[vsnet-chat 2103] Re: faster read-out is really better ?

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

On Fri, 9 Jul 1999 aah@nofs.navy.mil wrote:

> ...(omissis) ...
> However, I'm surprised that you take >3min for reading a 1024x1024.
> That would imply a 5.1Kpix/sec read rate.  I would be more
> concerned about the increased liklihood of cosmic ray hits
> and the differential dark current during readout, in addition
> to the loss of efficiency.  Have you checked your readnoise
> curve for your detector to see where the optimal read rate resides?

Hi Arne !

this has been carefully evaluated by local engeneers (who built the
camera and the associated electronics):
-) dark current is not a concern because in our case the rate is
   6 counts/pixels/hour (or 15 electrons/pixels/hour)
   with a very uniform pattern over the CCD (we cool with
   liquid N_2). The direction of reading over the CCD is along the
   spectral dispersion, so sky-subtraction go toward 
   alleviation of the problem 
-) the cosmic hit rates is equally not a concern, particoularly
   after the substitution of the coating over one of the surfaces
   of the flattening lenses

We use faster read-out only for test focusing images and thorium
comparison lamp (when the outmost accuracy in the wavelength 
scale is not looked for).

A source of some concern (apart from the stright-light typical of echelle
designs) could the moon light when it illuminate directly the spectrograph 
and the attached CCD camera. Years ago I found this contributing
several counts in a VERY DISUNIFORM pattern over the spectrograph
focal plane. Some replacements of the light-sealing glues
(guided by the trial-and-error approach) fixed the problem.

I have no firm number to offer here, but 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.

On the other hand (on professional CCDs only, of course), 
one could preserve the slow read-out and go for fast down-load
using multiple reading electronics, as should be already implemented
in the giant CCD chip of the latest generation (those rushing
toward the 10,000x10,000 terrific dimensions), where 4096x4096
frames are read in a matter of a couple of minutes by 
a battery of identical electronic ADCs

>   Note that the above discussions are rarely important for most
> amateur work, especially if 0.01-0.02mag precision is the goal.
> Just look at the differential results between two well-exposed
> comparison stars...that will tell you how well you are currently
> doing photometry.

Over recent times, some amateurs passed over to me some of their
CCD direct imaging frames for comments. When asked to secure 
appropriately exposed flat fields, it turned out that in one case
one such camera had a large deposit of dust grains sticked to the
side of the camera window closest to the CCD (dust grains not 
visually spotted by the amateurs that were reporting to me
"very clean surface of the CCD window". They were possibly looking
only to the external side of the window).
The typical shadow of one such dust grain (similar in size to twice the
seeing disk) caused the response to  fall down to about 90% of adjacent
unaffected pixels, or a loose of 0.1 mag.
I wonder how much the photometric centers of CCD images of asteroids
could be affected by edge-proximity with shadows of dust grains,
and how the already interesting astrometry performed by the
amateurs could be made even better by flat-fielding (rarely done
by amateur astrometrists, at least for what I perceive is their
common practice).

Another practice the amateurs could seriously consider is to 
perform their CCD observations with filters. 

Take the example of the amateur astrometrists working on asteroid
discovery and orbit determination.
Without filter-photometry and photometric calibration, their
discovered asteroids will remain just a line of orbital elements.
In case they could have derived the "color" of the asteroid,
this would immediately translate into a possible surface composition,
which means an albedo and therefore dimensions. If you assume
the interior of the asteroid has the same composition of its surface,
you will infer the density of the material in the asteroid's body. 
An you will end up with a guess of the mass, dimensions 
and composition of the asteroid !
What a marvelous improvment compared with just a line of 
orbital elements related to an uncharacterized piece of rock 
orbiting somewhere out there !

One could argue that filter photometry will excessively 
sacrifice the limiting magnitude (and decrease the chance 
to label one such piece of rock with his/her name !). 
If indeed quite true in the blue, I suspect that with current
red-sensitive amateur CCDs the effect should be tollerable
in V and go just barely noticed in the R and I bands.

Note that V,R,I band photometry is already sufficient to distinguish
among at least several possible surface compositions of asteroids:
looking at the figure at page 236 in the first edition of The New Solar
System (or the same figure at page 344 of the latest edition),
where the reflectance spectra of several asteroids are compared,
it is easy to get convinced how V,R,I photometry could easily
distinguish the surface composition of Vesta (which shows a deep
and wide absorption band of Eucrite compounds at 9000 Ang)
from that of Cerere (flat over the wavelength range of interest).
(central wavelengths for V, R and I bands are respectively 
about 5300, 6100 and 8500 Ang)

Ops! Sorry, I see I came too long ! I stop here.

Ulisse

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