, one possible
way out is to guide so accurately to maintain your stellar field
FIXED on the same pixels, and therefore the variations you are going to
see in your photometry will possibly look more intrinsic than spurious


  Ulisse Munari

  Osservatorio Astronomico di Padova  tel  (+39) 0424 600033
  Sede di Asiago                      fax  (+39) 0424 600023
  via dell'Osservatorio 8             munari@astras.pd.astro.it
  I-36012 Asiago (VI), Italy          http://ulisse.pd.astro.it/

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